17.3B: Wastewater and Sewage Treatment - Biology

Wastewater is treated in 3 phases: primary (solid removal), secondary (bacterial decomposition), and tertiary (extra filtration).

Learning Objectives

  • List the steps of wastewater/sewage treatment

Key Points

  • Primary treatment is the first phase of sewage treatment: wastewater is placed in a holding tank and solids settle to the bottom where they are collected and lighter substances like fats and oils are scraped off the top.
  • Secondary treatment is where waste is broken down by aerobic bacteria incorporated into the wastewater treatment system.
  • Tertiary treatment is designed to filter out nutrients and waste particles that might damage sensitive ecosystems; wastewater is passed through additional filtering lagoons or tanks to remove extra nutrients.

Key Terms

  • Effluent: Sewage water that has been partially treated and is released into a natural body of water; a flow of any liquid waste.
  • zooplankton: Small protozoa, crustaceans (such as krill), and the eggs and larvae from larger animals.
  • aerobic: Living or occurring only in the presence of oxygen.

Sewage is generated by residential and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world. Greywater is water generated from domestic activities such as laundry, dishwashing, and bathing, and can be reused more readily. Blackwater comes from toilets and contains human waste.

Sewage treatment is done in three stages: primary, secondary and tertiary treatment.

Primary Treatment

In primary treatment, sewage is stored in a basin where solids (sludge) can settle to the bottom and oil and lighter substances can rise to the top. These layers are then removed and then the remaining liquid can be sent to secondary treatment. Sewage sludge is treated in a separate process called sludge digestion.

Secondary Treatment

Secondary treatment removes dissolved and suspended biological matter, often using microorganisms in a controlled environment. Most secondary treatment systems use aerobic bacteria, which consume the organic components of the sewage (sugar, fat, and so on). Some systems use fixed film systems, where the bacteria grow on filters, and the water passes through them. Suspended growth systems use “activated” sludge, where decomposing bacteria are mixed directly into the sewage. Because oxygen is critical to bacterial growth, the sewage is often mixed with air to facilitate decomposition.

Tertiary Treatment

Tertiary treatment (sometimes called “effluent polishing”) is used to further clean water when it is being discharged into a sensitive ecosystem. Several methods can be used to further disinfect sewage beyond primary and secondary treatment. Sand filtration, where water is passed through a sand filter, can be used to remove particulate matter. Wastewater may still have high levels of nutrients such as nitrogen and phosphorus. These can disrupt the nutrient balance of aquatic ecosystems and cause algae blooms and excessive weed growth. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulate organisms that store phosphate in their tissue. When the biomass accumulated in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Nitrogen can also be removed using nitrifying bacteria. Lagooning is another method for removing nutrients and waste from sewage. Water is stored in a lagoon and native plants, bacteria, algae, and small zooplankton filter nutrients and small particles from the water.

Sludge Digestion

Sewage sludge scraped off the bottom of the settling tank during primary treatment is treated separately from wastewater. Sludge can be disposed of in several ways. First, it can be digested using bacteria; bacterial digestion can sometimes produce methane biogas, which can be used to generate electricity. Sludge can also be incinerated, or condensed, heated to disinfect it, and reused as fertilizer.

Sewage treatment

Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage. Sewage contains wastewater from households and businesses and possibly pre-treated industrial wastewater. Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment. A by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge. The sludge has to undergo further treatment before being suitable for disposal or application to land. The term "sewage treatment plant" is often used interchangeably with the term "wastewater treatment plant". [2]

Sewage treatment
SynonymWastewater treatment plant (WWTP), water reclamation plant
Sewage treatment plant in Massachusetts, US
Position in sanitation chainTreatment
Application levelCity, neighborhood [1]
Management levelPublic
InputsBlackwater (waste), sewage [1]
OutputsSewage sludge, effluent [1]
TypesList of wastewater treatment technologies (not all are used for sewage)
Environmental concernsWater pollution , Environmental health, Public health, sewage sludge disposal issues

For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant that has usually received pre-treatment at the factories to reduce the pollutant load. If the sewer system is a combined sewer, then it will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage is conveyed in sewerage which comprises the drains, pipework and pumps to convey the sewage to the treatment works inlet. The treatment of municipal wastewater is part of the field of sanitation. Sanitation also includes the management of human waste and solid waste as well as stormwater (drainage) management. [3]

At the global level, an estimated 52% of municipal wastewater is treated. [4] However, wastewater treatment rates are highly unequal for different countries around the world. For example, while high-income countries treat approximately 74% of their municipal wastewater, developing countries treat an average of just 4.2%. [4] Wastewater that is discharged untreated into the environment can cause water pollution. [5]

In developing countries and in rural areas with low population densities, sewage is often treated by various on-site sanitation systems and not conveyed in sewers. These systems include septic tanks connected to drain fields, on-site sewage systems (OSS), vermifilter systems and many more. A typical sewage treatment plant in a high-income country may include primary treatment to remove solid material, secondary treatment to digest dissolved and suspended organic material as well as the nutrients nitrogen and phosphorus, and – sometimes but not always – disinfection to kill pathogenic bacteria. Sewage can also be treated by processes using "Nature-based solutions".

Wastewater Treatment Water Use

Wastewater is used water. It includes substances such as human waste, food scraps, oils, soaps and chemicals. In homes, this includes water from sinks, showers, bathtubs, toilets, washing machines and dishwashers. Businesses and industries also contribute their share of used water that must be cleaned.

What is wastewater, and why treat it?

The Central Wastewater Treatment Plant, Nashville, Tennessee.

We consider wastewater treatment as a water use because it is so interconnected with the other uses of water. Much of the water used by homes, industries, and businesses must be treated before it is released back to the environment.

If the term "wastewater treatment" is confusing to you, you might think of it as "sewage treatment." Nature has an amazing ability to cope with small amounts of water wastes and pollution, but it would be overwhelmed if we didn't treat the billions of gallons of wastewater and sewage produced every day before releasing it back to the environment. Treatment plants reduce pollutants in wastewater to a level nature can handle.

Wastewater also includes storm runoff. Although some people assume that the rain that runs down the street during a storm is fairly clean, it isn't. Harmful substances that wash off roads, parking lots, and rooftops can harm our rivers and lakes.

Why Treat Wastewater?

It's a matter of caring for our environment and for our own health. There are a lot of good reasons why keeping our water clean is an important priority:

FISHERIES: Clean water is critical to plants and animals that live in water. This is important to the fishing industry, sport fishing enthusiasts, and future generations.

WILDLIFE HABITATS: Our rivers and ocean waters teem with life that depends on shoreline, beaches and marshes. They are critical habitats for hundreds of species of fish and other aquatic life. Migratory water birds use the areas for resting and feeding.

RECREATION AND QUALITY OF LIFE: Water is a great playground for us all. The scenic and recreational values of our waters are reasons many people choose to live where they do. Visitors are drawn to water activities such as swimming, fishing, boating and picnicking.

HEALTH CONCERNS: If it is not properly cleaned, water can carry disease. Since we live, work and play so close to water, harmful bacteria have to be removed to make water safe.

Effects of wastewater pollutants

Epic September 2009 flooding around Atlanta, Georgia. An overflowing sewer on Riverside Road, Roswell, Georgia. Likely this is a storm sewer, designed to carry stormwater runoff off of streets, that cannot handle the volume of runoff.
In older sections of Atlanta there are combined sewer systems that are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. These overflows, called combined sewer overflows ( CSOs ) contain not only stormwater but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems (EPA). The City of Atlanta is spending about $3 billion dollars to put in separate storm and waste systems in the metro Atlanta area.

Credit: Alan Cressler , USGS

If wastewater is not properly treated, then the environment and human health can be negatively impacted. These impacts can include harm to fish and wildlife populations, oxygen depletion, beach closures and other restrictions on recreational water use, restrictions on fish and shellfish harvesting and contamination of drinking water. Environment Canada provides some examples of pollutants that can be found in wastewater and the potentially harmful effects these substances can have on ecosystems and human health:

  • Decaying organic matter and debris can use up the dissolved oxygen in a lake so fish and other aquatic biota cannot survive
  • Excessive nutrients, such as phosphorus and nitrogen (including ammonia), can cause eutrophication, or over-fertilization of receiving waters, which can be toxic to aquatic organisms, promote excessive plant growth, reduce available oxygen, harm spawning grounds, alter habitat and lead to a decline in certain species
  • Chlorine compounds and inorganic chloramines can be toxic to aquatic invertebrates, algae and fish
  • Bacteria, viruses and disease-causing pathogens can pollute beaches and contaminate shellfish populations, leading to restrictions on human recreation, drinking water consumption and shellfish consumption
  • Metals, such as mercury, lead, cadmium, chromium and arsenic can have acute and chronic toxic effects on species.
  • Other substances such as some pharmaceutical and personal care products, primarily entering the environment in wastewater effluents, may also pose threats to human health, aquatic life and wildlife.

Wastewater treatment

The major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, called effluent, is discharged back to the environment. As solid material decays, it uses up oxygen, which is needed by the plants and animals living in the water.

"Primary treatment" removes about 60 percent of suspended solids from wastewater. This treatment also involves aerating (stirring up) the wastewater, to put oxygen back in. Secondary treatment removes more than 90 percent of suspended solids.


The term "sewage treatment plant" (or "sewage treatment works" in some countries) is nowadays often replaced with the term wastewater treatment plant or wastewater treatment station. [2] Strictly speaking, the latter is a broader term that can also refer to industrial wastewater.

Sewage can be treated close to where the sewage is created, which may be called a "decentralized" system or even an "on-site" system (in septic tanks, biofilters or aerobic treatment systems). Alternatively, sewage can be collected and transported by a network of pipes and pump stations to a municipal treatment plant. This is called a "centralized" system (see also sewerage and pipes and infrastructure).

Sewage is generated by residential, institutional, commercial and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, and sinks draining into sewers. In many areas, sewage also includes liquid waste from industry and commerce.

Sewage contains organic matter that can cause odor and attract flies. It also has high concentrations of ammonium, nitrate, nitrogen, phosphorus, high conductivity (due to high dissolved solids), high alkalinity, with pH typically ranging between 7 and 8. Sewage contains human feces, and therefore often contains pathogens. [6] [7]

Sewerage (or sewage system) is the infrastructure that conveys sewage or surface runoff (stormwater, meltwater, rainwater) using sewers. It encompasses components such as receiving drains, manholes, pumping stations, storm overflows, and screening chambers of the combined sewer or sanitary sewer. Sewerage ends at the entry to a sewage treatment plant or at the point of discharge into the environment. It is the system of pipes, chambers, manholes, etc. that conveys the sewage or storm water.

In many cities, sewage (or municipal wastewater) is carried together with stormwater, in a combined sewer system, to a sewage treatment plant. In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to these systems, for maintenance purposes, is typically through a manhole. During high precipitation periods a sewer system may experience a combined sewer overflow event or a sanitary sewer overflow event, which forces untreated sewage to flow directly to receiving waters. This can pose a serious threat to public health and the surrounding environment.

The system of sewers is called sewerage or sewerage system in British English and sewage system in American English.

Overview Edit

Sewage treatment is the process of removing the contaminants from sewage to produce liquid and solid (sludge) suitable for discharge to the environment or for reuse. It is a form of waste management. A septic tank or other on-site wastewater treatment system such as biofilters or constructed wetlands can be used to treat sewage close to where it is created.

Sewage treatment results in sewage sludge which requires sewage sludge treatment before safe disposal or reuse. Under certain circumstances, the treated sewage sludge might be termed "biosolids" and can be used as a fertilizer.

In most countries, sewage collection and treatment is typically subject to local and national regulations and standards.

Before the 20th century, sewers usually discharged into a body of water such as a stream, river, lake, bay, or ocean. There was no treatment, so the breakdown of the human waste was left to the ecosystem. Today, the goal is that sewers route their contents to a sewage treatment plant rather than directly to a body of water. In many countries, this is the norm in many developing countries, it may be a yet-unrealized goal.

The aim of treating sewage is to produce an effluent that will do as little harm as possible when discharged to the surrounding environment, thereby preventing pollution. [8]

The main processes involve removing as much of the solid material as possible, and then using biological processes to convert the remaining soluble material into a floc that entraps any remaining fine solids and which can then be settled as a sludge, leaving a liquid substantially free of solids, and with a greatly reduced concentration of pollutants.

Sewage treatment generally involves three main stages, called primary, secondary and tertiary treatment but may also include intermediate stages and final polishing processes.

Pretreatment Edit

Pretreatment removes all materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines of primary treatment clarifiers. Objects commonly removed during pretreatment include trash, tree limbs, and other large objects.

The influent in sewage water passes through a bar screen to remove all large objects like cans, rags, sticks, plastic packets etc. carried in the sewage stream. [9] This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, while in smaller or less modern plants, a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill, or incinerated. Bar screens or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving parts of the treatment plant, and can cause substantial damage and inefficiency in the process. [10] : 9

Grit removal Edit

Grit consists of sand, gravel, cinders, and other heavy materials. Pretreatment may include a sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand and grit. Grit removal is necessary to (1) reduce formation of heavy deposits in aeration tanks, aerobic digesters, pipelines, channels, and conduits (2) reduce the frequency of digester cleaning caused by excessive accumulations of grit and (3) protect moving mechanical equipment from abrasion and accompanying abnormal wear. The removal of grit is essential for equipment with closely machined metal surfaces such as comminutors, fine screens, centrifuges, heat exchangers, and high pressure diaphragm pumps. Grit chambers come in 3 types: horizontal grit chambers, aerated grit chambers and vortex grit chambers. Vortex type grit chambers include mechanically induced vortex, hydraulically induced vortex, and multi-tray vortex separators. Given that traditionally, grit removal systems have been designed to remove clean inorganic particles that are greater than 0.210 millimetres (0.0083 in), most grit passes through the grit removal flows under normal conditions. During periods of high flow deposited grit is resuspended and the quantity of grit reaching the treatment plant increases substantially. It is, therefore important that the grit removal system not only operate efficiently during normal flow conditions but also under sustained peak flows when the greatest volume of grit reaches the plant. [2]

Flow equalization Edit

Clarifiers and mechanized secondary treatment are more efficient under uniform flow conditions. Equalization basins may be used for temporary storage of diurnal or wet-weather flow peaks. Basins provide a place to temporarily hold incoming sewage during plant maintenance and a means of diluting and distributing batch discharges of toxic or high-strength waste which might otherwise inhibit biological secondary treatment (including portable toilet waste, vehicle holding tanks, and septic tank pumpers). Flow equalization basins require variable discharge control, typically include provisions for bypass and cleaning, and may also include aerators. Cleaning may be easier if the basin is downstream of screening and grit removal. [11]

Fat and grease removal Edit

In some larger plants, fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface. Air blowers in the base of the tank may also be used to help recover the fat as a froth. Many plants, however, use primary clarifiers with mechanical surface skimmers for fat and grease removal.

Primary treatment Edit

Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. Some sewage treatment plants that are connected to a combined sewer system have a bypass arrangement after the primary treatment unit. This means that during very heavy rainfall events, the secondary and tertiary treatment systems can be bypassed to protect them from hydraulic overloading, and the mixture of sewage and storm-water only receives primary treatment. [12]

In the primary sedimentation stage, sewage flows through large tanks, commonly called "pre-settling basins", "primary sedimentation tanks" or "primary clarifiers". [13] The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. [10] : 9–11 Grease and oil from the floating material can sometimes be recovered for saponification (soap making).

Secondary treatment Edit

Secondary treatment is a treatment process for wastewater (for example for sewage but also for some types of industrial wastewaters) to achieve a certain degree of effluent quality by using a sewage treatment plant with physical phase separation to remove settleable solids and a biological process to remove dissolved and suspended organic compounds. After this kind of treatment, the wastewater may be called as secondary-treated wastewater. Secondary treatment is the portion of a sewage treatment sequence removing dissolved and colloidal compounds measured as biochemical oxygen demand (BOD). Secondary treatment is traditionally applied to the liquid portion of sewage after primary treatment has removed settleable solids and floating material. Secondary treatment is usually performed by microorganisms in a managed aerobic habitat (however, it can also be an anaerobic process). Bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) while reproducing to form cells of biological solids. Secondary treatment by biochemical oxidation of dissolved and colloidal organic compounds is widely used in sewage treatment and is applicable to some agricultural and industrial wastewaters.

Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants use aerobic biological processes as a secondary treatment step. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules) and bind much of the less soluble fractions into floc.

Tertiary treatment Edit

The purpose of tertiary treatment is to provide a final treatment stage to further improve the effluent quality before it is discharged to the receiving environment (sea, river, lake, wet lands, ground, etc.) or reused. More than one tertiary treatment process may be used at any treatment plant. If disinfection is practiced, it is always the final process. It is also called "effluent polishing".

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow discharge into a highly sensitive or fragile ecosystem such as estuaries, low-flow rivers or coral reefs. [14] Treated water is sometimes disinfected chemically or physically (for example, by lagoons and microfiltration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, greenway or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

Filtration Edit

Sand filtration removes much of the residual suspended matter. [10] : 22–23 Filtration over activated carbon, also called carbon adsorption, removes residual toxins. [10] : 19 Micro filtration or synthetic membranes are also used. After membrane filtration, the treated wastewater is nearly indistinguishable from waters of natural origin of drinking quality (without its minerals).

Lagoons or ponds Edit

Settlement and further biological improvement of wastewater may be achieved through storage in large man-made ponds or lagoons. These lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter-feeding invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.

Biological nutrient removal Edit

Biological nutrient removal (BNR) is regarded by some as a type of secondary treatment process, [2] and by others as a tertiary (or "advanced") treatment process.

Wastewater may contain high levels of the nutrients nitrogen and phosphorus. Excessive release to the environment can lead to a buildup of nutrients, called eutrophication, which can in turn encourage the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). This may cause an algal bloom, a rapid growth in the population of algae. The algae numbers are unsustainable and eventually most of them die. The decomposition of the algae by bacteria uses up so much of the oxygen in the water that most or all of the animals die, which creates more organic matter for the bacteria to decompose. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. Different treatment processes are required to remove nitrogen and phosphorus.

Nitrogen removal Edit

Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Nitrogen gas is released to the atmosphere and thus removed from the water.

Nitrification itself is a two-step aerobic process, each step facilitated by a different type of bacteria. The oxidation of ammonia (NH3) to nitrite (NO2 − ) is most often facilitated by Nitrosomonas spp. ("nitroso" referring to the formation of a nitroso functional group). Nitrite oxidation to nitrate (NO3 − ), though traditionally believed to be facilitated by Nitrobacter spp. (nitro referring the formation of a nitro functional group), is now known to be facilitated in the environment almost exclusively by Nitrospira spp.

Denitrification requires anoxic conditions to encourage the appropriate biological communities to form. It is facilitated by a wide diversity of bacteria. Sand filters, lagooning and reed beds can all be used to reduce nitrogen, but the activated sludge process (if designed well) can do the job the most easily. [10] : 17–18 Since denitrification is the reduction of nitrate to dinitrogen (molecular nitrogen) gas, an electron donor is needed. This can be, depending on the waste water, organic matter (from feces), sulfide, or an added donor like methanol. The sludge in the anoxic tanks (denitrification tanks) must be mixed well (mixture of recirculated mixed liquor, return activated sludge [RAS], and raw influent) e.g. by using submersible mixers in order to achieve the desired denitrification.

Sometimes the conversion of ammonia to nitrate alone is referred to as tertiary treatment. Nitrate can be removed from wastewater by natural processes in wetlands but also via microbial denitrification. [15]

Over time, different treatment configurations have evolved as denitrification has become more sophisticated. An initial scheme, the Ludzack–Ettinger Process, placed an anoxic treatment zone before the aeration tank and clarifier, using the return activated sludge (RAS) from the clarifier as a nitrate source. Influent wastewater (either raw or as effluent from primary clarification) serves as the electron source for the facultative bacteria to metabolize carbon, using the inorganic nitrate as a source of oxygen instead of dissolved molecular oxygen. This denitrification scheme was naturally limited to the amount of soluble nitrate present in the RAS. Nitrate reduction was limited because RAS rate is limited by the performance of the clarifier.

The "Modified Ludzak–Ettinger Process" (MLE) is an improvement on the original concept, for it recycles mixed liquor from the discharge end of the aeration tank to the head of the anoxic tank to provide a consistent source of soluble nitrate for the facultative bacteria. In this instance, raw wastewater continues to provide the electron source, and sub-surface mixing maintains the bacteria in contact with both electron source and soluble nitrate in the absence of dissolved oxygen.

Phosphorus removal Edit

Every adult human excretes between 200 and 1,000 grams (7.1 and 35.3 oz) of phosphorus annually. Studies of United States sewage in the late 1960s estimated mean per capita contributions of 500 grams (18 oz) in urine and feces, 1,000 grams (35 oz) in synthetic detergents, and lesser variable amounts used as corrosion and scale control chemicals in water supplies. [16] Source control via alternative detergent formulations has subsequently reduced the largest contribution, but the content of urine and feces will remain unchanged. Phosphorus removal is important as it is a limiting nutrient for algae growth in many fresh water systems. (For a description of the negative effects of algae, see Nutrient removal). It is also particularly important for water reuse systems where high phosphorus concentrations may lead to fouling of downstream equipment such as reverse osmosis.

Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate-accumulating organisms (PAOs), are selectively enriched and accumulate large quantities of phosphorus within their cells (up to 20 percent of their mass). When the biomass enriched in these bacteria is separated from the treated water, these biosolids have a high fertilizer value.

Phosphorus removal can also be achieved by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminum (e.g. alum), or lime. [10] : 18 This may lead to excessive sludge production as hydroxides precipitate and the added chemicals can be expensive. Chemical phosphorus removal requires significantly smaller equipment footprint than biological removal, is easier to operate and is often more reliable than biological phosphorus removal. [17] Another method for phosphorus removal is to use granular laterite.

Some systems use both biological phosphorus removal and chemical phosphorus removal. The chemical phosphorus removal in those systems may be used as a backup system, for use when the biological phosphorus removal is not removing enough phosphorus, or may be used continuously. In either case, using both biological and chemical phosphorus removal has the advantage of not increasing sludge production as much as chemical phosphorus removal on its own, with the disadvantage of the increased initial cost associated with installing two different systems.

Once removed, phosphorus, in the form of a phosphate-rich sewage sludge, may be sent to landfill or used as fertilizer in admixture with other digested sewage sludges. In the latter case, the treated sewage sludge is also sometimes referred to as biosolids.

Disinfection Edit

The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite. [10] : 16 Monochloramine, which is used for drinking water, is not used in the treatment of waste water because of its persistence. After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of the nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday human uses.

Chlorination remains the most common form of waste water disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom, UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Some sewage treatment systems in Canada and the US also use UV light for their effluent water disinfection. [18] [19]

Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated on-site as needed from the oxygen in the ambient air. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for special operators.

Ozone wastewater treatment requires the use of an ozone generator, which decontaminates the water as ozone bubbles percolate through the tank.

Fourth treatment stage Edit

Micropollutants such as pharmaceuticals, ingredients of household chemicals, chemicals used in small businesses or industries, environmental persistent pharmaceutical pollutants (EPPP) or pesticides may not be eliminated in the conventional treatment process (primary, secondary and tertiary treatment) and therefore lead to water pollution. [20] Although concentrations of those substances and their decomposition products are quite low, there is still a chance of harming aquatic organisms. For pharmaceuticals, the following substances have been identified as "toxicologically relevant": substances with endocrine disrupting effects, genotoxic substances and substances that enhance the development of bacterial resistances. [21] They mainly belong to the group of EPPP. Techniques for elimination of micropollutants via a fourth treatment stage during sewage treatment are implemented in Germany, Switzerland, Sweden [ citation needed ] and the Netherlands and tests are ongoing in several other countries. [22] Such process steps mainly consist of activated carbon filters that adsorb the micropollutants. The combination of advanced oxidation with ozone followed by granular activated carbon (GAC) has been suggested as a cost-effective treatment combination for pharmaceutical residues. For a full reduction of microplasts the combination of ultrafiltration followed by GAC has been suggested. Also the use of enzymes such as the enzyme laccase is under investigation. [23] A new concept which could provide an energy-efficient treatment of micropollutants could be the use of laccase secreting fungi cultivated at a wastewater treatment plant to degrade micropollutants and at the same time to provide enzymes at a cathode of a microbial biofuel cells. [24] Microbial biofuel cells are investigated for their property to treat organic matter in wastewater. [25]

To reduce pharmaceuticals in water bodies, "source control" measures are also under investigation, such as innovations in drug development or more responsible handling of drugs. [21] [26] In the US, the National Take Back Initiative is a voluntary program with the general public, encouraging people to return excess or expired drugs, and avoid flushing them to the sewage system. [27]

Sludge treatment and disposal Edit

Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.

Sludge treatment is focused on reducing sludge weight and volume to reduce disposal costs, and on reducing potential health risks of disposal options. Water removal is the primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during thermophilic digestion, composting, or incineration. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options. Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.

Secondary treatment

Secondary treatment removes the soluble organic matter, nutrients such as nitrogen and phosphorus, and most of the suspended solids that escape primary treatment. Most often, biological processes are used in which microbes metabolize organic compounds and nutrients to grow and reproduce. The two most common biological secondary treatment processes are attached growth and suspended growth systems. A suspended growth process fosters the growth of suspended microorganism flocs from individual organisms already present in the wastewater and in the return activated sludge. The flocs contain organisms that can remove the pollutants through aerobic, anoxic, and anaerobic environments. Once the pollutants are removed, the flocs are sent to a secondary clarification process where they separate from the water via gravity. A portion of sludge in the bottom of the secondary clarifier is then directed back upstream to blend with the primary effluent (Return Activated Sludge) to create mixed liquor. The remainder of the sludge is removed from the process (Waste Activated Sludge) to create the ideal ecology of microorganisms. Attached growth systems rely on the microorganisms to attach to a media, and create a biofilm. The settled sewage is either mixed or sprinkled over the biofilm coated media where the microorganisms remove the pollutants. Like the suspended growth process, biofilm fragments and suspended flocs are sent to a secondary clarifier for separation where sludge is recycled and wasted and clean water is discharged to the next process.

For biological treatment to function efficiently, organisms require nutrients in a balanced ratio, including carbon, nitrogen, and phosphorus (referenced as C:N:P), as well as trace elements including iron, copper, zinc, nickel, manganese, potassium, sulfur, and other components which are typically present in wastewater. The commonly accepted C:N:P Ratio is 100:5:1, although some facilities thrive outside of this ratio, while others experience polysaccharide slime formation or filamentous bacteria growth that inhibit the biology and settling in the secondary clarifier.

Multiple biological processes can be employed to complete secondary treatment, including plug flow aeration basins, complete mix aeration tanks, sequencing batch reactors, oxidation ditches, trickling filters, moving bed biological reactors, integrated fixed film activated sludge, and others.

Biological Nutrient Removal (BNR) alters the environment of the microorganisms to remove nitrogen and phosphorus from the water. A BNR process consists of anaerobic (no oxygen or nitrate), anoxic (no oxygen, nitrate is present), and aerobic (oxygen present) stages, during which the water is moved through a series of chambers to perform various biological functions.

Chemical treatment processes can also be used, such as the chemical removal of phosphorus. By introducing a chemical precipitant to within the aeration basin and clarifiers, phosphorus is removed by flocculation, binding into insoluble compounds that settle out and can be removed as sludge.

Alternative systems

Sometimes the cost of conventional gravity sewers can be prohibitively high because of low population densities or site conditions such as a high water table or bedrock. Three alternative wastewater collection systems that may be used under these circumstances include small-diameter gravity sewers, pressure sewers, and vacuum sewers.

In small-diameter gravity systems, septic tanks are first used to remove settleable and floating solids from the wastewater from each house before it flows into a network of collector mains (typically 100 mm, or 4 inches, in diameter) these systems are most suitable for small rural communities. Because they do not carry grease, grit and sewage solids, the pipes can be of smaller diameter and placed at reduced slopes or gradients to minimize trench excavation costs. Pressure sewers are best used in flat areas or where expensive rock excavation would be required. Grinder pumps discharge wastewater from each home into the main pressure sewer, which can follow the slope of the ground. In a vacuum sewerage system, sewage from one or more buildings flows by gravity into a sump or tank from which it is pulled out by vacuum pumps located at a central vacuum station and then flows into a collection tank. From the vacuum collection tank the sewage is pumped to a treatment plant.

Wastewater: Problem and its Treatment | Ecology

According to the World Bank, “the greatest challenge in the water and sanitation sector over few decades will be the implementation of low cost sewage treatment that will at the same time permit selective reuse of treated effluents for ag­ricultural and industrial purpose”.

It is crucial that sanitation systems have high levels of hygienic standards to prevent the spread of diseases. Other treatment goals include the recovery of nutrient and water resources for reuse in agricultural pro­duction and to reduce the overall user-demand for water resources.

In order to achieve ecological wastewater treat­ment, a closed-loop treatment system is recom­mended. Many present day systems are a “disposal-based linear system”. The traditional linear treat­ment systems must be transformed into the cycli­cal treatment to promote the conservation of wa­ter and nutrient resources.

Using organic waste nutrient cycles, from point-of-generation to point- of-production, closes the resource loop and pro­vides an approach for the management of valu­able wastewater resources. Failing to recover or­ganic wastewater from urban areas means a huge loss of life-supporting resources than instead of being used in agricultural for food production, Fill Rivers with polluted water.

The development of ecological wastewater management strategies will contribute to the reduction of pathogens in sur­face and groundwater to improve public health. “The goal of ecological engineering is to attain high environmental quality, high yields in food and fiber, low consumption, good quality, high effi­ciency production and full utilisation of wastes”.

In the growing number of conflicts between agricultural and domestic use of scarce water re­sources, an increased use of treated wastewater for irrigation purpose is vital. Wastewater is com­posed of over 99% water. In a developing urban society, the wastewater generation is usually ap­proximately 30-70 m 3 per person per year.

In a city of one million people, the wastewater gener­ated would be sufficient to irrigate approximately 1500-3500 hectare. Innovative and appropriate technologies can contribute to urban wastewater treatment and reuse.

Based on extensive success­ful experience in Canada and elsewhere on cost effective and environmentally sound practices of sludge application on agricultural land, there is tre­mendous potential for the safe disposal of sew­age sludge on agricultural land.

Problem Statement of Wastewater Treatment:

Problems concerning water sanitation stem from the rise in urban migration and the practice of discharging untreated wastewater. The uncon­trolled growth in urban areas has made planning and expansion of water and sewage systems very difficult and expensive to carry out. In addition, many of those moving to the city have low in­comes, making it difficult to pay for any water sys­tem upgrades.

In developing countries, 300 mil­lion urban residents have no access to sanitation and it is mainly low-income urban dwellers who are affected by lack of sanitation infrastructure. Approximately two-thirds of the population in the developing world has no hygienic means of dis­posing excreta and an even greater number lack adequate means of disposing of total wastewater.

It is a common practice to discharge untreated sewage directly into bodies of water or put onto agricultural land, causing significant health and economic risks. While the number of households with access to drinking water supply has increased (approximately eighty per cent in Latin America and the Caribbean), the per cent connected to ur­ban sewage collection systems is only five per cent.

The effects of inadequate treatment can be detrimental to a community on economic, cultural and health-levels. The costs of poorly managed domestic waste are very high. In India, the 1994 plague epidemic resulted in a loss of tourism rev­enue estimated at $200 USD million in Peru, a recent cholera epidemic resulted in an estimated loss amounting to three times the expenditure on water and sanitation for the entire country over the preceding 10 years and in Shanghai, China, a recent major outbreak of Hepatitis A was attrib­uted to sewerage contamination.

Water contami­nated by human, chemical or industrial wastes can cause a number of diseases through ingestion or physical contact. Water-related diseases include dengue, filariasis, malaria, onchocerciasis, trypano­somiasis and yellow fever. Consequently, no other type of intervention has greater impact upon a country’s development and public health than the condition of clean drinking water and the appro­priate disposal of human waste.

The benefits of reusing treated wastes must also be measured against the cost of not doing so at both the economic and environmental level. The costs of implementing zero-discharge organic waste to agriculture recycling schemes may not be expensive. Full-scale implementation of urban or­ganic waste to agriculture systems could cost as little as $5-6 USD million for a city of 1 million people.

The problem with the current treatment tech­nologies is they lack sustainability. The conven­tional centralised system flushes pathogenic bacteria out of the residential area, using large amounts of water and often combines the domestic waste­water with rainwater, causing the flow of large volumes of pathogenic wastewater.

In fact, the conventional sanitary system transfers a concen­trated domestic health problem into a diffuse health problem for the entire settlement and/or region. In turn, the wastewater must be treated where the cost of treatment increases as the flow increases.

The abuse of water use for diluting hu­man excreta and transporting them out of the settlement is increasingly questioned and being considered unsustainable. The negative effects of centralised treatment are sum­marised in Table 19.1.

Another reason is that many treatment sys­tems in developing countries are not successful and therefore unsustainable are that they were sim­ply copied from western treatment systems with­out considering the appropriateness of the tech­nology for the culture, land, and climate.

Often local engineers educated in the western develop­ment programs supported the choice for the in­appropriate systems. Many of the implemented installations were abandoned due to the high cost of running the system and repairs.

On the other hand, conventional systems may even be technologically inadequate to handle the locally produced sewage. For example, in compari­son to the US and Europe, domestic wastewater in arid areas like the Middle East are up to five times more concentrated in the amount of oxygen demand per volume of sewage. This is extremely high and may cause a large amount of sludge production.

Appropriate Treatment Technology:

Based on experience from past mistakes in sew­age treatment technology, the definition of what is sustainable is clearer. Developers should base the selection of technology upon specific site con­ditions and financial resources of individual com­munities. Although site-specific properties must be taken into account, there are core parts of sus­tainable treatment that should be met in each case. The criteria for sustainable technology are summarised in Table 19.2.

One approach to sustainability is through de­centralisation of the wastewater management sys­tem. This system consists of several smaller units serving individual houses, clusters of houses or small communities. Black and gray water can be treated or reused separately from the hygienically, more dangerous excreta.

Non-centralised systems are more flexible and can adopt easily to the local conditions of the urban area as well as grow with the community as its population increases. This approach leads to treatment and reuse of water, nutrients, and by-products of the technology (i.e., energy, sludge, and mineralized nutrients) in the direct location of the settlement.

Communities must take great care when reus­ing wastewater both chemical substances and bio­logical pathogens threaten public health as well as accumulate in the food chain when used to irri­gate crops or in aquaculture. In most cases, Indus­trial pollution poses greater risk to public health than pathogenic organisms.

Therefore, more em­phasis is being placed on the need to separate do­mestic and industrial waste and to treat them indi­vidually to make recovery and reuse more sustain­able. The system must be able to isolate industrial toxins, pathogens, carbon, and nutrients.

1. Sustainable Treatment Types:

Now that the requirements for a sustainable waste­water treatment system have been presented, there are several options one can choose from in order to find the most appropriate technology for a par­ticular region. This paper will discuss sustainable. Wastewater treatment systems including la­goons/wetlands, UASB (anaerobic digesters), Hy­brid reactor, and SAT technologies.

2. Lagoons and Wetlands:

In wetland treatment, natural forces (chemical, physical, and solar) act together to purify the waste­water, thereby achieving wastewater treatment. A series of shallow ponds act as stabilisation lagoons, while water hyacinth or duckweed act to accumu­late heavy metals, and multiple forms of bacteria, plankton, and algae act to further purify the water.

Wetland treatment technology in developing coun­tries offers a comparative advantage over conven­tional, mechanised treatment systems because the level of self-sufficient ecological balance, and eco­nomic viability is greater.

The system allows for to­tal resource recovery. Lagoon systems may be con­sidered a low-cost technology if sufficient, non-arable land is available. However, the availability of land is not generally the case in big cities. The de­mand of flat land is high for the expanding urban developments and agricultural purposes.

The decision to use wetlands must consider the climate. There are disadvantages to the system that in some locations may make it unsustainable. Some mechanical problems may include clogging with sprinkler and drip irrigation systems, particu­larly with oxidation pond effluent.

Biological growth (slime) in the sprinkler head, emitter ori­fice, or supply line cause plugging, as do heavy concentrations of algae and suspended solids. Other disadvantages are listed in Table 19.3.

3. Anaerobic Digestion:

Another treatment option available, if there is little access to land, is anaerobic digestion. Anaerobic bacteria degrade organic materials in the absence of oxygen and produce methane and carbon di­oxide. The- methane can be reused as an alterna­tive energy source (biogas).

Other benefits include a reduction of total bio-solids volume of up to 50-80% and a final waste sludge that is biologi­cally stable can serve as a rich humus for agricul­ture have low sludge production and low energy needs. Since nitrogen and phosphorus are not ef­fectively reduced in anaerobic technologies, this primary treatment approach works well with agri­culture or aquaculture.

However, they are not com­pletely effective at removing all pathogens, the wastewater needs a post treatment option to meet discharge standards, such as composting digested sludge, wetland systems, or stabilisation ponds (Table 19.4).

The UASB reactor essentially consists of a gas- solids separator (to retain the anaerobic sludge within the reactor), an influent distribution sys­tem, and effluent draw-off facilities, See Fig. 19.1 below for a schematic of UASB re­actor.

It is constructed with entrance pipes deliv­ering influent to the bottom of the unit and a gas solids separator at the top of the reactor to Sepa­rate the biogas from the liquid phase (water and sludge) overall, this prevents sludge washout.

The UASB system with a stabilisation pond for secondary treatment can cost $4 USD per person equivalent compared to $8 USD per person equiva­lent for activated sludge treatment. These costs would be for a system scale of 50,000 person equiva­lents if the land cost is less than $20 USD.

The hybrid reactor is an improved version of the UASB system and combines the merits of the up-flow sludge blanket and the fixed film reactors. The advantages include simplicity of design and operation it also is- more economical than a fixed bed system.

Wastewater treatment by the hybrid reactor sys­tem has become widespread as it provides advan­tages of both the suspended and attached growth phase at the same time. It may be used to treat some rate-limiting substrate, priority pollutants, volatile organic compounds etc. as well as for ni­trification.

This versatile nature of hybrid reactor demands for a detailed investigation on the mecha­nism, mode of operation, different applications and major configurations available. The present article is devoted to explore these issues with re­spect to previous background and successive de­velopment in this area.

Apart from the laboratory and pilot-scale study, some industrial applications have been overviewed to understand the perfor­mance of hybrid reactor in the concerned field. The approach of modelling for the hybrid reactor system is also demonstrated with the hypothetical data set. A comprehensive details about major hybrid processes is presented along with their sche­matic diagrams.

The review on hybrid process revealed that it would be economic for upgradation of existing activated sludge system, ensuring car­bonaceous oxidation and nitrification in a single reactor and treatment of slowly biodegradable substances also.

5. Soil Aquifer Treatment:

Soil aquifer treatment (SAT) is a geo-purification system where partially treated sewage effluent ar­tificially recharges the aquifers, and then withdrawn for future use. By recharging through unsaturated soil layers, the effluent achieves additional purifi­cation before it is mixed with the natural ground­water.

In water scarce areas, treated effluent be­comes a considerable resource for improved groundwater Sources. The Gaza Coastal Aquifer Management program includes treated effluents to strengthen the groundwater, in terms of both quantity and quality. With nitrogen reduction in the wastewater treatment plants, the recharged ef­fluent has a potential to reduce the concentration of nitrates in the aquifer.

In water scarce areas such as in the Middle East and parts of Southern Africa, wastewater has become a valuable resource that, after appropriate treatment, becomes a commer­cially realistic alternative for groundwater recharge, agriculture, and urban applications (Fig. 19.2).

SAT systems are inexpensive, efficient for pathogen removal, and operation is not highly tech­nical. Most of the cost associated with an SAT is for pumping the water from the recovery wells, which is usually $20-50 USD per m 3 .

In terms of reductions, SAT systems typically remove all BOD, TSS, and pathogenic organisms from the waste and tend to treat wastewater to a standard that would generally allow unrestricted irrigation. The biggest advantage of SAT is that it breaks the pipe-to-pipe connection of directly reusing treated wastewater from a treatment plant. This is positive attribute for those cultures where water reuse is taboo.

The pretreatment requirements for SAT vary depending on the purpose of groundwater recharge, sources of reclaimed water, recharge methods, and location. Some may only need primary treatment or treatment in a stabilisation pond. However pre­treatment processes should be avoided if they leave, high algae concentrations in the recharge water. Algae can severely clog the soil of the infiltration basin.

While the water recovered from the SAT sys­tem has much better water quality than the influ­ent, it could still be lower quality than the native groundwater. Therefore, the system should be de­signed and managed to avoid intrusion into the na­tive groundwater and use only a portion of the aqui­fer.

The distance between infiltration basins and wells or drains should be as large as possible, usu­ally at least 45 to 106 m to allow for adequate soil-aquifer treatment. All the systems described allow for the reuse of treated wastewater in order to have a cyclic, sustainable system.

These treated wastewater pro­vide essential plant, nutrients (nitrogen, phospho­rus, and potassium) as well as trace nutrients. Phos­phorus is an especially important nutrient to re­cycle, as the phosphorus in chemical fertiliser comes from limited fossil sources.

The applica­tion of treated wastewater, as well as sludge, has considerable potential in a cyclical approach to crop applications, provided health risks and quality re­strictions are taken into consideration. Public health is the most critical issue regarding reclaimed wastewater.

Treated Wastewater Reuse:

Wastewater reuse must meet certain controls. First, wastewater treatment to reduce pathogen concen­trations must meet the WHO (1989) guidelines in Table 19.5. Second, crop restrictions must be speci­fied to prevent direct exposure to those consum­ing uncooked crops as well as defining application methods (irrigation) that reduce the contact of wastewater with edible crops.

Finally, control of human exposure is needed for workers, crop- handlers and final consumers.

It is well known that human waste is very much rich in nutrients (Table 19.6). But it field applica­tion need for societal support. Engineers and the local residents is necessary early on in the project, and if local participation is extensive, capital costs can ultimately be reduced.

According to the Inter- American Development Bank, “Citizen participa­tion, properly channeled, generates savings, mo­bilizes financial and human resources, promotes equity and makes a decisive contribution to the strengthening of society and the democratic sys­tem”.

There is a strong sense of ownership by mem­bers of the community in their projects. This pride in the new development helps to ensure the sustainability of the water supply and sanitation systems. Once the project is implemented, local participation contributes to the community’s con­fidence in the new technology and allows them to take on other challenges such as accessing finan­cial aid for other infrastructure projects.

On the governmental level, institutional strengthening is usually needed to assist small to medium-sized cities in dealing with new adminis­trative and financial management responsibilities. One program that has been developed to address the problems associated with decentralisation is RIADEL (Local Development Research and Ac­tion Network).

It is a network for sharing infor­mation about local community development in Latin America. It includes decentralisation and the training of social leaders and civil servants.

Case Studies and Current Research Activities:

There are several research and development projects on wastewater treatment, some have been successful and sustainable and some have not. The reasons for success or failure most often depend on the appropriateness of the implemented tech­nology. The following description is a perfect example of the in-appropnateness of adapting West­ern technology without making adjustments for the local environment.

In the 1970s, a foreign country donated a conventional activated sludge plant to the city of Amman, Jordan. Due to the arid climate, however, sewage in Jordan has ex­tremely high concentrations of organic matter.

This caused several problems in the plant such as: high-energy consumption for aeration, high vol­ume of sludge production, operational problems in the operational plant, and high consumption of polymers and clean water for drying the sludge after digestion.

Next, they implemented another unsustainable technology by constructing one of the world’s largest stabilisation ponds. Soon after the pond was installed, the plant was operating at loading rates double that of the design load caus­ing very poor effluent quality. Recently, another Western program installed off-gas treatment to prevent odor by placing surface aerators in the maturation ponds.

However, operation costs of the aerators were too high and the system stopped after two months. Not only was it expensive, but it also didn’t fix the odor problems since the odor­ous gases were coming off the anaerobic ponds and there was little improvement in effluent qual­ity.

One alternative treatment technology that would have supported the high COD quality of the effluent would have been anaerobic digestion. As explained previously, anaerobic digesters are generally low-tech, have low energy usage, and are less expensive to maintain.

The next case study, on the other hand, at­tempts to find a proper system for the country at a low cost to the community, and shows that in areas like the Middle East and Southern Africa where there is a shortage of water, groundwater recharge and agricultural/urban applications of treated effluent can be sustainable solutions.

In this case, Windhoek, Namibia is the location for a successful project implementing treated wastewa­ter reuse. Because the arid climate and water short­age were taken into account when determining the technology, the project incorporated SAT systems to recharge the groundwater and water demand management, based on IWRM (Integrated Waste­water and Recycling Management).

The required volume of water used to irrigate parks, sports fields, etc. has lowered since 1987, even though the population has doubled from approximately 105,000 to 202,000 over the same time. The artifi­cial recharge of aquifers was beneficial due to the lower evaporation, which allowed for water sup­ply during droughts. In addition, a feasibility study showed the system’s total investment cost would be recovered within five years.

Thus it is essential to treat wastewater by cheap­est methods. The first was by decentralizing the treatment rather than installing expensive sewer systems that combine and increase the volume of the waste. The next involved choosing an appro­priate treatment technology for the community, where several types of proposal included lagoons/ wetlands, UASB (anaerobic digester), hybrid reac­tor, and SAT.

The common characteristic of all of the described types is that they encourage “zero-discharge” technology. This cyclical, rather than linear approach includes the reuse of the treated effluent for agricultural reuse. The reuse of the wastewater decreases the money spent on fertilisers and it is considered safe, since it has been treated for pathogens.

The urban areas of many developing countries are growing rapidly, ecological sanitation systems must be implemented that are sustainable and have the ability to adopt and grow with the community’s sanitation needs. In order to decide what the ap­propriate treatment system is, the developer must consider the area’s climate, topography, and socio­economic factors.

There are still plenty of needs in this area for research to improve or optimize the current methods of wastewater treatment. The re­sult of increased attention to this topic will improve the health, economic, and agricultural factors of a developing community.

Frequently asked questions (FAQ) on wastewater / sewage treatment plants (STP)

The Wastewater/ STP FAQ, provides a primer on the basics related to all aspects of wastewater/ STP. The most popular FAQs are listed below. Please click on a topic to view more detailed information:

What is Waste Water? How is it generated?

Waste water is the water that emerges after fresh water is used by human beings for domestic, commercial and industrial use. This document will restrict itself only to the waste water generated due to domestic use.

By and large,it is fresh water that is used for a variety of domestic uses such as washing, bathing & flushing toilets. Washing involves the washing of utensils used in cooking, washing vegetables and other food items, bathing, washing hands, washing clothes.

The water that emerges after these uses contains, vegetable matter, oils used in cooking, oil in hair, detergents, dirt from floors that have been washed , soap used in bathing along with oils/greases washed from the human body. This water is referred to as “ Grey Water” or sullage.

Water used to flush toilets to evacuate human excreta is called “ Black Water” or Sewage.

Grey water is easier to purify as compared to black water, i.e sewage. However, the practice predominantly followed in India is to combine these two wastes to discharge into a public sewer or into a sewage treatment plant in a residential community/ building that has no access to a public sewer.

How much waste water is generated in a residential complex?

As per standards laid down by the CPHEEO (Central Public Health Environmental & Engineering Organisation), the fresh water consumption per day per person should be between 135 to 150 litres per day. It is officially expressed as “litres per capita daily” (lpcd). By and large public water supply and sewerage bodies/authorities all across the country use the former figure to work out probable water consumption.

Waste generation in a residential complex:

When water is consumed by people living in a residential complex without access to an underground sewerage/drainage system , the amount consumed is estimated to be 135 lpcd. The total quantity (No. of residents X 135 litres) comes into a sewage treatment plant(STP) in the premises, and , this total volume has to be treated by the STP.

In a vast majority of cases, the actual waste generated exceeds this figure comfortably leading to overloading of the STP. This happens routinely because almost all residential complexes do not install water meters or similar water volume and flow measurement devices to keep track of water consumption in a residential complex/ gated community.

Consequently, when a device is installed and readings monitored, consumption has been found to be double and some times triple the suggested figure of 135 lpcd.

Waste generation in a commercial complex:

Human occupation of this kind of building is only during “duty hours”,i..e for approximately 8 to 10 hours per shift if there is more than a single shift. In this case water consumption is considered as 50 lpcd per person per shift.

What are the constituents of waste water (sewage) ?

Waste water contains all the dissolved minerals present in the fresh water that was used and which became waste water as well as all the other contaminants mentioned above. These are proteins, carbohydrates, oils & fats. These contaminants are degradable and use up oxygen in the degradation process.

Therefore, these are measured in terms of their demand for oxygen which can be established by certain tests in a laboratory. This is called Bio Degradable Oxygen demand(BOD). Some chemicals which also contaminate the water during the process of domestic use also degrade and use oxygen and the test done to establish this demand which is called Chemical Oxgen demand (COD).

Typically a domestic sewage would contain approximately 300 to 450 mg/litre of BOD and COD on an average. Sewage also contains coliform bacteria (e coli) which is harmful to human beings if water containing such bacteria is consumed(drunk). E coli is bacteria that thrives in the intestines of warm blooded creatures such as humans, animals and birds.

Another feature of sewage is the high level of Total Suspended Solids (TSS). This is what gives the sewage a black colour ,hence the name “ black water”. If sewage is allowed to turn septic, it then also has a strong, unpleasant odour.

Why treat waste water ?

Much of the water used for domestic purposes does not require potable ( suitable for drinking) water quality. For instance, water used for flushing toilets or for washing floors, yards or roads & gardening does not require to be potable. In a scenario where fresh water is getting increasingly scarce and when enormous volumes of sewage generated in the country are not being treated ,but goes unchecked to pollute fresh water from lakes, rivers and the ground water table, it must be treated.

Discharging untreated sewage into any drains other than an underground sewerage system, or into open land , is an offence and invites prosecution under the laws of all Pollution Control Boards in the country.

Sewage must necessarily be treated correctly and then re-used/re-cycled for various uses that do not need potable water quality. Recycling/re-using treated sewage can reduce fresh water requirements very substantially, by almost 50-60%.

In a scenario where fresh water availability itself is increasingly in doubt this is critical.

How can treated sewage be re-used/re-cycled ?

This requires plumbing to be laid so as to serve two sets of storage tanks on the roofs of any residential/commercial building. One set of storage tanks will be used to receive and store fresh water which will flow through plumbing laid to take it to bathrooms and kitchens where it can be used for drinking, cooking, washing & bathing.

The second set of tanks will receive treated sewage which will be connected by plumbing to all the flush tanks in toilets and to other points where the water can be used for washing yards, floors and also for gardening.

How is waste water treated ?

Sullage (grey water) which is mentioned above, if collected in a storage tank separately can be treated by aerating it to prevent it from turning septic, and then dosed with a coagulant, chlorinated and then subjected to filtration by pressure sand filtration followed by activated carbon filtration and stored in a separate overhead tank or tanks from which it can be used for flushing toilets and other uses where fresh or potable water is not required.

However, the current practice is to combine sullage and sewage (black water) and treat the mixture in an STP (Sewage treatment plant). This practice has come in predominantly to reduce the cost of construction of two separate plants and because space is now at a premium in any building.

Why not consider grey water treatment seriously in spite of the extra space it requires ?

From the point of view of a resident it is worth considering as it enhances the water security of the resident. A builder’s priority is totally different, since the space taken up by the treatment system can not be ‘sold’ to a buyer, he will just not consider it, instead the builder will combine greywater with sewage in an STP. This enables the builder to save costs.

However if looked at from the residents’ view point, a separate grey water treatment system being easier to operate provides a facility to ‘fall back on’ when the STP fails.

Why STPs fail becomes clearer as you read on.

What are the common methods for sewage treatment ?

Treatment of sewage is based on a method provided by nature, i.e by using microbial action. When a steady consistent supply of air is pumped into a tank containing sewage which has been screened to remove all floating debris and non soluble contents in sewage, microbes which are present in it get activated. These microbes are present in the sludge which makes up a substantial part of sewage, and they consume the pollutants in the sewage while the air supply brings them to life and keeps them alive and multiplying.

This is a time tested system called the world over as the 'Activated Sludge Process (ASP)'. It is the oldest system world wide and is the most used process world over. An STP based on this aerobic process will consist of the following major stages of treatment :

Primary Treatment: In this stage, raw sewage is screened to remove floating debris/ insoluble impurities such as plastic bags, leaves, twigs, paper,etc.

Secondary Treatment: In this stage, oxygen(air) is mixed into the sewage to activate the microbes which consume the pollution load and which then become sludge (biomass) . Aerated sewage and sludge are than separated so that the sludge can be removed and de-watered/ dried for disposal. The sludge can be used as compost in a garden. The water free from sludge is sent to a clear water tank(also called clarified water tank).

Tertiary Treatment: Clarified water is filtered through a pressure sand filter and an activated sand filter to remove any remaining suspended impurities and a substantial portion of the BOD & COD present in it. Finally it is disinfected to kill all the bacteria present in it by either chlorination or ozonation or with ultra violet light. This tertiary treated water is then pumped into a dedicated set of storage tanks from where it is used to flush toilets, wash roads, yards and for gardening.

A majority of the STPs in India are based on this ASP system in its most basic form. Such STPs are highly susceptible to input fluctuations (a frequent feature in India) and this results in a lot of untreated sewage and other related problems.

Other processes available :

  • A fairly recent system called the MBR system (membrane bio reactor system) which is a superior system is becoming very popular. It is a very compact waste treatment system that combines biological decomposition with membrane separation of the sludge (biomass).The membrane compacts & concentrates the sludge making for a far more compact design than the ASP system described above(it combines the secondary and tertiary treatment in one single step). Further it produces far less sludge too. Best of all ,it is not so sensitive to input load fluctuations like the ASP system.However, an MBR system requires skilled operators, though it can be automated to cut out operator error and poor operation.
  • DEWATS (Decentralised Water Treatment System) is a combination of anaerobic treatment with aerobic treatment. It is a low cost system and requires no operator intervention unlike the ASP. This process has no moving parts and it can even provide methane gas from the anaerobic part of the system which can be used for cooking. The DEWATS system was developed keeping in mind the needs of developing economies where it is difficult to get skilled operating personnel for conventional ASP based STPs. Treated water from DEWATS will still require tertiary treatment for re-use/recycle,and this part of the system (tertiary treatment) needs operators.
  • Reed Bed Sewage Treatment System is an extremely eco friendly system where sewage is allowed into a constructed water body where certain kinds of aquatic plants are planted which absorb atmospheric oxyen and let this out through their roots thereby providing the oxygen to feed the microbes which will clean up the sewage. This system does however require a tertiary treatment system if the treated sewage is required for anything more than gardening. Its drawback is that it requires a lot of land to function and this is a major disadvantage in a world where space is at a premium. One major advantage is that it requires virtually no electricity to operate if the flow of raw and treated sewage is by gravity.

What is the difference between “ Aerobic” and “ Anaerobic” processes in sewage treatment?

The aerobic process as explained above is one where the microbes which clean up the sewage need to be supplied with air(oxygen) to function and multiply so that the sewage is ‘cleaned up’. An anaerobic process is one where a different kind of bacteria comes into action. It is a bacteria/microbe that does not need air and operates in an atmosphere without air ( hence the term ‘anaerobic’).This kind of bacteria produces methane and in the waste/environmental engineering industry, they are called “ methanogens”.

Mostly, anaerobic treatment is usually followed by the aerobic process and this combination is used where the waste water has very high values of BOD and COD. In such situations, the anaerobic system reduces the BOD & COD down to a level where the aerobic process completes the job of reducing it down to the levels where a tertiary treatment stage can do the final ‘polishing’ of the treated sewage as stated above.

What are the problems that can be expected with an installed STP?

The most common problems encountered are listed below and are based on an informal survey of STPs (including Water Treatment plants also) carried out over the last 4 years.

  • Initial Start up of an STP failing to treat sewage: An STP is normally designed for the total sewage that can be expected when a building or premises is fully occupied. Full occupation in most cases usually takes up to a year or more. During this period when occupancy can be as low as 30% or so and gradually increases over a year or more, consequently the sewage that comes in initially does not provide the minimum load needed for satisfactory operation of an STP. It results in a situation which can only be called “sewage in sewage out”. Many STPs which face this situation take a long time to stabilize and provide treated sewage, very often, due to poor or wrong operation, STPs do not stabilize.
  • Poor design/underdesign of the STP : Often STPs which initially ‘struggle’ to overcome the first problem described above also can not function because
    1. the balancing tank is undersized
    2. aeration tank is undersized or clarifier is badly designed
    3. the total inflow of sewage is higher than the volume the STP was designed to handle.

The tanks mentioned in 1) & 2) above are part of the primary and secondary treatment portions of an ASP system.

  • Consistent mal-operation of the STP : Another very common feature is that a majority of plant operating personnel employed by agencies that take on Operation & Maintenance(O&M) contracts are illiterate, un trained and supervised by people with little or no knowledge of what O&M involves. Such agencies generally charge O&M charges that residents’ associations consider affordable. Companies with well trained operation personnel and experienced supervisory staff charge for services an amount that reflect their skill and expertise which residents’ Associations are reluctant to pay and thereby lose out on a well run/operated water infrastructure. They often do not realize that even the charges which they consider as cheaper/lower are going to waste if the sewage is only partially treated.
  • Strong smell/odour from the STP: This is a very common complaint from numerous housing communities and even commercial buildings which have an STP in operation. The smell is often very strong and quite often unbearable. It is caused by any one or all of the problems listed above.
  • Very high noise levels from the STP: Quite often residents of an apartment building have sought help from experts to minimize the very high noise levels from their STP which they find unbearable throughout the day and more so at night, thereby preventing the residents from sleeping in peace.

How can common problems in STP be avoided and/or resolved?

  • Modern designs for STPs which are modular are available from reputed companies which are in the field of water and waste water treatment. Such companies have standardized designs where,for instance an STP to handle 150 KLD ( 150,000 Litres per day) of sewage can be made up of 3 modular STPs each of 50KLD capacity. Such an installation would be able to handle the initial lower load of sewage with one module in operation with remaining modules being commissioned/started up as the sewage volume increases. Such a modular approach also makes it possible to handle sewage in the case of a break-down of the STP as it is extremely rare for all modules to break-down together. In short, there is a stand-by always available. For several years now a few companies have been offering microbial agents which can help overcome these problems if these microbial agents are added to the incoming sewage. Go in for Modular STPs & use microbial agents regularly.
  • It is equally important to know and be able to control the volume of fresh water used in a community so that it does not exceed the design capacity of an STP. This involves installing water meters at all crucial points to measure water flow (consumption) & thereafter taking action to curb excess consumption of fresh water to prevent overloading the STP. Control excess consumption of fresh water and thereby prevent overloading of the STP
  • Builders are not expected to be experts in water or sewage treatment plant design, manufacture etc. They can however have tie-ups with reputed environmental engineering companies with sound technical experience and a proven track record, to make up for their lack of knowledge. This seldom happens since a builder’s interest ends with selling a completed project and then handing over the project to the Resident’s Association as soon as possible, often without even demonstrating actual, successful operation of the water infrastructure. Most builders link up with small, obscure local companies with inadequate knowledge and expertise in waste and water treatment,but will put up something for an extremely low price. The result is poor/ wrong operation of an STP leading to untreated sewage and unpleasant odours from it. Ensure supply of an STP from a reputed supplier and entrust operation & maintenance to a well trained professional team.
  • One of the major reasons for STPs not working properly is the fluctuations in input loads. Flow of sewage in a residential community is never uniform. It varies with peak flows in the morning (residents getting ready to go to work), very low or almost no flows later in the day with another peak in the evening. Raw sewage is collected in a sewage balancing tank(mentioned above) which should be sized to hold at least 6 to 8 hours flow of sewage. This ensures that the sewage collected in the balancing tank is homogenized, thereby avoiding input fluctuations in input load on the STP. Do not compromise on the size of a raw sewage balancing tank.
  • High noise levels from an STP are due to the operation of electric motor driven equipment such as pumps, air blowers, air compressors, etc. Old designs/makes of pumps, blowers , compressors , etc are still available at very low prices in the market and these are used in most of the STPs that have been put up. The noise levels of such equipment is very high as compared to modern, world class pumps and rotary motor driven equipment now available in India. These modern makes are almost noiseless and extremely efficient. The old designs are also the cause of high energy consumption in addition to very high noise levels. As per the laws in force in India, the noise level permitted in a residential area is 55 dB (dB= decibels of sound) during day time,i.e from 6:00 am to 10:00 pm and 45 dB during night time(10:00pm to 6:00 am).As compared to these limits, the actual noise levels are likely to be as high as 75 dB or higher. To reduce noise levels and high energy consumption, it will be necessary to replace most of the critical rotary motor driven equipment with the latest noiseless high efficiency equipment. Here it is advisable to choose a reputed company with an established reputation in sewage/waste water treatment to buy an STP. Such companies have constantly improved their designs to reduce the foot prints (space occupied) of their equipment and reduction in the power consumption of power by a very appreciable amount. Unfortunately, residents have no say in this as they face up to this crucial fact when it is too late as the STP has been ordered probably even before the residents bought a home in the property.

What would STPs of different capacities cost?

The prices given here are only indicative and meant to give an idea. All capacities given are in KLD ( Kilo litres /day, kilo= 1000 litres).

  • 5.0 KLD STP = Rs.5.0 lakhs.
  • 10 to 15 KLD = Rs.8.0 lakshs.
  • 25 KLD = Rs.15.0 Lakhs.
  • 35 KLD = Rs.18.0 Lakhs.
  • 50 KLD = Rs.35.0 Lakhs.
  • 75 KLD = Rs.40.0 Lakhs.
  • 100 KLD = Rs.30.0 Lakhs. (All civil work for this size to be built by buyer)
  • Prices given are exclusive of Value Added Tax and excise duties (if applicable)
  • Supplier will charge a separate amount for installation and starting up the STP. This can cost an additional 5 to 10%
  • Prices for MBR systems are not given since they involve a substantial import content and hence it would be better to approach companies that offer such a sytem for a price directly.

What about the operating costs for an STP?

The operating costs, including maintenance for:

  • STP of 75KLD capacity and above : 1.2 paise per litre of sewage treated.
  • STP of 50KLD and below : 1.5 paise per litre of sewage treated.

These costs do not include the cost of plant operating personnel. If O&M is provided by a reputed company which would use well trained operators with one operator per shift and one supervisor during general shift, they would charge approximately Rs.60,000. per month. Other obscure agencies who take on O&M contracts would charge approximately Rs.20,000.00 to 30,000.00 for providing the same number of personnel, but without the necessary training.

Is treatment of sewage rather complicated to understand for an average resident/ owner of an apartment or villa ?

Yes, unfortunately, it is true.If sewage treatment was simple and easy for all to understand, this entire write up with FAQs would be much smaller and easy to follow.

What can a Resident’s Association do if it needs expert help to sort out problems with sewage treatment?

Go to the “Ask the Expert” service and you will find that there are several persons with the necessary expertise based in different cities in India. Contact them and ask if they will help and the terms on which they will do so if it involves visiting your location. You will get one of them to definitely offer to help.

Any important DOs & DON’Ts for sewage treatment plants?

Yes, there are a few. Unfortunately, it is too late for Residents’ Associations to do anything about these because they take over a property after it is all over and done. It is the builders who should read this section and hopefully do what is suggested if they have their buyers’ interests at heart.

  • Sewage treatment is one of the most crucial features for the residents of an apartment complex/gated community. It needs to be installed in the premises in a location where it is above ground and hence can get all the air it needs for it to function and to facilitate easy maintenance. Never install an STP in a basement.
  • Almost all STPs in multi storied apartment complexes are installed as deep underground as possible! From the point of view of an agency that manufactures, installs and probably also operates it, an underground STP is a night mare! If it stops working , emptying the collected sewage so as to be able to repair it is a terribly unpleasant task. Equally important is the handling of sludge which is generated in appreciable volumes during the normal operation of an STP. This sludge needs to be manually handled as it is coming from the basement. Regular maintenance therefore can become a recurring night mare for the Operation & Maintenance team.

Why should an STP not be in a basement ?

Something that is extremely important but never done till it becomes too late. A residents’ association must insist on the builder furnishing the association with all documentation of what the property has installed on it, eg, As built drawings with criteria used for designing/selecting, as well as detailed technical specifications for the electrical installations, power generation equipment, complete water infrastructure, piping for fresh water and sewage with drawings showing the routings and this must include the piping for the waste water to and from the STP.

Details of all the pumps and other motor driven rotary equipment as installed with information on how these have been selected, as, this will have a crucial bearing on the power consumption in the community. Proper documentation is a must for all the engineering incorporated in a property Here, ignorance is not bliss,it is an unmitigated disaster! Without this, maintenance/repairs can become a major problem due to sheer lack of information on all the equipment which is required to undertake planned maintenance.

It looks like a buyer of a home in any residential complex is going to be a big loser no matter what, what can be done to rectify this and protect the home buyers?

One option is for the Government (either Central or State) to enact legislation to protect buyers of homes. Another option would be for the various associations of builders to themselves evolve a code of ethics that would ensure protection with regard to providing water security in a totally transparent manner. Neither of these is likely to happen in a hurry ,so, it may be necessary for Residents Associations countrywide to come together and put pressure on the governments and builders to ‘clean up their act’ and do things in a more transparent manner concerning the crucial aspect of water security.

Research and Compilation by

Mr.S.S.Ranganathan, a Bangalore based water expert

India Water Portal requests users to view the same as a starting point in collating information on Waste water / Sewage Treatment Plants (STP) and to add more suggestions, information as responses in this thread.