Sewage Treatment

Sewage Treatment

Sewage is a contaminated water discharged from domestic, industrial, municipal and other sources. Sewage treatment plant is designed to remove the contaminants from the water so that they can be recycled and reused. Both organic and inorganic contaminants are removed by using different physical, chemical and biological processes. The treatment plant produces waste stream which is a treated effluent and a solid waste or sludge which are reused.
 
Sewage is collected and transported via a network of pipes and pump stations to a municipal treatment plant. Normally three stages are involved for treating sewage- primary, secondary and tertiary. After being treated the effluent can be reused to flush toilets, greening parks, or for recharging ground water.
 
Stages of Sewage Treatment
 
Sewage treatment is accomplished in three stages which are as follows:
 
 
 
 
Primary Treatment
 
In a primary treatment wastewater is relieved from the contaminants like coarse sands, pebbles, organic and inorganic wastes, fatty substances etc. The wastewater consist of large objects like rags, cans, tin etc which are removed by screen. This is necessary because the presence of these objects can harm the sensitive parts of the plant. Followed by the screen there is a a grit chamber that are used for removing sand and other heavy inorganics. The materials so collected in the grit chamber are collected and bagged for disposal. 
 
The liquid being freed from grit is passed to through the fixed or rotating screens in order to get rid of floating and larger materials. Some plants use sedimentation stage in which the sewage is passed through large tanks which are called as primary clarifiers or primary sedimentation tanks. In these tanks floating materials are removed and homogeneous liquid is ready for the biological treatment whereas, the sludge is is separated for the separate treatment. Primary tanks consist of scrapers that drive the sludge towards a hopper in the base of the tank. From here the sludge is pumped out for treatment.
 
Secondary Treatment
 
Secondary treatment method uses aerobic biological processes to degrade the biological content of the sewage like human waste, food waste, soaps and detergent. In this method bacteria and protozoa convert the biodegradable soluble organic contaminants into floc.
 
In Secondary treatment systems two methods viz. fixed film and suspended growth are used. In these methods sewage passes over the surface of the biomass which grows on the media. Of these two systems fixed film system is preferred as it can easily cope with drastic changes in the amount of biological material and can provide higher removal rates for organic material and suspended solids than suspended growth systems.
 
For strong organic loads sometimes Roughing filters used. These filters usually consists of tall, circular filters which are filled with open synthetic filter media where sewage is applied at a relatively high rate. These filters are fabricated to for high hydraulic loading and a high flow-through of air. 
 
Activated sludge plants are also used in secondary treatment that use use dissolved oxygen to promote the growth of biological floc which aids in the removal of the organic material. It is also capable of trapping particulate material and can, convert ammonia to nitrite and nitrate ultimately to nitrogen gas. If plant is receiving more variable loads then trickling filter beds can be used.
 
In secondary treatment Biological Aerated Filter (BAF) is also used which consist of reactor filled with a filter media. The media supports highly active biomass that is attached to it and filter suspended solids. Carbon is reduced and ammonia is converted in aerobic mode.
 
In the final stage of secondary treatment biological floc is settled and material is filtered. The sewage water so produced after this treatment contains very low levels of organic material and suspended matter.
 
When the sewage is passed through rotating biological contractors (RBCs) the microorganism which covered the surface of the RBCs reduce the BOD and further remove suspended matter from the wastewater.
 
Tertiary treatment
 
Wastewater is given tertiary treatment to further enhance its quality before they are discharged in the environment. The popular methods that are used in this stage are filtration, lagooning, removal of chemicals like nitrogen and phosphorus etc.
 
Different treatment processes are required to remove nitrogen and phosphorus. Nitrogen is removed through the biological oxidation of nitrogen from ammonia to nitrate, followed by denitrification, the reduction of nitrate to nitrogen gas which is removed to the atmosphere. Phosphorus on the other hand can be removed by using two process that is by the use of specific bacteria, called polyphosphate that accumulate large quantities of phosphorus within their cells they are removed from the water ans are used as fertilizer. Chemical precipitation can also be used to remove phosphorus. Wastewater is sometimes further disinfected by using chlorine, ozone gas and ultraviolet light.

 

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