Environmental Science (2+1)

Sludge treatment and disposal

 
• The sludges accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Incineration is also used albeit to a much lesser degree.


• Choice of a wastewater solid treatment method depends on the amount of solids generated and other site-specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications.

Anaerobic digestion

• Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.


• Anaerobic Digestion is the most common (mesophilic) treatment of Domestic Sewage in Septic Tanks, which normally retain the sewage, from one day to two days, reducing the B.O.D. by about 35 to 40%. This reduction can be increased by a combination of anaerobic and aerobic by installing '[Aerobic Treatment Units]' (ATUs) in the Septic Tank. One major feature of anaerobic digestion is the production of biogas, which can be used in generators for electricity production and/or in boilers for heating purposes.

Aerobic digestion

• Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs used to be characteristically much greater for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add oxygen to the process. However, since the recent advent of stone fibre filter technology which uses natural air currents for oxygenation, this no longer applies. Aerobic digestion can also be achieved by using jet aerators to oxidize the sludge.

Composting

• Composting is also an aerobic process that involves mixing the sludge with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.
• Incineration
• Sludge disposal
• Treatment in the receiving environment