Sewage is the municipal waste water containing large quantity of human excreta. Every day large quantity of sewage produced in cities & towns. Sewage is formed of 99.9% water & only 0.1% of solid impurities which are formed of suspended solids etc. There is need of sewage in sewage treatment plants before its discharge into natural water bodies like rivers etc.                                                                                                                                             

SEWAGE TREATMENT:  it is of two modes that are following                                                                                        Primary treatment: in this method mechanical screening & sedimentation of undissolved solids in raw sewage (As floating polythene bag,sand, etc.) is done. After screening sewage is passed through the grit chamber to separate sand. Finally sewage is passed into the primary settling tank where most of suspended particles settle down to form primary sludge. It fails to remove any dissolved substance in water. It does not remove pathogen.   

In the primary sedimentation stage, sewage flows through large tanks, commonly called “primary clarifiers” or “primary sedimentation tanks.” The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Grease and oil from the floating material can sometimes be recovered for Saponification.

The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical sedimentation tank may remove from 50 to 70 percent of suspended solids, and from 30 to 35 percent of biochemical oxygen demand (BOD) from the sewage.

Secondary treatment:sewage treated in primary treatment is brought in contact with O2& aerobic m/o. They breakdown the organic material into harmless material as CO2& H2O. Then chlorination is done to kill bacteria. It further treated to tertiary level. There is lesser B.O.D. in compared to primary treatment.  

It is designed to substantially degrade the biological content of the sewage which is derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes. To be effective, the biota require both oxygen and food to live. The bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc. Secondary treatment systems are classified as fixed-film or suspended-growth systems.

  • Fixed-film or attached growthsystems include trickling filters, biotowers, and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface. The fixed-film principal has further developed into Moving Bed Biofilm Reactors (MBBR), and Integrated Fixed-Film Activated Sludge (IFAS) processes. An MBBR system typically requires smaller footprint than suspended-growth systems. (Black & Veatch)
  • Suspended-growthsystems include activated sludge, where the biomass is mixed with the sewage and can be operated in a smaller space than trickling filters that treat the same amount of water. However, fixed-film systems are more able to 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.

Roughing filters are intended to treat particularly strong or variable organic loads, typically industrial, to allow them to then be treated by conventional secondary treatment processes. Characteristics include filters filled with media to which wastewater is applied. They are designed to allow high hydraulic loading and a high level of aeration. On larger installations, air is forced through the media using blowers. The resultant wastewater is usually within the normal range for conventional treatment processes.

A filter removes a small percentage of the suspended organic matter, while the majority of the organic matter undergoes a change of character, only due to the biological oxidation and nitrification taking place in the filter. With this aerobic oxidation and nitrification, the organic solids are converted into coagulated suspended mass, which is heavier and bulkier, and can settle to the bottom of a tank. The effluent of the filter is therefore passed through a sedimentation tank, called a secondary clarifier, secondary settling tank or humus tank.


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