Quantification of soluble recalcitrant compounds in commercial thermal hydrolysis digestates.

: Solid residues such as primary sludge (PS), waste activated sludge (WAS) and food waste (FW) can be stabilized through anaerobic digestion (AD). Application of the thermal hydrolysis process (THP) prior to AD results in several benefits in AD and dewatering. However, soluble recalcitrant compounds associated with Maillard reactions have been identified after THP which can impact downstream processes and water discharge limits. In this study, the soluble colloidal chemical oxygen demand, color, ultraviolet absorbance at 254 nm and dissolved organic nitrogen in seven full-scale THP facilities were quantified and compared. The THP substrate influenced the concentration of soluble melanoidin-associated compounds in the digestates. THP implementation in five water resource recovery facilities (WRRFs) was modelled and found to give a 3-8 mg/L increase on the water effluent COD concentration depending on the PS/WAS ratio. The results provide novel information useful in planning new WRRFs and optimization of existing facilities.