Effects of temperatures and extracellular proteins on dewaterability of thermophilically digested biosolids

Abstract

Thermophilic processes digest sludge at high temperatures to produce Class A biosolids. Recent research work revealed that digestion temperature is the predominant factor affecting dewaterability of thermophilic biosolids. This paper presents findings of a laboratory study that investigated how various digestion temperatures affect dewaterability of digested biosolids, studied the phase partition of the substances affecting dewaterability in digested biosolids, and tested the role of extracellular proteins in affecting dewaterability. Secondary sludges were digested at 40°C, 50°C, 60°C, 70°C, or 22°C for up to 12 d. Filtrates from thermophilically digested biosolids were treated with protease and boiling. This study found that, during the first few hours of digestion, higher temperatures resulted in more rapid and more significant deterioration in dewaterability than lower digestion temperatures. Continued digestion resulted in either improved (60°C or 70°C), unchanged (40°C or 50°C), or gradually deteriorated dewaterability (22°C). The substances affecting dewaterability were primarily located in the liquid phase of thermophilically digested biosolids. Boiling treatment did not result in significant changes in dewaterability. Protease treatment of the liquid phase of thermophilic biosolids improved dewaterability by 13–19%. Such an improvement confirmed the role of proteins in affecting dewaterability. Key words: dewaterability, thermophilic, mesophilic, digestion, biosolids, protein, protease.