Abstract
Background To understand the intrinsic role of hydrolytic enzymes in sludge treatment, particularly their effect on the digestibility and dewaterability of sludge, activated sludge flocs were disintegrated using various techniques that included different enzymes (amylase, cellulase, proteinase, DNase, and polygalacturonase), pH adjustment, and temperature adjustment. The effectiveness of each enzyme treatment was pinpointed by quantifying the spatial distribution of each type of organic matters (protein, polysaccharide, DNA, fluorescent organics) in outer layer extracellular polymeric substances (EPS), inner layer EPS, and cells. ResultsMost hydrolytic enzymes functioned only owing to a temperature or pH effect. The release of organic matter from the interior fraction of EPS to the exterior fraction was prompted under high pH and temperature even without enzyme addition. The effectiveness of enzyme addition was only significant for cellulase and polygalacturonase treatments. Polygalacturonase unexpectedly increased the total EPS polysaccharides up to seven times, accompanied with improved dewaterability, while the amount of EPS proteins was almost unchanged. Combining chemical and morphological evidence, a new conceptual model considering the spatial distribution of polygalacturonic acid-like matter, proteins, cellulose, and other organics in EPS was proposed.ConclusionsPolygalacturonic acid-like matter hydrolysis caused significant release of polysaccharides. Polygalacturonase released polysaccharides while keeping proteins unreleased. Temperature and pH adjustment were as effective as enzyme at sludge disintegration. Cellulose hydrolysis led to massive release of all kinds of organic matters. A new conceptual sludge structure model regarding organic components is proposed.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0444-y) contains supplementary material, which is available to authorized users.
Highlights
To understand the intrinsic role of hydrolytic enzymes in sludge treatment, their effect on the digestibility and dewaterability of sludge, activated sludge flocs were disintegrated using various techniques that included different enzymes, pH adjustment, and temperature adjustment
The choice of enzymes mainly has been usually limited to amylase and protease, so massive quantities of enzymes are needed to hydrolyze the proteins and polysaccharides that comprise the huge amount of extracellular polymeric substances (EPS) (200–500 mg/g volatile solids) in sludge [1]
Three enzymes were used to attack different polysaccharides: α-amylase (Sigma, Product No.: 10070-10G, about 50 U/mg derived from Bacillus subtilis, to be used at pH 6.9 and 25 °C), cellulase (Sigma, Product No.: C260550 mL, at least 1000 U/g derived from Aspergillus sp., to be used at pH 8.0–8.2 and 40 °C), and polygalacturonase (Sigma, Product No.: 17389-10G, greater than 1 U/mg derived from Aspergillus niger, to be used at pH 4.0 and 50 °C)
Summary
To understand the intrinsic role of hydrolytic enzymes in sludge treatment, their effect on the digestibility and dewaterability of sludge, activated sludge flocs were disintegrated using various techniques that included different enzymes (amylase, cellulase, proteinase, DNase, and polygalacturonase), pH adjustment, and temperature adjustment. The effectiveness of each enzyme treatment was pinpointed by quantifying the spatial distribution of each type of organic matters (protein, polysaccharide, DNA, fluorescent organics) in outer layer extracellular polymeric substances (EPS), inner layer EPS, and cells. The choice of enzymes mainly has been usually limited to amylase and protease, so massive quantities of enzymes are needed to hydrolyze the proteins and polysaccharides that comprise the huge amount of EPS (200–500 mg/g volatile solids) in sludge [1]. Studies on hydrolyzing EPS lack specific details, such as what roles various types of organic matter have in EPS and what kind of enzyme can disintegrate EPS efficiently
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