Activated Sludge Process

Activated Sludge Process

	Activated Sludge Process This is the most common option in secondary treatment and is also a method of suspended cell sewage treatment method. It starts with aeration that encourages the growth of microbes in the waste. The microbes feed on the organic material, which then allows solids to settle out. Bacteria-containing ‘activated sludge' is continually re-circulated back to the aeration basin to increase the rate of organic decomposition.
After primary settling, sewage containing dissolved organic compound is introduced into an aeration tank. Aeration is provided by air injection or mechanical stirring by allowing compressed air continuously from the bottom of the aeration tank. Microbial activity is maintained at high levels by introduction of most of the settled activated sludge (also called returned sludge which is rich in culture of aerobic bacteria) thus the name activated sludge process. During the process of aeration, organic matter of the sewage gets oxidised with the help of aerobic bacteria in the tank which are suspended in the sewage water. The typhoid and cholera organisms are definitely destroyed and the coliforms are greatly reduced. During the holding period in the tank, there is vigorous development of diverse heterotrophic bacterial population like Micrococcus, Achromobacter, Athrobacter, Flavobacterium, Zoogloea. Filamentous bacteria like Sphaerotilus, mycobacteria are also common. Filamentaous fungi and yeast occur in low numbers. Protozoa are represented by ciliates. They are important predators of bacteria.
Bacteria occur individually in free suspension aggregates as floccules by Zoogloea ramigera. Floc is too large to be ingested by protozoa hence considered as a defence mechanism. In raw sewage, bacteria predominate during the holding time in the aeration tank. The number of suspended bacteria decrease but those associated with floc greatly increase in number. During the holding period, a portion of dissolved organic substrate is mineralised. Another portion is converted to microbial biomass. In the advanced stage of aeration, most of the microbial biomass is associated with floc that can be removed from suspension by settling.
Poor settling characteristics are associated with the bulking of the sewage sludge, a problem caused by proliferation of filamentous bacteria like Sphaerotilus, Beggiatoa, Thiothrix and Bacillus, filamentous fungi such as Geotrichum, Cephalosporium, Cladosporium and Penicillium. Bulking may be caused by a high C : N and C : P ratios and low dissolved oxygen concentration. A portion of the settled sewage is recycled for inoculation of incoming raw sewage. Excess sludge requires incineration or addition of treatment by anaerobic digestion and composting or disposal as landfills. ASP tends to reduce the BOD to 5-15 % of raw sewage. Treatment drastically reduces the number of intestinal pathogens in sewage (through competition, adsorption, predation and settling). Numbers of E.coli and enteroviruses are 90-99% lowered in the effluent by the ASP than in the incoming raw sewage. ASP is essentially a continuous culture process. In a steady stable ASP, growth of sludge bacteria (floc) must be equal to the sludge wastage.
	The presence of attached ciliates is critical for the control of disposed bacteria and removal of portion of BOD. The types and number of protozoa associated with the floc can be used as an indicator of sludge condition and thus treatment performance can be monitored. Sludge is in poor condition when few ciliates and many flagellates are seen. Ciliates predominate in good sludge. ASP is efficient and flexible and is able to withstand variation in sewage flow rate and concentration and is widely used for the treatment of domestic waste and industrial effluent. It produces large volume of sludge.

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