Anaerobic digestion is quickly becoming the choice technology for sewage sludge treatment at wastewater treatment plants, as the methane rich biogas produced is a valuable resource which can be used to generate electricity. In the oxygen restricted environment, a natural by-product of the anaerobic process is hydrogen sulfide, a harmful contaminant. Also present in the biogas are organosilicons, or siloxanes, which originate from cosmetic products. Efficient removal of these compounds is essential to support economic recovery and use of the biogas. This paper discusses various technologies and the benefits and economics of biogas conditioning.
Biogas, Hydrogen Sulfide, Siloxanes, Conditioning, Benefits of Conditioning
Biogas Production at Wastewater Treatment Plants
Treatment of wastewater in municipalities consists of a screening process, primary settling and secondary treatment, all of which remove solids from the wastewater. The solids from primary and secondary treatment, referred to as sewage sludge, require further treatment before disposal in landfills or agricultural land application. As wastewater treatment plants become larger, the quantity of sludge produced requires a more extensive treatment than aerobic digestion or composting, both of which have low capital costs but higher operating costs due to the energy costs associated with aeration. Anaerobic digestion can easily be designed to handle higher loadings of sludge and after processing produces a small quantity of biosolids.
Anaerobic digestion is the decomposition of organic material in the absence of oxygen. Depending on the operating temperature, retention times vary from 12-40 days, which allows a variety of bacteria to breakdown the organics into essentially methane (CH4), carbon dioxide (C02) and water (H20). Due to the physical properties of these compounds, they form a gaseous mixture, biogas, consisting of 40-70% methane.
In addition to carbon, hydrogen and oxygen molecules, sewage sludge also contains organic sulfur from feces and inorganic sulfur from sulfate ions present in the water. Under anaerobic conditions, sulfate reducing bacteria produce hydrogen sulfide (H2S), which volatilizes into the biogas.
As siloxanes become an increasingly popular compound in cosmetics such as shampoos and conditioners, they are also being identified in higher concentrations in wastewater. In the anaerobic digestion process the siloxanes volatilize into the biogas.