Variation of Chemical and Structural properties of Waste Aerobic Granular Sludge (WAGS) and Flocculent Sludge in Anaerobic Digestion

Abstract

A comparative batch experiment as well as a biochemical methane potential(BMP) test were conducted over 44d for aerobic granular sludge (WAGS) and waste flocculent activated sludge(WAS) on sludge anaerobic digestion (AD), to give an insight into the chemical and mechanical properties change of the EPS in different sludge source during AD process, via the analysis of polysaccharides (PS), protein (PN), loosely bound extracellular polymeric substance (LB-EPS), tightly bound extracellular polymeric substance (TB-EPS) change and its gel-forming property with multi-valent ions. The EPS, who play a vital role in forming the compact gel-like sludge structure, was extracted from WAGS and AS and analyzed for several test.
During AD process, WAGS demonstrated a lower hydrolysis rate than AS over the first 6 days and reached a comparable overall methane production potential afterwards, suggesting the necessity of pre-treatment of WAGS to enhance sludge hydrolysis. The isolated SEP was slowly degraded over the AD process with a biodegradability of 31% for WAGS and 39% for WAS and this degradation of EPS mainly contributed from the PS and PN it contained. This EPS degradation led to the loss of gel-like properties for both sludge after AD process. Although the results from fourier transformed infrared (FT-IR) spectra exhibited the similar typical brands within the range of 1000~3000 cm-1 in all samples, the second-derivative spectra manifested the occurrence of polysaccharides degradation, especially for homopolymannuronic acid blocks (MM) and homopolyguluronic acid blocks during sludge digestion. Meanwhile, the dewaterability of both sludge was observed to deteriorate with AD ongoing due to the internal transmission from the compact TB-EPS to porous LB-EPS as well as the increase in PN/PS ratio in EPS. Moreover, the mechanical property variation of different sludge was examined by the crosslink of EPS solution with ion to form hydro-bead. Reduction on mechanical strength of EPS formed hydro-beads was seen for both WAGS and WAS but mechanical structure loss of WAGS was less than that of WAS after digestion which can be attributed to higher EPS residues in WAGS. However, no matter for WAGS or WAS, only a limited amount of EPS was degraded during AD process and this provided a possibility of recovery EPS as a coating material from digested sludge.