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The composition of municipal waste water varies from place to place. Sometimes industrial wastes also mix with sewage. The type of treatment of waste water thus depends upon its characteristics

and the desired quality of water after treatment. The waste water treatment plants are generally primary, secondary or for advanced treatment.

The purpose of waste water treatments is to remove/reduce organic and inorganic substances, nutrients, toxic substances, kill pathogenic organisms, etc., so that the quality of discharged water is improved to meet the permissible level of water to be discharged in some water body, on land or agricultural field. Treatment of water thus aims at reduction of BOD, COD, eutrophication, etc. of receiving water bodies and prevention of bio-magnification of toxic substances in food chain and prevention of disease due to pathogenic organisms present in the waste water.

Various steps involved in treatment of waste water are shown in Fig. 12.25.

Screen Grit chamber Waste water Primary settling tank Aeration Air Secondary settling tank Effluent Chlorination Activated sludge Primary sludge Sludge digestion tank

Digested sludge Sludge

dewatering Sludge disposal Anaerobic

digester Supernatent

Primary Treatment Secondary Treatment

Fig. 12.25. Flow diagram of sewage (waste water) treatment plant

Primary treatment: Primary treatment: Primary treatment: Primary treatment:

Primary treatment: It is a physical process for removal of debris, large particles with the help of screen. The waste water after screening is passed through grit chamber where sand, grit and other solids settle down. The water is then passed through the sedimentation tank or clarifier where most of the suspended solids settle down due to gravity. For better removal of suspended solids, sometimes chemically treated polymers are used. About 35% BOD and 60% of suspended solids are removed during primary treatment.

Secondary treatment: Secondary treatment: Secondary treatment: Secondary treatment:

Secondary treatment: It is biological process which involves micro-organisms. It removes up to 90% of the BOD and 90% of suspended solids. Biodegradable oxygen demanding wastes are stabilised. Following are the various approaches adopted in secondary treatment.

T T T T

Trickling filters:rickling filters:rickling filters:rickling filters: These consist of a bed of crushed stones/pebbles covered with slimerickling filters: which consists mainly of aerobic bacteria, algae, fungi, protozoa, worms and insect larvae. Sewage is degraded by the aerobic bacteria when it passes through the bed and is collected at the bottom of the filter. Some of the treated sewage may be recirculated along the influent. It helps in better removal of organic matter and also keeps the filter moist when the flow rate over the filter is slow.

Activated Sludge Process: Activated Sludge Process: Activated Sludge Process: Activated Sludge Process:

Activated Sludge Process: The effluent from the primary clarifier goes to aeration tank. Aeration tank also receives micro-organisms from the secondary settling tank known as activated sludge. Oxygen is pumped into aeration tank for maintaining aerobic conditions. After few hours of agitation, the waste water goes to secondary settling tank where solids settle at the bottom. The sludge is produced, dewatered and disposed off. This can be used for landfills or disposed off in ocean or used in croplands, pastures, etc.

Effluent Influent (waste water) Rotating arm Influent spray Pebbles with aerobic organisms

Fig. 12.26. Cross section of a trickling filter

Rotating Biological Contactor (RBC): Rotating Biological Contactor (RBC): Rotating Biological Contactor (RBC): Rotating Biological Contactor (RBC):

Rotating Biological Contactor (RBC): It consists of circular plastic discs which are arranged on a rotating shaft. Circular discs have micro-organisms grown on them. The discs are contained in a waste water holding tank. About 40% area of the discs is submerged in the tank. The discs rotate in and out of water as the RBC rotates. The micro-organisms present on the discs absorb organic matter when they are in water and obtain the required oxygen when the discs are out of water. Thus a high degree of organic matter removal is achieved.

Secondary setting tank Primary setting tank Effluent Waste- water Sludge

Fig. 12.27. Rotating biological contactor

Advanced Sewage T Advanced Sewage T Advanced Sewage T Advanced Sewage T

Advanced Sewage Treatment:reatment:reatment:reatment: After the primary and secondary treatments many undesirablereatment: substances still remain in the effluent. Advanced water treatment involves the removal of such substances. The treatment, therefore, involves specific steps depending upon the type of substances to be removed. The materials to be removed in such treatments may include nitrates and phosphates (which cause eutrophication of receiving waters), colour, bacteria, viruses, pesticides, toxic metals, etc. Chlorination of water is generally done to kill harmful bacteria and some viruses. However,

chlorine can produce cancer-causing chlorinated hydrocarbons by reacting with organic matter. Other, but costly methods of disinfection are the use of ultraviolet light and ozone treatment. The sludge produced after such treatment is used as a fertilizer in the fields. However, there are chances of toxic metals and other untreated substances to build up in the food chain or leach to the ground water.

Upflow Anaerobic Sludge Blanket (UASB) Reactor: Upflow Anaerobic Sludge Blanket (UASB) Reactor: Upflow Anaerobic Sludge Blanket (UASB) Reactor: Upflow Anaerobic Sludge Blanket (UASB) Reactor:

Upflow Anaerobic Sludge Blanket (UASB) Reactor: Anaerobic treatment is gaining importance as a method of waste water treatment due to its effectiveness in treating waste water and economic advantages. The UASB process consists of four stages of anaerobic digestion: hydrolysis, acidogenesis, acetogenesis and methanogenesis. A dense blanket of granular anaerobic biomass is used to convert organic compounds that are passed through the sludge blanket continuously. Biogas produced is collected at the top of the reactor. The waste water to be treated is fed into the reactor at the bottom. As the influent flows through the loops and enters the reactor chamber, hydrolysis occurs. The above mentioned four anaerobic processes convert the influent into H₂, CO₂, CH₄, acetate, new cell-matter etc. COD removal of up to 80% is achieved. Methane and CO₂ produced are separated from the reactor.

12.8

LAND POLLUTION