Environment Counts | Wastewater and sewage as an environmental issue
Author: Rick Higgins – Published At: 2013-02-18 22:18 – (2736 Reads)
Wastewater includes human generated sewage and grey water, liquid waste from industry and mining, and urban and agricultural runoff. It is a large and persistent global environmental problem. The UN estimated that in 1995 the total volume of waste water generated globally from all sources was in the order of 1,500 km3/year. Current global estimates of â€œurbanâ€ wastewater (this does not include wastewater from manufacturing, mining, agriculture and runoff) are in the order of 650 km3/year. The World Health Organization (WHO) estimates that each year 1.8 million children die as a result of water related diseases; that is one child dies every 20 seconds from water related diseases. The UN estimates that â€œup to 90% of wastewater is dumped untreated into water bodies polluting usable water resourcesâ€. Many urban and rural areas have no effective wastewater and sewage collection systems, let alone efficient treatment systems. Untreated and poorly treated wastewater end up back in the natural (hydrological) systems and contribute to the loss of wetlands, the contamination of groundwater and increase the number and extent of coastal marine dead zones. In many areas the impact on inland and coastal waterways increases the environmental impact of water-related disasters such as flooding and reduces the ability of local populations to derive a livelihood from their traditional industries.
Wastewater â€“ theÂ issue
Freshwater in all forms is only about 3% of the entire worldâ€™s water. The other 97% is salt water. Of all the worldâ€™s freshwater almost 70% is locked up in the Antarctic and Greenland icecaps, and most of the remaining freshwater is too deep underground to be accessible or is contained in soil moisture. This leaves only 1% of the worldâ€™s freshwater available for withdrawal and human use. The Water Cycle – A Summary
What happens to this 1% after it has been used or otherwise affected by humans is the subject of this article. This article starts with a more thorough definition of wastewater and summarises its origins and composition, what happens to it after collection and treatment (including when it is not collected or treated) and indicates some key environmental impacts. Most cities and countries (perhaps surprisingly) do not collect data on the volume of wastewater â€“ see Editorâ€™s comments above. Although detailed global data on wastewater is not available, estimates are provided by various international and national authorities (eg UNEP, OECD, EPA) and these provide a useful starting point.
Wastewater is any water that has been adversely affected in quality by human activity (anthropogenic). It is made up of liquid waste discharged by domestic residences (commonly referred to as sewage), commercial properties, manufacturing industry, mining and agriculture.
Sewage (or sewerage – see editorâ€™s comments) is a subset of wastewater that is contaminated with faeces or urine, but is often used to mean any wastewater. The term â€œsewageâ€ is also used by many authorities to include the physical infrastructure, including pipes, pumps, screens, channels and so on used to convey sewage from its origin to the point of eventual treatment or disposal into the hydrological system (including inland and urban waterways and eventually the oceans).
Origins and composition
The origins and composition of wastewater are many and diverse. These can include all the following.
- Human waste (urine, faeces, toilet paper and other bodily fluids). This is sometimes referred to as â€œblack waterâ€
- Septic tank discharge and cesspit leakage
- Sewage treatment plant discharge (following varying levels of primary, secondary and tertiary treatment)
- Disease causing agents or pathogens such as germs, bacteria, parasitic worms and non-pathogenic bacteria of various origins
- Washing water (personal, clothes, cleaning of all types using water with or without chemicals) commonly known as â€œgrey waterâ€ or â€œsullageâ€
- Urban rainfall runoff, from roofs, roads and other surfaces, often including traces of oils and fuels as well as litter, animal faeces and residues from vehicle exhausts, de-icing agents, rubber tires and metals
- Manufactured liquids including cooking oil, pesticides, paint, cleaning liquids, etc
- Industrial site drainage including water used in processes, waste from abattoirs, dairy processing, and a wide range of organic and inorganic and often toxic wastes from industry
- Mine site drainage both direct and diffuse from treated and untreated mines and mine
- Agriculture drainage, both direct and diffuse into waterways and groundwater
- Drugs and pharmaceuticals, which are becoming increasingly common and dangerous additions to wastewater
- Soluble organic material (in addition to drugs) including urea, soluble proteins and fruit sugars
- Toxins from a wide range of sources including pesticides, herbicides and poisons
- Soluble inorganic materials including road salts, cyanide, hydrogen sulfide
- Emulsions including paints, oils and adhesives
Greater than 95% of wastewater by volume is water which either has been used in a process or used to flush waste away from the origin of contamination.
Wastewater and sewageÂ treatment
In many cities and urban areas (in both developed and developing countries) the problems of treatment start with the physical collection systems. Wastewater and sewage collection systems are sometimes separate, sometimes combined and sometimes occasionally separate and occasionally combined (as during heavy rain and local flooding).
There are normally three levels of treatment; primary, secondary and tertiary.
Ideally treatment should follow an integrated system of all three components, but the cost of doing this has prevented it in many urban areas, particularly in developing countries.
The following diagram is a simplified schematic of a typical primary, secondary, tertiary treatment process for wastewater and sewage treatment. Specialised treatment processes for mining and manufacturing industries may follow similar steps, but are designed specifically for the particular industrial process or mine.
Primary treatment refers to the treatment of
wastewater by the gravity-driven settling of solids and flotation of scum. The preliminary steps in this stage include the removal of coarse materials such as sand, grit and rock and other materials such as sticks, rags, and metals by using bar screens and grit removal systems. Oxygen is added by aeration, up to 95% of settled solids are removed and the initial biodegradation by microorganisms begins.
- Secondary treatment refers to the treatment of wastewater to remove dissolved solids (e.g. sugars) from the wastewater; this is done using micro-organisms to consume the wastes in aerated tanks, followed by settling of the micro-organisms and associated solids in a clarifier (a quiet settling tank) or pond. This stage of treatment produces secondary sludge (biomass) and microorganisms feed on the sludge and convert significant proportions of it into methane (CH4) and carbon dioxide (CO2). In turn the CH4 and CO2 create additional environmental problems if these are released into the atmosphere (they are often flared instead of being released). The remaining sludge is usually sent to landfills.
Tertiary treatment (advanced) refers to additional
processes undertaken with the goal of creating cleaned water suitable for subsequent use. Common tertiary treatments include additional removal of nutrients (nitrogen, phosphorus), chemical treatments, pressure filtration, and polishing (eg through a wetlands system). Tertiary treatment can even take the final stage of treatment to the point of yielding potable water, though this is still rarely done on a large urban scale.
- Septic systems are widespread in rural and less densely populated areas. These are separate from the above primary, secondary, tertiary systems. They are local, on-site forms of treatment in a tank which releases liquid waste after settling from the top of the tank into the surrounding soil, often through a leach field.
Worldwide there are extreme variations in the extent to which wastewater and sewage is
collected and treated. The above graph (Fig 8.5) indicates the wide variations in treatment across OECD countries.
Global variations in treatment are considerably wider than within the OECD. The following graphic indicates regional variations in the ratio of treatment (treated vs. untreated).
Global estimates ofÂ wastewater
Although reliable data on the extent and severity of pollution is incomplete, UN agencies and development banks most frequently cite the estimate that annual global wastewater production is in the order of 1,500 km3. This estimate comes from the work of Shiklomanov â€“ source below.
The UN also estimates that in 1995 the volume of waste water in Europe was 326 km3/year, in North America it was 431 km3/year, in Asia it was 590 km3/year and in Africa it was 55 km3/year. These estimates are somewhat old but are still widely used and quoted by the UN and other relevant international organizations.
At issue is how these estimates are derived (see Editorâ€™s comments). Underlying these estimates are further estimates of the average per capita daily volume of wastewater generated. These estimates in turn come from numerous studies at the city level of per capita wastewater generation.
Estimated average per capita daily generation of wastewater by country varies widely from more than 650 litres/capita/day (eg Canada) to less than 120 litres/capita/day (eg India). Current global wastewater estimates are based on an average of 260 litres/capita/day. This is approximately 95,000 litres or 95 cubic metres/capita/year. Note: 1 km3 (cubic km) equals 1 billion m3 (cubic metres.)
By way of reference, the total volume of the worldâ€™s freshwater contained in all the worldâ€™s lakes and rivers is in the order 105,000 km3. UNWater
The estimated global volume of wastewater generated annually is therefore in the order of 1.5% of the volume of water in all the worldâ€™s freshwater lakes and rivers.
The following table (data from WHO/UNICEF Global Water Supply and Sanitation Assessment 2000 Report) indicates the wide regional variations in sewage collection (# of connections) and the extreme variation in the proportion of wastewater treated to the
secondary level. Some countries discharge most of their waste water containing harmful substances into the hydrological system with no preliminary treatment. Prime water resources are thus polluted and their subsequent use becomes unsuitable for most uses, especially as potable water supplies.
The UN estimates that worldwide up to 90% of wastewater is dumped untreated into water bodies polluting usable water resources.
One particular body of research on water and wastewater volumes at the global level that is consistently cited by most agencies and authorities on the subject is the work of Shiklomanov.
Health andÂ wastewater
Poor sanitation and inadequate wastewater management in developing countries results in contamination of freshwater sources and is a major cause of disease and death, particularly among children. This is mainly, but not exclusively a problem in developing countries. The greatest short-term danger to humans from untreated wastewater is from pathogens and other micro-organisms that can cause disease. WHO estimates that worldwide:
â€¢ At least 1.8 million children under five years die every year due to water related disease. That is one child dies every 20 seconds from water related diseases. (WHO, 2008).
â€¢ Approximately 900 million people currently lack access to safe drinking water. (WHO/UNICEF, 2010)
â€¢ An estimated 2.6 billion people lack access to basic sanitation. (WHO/UNICEF,
â€¢ An estimated 3.7% of all deaths and at any one time over half of the worldâ€™s hospitals beds are filled with people suffering from water related diseases (UNDP 2006).
For an estimated 88 per cent of diarrhoea cases the underlying cause is unsafe water, inadequate sanitation and related poor hygiene. The WHO also estimates that 50 per cent of malnutrition is associated with repeated diarrhoea or intestinal worm infections. (WHO, 2008).
Pollutants inÂ wastewater
The section on â€œOrigins and Compositionâ€ above lists some of the many of the types of pollutants found in wastewater. Treatment systems can deal with many pollutants. There are some toxic pollutants (eg heavy metals and some pharmaceuticals) which are not readily removed or effectively treated by standard municipal treatment systems. Many of these are from manufacturing industry and mining rather than domestic sources. This field receives considerable research attention. While there are many studies on aspects of this issue. The European Commission (EC) released a major study in 2001 titled Pollutants in Urban Wastewater and Sewage Sludge (prepared for the EC by I C Consultants Ltd, London) which serves as a relatively comprehensive reference on the subject.
Wastewater contribution to greenhouseÂ gases
The US EPA reports that wastewater is the fifth largest source
of anthropogenic methane (CH4) emissions, contributing approximately 9% of total global CH4 emissions in 2000. India, China, the United States and Indonesia combined account for 49 percent of the worldâ€™s CH4 emissions from wastewater. Worldwide CH4 from wastewater accounted for more than 523 MtCO2eq in 2000.
Wastewater is also the sixth largest contributor to nitrous oxide (N2O) emissions,
accounting for approximately 3% of total global N2O emissions from all sources. China, the United States and Brazil together accounted for approximately 43% of total N2O emissions from domestic wastewater in 2000. Worldwide, N2O emissions from wastewater accounted for approximately 91 MtCO2eq in 2000. EPA – III Waste
Impact on inland and coastalÂ waters
The UN estimates that worldwide 90 per cent of wastewater is dumped untreated into
water bodies polluting usable water resources. This worsens the impact of water-related disasters such as floods when they invade low lying settlements. It also directly contributes to growing marine dead zones, which already cover an area of 245,000 km2. By way of reference this is approximately the same area covered by all the worldâ€™s coral reefs. UNSGAB
Any oxidizable material present in a natural waterway or in industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. As a result the oxygen content of the water is generally decreased. As all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into such waterways will initiate biochemical reactions. To date, global estimates of the impact on inland and coastal waters are not available.
Wastewater is a complex issue and the nature and significance of it as an environmental problem varies considerably from country to country. This article indicates some of the country and regional differences. The following chart summarises graphically at a high level some of the differing regional issues in dealing with wastewater.
Source andÂ reference
â€¢ World Water Resources at the Beginning of the Twenty-First Century
Edited by Shiklomanov and Rodda. International Hydrology Series, UNESCO and Cambridge University Press