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Frequently Asked Questions


1. What is wastewater / sewerage?

Wastewater is any water that has been adversely affected in quality by human activities or influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations. In the most common usage, it refers to the municipal wastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewaters from different sources.
Sewerage is correctly the subset of wastewater that is contaminated with faeces or urine, but is often used to mean any waste water. "Sewerage" includes domestic, municipal, or industrial liquid waste products disposed of, usually via a pipe or sewer or similar structure, sometimes in a cesspool emptier.

The physical infrastructure, including pipes, pumps, screens, channels, etc. used to convey sewage from its origin to the point of eventual treatment or disposal is termed sewerage.

Wastewater or sewerage can come from:

  • Human waste (faeces, used toilet paper or wipes, urine, or other bodily fluids), also known as black water, usually from lavatories;
  • Cesspit leakage;
  • Septic tank discharge;
  • Sewerage treatment plant discharge;
  • Washing water (personal, clothes, floors, dishes, etc.), also known as grey water or sullage;
  • Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel);
  • Groundwater infiltrated into sewage;
  • Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.);
  • Urban rainfall runoff from roads, carparks, roofs, sidewalks, or pavements (contains oils, animal faeces, litter, fuel or rubber residues, metals from vehicle exhausts, etc.);
  • Seawater ingress (inflow) (high volumes of salt and micro-biota);
  • Direct ingress of river water (high volumes of micro-biota);
  • Direct ingress of manmade liquids (illegal disposal of pesticides, used oils, etc.);
  • Highway drainage (oil, de-icing agents, rubber residues);
  • Storm drains (almost anything, including cars, shopping trolleys, trees, cattle, etc.);
  • Black water (surface water contaminated by sewerage);
  • Industrial waste
  • industrial site drainage (silt, sand, alkali, oil, chemical residues);
  • Industrial cooling waters (biocides, heat, slimes, silt);
  • Industrial process waters;
  • Organic or bio-degradable waste, including waste from abattoirs, creameries, and ice cream manufacture;
  • Organic or non bio-degradable/difficult-to-treat waste (pharmaceutical or pesticidal manufacturing);
  • Extreme pH waste (from acid/alkali manufacturing, metal plating);
  • Toxic waste (metal plating, cyanide production, pesticide manufacturing, etc.);
  • Solids and Emulsions (paper manufacturing, foodstuffs, lubricating and hydraulic oil manufacturing, etc.);
  • Agricultural drainage, direct and diffuse.

The composition of wastewater varies widely. This is a partial list of what it may contain:

  • Water ( > 95%) which is often added during flushing to carry waste down a drain;
  • Pathogens such as bacteria, viruses, prions and parasitic worms;
  • Non-pathogenic bacteria (> 100,000 / ml for sewage);
  • Organic particles such as faeces, hairs, food, vomit, paper fibers, plant material, humus, etc.;
  • Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.;
  • Inorganic particles such as sand, grit, metal particles, ceramics, etc.;
  • Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.;
  • Animals such as protozoa, insects, arthropods, small fish, etc.;
  • Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants, body parts, etc.;
  • Gases such as hydrogen sulfide, carbon dioxide, methane, etc.;
  • Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.;
  • Toxins such as pesticides, poisons, herbicides, etc.


2. What is grey water?

Grey water is wastewater generated from domestic activities such as dish washing, laundry and bathing. Grey water comprises 50-80% of residential wastewater generated from all of the house's sanitation equipment except for the toilets. Water from the toilets is designated sewage or black water to indicate it contains faecal matter and urine. Grey water gets its name from its cloudy appearance and from its status as being neither fresh (white water from groundwater or potable water), nor polluted (sewage). According to this definition, wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners, etc., may be considered "dark grey" or dirty water.

In recent years, concerns over dwindling reserves of groundwater and overloaded or costly sewage treatment plants have generated much interest in the reuse or recycling of grey water, both domestically and for use in commercial irrigation. However, concerns over potential health and environmental risks mean that many jurisdictions demand such intensive treatment systems for legal reuse of grey water that the commercial cost is higher than for fresh water.

Despite these obstacles, grey water is often reused for irrigation, illegally or not. In drought zones or areas hit by hose pipe bans (irrigation restrictions), grey water can be harvested informally by manual bucketing. In the third world, reuse of grey water is often unregulated and is common. At present, the recycling of grey water is poorly understood compared with elimination.

Recycling of grey water

Most grey waters are much easier to treat and recycle than black waters, because of their lower levels of contamination. However, entirely untreated grey water is still considered to be a potential health and pollution hazard, because studies have established the presence of the same micro-organisms within grey water as found in sewerage (albeit in much lower concentrations).

Nevertheless, while all grey water will contain micro-organisms, the health hazards associated with grey water from a multiple dwelling source should be considered different from that of a single dwelling grey water source. Within single dwellings, inhabitants and their clothing are mutually exposed to each other’s grey water and their shared living arrangements will likewise expose them to the existing reservoir of micro-organisms within the dwelling, whereas grey water from multiple dwelling sources provides scope for exposure to a broader reservoir of micro-organisms thus increasing the risk of disease spread between dwelling unit inhabitants.

If collected using a separate plumbing system from black water, domestic grey water can be recycled directly within the home, garden or agricultural company and used either immediately or processed and stored. Recycled grey water of this kind is never clean enough to drink, but a number of stages of filtration and microbial digestion can be used to provide water for washing or flushing toilets; relatively clean grey water may be applied directly from the sink to the garden or container field, as it receives high level treatment from soil and plant roots. Given that grey water may contain nutrients (e.g. from food, fertiliser, etc.), pathogens (e.g. from your skin), and is often discharged warm, it is very important not to store it before using it for irrigation purposes, unless it is treated first.


3. Where does wastewater go and what happens to it?

Sewerage disposal

Sewerage or wastewater goes down the drain and into pipes where it is transported to a sewerage or wastewater works. The methods for cleaning wastewater from urbanised areas vary, but on the whole include similar steps. The following outlines the basic method for cleaning wastewater at a municipal waste water works:

  • The wastewater arrives at the facility and large objects like litter and solid waste are removed.
  • The wastewater then flows into chambers where heavy objects like sand and stones sink to the bottom and are removed.
  • In the next step the waste water flows into primary settling tanks where fine solids containing germs sink to the bottom to form a layer of sludge.
  • Organisms known as Bio-reactors are added to eat dissolved nutrients in the water.
  • In secondary settling tanks the bio-reactors along with some germs sink to the bottom to form a layer of secondary sludge.
  • The water is then treated with chlorine gas to kill any remaining germs and bio-reactors.
  • After this the water is then put back into the nearest drainage basin.

In some urban areas, sewerage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these is typically through a manhole. During high precipitation periods a sanitary sewer overflow can occur, causing potential public health and ecological damage.

Wastewater may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have serious impacts on the quality of an environment and on the health of people. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.


There are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination. Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes. However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in rural areas. The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to absorb and adsorb the organic matter carried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes.

After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with a source of carbon. Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozone generator, which decontaminates the water as Ozone bubbles percolate through the tank.
Disposal of wastewaters from an industrial plant is a difficult and costly problem.

Most petroleum refineries, chemical and petrochemical plants have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans. Other Industrial processes that produce a lot of waste-waters such as paper and pulp production has created environmental concern leading to development of processes to recycle water use within plants before they have to be cleaned and disposed of.


Treated wastewater can be reused as drinking water, in industry (cooling towers), in artificial recharge of aquifers, in agriculture (70% of Israel's irrigated agriculture is based on highly purified wastewater) and in the rehabilitation of natural ecosystems.


4. What is a sewerage spill?

Sewerage spills are simply an overflow of untreated or partially-treated sewage from the sewer system (i.e., the raw sewage overflows from a sewer line before it reaches the wastewater treatment plant). The sewerage can overflow from the manholes in the streets, from open cleanout lines, or from toilets and drains in your home. In really bad situations, someone else’s sewerage could spill out of your toilet or shower and flood your home. This may not happen to you but what you do in your home could cause it to happen to someone else living further down the sewer line!

Ten Terms to Help You Better Understand Your Sewer System:

1. “Sewerage” Or “Wastewater.”
This is the “used” water that contains human wastes from toilets and water from other sources such as sinks, showers, washing machines, etc. In addition to being odorous, sewerage can contain large amounts of germs that cause disease. The term “wastewater” is often used in place of “sewerage” to make things sound more pleasant when discussing this unpleasant subject.

2. “Sanitary Sewer System,” Also Known As “Wastewater Collection System,” Or “Sewers.”
These are pipes through which sewerage is carried from homes and businesses to a treatment plant. The sanitary sewer system includes the main sewer lines in the streets and the branch lines to individual sewer customers called “sewer laterals.”

Sewer systems are generally designed to flow by gravity through sloped pipes until it reaches either the treatment plant or a sewerage pumping station (which pumps the sewerage up to another higher sewer or a treatment plant).

Although sewerage is very unsanitary, the term “sanitary sewer” is used because the sewer pipes are separate from the pipes used for storm water drainage. This helps protect public health and the environment. In some older cities, sewerage and rainwater flow through the same pipes. This can cause major environmental and public health problems because untreated or partially treated-sewage is discharged into streams, rivers and other water bodies during heavy rain.

3. “Sewer Lateral.”
This is the sewer pipe that connects a building’s plumbing system to the main sewer line in the street. Maintenance of sewer lateral pipes located within private property is generally the responsibility of the property owner.

Sewer laterals are also called “service laterals,” “house laterals,” or simply “laterals.”

4. “Sewer Cleanout.”
This is a pipe rising from the sewer lateral to the ground surface with a removable cap or plug. It is used to access the sewer lateral to free blockages. A sewer cleanout is usually located just inside the property line. There may be additional sewer cleanouts at various other
locations in your property.

5. “Wastewater Treatment Plant” or “Wastewater Reclamation Facility”.
These are facilities where organic matter, bacteria, viruses and solids are removed from sewerage through physical, biological and chemical processes. The treated wastewater (called effluent) may be disposed of by discharging it to water bodies, injecting it into the ground, or reusing it for irrigation or other beneficial non-potable (non-drinking) uses.

6. “Infiltration.”
This refers to groundwater (water found below the ground surface) that enters sewer pipes through cracks, pipe joints, and other system leaks. Because sewers in coastal areas are typically buried deep, they are often located below the water table. Since most sewer lines do not flow full (under pressure), groundwater “infiltrating” into the sewer line is actually more of a problem than sewage leaking out of the line. Storm events can raise groundwater levels and increase infiltration of groundwater into sewer pipes. The highest infiltration flows are observed during or right after heavy rain. Too much infiltration will overload the sewers and cause spills.

7. “Inflow.”
This is rainwater that enters the sewer system from sources such as yard and patio drains, roof gutter downspouts, uncapped cleanouts, pond or pool overflow drains, footing drains, cross-connections with storm drains, and even holes in manhole covers. Inflow is greatest during heavy rainfall and like infiltration, can cause excessive flows and sewage spills.

8. “Pathogens.”
These are harmful germs in raw sewerage that cause diseases such as cholera, dysentery, hepatitis and gastroenteritis.

9. “Manholes.”
Sewer manholes are underground structures used to provide access to underground sewer lines and are usually found in a street, parking area or sidewalk. Access is required to periodically inspect and clean the lines. Sewer manholes typically have heavy round covers with the words “Sanitary Sewer” on the cover.

10. “Sanitary Sewer Overflow.”
Sewerage spills are technically called “sanitary sewer overflows” since it involves the overflow of sewage from the sanitary sewer system. The word “sanitary” is used only because the overflow is from the sanitary sewer system, and not because the raw sewerage is sanitary. For simplicity, we will use the term “sewerage spill” or “sewage overflow.”

Sewerage overflows often occur from sewer manholes in the streets. Sewerage can also backup into homes through your toilets, showers and floor drains. Sewerage spills are caused by sewage filling the sewer pipes behind the clog to the point where it spills out of an opening in the system (generally the lowest manhole, shower drain or other plumbing fixture).

5. How can wastewater cause water pollution?

A sewerage or wastewater spill can occur if a sewerage system is overloaded and cannot handle the volume of waste water coming through or it. The spill usually happens at the nearest manhole on the sewerage line and raw sewerage flows out onto the surface and into the nearest drainage basin via the roads and storm water drains. A sewerage spill can also occur at a sewerage works where mismanagement is present or if flooding from high rainfall occurs. Raw or partly treated sewerage can flow out of the settling tanks and into the nearest drainage basin.

Raw sewerage contains high levels of nutrients such as nitrates and phosphates, disease causing bacteria such as E. coli and large amounts of suspended sediment from human and household waste. These cause serious pollution problems in water bodies.

Raw sewerage contains biological agents such as bacteria, viruses, fungi and parasites that can cause serious illness and even death. There is also a risk from contamination with unknown chemicals (such as solvents, carcinogens, pesticides) and from toxic, irritant, asphyxiating or flammable gases in confined spaces.

Always assume that floodwater is contaminated with sewage. Immediate clean up is essential to reduce the risk of infection and/or mould growth.

6. What are the potential health effects of a sewerage spill?

Exposure to a sewerage spill can cause various health risks to the individual. It is important to always exercise caution in the event of exposure to raw sewerage. The risk to health depends on the microbes present, duration of exposure and method of exposure. Microbes in raw sewerage can enter the body via the nose, mouth, open wounds or by inhalation of aerosols or dusts. The most common modes of infection are through drinking contaminated water or hand to mouth transmission. Skin contact alone does not pose a health threat unless you have an open wound.

The survival of pathogens depends on a number of factors: location, type of surface contaminated, whether disinfectants are used and environmental conditions. UV radiation reduces the survival rate of pathogens. Mild temperatures and higher humidity increase survival times. The risk of exposure when handling sewerage can be reduced significantly by effective and immediate clean-up and by taking appropriate safety precautions.
The following are some examples of the risks:

Tetanus is caused by a toxin produced by the bacterium Clostridium tetani that is common in soil and in sewage. The bacterium enters the body via open wounds. There is a high risk of death occurring if infected. Anyone who may be exposed to sewage or soil should have prophylaxis tetanus vaccinations every ten years.

Leptospirosis is caused by the parasitic worm Leptospira icterohaemorrhegiae and is transmitted from water and damp earth contaminated primarily by rats that harbour the organism. The initial septicemia phase lasts for 4-7 days and causes acute headache, chills, fever, severe muscle aching, anorexia, nausea and vomiting. The immune phase, characterised by aseptic meningitis, follows a 24-72 hour asymptomatic period. Approximately 10-15% of patients present with Weil's disease, jaundice, hemorrhage and renal damage.

Hepatitis A is caused by the Hepatitis A virus (HAV) that is transmitted primarily by ingestion. The virus must be present in sufficient quantities to cause infection. Infection occurs after an incubation period of three to four weeks. Hepatitis A is often mild, but can be severe or even fatal in some cases. Symptoms are fever, headache, nausea and pain in the abdomen, dark urine and jaundice. People can spread the disease to others in the immediate period before they become ill and while they are ill. Recovery from Hepatitis A can be slow and require several weeks or months of increased rest. A majority of patients make a complete recovery but the disease can be more severe in older patients.

Giardia and Cryptosporidium are protozoan parasites, commonly found in sewage and surface waters, that can cause diarrhea, stomach cramps, nausea and sometimes fever. Symptoms may last for only a few days or can last for months or years. Many people, especially children, have no symptoms. Cysts from infected persons or animals enter sewage and if untreated may infect other people who ingest the cysts.

Gram-negative bacteria such as E.coli can cause gastro-intestinal diseases if ingested or airway problems, headache, tiredness and nausea if inhaled. Substances called endotoxins that are released at the time of death of the bacterium have been suggested as the cause of a wide variety of occupational diseases such as mill fever and grain fever.


7. What is the influence of a sewerage spill on the water quality, ecology and habitat functioning of a river?

Water Quality:

Raw sewerage will have a severe impact upon the water quality if it enters a river. The sewerage contains elevated levels of nutrients (nitrates and phosphates), disease causing bacteria (in particular E. coli) and large amounts of waste matter.

The elevated levels of nutrients will provide food for the bacteria to thrive and spread in the water. This will make the water undrinkable as the permitted amount of E. coli bacteria in potable water is 10 – 20 colonies per 100ml of water. High levels of nitrates will also cause burning of mucous membranes in the mouth and stomach and can cause ‘Blue Baby Syndrome,’ a condition in infants who drink nitrate rich water, where the blood starts to absorb nitrogen instead of oxygen and the baby slowly suffocates.

The large amount of waste matter will increase the turbidity and provide a habitat for bacteria to breed and feed on the suspended material.

Ecology and Habitat Functioning:

Sewerage contains large amounts of matter which adds to the suspended material in the water. This increases the turbidity of the water, blocking out sunlight which is necessary for all forms of life to exist in the water. It also blocks the gills of aquatic organisms, making it difficult to breathe and it makes it difficult for them to hunt and catch food.

The excess nutrients cause massive algal growth which uses up the dissolved oxygen in the water and causes eutrophication. All forms of life except for the very hardy will die or be chased away by these conditions. Breeding cycles will be disrupted and populations of sensitive organisms will decline.

Eutrophic conditions also promote the invasion of alien plants, for example the Water Hyacinth (Eichhornia crassipes), which thrives in low oxygen, nutrient rich water bodies. In general, a healthy, functioning aquatic eco-system can be drastically transformed into a choked, unhealthy system that is low in bio-diversity, if a sewerage spill affects it.

8. What is the influence of a sewerage spill on the economy of the community in the area?

Sewerage spills are a big problem because:

  • Sewerage spills cause public health problems. Spills can expose people to disease causing germs (pathogens) such as E. coli and Cryptosporidium that are present in sewerage.
  • Sewerage spills can pollute our streams, the ocean and other bodies of water. In addition to being a public health problem, sewerage can add unwanted nutrients to our water environment and cause excessive growth of algae that disrupts the ecosystem.
  • Sewerage spills can pollute the groundwater, which in many inland areas, is our source of drinking water. Sewerage spills hurt our economy. Sewerage spills are costly to clean up and this affects our sewer bills.

9. What are the main causes of sewerage spills and how can they be prevented?

  • Sewerage spills are caused by the clogging of pipes and/or too much flow. Clogging is caused by blockages from fats, oils and grease as well as rubbish, roots and other foreign or unwanted objects in the sewer system. Too much flow is caused by infiltration and inflow (i.e., groundwater and rainwater getting into the sewer system).
  • Preventing sewage spills is quite simple. Spills are simply caused by clogged pipes and/or too much flow. All everyone needs to do is keep unwanted things out of our sewer pipes such as grease, trash, rainwater and tree roots.
  • Fats, oils, and grease, and other byproducts of cooking come from meat, lard, shortening, butter, margarine, food scraps, sauces, and dairy products. They present a significant clogging problem for sewer systems. Fats, oils and grease stick to the inner walls of sewer pipes and reduce the diameter of the pipes over time. This eventually causes clogged sewer pipes and sewage spills.
  • Clogging is further caused by chunks of grease breaking away from the pipe walls and becoming stuck further down the line. Grease balls that form when grease combines with sand, grit, and other sewage debris can even become large and hard enough to clog sewage pumps.
  • Fats, oils and grease also flow down to the wastewater treatment plants where it disrupts operations and increases maintenance costs. Regulations require restaurants and other commercial food handling facilities to install large grease separation devices to protect sewers from grease problems. Folks at home need to do their part.

Do your share to keep rubbish from clogging our sewers by following these simple
Do’s and Don’ts:


  • Place and use a wastebasket in the bathroom to dispose of rubbish (includingdisposable diapers and personal hygiene products)
  • Use sink and shower drain strainers.
  • Scrape food scraps into sealed containers or bags and throw them out in the garbage.
  • Educate each other on minimizing disposal of rubbish to our sewers.


  • Don’t use the sewer as a convenient means to dispose of food scraps.
  • Don’t use the toilet as a wastebasket.


10. How should we properly dispose of grease and oils?

Everyone can do their share to prevent clogged sewers by following these simple Do’s and Don’ts:


  • Collect oil and grease in a container filled with absorbent material (shredded newspaper, napkins, paper towels, rags, etc.) and properly dispose of it in the garbage.
  • Scrape grease and food scraps off cooking/serving utensils and plates for proper disposal. Better yet, wipe them with used napkins and paper towels before washing.
  • Encourage friends and neighbors to practice similar habits of proper oil and grease disposal.


  • Do not pour grease or oil down the drain or toilet.
  • Do not dump greasy or oily food waste into the drain. (Minimize the use of your garbage disposal and better yet, compost your vegetable scraps).
  • Be sure to put your oil and grease in a suitable container or bag with absorbent material. The reason for using the absorbent material is so that your grease and oils do not leak out of garbage trucks and cause a big mess. Also, remember that solid grease can turn to liquid in our hot climate so use absorbent material for solid or semi-solid fats too.
  • If you have a large amount of cooking oil, consider using a disposable automotive oil change box filled with absorbent material. For even larger quantities, take your used cooking oil to a recycler.


11. How do you keep rubbish out of the sewer system?

Your toilet and sewer system are only designed to dispose of human wastes and toilet paper (which quickly breaks down). Unfortunately, people use the toilet as a wastebasket out of convenience. It is a huge “out of sight, out of mind” problem because people often don’t see the mess sewer overflows cause and the problems that sewer workers need to deal with. Almost any type of rubbish may restrict sewerage flow, clog sewers, and cause sewerage overflows. Keep the following from going down your toilet and sinks:

  • Paper (paper towels, facial tissue (Kleenex), paper napkins, wrappers, etc.). Only toilet tissue is okay.
  • Plastics (bags, wrappers, bottles, cotton-tip shafts),
  • Rubber (gloves, condoms, underclothes elastic, etc.),
  • Cloth and fibers (cotton balls, tampons, cigarette filters, stockings, rags, etc.).
  • Food scraps (greasy items are the worst but minimize throwing down non-greasy items too. Try to even keep out smaller food items such as tea-leaves, coffee grounds or eggshells. Garbage grinders help but its even better not to use it where possible -- compost what you can and throw the rest in the trash. Place food scraps in tightly sealed bags or other containers so it does not become an odor or rodent problem.)
  • Toys, cans, sticks, pebbles and sand, and pretty much all other solids except for human wastes and toilet tissue. Why is it a problem? Rubbish and other objects often combine with hair, grease and other debris to cause clogging of the sewer system. Even something as small as a cotton tip swab with other attached debris can cause a blockage in sewer pipes. Rags and stringy material can clog sewage pumps. Malfunctioning sewerage pumps, like clogged pipes, prevent sewage from flowing through the system and are a cause of spills. Any rubbish type items that you dump in toilets and sinks at home, work, schools, shopping centres, movie theaters, or parks can contribute to sewerage spills.


12. How do you keep rainwater and other excess water out of the sewer system?

What is infiltration and inflow?

Infiltration and inflow are the technical terms referring to rainwater and/or groundwater that enters the sewer system through such sources as cracked pipes, leaky manholes, or improperly connected storm drains and roof gutter downspouts. Most infiltration comes from groundwater and most inflow comes from rainwater.

Why are infiltration and inflow big problems?

In addition to causing sewerage spills, the additional flow from infiltration and inflow results in the need for larger sewers and treatment plants. This raises the sewer fees that residents and businesses must pay the government or private sewer agency to build, operate and maintain the sewers and wastewater treatment plants. Sewer systems (sewer pipes and pumping stations) are designed to handle sewerage flows from houses and businesses plus some additional flow from infiltration and inflow.

Sewerage flow rates used to design sewers have been developed over the years based on information obtained from water usage within the household and workplace. The exact volume of groundwater and rainwater (infiltration and inflow) entering the system, however, varies with location and is virtually impossible to predict. Infiltration and inflow entering the system can be much higher than the system’s capacity when there is too much leakage due to infiltration from deteriorated sewer pipes or significant sources of rainwater inflow. The infiltration and inflow that enters the sewer system is transported to wastewater treatment plants along with the sewage.

The groundwater and/or rainwater mixed with the sewerage can double and even triple the design capacity of the treatment plant. Like the sewer system, the treatment plants are generally designed and constructed to accommodate the expected sewage flows plus some infiltration and inflow, but not large volumes of groundwater and rainwater.

When large volumes of infiltration and inflow increase the wastewater flow, the sewer system is overwhelmed to the point where a sewage spill can occur. The extra flow from infiltration and inflow simply causes the sewer system capacity to be exceeded. Sewerage spills pose a public health risk due to increased probability of human contact with harmful pathogens as the sewage runs down the street to the storm drains, the streams, and eventually our recreational waters. Devastating backups of sewage into homes can also occur. In addition to causing sewage spills, the high flows can also affect the ability of the treatment plant to adequately treat the wastewater.

How does this affect the sewer fees that everyone pays? In many cases, your sewer
agency will deal with heavy infiltration and inflow by increasing the size of the sewer pipes, pumping stations, and treatment plants.

Constructing large sewer lines to handle high infiltration and inflow is very expensive and has its own problems associated with it. For example, large sewer pipes tend to result in sluggish flow during normal low dry weather flows. This causes the organic matter to putrefy and generate gases that are both odorous and corrosive to the sewer pipes. The corrosive gases shorten the life of the sewer lines and manholes, which increases your sewer bill even more.

At the sewage treatment plant, high infiltration and inflow can result in a significant amount of money being spent to construct facilities that are rarely used. The sewer users pay for the higher maintenance costs as well as the added construction costs. Once again, this increases your sewer bill.


13. What can you do to prevent and reduce infiltration and inflow?

The following are important actions that sewer users can take to help reduce infiltration and inflow:

  • Inspect the rain gutters on your house to see if the downspout connects to a sewer line. Such connections are illegal (violation of the plumbing code)! If the gutter downspouts are connected to the sewer line, have them disconnected—the large amount of water from the roof can cause a sewage spill. The rainwater needs to be directed onto your lawn and/or to the storm drain system.
  • Look for and check your sewer cleanout. The cleanout is usually a small pipe, about 4- inches in diameter, outside your house that is used to access the sewer lateral for cleaning. You will normally find it near the house (where the sewer lateral comes out) and/or near the property line (where the sewer lateral connects to the main sewer line). Make sure the cap to the cleanout pipe is not missing and has not been damaged (such as by a lawn mower). Replace missing caps so that rainwater cannot get into the sewer line. Kids love to throw rocks, toys and other nasty things down an uncapped cleanout! By keeping the cleanout capped, you can also prevent unpleasant sewer odors and gases from escaping.
  • Check to see that outdoor patio, deck or yard drains are not connected to the sewer.
  • Also, be sure that pool or pond overflow drains are not connected to the sewer. These connections are not allowed by the plumbing code. You may want to call your plumber to assist you in checking your connection. You can also try calling your sewer agency for assistance since they often have personnel that can trace lines and have a strong interest in keeping rainwater out of the sewers. If you are voluntarily taking steps to find and correct the problem, it is unlikely that you will be fined for the illegal connection(s).
  • If you live in a low area with a high water table, and/or experience a lot of settlement on your property, you may want to have your sewer line checked for cracks, separated joints, or “sags” that could cause entry of rainwater or clogging problems. Many plumbers now have miniature video cameras that can be sent down your line to check if the line has any significant damage or other problems.
  • Avoid planting trees and shrubs over or near the sewer laterals. This also applies to sewer mains that may be in yard easements. Roots can enter and damage sewers. This allows groundwater and rainwater to enter the sewer and also causes costly ongoing problems with sewer clogging, backups and spills.
  • If you have a basement sump pump to pump out groundwater or rainwater leakage, be sure that it does not connect to your sewer pipes or to a sink or floor drain in your basement. This would be another source of unwanted excess flows that can overload the sewer system.
  • If your area is experiencing flooding, NEVER try to drain the areas by removing the sewer manhole covers in the street or covers from your cleanouts. The huge amount of flow that would enter the sewer system will definitely cause a problem downstream. Notify your sewer agency if you observe or know of someone doing this.


14. Is it possible to purify wastewater / grey water for human consumption?

It is possible to purify wastewater and use it again for domestic consumption, but it is often very costly. There are different technologies that are still being developed for the purification of wastewater because often the method depends on the type of waste that is in the water. For example wastewater from a dairy needs different technology to grey water from residential areas. A common method is one that uses the process of reverse osmosis that removes dissolved salts from waste water. This process can also be used where water has been polluted by mining and other industrial activities.

The following is an example of the purification of wastewater:



NEWater is the brand name given to reclaimed water produced by Singapore's Public Utilities Board. More specifically, it is treated wastewater (sewage) that has been purified using dual-membrane (via microfiltration and reverse osmosis) and ultraviolet technologies, in addition to conventional water treatment processes. The water is potable and is consumed by humans, but is mostly used for industry requiring high purity water.


Water recycling in Singapore began in 1974 but the experimental treatment plant was closed a year later due to cost and reliability issues.

The Singapore Water Reclamation Study (NEWater Study) was initiated in 1998 by the Public Utilities Board (PUB) and the Ministry of the Environment and Water Resources (MEWR). The aim of this study was to determine if NEWater was a viable source of raw water for Singapore's needs. NEWater and desalination were explored as means to reduce reliance on water imported from Malaysia, which has been a source of friction over the years. Also, while the Malaysian government is treaty-bound to sell Singapore water until 2011, it is under no obligation to do so after this. It can still sell water until 2061.

In 2001, PUB began an effort to increase water supplies for non-potable use. Using NEWater for these applications would reduce the demand on the reservoirs for potable water.


Singapore has a total of four operational NEWater factories, at Bedok, Kranji, Ulu Pandan and Seletar Water Reclamation Plants, with the former two completed at the end of 2002, and the latter in February 2004 and the Ulu Pandan plant on March, 2007. There is a Visitor Centre near Tanah Merah MRT Station, which contains a working NEWater factory for educational purposes.


NEWater is the product from a multiple barrier water reclamation process:

  • The first barrier is the conventional wastewater treatment process whereby the used water is treated in the Water Reclamation Plants.
  • The second barrier, and first stage of the NEWater production process, uses microfiltration/ultrafiltration to filter out suspended solids, colloidal particles, disease-causing bacteria, some viruses and protozoan cysts. The filtered water that goes through the membrane contains only dissolved salts and organic molecules.
  • The third barrier, and second stage of the NEWater production process, utilizes reverse osmosis (RO). In RO, a semi-permeable membrane filters out undesirable contaminants such as bacteria, viruses, heavy metals, nitrate, chloride, sulphate, disinfection by-products, aromatic hydrocarbons, and pesticides that cannot pass through the membrane. Hence, NEWater is free from viruses and bacteria and contains very low levels of salts and organic matter. At this stage, the water is of a very high quality.
  • The fourth barrier, and third stage of the NEWater production process, acts as safety precaution. UV disinfection is used to ensure that all organisms are inactivated and the purity of the product water guaranteed. With the addition of some alkaline chemicals to restore the pH balance, the NEWater is ready for use.


At present, the total capacity of the three factories is about 20 million US gallons per day (75,700 m³/day). About 6% of this is used for indirect potable use, which contributes 1% of Singapore's potable water requirements of 300 million US gallons per day (13 m³/s). The rest of the water is used at wafer fabrication plants and other non-potable applications in industries in Woodlands, Tampines, Pasir Ris, and Ang Mo Kio.


PUB claims that the main reason why NEWater is not used as potable water directly is the psychological barrier of consuming (treated) sewage. Adding treated NEWater into the reservoirs helps to ameliorate this problem. The second reason PUB gives is that NEWater lacks minerals removed during the reverse osmosis process, and this changes the taste of the water.
The quality of NEWater consistently exceeds the requirements set by USEPA and WHO guidelines and is, in fact, cleaner than the other sources of Singapore's water.

Plans are under way to increase the amount of NEWater in indirect potable use up to 3.5% by 2011.
Besides water fabrication plants and other non-potable applications in industries, NEWater has recently been used for contest purposes. Notably, local universities like the National University of Singapore (NUS) and Singapore Management University (SMU) has encouraged its students to organise and participate in wet t-shirt contests, using NEWater as the main source of splashing fun. The contests are known to be aimed at raising money for charitable projects.


1. Water Management Issues in Singapore
2. NEWater
3. Public Utilities Board, NEWater FAQ, accessed 8 Jan 2007.