Effects of biomass addition on organic composition of supernatant in sludge digestion process

Abstract

A mesophilic (35°C) continuous co-digestion experiment of sewage sludge and softened pre-treated rice straw (RS) was conducted in the present study to evaluate the impact on the dissolved organic matters (DOMs) in the digested sludge due to the addition of RS. Sludge retention time was 25 d; total solids (TS) of sewage sludge and RS were 3% and 1.5%, respectively. Methane yields of sludge digestion, RS digestion and co-digestion were 0.27, 0.11 and 0.25 L/g-CODCr, respectively. The degradation of RS caused significant increase in dissolved organic carbon (DOC) in digested sludge, and humic matter concentration increasing by over 10 folds. Results of excitation-emission matrix (EEM) fluorescence spectroscopy revealed that the addition of RS caused additional fluorescent peak that related to humic acid-like substances in the digested sludge. The digested supernatant was then mixed with activated sludge [mixing ratio: 10: 1 (vol./vol.)] and cultured under aerobic condition to study the biodegradability of DOMs. Results of DOMs analysis and EEM spectra of filtrate samples after aerobic treatment showed that organic content in digested sludge was resistant to biodegrade.