Characterization of organic matter in the thermal hydrolysis pretreated anaerobic digestion return liquor

Disposal and treatment of the return liquor produced by the dewatering of the thermal hydrolysis pretreated anaerobic digestion biosolids is a major concern for treatment plants because the presence of UV quenching substances in the return liquor might interfere with UV disinfection. Also, with the lowering of effluent nitrogen standards in some locations, development of treatment techniques for nitrogen-containing organics in the return liquor is required. In this study, the molecular weight (MW) and chemical nature based fractions were characterized by their total organic carbon, UV absorbance, organic nitrogen and protein content. The humic acids and the hydrophilic fraction were found to be the major contributors to the organic matter and organic nitrogen in the return liquor, whereas the humic substances (humic and fulvic acids) were found to be mainly responsible for the UV254 quenching. Humic substances were observed to be the larger molecular weight (mainly >1 kDa) fractions and could not be efficiently removed by aerobic biological treatment.