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. It’s formed by a number of activities including bathing, washing, using the toilet, and rainwater runoff. Wastewater is full of contaminants including bacteria, chemicals and other toxins.
Wastewater treatment is becoming increasingly complex because of the wide spectrum of pollutants generated by industries including chemical, biomedical, pharmaceutical, textile, and other sectors.
There are two basic wastewater management strategies. They are onsite (non-sewered) disposal and off-site (sewered) disposal with or without centralised treatment facilities.
Wastewater treatment process can be divided into anaerobic systems (septic tanks, anaerobic ponds and anaerobic digesters), aerobic systems with attached growth (typically trickling filters) and aerobic suspended growth systems (typically activated sludge).
A primary sedimentation tank usually precedes these systems.
Untreated wastewater causes major damage to the environment and to human health. Almost always, therefore, wastewater should be treated in order to:
The continued discharge of untreated wastewater with its resultant damage to the environment and human health. Currently the global burden of excreta-related disease is extremely high. The world’s rivers, lakes and coastal waters are seriously polluted by untreated domestic, industrial and agricultural wastewaters, and they contain high numbers of faecal bacteria. Effective wastewater treatment needs to be recognized, therefore, as an environmental and human health imperative.
Domestic (sanitary) wastewater is the water that has been used by a community and which contains all the materials added to the water during its use. It is thus composed of human body wastes (faeces and urine) together with the water used for flushing toilets, and sullage, which is the wastewater resulting from personal washing, laundry, food preparation and the cleaning of kitchen utensils.
Basically, two types of wastewater are created in a home: greywater and blackwater.
Greywater is wastewater from non-toilet plumbing fixtures such as showers, basins and taps.
Blackwater is water that has been mixed with waste from the toilet. Because of the potential for contamination by pathogens and grease, water from kitchens and dishwashers should be excluded from greywater and considered as blackwater.
Sullage contributes a wide variety of chemicals: detergents, soaps, fats and greases of various kinds, pesticides, anything in fact that goes down the kitchen sink, and this may include such diverse items as sour milk, vegetable peelings, tea leaves, soil particles (arising from the preparation of vegetables) and sand (used to clean cooking utensils). The number of different chemicals that are found in domestic wastewater is so vast that, even if it were possible, it would be meaningless to list them all. For this reason wastewater treatment engineers use special parameters to characterize wastewaters.
Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use. The different types of contamination of wastewater require a variety of strategies to remove the contamination.
The main objective of wastewater treatment is to reduce the concentrations of specific pollutants to the level at which the discharge of the effluent will not adversely affect the environment or pose a health threat. Moreover, reduction of these constituents need only be to some required level. Although water can technically be completely purified by distillation and deionization, this is unnecessary and may actually be detrimental to the receiving water. Fish and other organisms cannot survive in deionized or distilled water.
In the early 1900s, wastewater treatment mainly involved filtration, settling, and septic tank use. The design of wastewater treatment plants was also considered, along with the application of disinfection methods, largely in the form of chlorination. The biological treatment method also emerged during this period.
Fresh wastewater is a grey turbid liquid that has an earthy but inoffensive odour. It contains large floating and suspended solids (such as faeces, rags, plastic containers, maize cobs), smaller suspended solids (such as partially disintegrated faeces, paper, vegetable peel) and very small solids in colloidal (ie non-settleable) suspension, as well as pollutants in true solution.
The nature of domestic wastewater is so complex that it precludes its complete analysis. However, since it is comparatively easy to measure the amount of oxygen used by the bacteria as they oxidize the wastewater, the concentration of organic matter in the wastewater can easily be expressed in terms of the amount of oxygen required for its oxidation. Thus, if, for example, half a gram of oxygen is consumed in the oxidation of each litre of a particular wastewater, then we say that this wastewater has an ‘oxygen demand’ of 500 mg/l, by which we mean that the concentration of organic matter in a litre of the wastewater is such that its oxidation requires 500 mg of oxygen.
Naturally occurring microorganisms are the workhorses of wastewater treatment. Consisting of bacteria, fungi, protozoa, rotifers, and other microbes, these organisms thrive on many of the complex compounds contained in domestic wastewater. Aerobic treatment of domestic wastewater has been around for years and is a proven way of quickly and economically treating domestic waste water.
Secondary-treatment processes are highly engineered bioreactors (found at municipal wastewater treatment plants). These bioreactors are designed to provide the microbes with the optimum conditions to assist in the renovation of domestic wastewater. With the mechanical addition of dissolved oxygen, aerobic and facultative microbes can rapidly oxidized soluble, bioavailable organic and nitrogenous compounds.
Aerobic treatment units can be an option when insufficient soil is available for the proper installation of a traditional septic tank and soil absorption area.
Increasingly, homes and small commercial establishments are being constructed in rural areas with no central sewer and on sites with marginal soils. In these situations, wastewater must receive a high-level of pretreatment before being discharged into the soil environment. Depending on local regulations, the use of an aerobic treatment unit may allow.
The final effluent from treatment plants can generally be released safely by irrigation or into the environment such as receiving streams or rivers under a local permit. The treated effluent is non-potable water, however it can be used in a variety of water conservation methods, such as agricultural irrigation, industrial uses, and other water reclamation uses
Most people consider bacteria and other microorganisms to be undesirable components of wastewater. In fact, only a small fraction of the microbes found in wastewater are truly pathogenic. Aerobic wastewater treatment encourages the growth of naturally-occurring aerobic microorganisms as a means of renovating wastewater. Such microbes are the engines of wastewater treatment plants. Organic compounds are high-energy forms of carbon. The oxidation of organic compounds to the low-energy form (carbon dioxide) is the fuel that powers these engines. Understanding how to mix aerobic microorganisms, soluble organic compounds and dissolved oxygen for high-rate oxidation of organic carbon is one of the fundamental tasks of wastewater engineers.
Aerobic treatment units are high-rate oxidizers of soluble organic and nitrogenous compounds. From a biological perspective, ATUs do not employ any new processes that are not already utilized in large-scale wastewater treatment plants. The technology that is unique to ATUs is the design and packaging of these systems for small flow situations. These devices are essentially miniature wastewater treatment plants. In addition to the reduction of BOD by aerobic digestion and the conversion of ammonia by nitrification, many commercially available ATUs have additional chambers that promote the removal of nutrients, suspended solids and pathogens from the effluent.
Other unique aspects to the design of ATUs are the ease of installation at remote locations and the ease of maintenance for semi-skilled maintenance providers. ATUs installed at homesites and small commercial locations must be dependable and maintenance-friendly.
Primary treated wastewater enters the aeration unit and is mixed with dissolved oxygen and suspended and/or attached microbes. The aerobic microbes convert organic compounds into energy, new cells and residual matter. As the water moves through the clarifier, a portion of the biological solids are separated out of the effluent and are retained within the ATU. The biological solids settle back into the aeration chamber where they serve as seed for new microbial growth. Settled biomass and residuals will accumulate in the bottom of the chamber and must be removed with periodic maintenance
With drought conditions affecting more than 60 percent of the country this year, water management and conservation in buildings have become even more critical. Beyond water shortages due to extreme climate events, the environmental and economic benefits that water recycling and on-site wastewater treatment systems offer make them worthy topics for architects to understand.
For architects unfamiliar with the design and permitting process for on-site wastewater treatment systems, installing a graywater system in tandem with other methods of water reduction—such as low-flow appliances and rainwater collection—is a big step toward reducing a building’s overall consumption of potable water.
Regardless of their size and capacity, graywater treatment systems (Pre-engineered, modular pre-fabricated structures : means low cost and quick turnaround time for delivery and installation) generally comprise pipes for collection at the source and for transport to a filtering mechanism; a septic or storage tank where collected liquid is held for treatment or undergoes treatment; and a filtration system to remove waste and debris. Most systems also require a pump to move the filtered wastewater to its point of reuse, although many take advantage of gravity. For example, a building may send graywater to a lower grade outside for subsurface irrigation.
Modern cities in developed countries have established extensive systems for wastewater collection and disposal with centralised treatment facilities. Like other public infrastructure facilities such as transport, urban parks, water and electricity supply, these wastewater management facilities also play a major role in making these cities more habitable, contributing to high living standards enjoyed by citizens.
Developing country governments and their regulatory agencies, as well as local authorities (which may be city or municipal councils, or specific wastewater treatment authorities, or more generally water and sewerage authorities), need to understand that domestic and other wastewaters require treatment before discharge or, preferably, re-use in agriculture and/or aquaculture. They also need to act, but first they need to decide where, when and how much to invest in wastewater treatment
The first stage of wastewater treatment is the removal of large floating objects (such as rags, maize cobs, pieces of wood) and heavy mineral particles (sand and grit). This is done in order to prevent, for example, floating material accumulating on the surface of waste stabilization ponds and heavy solids entering the pond sludge layer, and to protect from damage the equipment used in the subsequent stages of treatment (for example the floating aerators in aerated lagoons or any pumps which may be used). This preliminary treatment comprises screening and grit removal.
Not all of these technologies are necessarily sustainable or always sensible. Some may not be sensible at all, but are included here because they are often advocated as being a good (sometimes even ‘the best’) solution. Professionals (and would-be professionals) need to understand these technologies so that they are able to make informed decisions about which is really the best technology, or combination of technologies, to implement in any given situation, and also which technologies to avoid.
On-site wastewater reuse can reduce water use in both urban and rural households. At present, most homes use potable (drinkable) after for practically everything in the house and garden. Greywater is ideal for garden watering, with the appropriate precautions, such as using low or no sodium and phosphorus products and applying the water below the surface. Appropriately treated greywater can also be reused indoors for toilet flushing and clothes washing, both significant water consumers. Blackwater requires biological or chemical treatment and disinfection before reuse. For single dwellings, treated and disinfected blackwater can be used only outdoors, and often only for subsurface irrigation.
Using treated wastewaters for crop irrigation or for fishpond fertilization is a very sensible thing to do, especially in water-short areas. However, it must not cause any excess transmission of excreta-related disease, and therefore the wastewaters must be treated to an appropriate microbiological quality.
The World Health Organization has produced guidelines for the microbiological quality of treated wastewaters used in agriculture and aquaculture
By using wastewater as a resource, you can
When sustainability is considered in relation to domestic wastewater treatment, the following issues are relevant:
The disadvantages of reusing wastewater also need to be considered. Currently, the main disadvantage for most households is the financial cost of installing and maintaining a reuse system.
Wastewater treatment must be done for a specified purpose – for example, to produce an effluent suitable for agricultural or aquacultural reuse (or both), or to produce an effluent that can be safely discharged into inland or coastal waters. Wastewater treatment plant designers have to know what is going to happen to the effluent – re-use or discharge – before they design the plant, as the effluent quality requirements will vary accordingly
In summary, wastewater treatment process is one of the most important environmental conservation processes that should be encouraged worldwide. Most wastewater treatment plants treat wastewater from homes and business places. Industrial plant, refineries and manufacturing plants wastewater is usually treated at the onsite facilities. These facilities are designed to ensure that the wastewater is treated before it can be released to the local environment. Some of the water is used for cooling the machines within the plants and treated again. They try to ensure that nothing is lost.
It illegal for disposing untreated wastewater into rivers, lakes, oceans or into the environment and if found culpable one can be prosecuted.
Mr.A Edirisinghe
Deputy Principal
Vidyartha College, Kandy
