Soluble microbial products (SMPs) in a sequencing batch reactor with novel cake filtration system

The formation, composition and characteristics of soluble microbial products (SMPs) were investigated in a novel system which coupled a sequencing batch reactor with a cake filtration system. Both suspended solids (SS) and turbidity were significantly removed, resulting in effluent SS of 0.12 mg L⁻¹ and turbidity of 0.72 NTU after cake filtration. The average concentrations of proteins and carbohydrates decreased respectively from 4.0 ± 0.4 and 7.1 ± 0.6 mg/L in the sequencing batch reactor (SBR) mixed liquor, to 0.85 ± 0.21 and 1.39 ± 0.29 mg/L in the cake filtration effluent. Analysis of the molecular weight (MW) distribution of SMPs revealed a substantial reduction in the intensity of high-MW peaks (503 and 22.71 kDa) after cake filtration, which implied the sludge cake layer and the underlying gel layer may play a role in the effectiveness of cake filtration beyond the physical phenomenon. Three-dimensional excitation emission matrix fluorescence spectroscopy indicated that polycarboxylate- and polyaromatic humic acids were the dominant compounds and a noticeable decrease in the fraction of these compounds was observed in the cake filtration effluent. Analysis with GC-MS set for detecting low-MW SMPs identified aromatics, alcohols, alkanes and esters as the dominant compounds. SMPs exhibited both biodegradable and recalcitrant characteristics. More SMPs (total number of 91) were accumulated during the SBR start-up stage. A noticeable increase in the aromatic fractions was seen in the SBR effluent accoutring for 39% of total compounds, compared to the SBR mixed liquor (28%). Fewer compounds (total number of 66) were identified in cake filtration effluent compared to the SBR effluent (total number of 75).