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The term water pollution, in general can be defined as any alteration in physical, chemical or biological properties of water by discharge of any sewage or industrial waste or of any liquid, gaseous or solid substances into water as may, or is likely to create nuisance or render such water harmful or injurious to public health of safety, or to domestic, commercial, industrial agricultural or other legitimate uses, or animal life and health.

1. The large number of water pollutants may be broadly classified under the following categories:
   1. Inorganic pollutants
   2. Organic pollutants
   3. Thermal pollutants
   4. Radioactive pollutants
   5. Sediments
   6. Infectious agents
   7. Plant nutrients

1. Inorganic pollutants

   Inorganic chemical pollutants are naturally found in the environment but due to human development these pollutants are often concentrated and released into the environment. The primary inorganic pollutants of concern are cadmium, copper, lead, zinc, nitrogen, nitrate, nitrite, ammonia, phosphorous and phosphate.

   Sources: Industrial effluent is the major source of Inorganic pollutants cause. E.g. sulfur dioxide from power plants, Ammonia from food processing waste and chemical waster from industrial byproducts. Agricultural fertilizers and surface runoffs are other sources of Inorganic pollutants. Harmful effects
   • Can kill the fish and other aquatic animals
   • Interfere with suitability of water for drinking and industrial use
   • Toxic pollutants tend concentrate in food chains
   • Degrade the soil microbial activity
   • Loss of vigilance ability
   • Loss of hand to eye coordination
   • NO₂ gas can cause coughing, breathlessness, irritation of upper airways, bronco spasms, nauseas and vomiting
2. Organic pollutants

   Organic pollutants are basically compounds which may be degraded by micro organisms. It usually uses up the available oxygen in the process of degradation. The optimum DO in natural water is 4-6 ppm and hence organic pollutants like sewage waste, oils etc must be removed to keep water free from such pollutants.

3. Thermal pollutants

   Heated water is discharged as industrial effluents. In order to maintain the ecological balance temperature needs to be equalized with the receiving water body. This is an important criterion as increased temperature lowers dissolved DO in water.

4. Radioactive pollutants

   Uranium and thorium wastes from mining, refining and its various industrial applications contribute to radioactive wastes. Nuclear power plants, medical and scientific research use are areas where such wastes may be created.

5. Sediments

   Erosion removes soil and minerals from crop lands, forests, residential and corporate communities and carries it as sediment. Sediments represent the most extensive pollutants of surface water.

   Bottom sediments are important source of inorganic and organic matter in streams, fresh water, estuaries and oceans. Sediments are also repositories for trace metals such as Co, Cr, Cu, Mn, Mo, Ni, etc.
6. Infectious agents

   Wastewater discharged from municipalities; sanatoria, tanning etc. may contain decease producing micro-organisms.

7. Plant nutrients

   Growth stimulating plant nutrients include like nitrogen and phosphorous add to the BOD of the water. Presence of nutrients encourages algal growth, which decreases DO levels, and creates problems in treatments.

The sources of water pollution are categorized as direct and indirect contaminant sources. Direct sources directly discharge contaminants into surface water which includes waste water from factories and sewage from housing colonies. Indirect sources include contaminants that enter the water supply from soil/groundwater systems and from the atmosphere via rain water. Soil and ground waters contain residue of agricultural practices (fertilizers, pesticides, etc.) and improperly disposed of industrial wastes. Atmospheric contaminants are also derived from human practices (such as gaseous emissions from automobiles, factories and even bakeries).

Water is polluted by different contaminants and it can be detected by analyzing water samples in laboratory. Titration test is to detect hardness, dissolve oxygen, carbon dioxide and alkalinity. Colorimetric tests are performed to check PH, Phosphate, silica, ammonia and Sulfides. Heavy metal limit test is to determine the level of heavy metals.

Increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aquatic ecosystem such as a lake is called eutrophication. This rise is nutrient level results in an accelerated life cycle of water plants like algae, water hyacinth, etc. Vegetative cover restricts entry of light and oxygen in the water. Moreover the degrading debris uses up all available dissolved oxygen in water rendering the water lifeless. Wastewater should be treated for removal of phosphorus and nitrates by various physiochemical and biological methods. Effective technologies are available in removal of nutrients along with BOD and COD. Conventional methods of separate nitrification and denitrification can also be adopted but with increased cost and area.

Waste water comes from sewage which includes black water (human waste) and grey water (kitchen and laundry waste), septic tank discharge, industrial waste which includes effluent and process water. Agricultural practices also produce waste water by contaminating water with pesticides and fertilizers

Proper treatment of wastewater reduces health risks to humans, animals and prevents surface and groundwater contamination. Wastewater treatment helps to reuse the waste water and reduces the consumption of fresh water.

Wastewater treatment is the process of removing contaminants from wastewater and household sewage, both runoffs (effluents), domestic, commercial and institutional. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer). Treatment system depends on the characteristics of wastewater and treated water quality to be achieved. The treatment plant includes primary treatment such as Screens, Grit Chambers & Clarifiers.

Aerobic wastewater treatment is the process where bacteria requires oxygen for their respiration while oxidizing or consuming organic matter present in the wastewater. Aerobic bacteria can only convert organic compounds when plenty of oxygen is present, because they need it to perform any kind of chemical conversion. Usually the products they convert the contaminants to are carbon dioxide and water

MLSS is Mixed Liquor Suspended Solids & MLVSS is Mixed Liquor Volatile Suspended Solids. MLSS is total suspende solids in the aeration tank whereas MLVSS is part of MLSS which actually concentration of biological matter present in aeration tank.

It is ration food to mass ratio ie BOD to the microorganism present in the aeration tank

C– Carbon source which is food for biomass.– Either COD or BOD. Normally BOD is considered for the design purpose of aerobic treatment and COD concentration considered for anaerobic treatment. N – Nitrogen (TKN) which is Nutrients P — Phosphorus. Normally it is in the ratio of C:N:P i.e. 100: 5: 1 if BOD as Carbon source and 500:5:1 if COD as carbon source for calculation.