Domestic Wastewater Treatment
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The process of removing contaminants from wastewater is known as sewage treatment or domestic wastewater treatment. A number of processes are used in this regard for treatment of such water, which includes biological, chemical and physical processes as well. The renowned organization of civil and infrastructure technology in Pakistan NESPAK organized the sewage treatment practical work for students to testify their theory skills in the practical field. Sewage is created by residences, hospitals and industries etc. Sewage collection and treatment is basically the matter of local, provincial or federal authorities. It is also notable that industrial sewage is treated in a different manner and requires specialized treatment process.
It is one of the leading environmental issues in Pakistan. Sewage water on main roads and pedestrian paths is the ugliest picture of the country. That is why I attended and beneficed myself from the Wastewater Treatment program organized by NESPAK.
The treatment consists of three main steps. These are called primary, secondary and tertiary treatment. Before starting primary treatment, a pretreatment is also done to remove easily collectables from raw sewage in order to prevent damage that they can cause to the treatment lines. The first step in pretreatment is screening. The sewage water is made to pass through mechanical bar that removes all objects like plastics, cans, rags, etc. Commonly mechanical bar is used for this purpose. The solids are collected and disposed in a landfill or incinerated. Sand or grit removal chambers are used to settle down broken pieces of glass, sand particles and grit. In some larger plants fat and grease are removed passing sewage from a tank, where skimmers collect floating fat.
In primary treatment, sewage flows through “pre-setting” basins. The tank is used for settlement of sludge, grease and oil which rise on the surface.
As for secondary treatment process, it cannot be described in detail, because it is too large process. Secondary treatment is designed to treat the content of the sewage which consists of the huan waste, food waste, soaps, detergent, etc. Secondary treatment is further classified as fixed film.
Suspended-growth system is based on activated sludge. Here the biomass is mixed with the sewage and is treated in a smaller space than trickling filters. The mechanism of secondary treatment involves various processes. New technology enhanced the working of plants. “The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling. The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates. Thus, the increased sludge retention times, usually exceeding 15 days, ensure complete nitrification even in extremely cold weather ”(Sewage Ministry of the Environment n.pag.).
The final step is secondary sedimentation which “is to settle the biological floc or filter material through a secondary clarifier and to produce sewage water containing low levels of organic material and suspended matter” (Sewage Ministry of the Environment n.pag.).
Now the final treatment of sewage is tertiary treatment, in which the sewage is raised to effluent quality and then and released in sea, river, lake, etc. Firstly, sand filtration removes the suspended matter, then lagooning provides settlement of sewage and it is further purified biologically. After this, harmful nutrients like phosphorus and nitrogen are removed by different chemical and biological processes. Nitrogen is mainly removed by oxidation of nitrogen from ammonia to nitrite. Then the process of denitrification is used and nitrogen is released in atmosphere. Phosphorus is removed by chemical precipitation of salts of iron with lime. Mainly it is removed biologically in process called EBPR (enhanced biological phosphorus removal); polyphosphate is used in this process. After these processes, disinfection and odor control is done in order to remove all microorganisms that create infection or odor in the water.
The final step before releaasing sewage water to environment is sludge treatment and disposal. The equipment used is ‘main sludge thickener’, ‘centrifugal sludge thickeners’ and ‘rotary drum sludge thickeners’. The process carried out depends upon size of sludge material. Aerobic and anaerobic digestions are for midsized and larger sized sludge respectively. These are bacterial processes with and without oxygen respectively. Then composting is carried out. In presence of oxygen, bacteria digest wastewater solids and the added carbon source and produce a large amount of heat. Finally, the sludge is disposed.
In many processes of a wastewater treatment plants are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water.
The outcomes of the domestic wastewater treatment processes are a great in number to discuss. The whole process is beneficial, because water is renewable resource, and if we do not clean it by these processes the amount of water on earth will soon sharply decrease. The removal of bacteria and contaminants assures our health guarantee and helps to avoid harmful diseases. The new technology enables to increase complexity and decrease effluent contamination by use 1990 MBR technology. Most of the developing countries discharge sewage with only primary treatment which is not enough.
The main lesson learned is that it is much better and beneficial to use constructed wetlands in sewage treatment because they reduce the risk profile of an STP. These wetlands are cost effective for nitrates removal. Moreover, there is no failure of work because minimum of mechanical work is needed. Sewage treatment plants are a source of job opportunities. In short, the domestic wastewater treatment can be done in a lot of ways, and it is necessary to treat raw sewage to make water re-useable. It is compulsory for health and cleaning of society as well.
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