‘Sewage’ is generated by residential, institutional, commercial and industrial establishments. Apart from wastewater generated by the process, it also includes waste liquid from toilets, baths, showers, kitchens, and sinks draining into sewers.
‘Sewage Treatment Plants’ is the process of removing contaminants from wastewater using physical, chemical and biological processes. The output water or the recovered water is safe for the environment. What remains is a semi-solid waste called sewage sludge which is further treated to make it suitable for disposal.
It includes physical, biological and sometimes chemical processes to remove pollutants. Its aim is to produce an environmentally safe sewage water, called effluent, and a solid waste, called sludge or biosolids, suitable for disposal or reuse. Reuse is often for agricultural purposes, but more recently, sludge is being used as a fuel source.
Water from the mains, used by manufacturing, farming, houses (toilets, baths, showers, kitchens, sinks), hospitals, commercial and industrial sites, is reduced in quality as a result of the introduction of contaminating constituents. Organic wastes, suspended solids, bacteria, nitrates, and phosphates are pollutants that must be removed.
To make wastewater acceptable for reuse or for returning to the environment, the concentration of contaminants must be reduced to a safe level, usually a standard set by the Environment Agency. Sewage can be treated close to where it is created (in septic tanks and their associated drainfields or sewage treatment plants), or collected and transported via a network of pipes and pump stations to a municipal treatment plant. The former system is gaining popularity for many new ECO towns, as 60% of the cost of mains sewerage is in the pipework to transport it to a central location and it is not sustainable. It is called ‘Decentralization‘ of sewage treatment systems.
The job of designing and constructing sewage works falls to environmental engineers. They use a variety of engineered and natural systems to meet the required treatment level, using physical, chemical, biological, and sludge treatment methods. The result is cleaned sewage water and sludge, both of which should be suitable for discharge or reuse back into the environment. Sludge, however, is often inadvertently contaminated with many toxic organic and inorganic compounds and diseases and the debate is raging over the safety issues. Some pathogens, for example, ‘Prion’ diseases (CJD or ‘Mad Cow Disease is a Prion disease) cannot be destroyed by the treatment process.
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Sewage can be treated at the site where sewage is generated or at a centralized treatment plant.
Pretreatment is where large objects, grit, fat and grease are removed to ensure the downstream treatment stages operate at the desired efficiency levels. With the help of a bar screen, materials such as trash, tree limbs, leaves, branches, and other large objects are removed from the raw sewage to prevent any damage or clog the pumps and sewage lines. Grit such as sand, gravel, cinders, and organic materials are removed using a grit channel or chamber where the velocity of the incoming sewage is adjusted to allow settlement. Grit removal helps reduce formation of heavy deposits in aeration tanks, aerobic digesters, pipelines, channels and protect moving mechanical equipment from abrasion and accompanying abnormal wear.
Equalization tanks temporarily hold incoming sewage and serve as a means of diluting and distributing batch discharges of toxic or high-strength waste. This is to ensure uniform flow conditions so that the clarifiers and mechanized downstream treatment are efficient. Fat and grease are removed by passing the sewage through a small tank where skimmers collect the fat floating on the surface.
Sewage is partly decomposed by anaerobic bacteria in a tank without the introduction of air, containing oxygen. This leads to a reduction of Organic Matter into Methane, Hydrogen Sulfide, Carbon Dioxide etc. It is widely used to treat wastewater sludge and organic waste because it provides volume and mass reduction of the input material to a large extent.. The methane produced by large-scale municipal anaerobic sludge treatment is currently being examined for use in homes and industry, for heating purposes. Septic tanks are an example of an anaerobic process, but the amount of methane produced by a septic tank (it is only the SLUDGE at the bottom that produces methane) serving less than 100 people is minuscule. In addition to this, septic tank effluent still contains about 70% of the original pollutants and the process smells very badly, due to the Hydrogen Sulfide, if not vented correctly. The effluent produced by this process is highly polluting and cannot be discharged to any watercourse. It must be discharged into the Aerobic layer of the soil (within the top meter of the ground) for the aerobic soil bacteria to continue the sewage treatment via the aerobic process below.
In this process, aerobic bacteria digest the pollutants. To establish an aerobic bacterial colony you must provide air for the bacteria to breathe. In a sewage treatment plant, air is continuously supplied to the Biozone either by direct Surface Aeration using Impellers propelled by pumps which whisk the surface of the liquid with air, or by Submerged Diffused Aeration using blowers for air supply through bubble diffusers at the bottom of the tank. (The most modern aerobic sewage systems use natural air currents and do not require electricity, though these are only used for small scale sewage systems at the moment. Once again, the general public leads the way!) Aerobic conditions lead to an aerobic bacterial colony being established. These achieve almost complete oxidation and digestion of organic matter and organic pollutants to Carbon Dioxide, Water and Nitrogen, thus eliminating the odour and pollution problem above. The effluent produced by this process is non-polluting and can be discharged to a watercourse
The Primary treatment consists of temporarily holding the sewage in a basin (“pre-settling basins”, “primary sedimentation tanks” or “primary clarifiers”) where heavy solids are allowed to 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 is subjected to secondary treatment. This is usually Anerobic. First, the solids are separated from the sewage. They settle out at the base of a primary settlement tank. The sludge is continuously being reduced in volume by the anerobic process, resulting in a vastly reduced total mass when compared to the original volume entering the system. The primary settlement tank has the sludge removed when it is about 30% of the tank volume.
This method removes dissolved and suspended biological matter. The biological content of the sewage is degraded by the indigenous, water-borne microorganisms in a managed habitat. The bacteria and protozoa consume biodegradable soluble organic contaminants and bind much of the less soluble fractions into floc. This is Aerobic. The liquid from the Primary treatment contains dissolved and particulate biological matter. This is progressively converted into clean water by using indigenous, water-borne aerobic micro-organisms and bacteria which digest the pollutants. In most cases, this effluent is clean enough for discharge directly to rivers.
The Tertiary treatment is sometimes defined as anything more than primary and secondary treatment in order to allow ejection into the environment. It is also called “effluent polishing.” Media Filtration removes residual suspended matter and residual toxins. Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas. Phosphorus can be removed by enhanced biological phosphorus removal and also by chemical precipitation, usually with salts of iron (e.g. ferric chloride), aluminium (e.g. alum), or lime. Chlorination, Ozonation and Ultraviolet (UV) light are one of many methods to waste water disinfection.
Treated water is disinfected chemically or physically prior to discharge for the use of irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.
Sludge treatment and Sewage disposal is an important stage in the sewage treatment plants. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used where necessary.
In some cases, the effluent resulting from secondary treatment is not clean enough for discharge. This may be because the stream it is being discharged into is very sensitive, has rare plants and animals or is already polluted by someone’s septic tank. The Environment Agency may then require a very high standard of treatment with a view to the new discharge being CLEANER than the water in the stream and to, in effect, ‘Clean it up a bit’. It is usually either Phosphorous or Ammonia-cal Nitrogen or both that the E.A. want reduced. Tertiary treatment involves this process. If Phosphorous is the culprit, then a continuous dosing system to remove it is the tertiary treatment. If Ammonia cal Nitrogen is the problem, then the sewage treatment plant process must involve a nitrifying and then de-nitrification stage to convert the ammonia cal nitrogen to Nitrogen gas that harmlessly enters the atmosphere.