Extracellular Enzymes

Hydrolysis of polymeric substances is the first and rate-limiting step in the process of wastewater treatment.22'23 Due to their large size, organic polymers such as proteins, polysaccharides, and lipids must be hydrolyzed to their subunit molecules before microorganisms can oxidize or recycle them back to inorganic C, N, S, and P. The microbial enzymes responsible for these extracellular hydrolysis reactions are referred to as extracellular enzymes or exoenzymes. Microbial metabolism of complex organic matter begins with reactions catalyzed by extracellular enzymes. Consequently, extracellular enzymes are essential components of many biochemical pathways utilized by microorganisms for growth. Microbial polysaccharases, esterases, lipases, and proteases are examples of extracellular enzymes that play an important role in the biodegradation of polysaccharides, lipids, and proteins, respectively.15,24 Exoenzymes also liberate inorganic phosphate (Pi), N, and S from organic molecules. Phosphatases (PO4ases) are a class of enzyme that liberates Pi from phosphate-containing organic compounds. PO4ases occur as intracellular enzymes where they control intracellular Pi concentrations needed for energy yielding phosphorylation reactions. They also occur in the cell envelope and as extracellular enzymes where they scavenge Pi from polymeric phosphorylated organic compounds to meet their inorganic phosphate requirements.

14.3.1 Forms of Phosphorus in Wastewater

Phosphorus removal from wastewater has become an important part of the overall treatment process. Enhanced biological P removal can achieve 90% to 95% reduction.25 Some of the steps in the biochemical pathway of enhanced P removal are known, but others remain to be elucidated,26 and the microorganisms responsible for some of the steps are still not known.27 Phosphorus entering the activated sludge process exists in both organic and inorganic forms (Table 14.1). These data suggest that one-third to one-half of the total P in the system exists as detrital organic-P. While much attention has been directed toward the microbial populations that accumulate poly-Pi, little attention has been directed to those populations in the activated sludge

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