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 modeled 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. PRACTITIONER POINTS: High amounts of WAS in substrate resulted in higher concentrations of CODsc, color and dissolved organic nitrogen in the digestate. Food waste treated at 145°C showed equal or lower concentrations of all components compared with sludge operated at 165°C. Installation of THP will increase the COD concentration in the water effluent of a water resource recovery facility. The characteristics of the THP substrate are important to consider in cost/benefit analysis when planning the installation of THP.