Engineering Chemistry S Datta

580 ENGINEERING CHEMISTRY
Control of Water Pollution
The problem of water pollution can be controlled by adopting the following procedures:
1. By reducing the waste at source.
2. Diluting the waste water from different sources before discharging since the dissolved
oxygen of diluting water helps in biodegradation of different toxic chemicals
by micro-organisms to inocuous chemicals before mixing with natural water.
3. By chemical treatment the polluted water can be made safe.
4. By physical methods like reverse osmosis, electrodialysis, ion-exchange etc., polluted
water can be purified.
5. By recycling of waste water before discharging involves treatments like use of activated
sludge, trickling filter etc.
6. Reclamation of waste water: sewage water can be made useable for irrigation, fishfarm
raising, as it contains essential nutrients.
Sewage and its Treatment
Sewage is the liquid waste from domestic and municipal waste water, industrial waste
water, ground wastes etc. Domestic-sewage has objectionable odour due to S and N containing
compounds and a grey to green color due to the presence of discharges from kitchens, bath and
lavatories.
Industrial wastes have different color or odour depending on types of industries present
and the chemical constituents also vary accordingly. The treatments consist of aerobic and
anaerobic oxidations, which are brought about by the aerobic or anaerobic bacteria present
in the sewage.
The aerobic bacteria oxidize the organic compounds present in the sewage by the dissolved
oxygen leading to the formation of nitrates, sulfates and phosphates which is called aerobic
oxidation.
Sewage
+
Air (to
supply dissolved
oxygen)
Oxidation by
Aerobic bacteria
Aerobic oxidation
Nitrates, sulfates,
phosphates etc.
In the absence of the supply of free oxygen, the anaerobic bacteria convert the organic
compounds in the sewage to methane, H 2
S, ammonium sulfide etc. (offensive odour).
In this type of anaerobic oxidation process the required oxygen is supplied by the nitrates,
sulfates and organic compounds.
Sewage
Oxidation by
Anaerobic bacteria
Anaerobic oxidation
Oxidation to
produce H2S,
PH , CH etc.
3 4