Advanced Waste Water Treatment

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Environmental Chemistry and Analysis

Prof. M.S.Subramanian

MODULE 1.5

Advanced Waste Water Treatment
Removal Of Suspended Solids Microstraining Coagulation and flocculation Filtration Removal of dissolved solids Ion exchange Reverse osmosis Electrodialysis Removal of nitrogen Phosphate removal (chemical treatment) Phosphate removal (biological treatment) Removal of dissolved organic compounds Adsorption Sludge treatment and disposal Disinfection 1 2 2 3 3 4 6 7 8 9 10 10 10 11 13

Indian Institute of Technology Madras

Environmental Chemistry and Analysis

Prof. M.S.Subramanian

MODULE 1.5 Advanced Waste Water Treatment
The effluent from a typical secondary treatment plant still contains 20-40 mg/L BOD which may be objectionable in some streams. Suspended solids, in addition to contributing to BOD, may settle on the stream bed and inhibit certain forms of aquatic life. The BOD if discharged into a stream with low flow, can cause damage to aquatic life by reducing the dissolved oxygen content. In addition the secondary effluent contains significant amounts of plant nutrients and dissolved solids. If the waste water is of industrial origin, it may also contain traces of organic chemicals, heavy metals and other contaminants. Different methods are used in advanced waste treatment to satisfy any of the several specific goals, which include the removal of (1) suspended solids (2) BOD (3) plant nutrients (4) dissolved solids and (5) toxic substances. These methods may be introduced at any stage of the total treatment process as in the case of industrial waterways or may be used for complete removal of pollutants after secondary treatment.

Removal Of Suspended Solids:
This treatment implies the removal of those materials that have been carried over from a secondary treatment settler. Many methods were proposed of which two methods were commonly used. The two methods are microstaining and chemical coagulation followed by settling and mixed media filtration.

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Indian Institute of Technology Madras

Environmental Chemistry and Analysis

Prof. M.S.Subramanian

Microstraining:
It is a special type of filtration procedure which makes use of filters oven from stainless steel wires with opening only 60-70 µm across to remove very small particles. High flow rates and low back pressures are normally achieved.

Coagulation and flocculation:
The object of coagulation is to alter these particles in such a way as to allow them to adhere to each other. Most colloids of interest in water treatment remain suspended in solution because they have a net negative surface charge that causes the particles to repel each other. The intended action of the coagulant is to neutralise that charge, allowing the particles to come together to form larger particles that can be more easily removed from the raw water. The usual coagulant is alum [Al2(SO4)2• 18H2O ], though FeCl3, FeSO4 and other coagulants, such as polyelectrolytes, can be used. Alum when added to water, the aluminium in this salt hydrolyses by reactions that consume alkalinity in the water such as: [Al(H2O)6 ]3 + + 3HCO3 − → Al(OH)3 (s) + 3CO2 + 6H2O ..............(1) The gelatinous hydroxide thus formed carries suspended material with it as it settles. In addition, however, it is likely that positively charged hydroxyl-bridged dimers such as

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Indian Institute of Technology Madras

Environmental Chemistry and Analysis

Prof. M.S.Subramanian

H O (H2O)4 Al O H Al (H2O)4

4+

and higher polymers are formed which interact specifically with colloidal particles, bringing about coagulation. Metal ions in coagulants also react with virus proteins and destroy upto 99% of the virus in water. Anhydrous ion (III) sulphate can also act as effective coagulant similar to aluminium sulfate. An advantage with iron (III) sulfate it that it works over a wide range of pH.

Filtration:
If properly formed, the addition of chemicals for promoting coagulation and...
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