The most important organics in wastewater are proteins, lipids and hydrocarbons. All can be utilized by acidogenic bacteria, which encompass a very large group of different, mostly facultative anaerobic bacteria.
Proteins are hydrolyzed into amino acids by proteases which function as exo-en-zymes. The amino acids can be taken up by diffusion through cell walls and membranes at a relatively high rate. This process is not rate-limiting for subsequent reactions (Seyfried 1979). A small amount of amino acids is used directly for growth (anabolism), while a large amount is converted to lower fatty acids, CO2, H2 as well as NH4+ and is excreted (catabolism).
Lipids are esters formed from glycerine, an alcohol with three valences, and fatty acids. These have been hydrolyzed previously by lipase enzymes. Glycerine can be partially used for anabolic reactions and is converted in part to lower alcohols (catabolism). The fatty acids cannot be used by acidogenic bacteria and are excreted.
Polymeric hydrocarbons are hydrolyzed into monomers (glucose and other sugars) by most facultative anaerobic bacteria via exo-enzymes (McInerney and Bryant 1981). One part is totally used for protein synthesis and bacterial growth, while another part is converted into lower fatty acids. An example is presented by Eq. (8.1), see Fig. 8.1:
This first reaction step results in a reduction of DOC (16.7%) and the formation of CO2 as well as H2. Mosey (1983) proposed a mechanism for the formation of fatty acids from glucose (Fig. 8.2).
The conversion of chemical energy into ATP during anaerobic fermentation is very low and only 1 mol ATP per 1 mol glucose is converted for growth.
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