INTRODUCTIONWater to be supplied for public use must be potable i.e., satisfactory for drinking purposes from the standpoint of its chemical, physical and biological characteristics. Drinking water should, preferably, be obtained from a source free from pollution. The raw water normally available from surface water sources is, however, not directly suitable for drinking purposes. Waste water consists of carbon, nitrogen, phosphorus, sulphur which lead to various diseases. The term “treatment” means removal of toxic or otherwise dangerous substances which are likely to disturb the biological cycles.The objective of water treatment is to produce safe and potable drinking water.

fig.Canal from water comes They can be broadly categorized into three different groups (on the basis of tasks involved).

1. Physical method 2. Chemical method 3. Biological method

Physical method When it comes to physical wastewater treatment, following physical processes are used for the treatment of water instead of resorting to chemicals or biological means.

Sedimentation Wherein coarse screening of waste water is done to remove contaminating objects after allowing them to settle at the base, when heavy contaminants settle down, the removal of cleared effluent or waste stream becomes relatively easy.

Aeration

Wherein air is added to the wastewater physically in order to provide oxygen to the contaminated water. Filtration

Wherein the contaminated water is passed through various filters to separate the contaminating solids from the water. Sand filter is by far the most common filter used in this process.

Chemical methodAs opposed to physical treatment of water, chemical treatment involves the use of chemicals to get rid of contaminants in it.

Various methods are used to treat water chemically, they are following;

Ø Chlorination

In this process, chlorine - a strong oxidizing chemical, is used to kill the bacteria which lead to decomposition of water.

fig.Chlorination of raw water

Oxidization This is a chemical water treatment method, wherein oxidizing agents - such as ozone, are used to treat polluted water. These oxidizing agents make water reusable by hampering the biological growth process of bacteria which happens to be the main cause of decomposition of water.

Neutralization

When it comes to industrial wastewater treatment, a chemical process known as Neutralization is quite common.

This process involves adding acid or base to the water to adjust its pH value and bring it back to neutral level. Lime (CaO), commonly known as quicklime or burnt lime, is one of the best examples of base used in the process of neutralization to neutralize acid wastes.

Polyvalent metals, i.e. metals having more than one valence, are very often used as coagulating chemicals in sewage treatment.

Iron and other metals containing compounds like ferric sulfate and aluminum sulfate are some of the best examples of coagulants.

Biological method In biological water treatment processes, bacteria and other such microorganisms are used to biochemically decomposes the wastewater and stabilizes the end product.

Biological water treatment is further categorized into two sub-divisions as following;

Aerobic process In the aerobic process, bacteria consume the organic matter and helps convert it to carbon dioxide in the presence of oxygen.

Anaerobic process In the anaerobic process, on the other hand, sludge is fermented at a particular temperature in the absence of oxygen.

WASTE WATER TREATMENT AND PUIRFICATION STAGES

Treatment and purification stages are categorized as following;

1. Primary treatment 2. Secondary treatment 3. Disinfection 4. Sludge treatment

Primary treatment

Primary treatment uses both physical as well as chemical method. Screening chamber The incoming wastewater, called influent, passes through screens consisting of upright bars, spaced one to three inches apart. These bars remove large pieces of trash including rags, sticks, newspaper, soft drink cans, bottles, plastic cups and other similar items. This protects the main sewage pumps and other equipment. The garbage is transported to landfills. The main sewage pumps then lift the wastewater from the screening chamber to the surface level of the plant.

The wastewater enters primary settling tanks, also called sedimentation tanks, for one to two hours.

fig.Screening chamberCoagulation

The materials which are suspended or found in the colloidal form in raw water or influent are removed by coagulation. Substance that is used to carry out coagulation is called coagulant.

The most important coagulants are;

oAl2(SO4)3·14H2O or Al2(SO4)3·18H2O(alum)o FeCl3o FeCl3 (with lime)o Fe2(SO4)3 (with lime)o FeSO4·7H2O (copperas) (with lime)

Alum or aluminium sulphate When alum is added to waste water in alkaline medium,aluminium hydroxide is precipitated out as reaction shows following;

K2SO4.Al2 (SO4)3·24H2O +3Ca(OH)2 3CaSO4+2Al(OH)3+K2SO4+24H2OSo, suspended particles get adsorbed on the surface of gelatinuous aluminium hydroxide.

Ferric salts Ferric salts are commonly used as coagulants but they are difficult to handle because an insoluble ferric oxide is produced in the pH range from 3 to 13.

Lime

The water may contain calcium and magnesium salts which make water hard. It is treated by adding lime in waste water as shown by reactions;

Partially treated waste water and primary sludge The flow of the water is slowed, allowing heavier solids to settle to the bottom of the tank and the lighter materials to float.

The settled solids, called primary sludge, are then pumped through cyclone degritters — devices that use centrifugal force to separate out sand, grit (such as coffee grinds) and gravel. This grit is removed, washed and taken to landfills.

o The degritted primary sludge is pumped to the plant's sludge handling facilities for further processing.

o The partially treated wastewater from the primary settling tanks then flows to the secondary treatment system.

Secondary treatmentSecondary treatment uses chemical, physical and biological methods.

The partially treated wastewater from the primary settling tanks then flows to the secondary settling tanks which are termed to be as aeration tanks (also called biological reactors or bubbling tanks).

Secondary treatment is also called the activated sludge process.

Aeration

Air pumped into large aeration tanks by means of aeration equipments which are in following forms and any of them can be used according to process feasibility ; Surface aerators Compressors connected to submerged air diffusers In this process, air is passed through partially treated waste water and following effects occurs such as;

Removal of dissolved foul smelling H2S Removal of Organosulpur compounds Removal of Volatile organic compounds Some organic materials are oxidized with air and CO2 is produced Removal of remaining organic materials by passing water over activated carbon Aeration process also oxidizes water soluble Fe+2 to Fe+3 which then forms insoluble Fe(OH)3 and is separated as solid

Aeration equipments mix the wastewater and sludge that stimulates the growth of oxygen-using bacteria and other tiny organisms that are naturally present in the sewage.

These beneficial microorganisms consume most of the remaining organic materials that are polluting the water and this produces heavier particles that will settle later in the treatment process.

Wastewater passes through these bubbling tanks in three to six hours, producing sludge which is called activated sludge.

The aerated wastewater then flows to the final settling tanks called clarifier which are similar to the primary settling tanks. Here the heavy particles and other solids settle to the bottom as secondary sludge. Some of this sludge is re-circulated back to the aeration tanks called return sludge which stimulates the activated sludge process.

The returned sludge contains millions of microorganisms that help maintain the right mix of bacteria and air in the tank and contribute to the removal of as many pollutants as possible.

The remaining secondary sludge is removed from the settling tanks and added to the primary sludge for further processing in the sludge handling facilities.

Wastewater passes through the settling tanks in two to three hours and then flows to a disinfection tank.

Disinfection

Even after primary and secondary treatment, disease causing organisms may remain in the treated wastewater.

Chlorination

To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes in chlorine-contact tanks. Hypochlorous acid HOCl is used as disinfecting agent.

Hypochlorous acid is not stable thus it cannot be stored. So it is generated by dissolving the water with any of the following;

Ø Molecular chlorine gas

Cl2 + H2O

HOCL + H+ +

Cl-

Ø Sodium hypochlorite

Ø Calcium hypochlorite

Sludge treatment The generation of activated sludge or floc in wastewater is a slow process and the amount so formed from any volume of wastewater during its period of treatment is small and inadequate for the rapid and effective treatment of the wastewater which requires large concentrations of activated sludge. Such concentrations are built up by collecting the sludge produced from each volume of wastewater treated and re-using it in the treatment of subsequent wastewater flows. The sludge so re-used is known as returned sludge.

The purpose of return sludge is to maintain a concentration of activated sludge in the aeration tank sufficient for the desired degree of treatment.

CONCLUSIONThrough this report we come to understand the working of waste water management and treatment. Due to the acute shortage of water resources, it is necessary to recycle and reuse waste water. In a waste water treatment plant water is treated through various treatment processes and is finally made fit for drinking purpose. In India there are only few large scale waste water treatment facilities and the municipal corporation is still not taking ample steps to effectively recycle the waste water. If resources are employed a lot more water can be reused and can cure water shortage problems in many parts of the Indian subcontinent.

fig.Power unit

References:1. Environmental Protection Agency (Water treatment Report)

2. Waste Water Treatment (EVS)

3. Wikipedia

4. Small scale waste water treatment plants (pdf)

5. http://www.defence.gov.au