The aim of wastewater treatment is the removal of undesirable substances and hazardous microorganisms in order that the water may safely enter a watercourse such as
Biodegradation is the term used to describe the natural processes of breakdown of matter by microorganisms.
a river or stream. Further purification procedures are required before it can be used as drinking water. Wastewater treatment is fundamental to any developed society, and greatly reduces the incidence of waterborne diseases such as cholera. Wastewater may come from domestic or commercial sources; highly toxic industrial effluents may require pre-treatment before entering a water treatment system. Sewage is the term used to describe liquid wastes that contain faecal matter (human or animal).
The effectiveness of the treatment process is judged chiefly by the reduction of the wastewater's biochemical oxygen demand (BOD). This is a measure of the amount of oxygen needed by microorganisms to oxidise its organic content. A high BOD leads to the removal of oxygen from water, a certain indicator of pollution.
Wastewater treatment usually occurs in stages, the first of which (primary treatment) is purely physical, and involves the removal of floating objects followed by sedimentation, a process that removes up to a third of the BOD value. Secondary treatment involves microbial oxidation, leading to a substantial further reduction in BOD. This may take one of two forms, both of which are aerobic, the traditional trickling filter and the more recent activated sludge process (Figure 16.6). In the former, the wastewater is passed slowly over beds of stones or pieces of moulded plastic. These develop a biofilm comprising bacteria, protozoans, fungi and algae, and the resulting treated water has its
Bar screen, grit tank
Chlorination/UV treatment \
Figure 16.6 Wastewater treatment. Wastewater treatment achieves a reduction of the biochemical oxygen demand of the water by primary (physical) and secondary (biological) treatment
BOD reduced by some 80-85 per cent. Activated sludge plants achieve an even higher degree of BOD reduction. Here the wastewater is aerated in tanks that have been seeded with a mixed microbial sludge. The main component of this is the bacterium Zoogloea, which secretes slime, forming aggregates called flocs, around which other microorganisms such as protozoans attach. Some of the water's organic content is not immediately oxidised, but becomes incorporated into the flocs. After a few hours' residence in the tank, the sludge is allowed to settle out, and the treated water passes out of the system. Before being discharged to a watercourse, it is treated with chlorine to remove any pathogenic microorganisms that may remain.
The principal operating problem encountered with activated sludge is that of bulking. This is caused by filamentous bacteria such as Sphaerotilus natans; it results in the sludge not settling properly and consequently passing out with the treated water.
Both secondary treatment processes result in some surplus sludge, which undergoes anaerobic digestion, resulting in the production of methane and CO2. The methane can be used as a fuel to power the plant, and any remaining sludge is dewatered and used as a soil conditioner. Care must be taken in this context, however, that the sludge does not have a high heavy metal content.
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