Intensification of Anaerobic Digestion: Co-Digestion of Grease Interceptor Waste (GIW) with Thermally Hydrolyzed (TH) Biosolids

Abstract

Intensifying methane production during anaerobic digestion (AD) is a key factor in making anaerobic digestion economically feasible. We explore two key ideas in biomethane production: using thermally hydrolyzed (TH) biosolids, and co-digestion with grease interceptor waste (GIW). We used modified biochemical methane potential (BMP) tests in several Phases in triplicate: Phase 1 investigated the optimal COD-based substrate to inoculum (S/I) ratio for mono-digestion of TH biosolids and GIW, Phase 2 investigated co-digestion, and Phase 3 investigated codigestion under conditions modified to be more like those of full-scale operations. Application of a TH pretreatment to Phase 1 biosolids, Phase 3 mixed and thickened (M&T) biosolids, and Phase 3 co-substrate (M&T biosolids and GIW) increased the fraction of soluble to total COD by 51.7%, 25.2%, and 25.5%. Modified Gompertz models identified maximum methane potential (MMP) and maximum daily methane production rate (MDMPR). The most productive Phase 1 GIW mono-digestion had a MMP three times that of Phase 1 TH biosolids but had a lag period that was almost seven times longer. In Phase 2 biosolids monodigestions, TH increased MMP and MDMPR in biosolids but no significant effect was observed in GIW. Inclusion of TH GIW and GIW without TH in Phase 2 co-digestions both increased MMP and MDMPR, compared to mono-digestions of biosolids, but there was no significant difference in MMP or MDMPR between these co-digestions. These are consistent with very similar microbial communities at the end of the incubations (analyzed using 16S gene sequencing). Modified Gompertz Model fitting of Phase 3 results also showed no significant difference between TH and raw co-digestions of M&T biosolids and GIW in terms of MMP or MDMPR. Again, these were consistent with the microbial community analysis results, that showed very similar communities at the end of the runs. One reason for the difference in TH effects between Phase 3 and other Phases was the inclusion of primary solids in M&T biosolids. The benefits of TH appeared to be less dramatic in substrates that contained primary solids than in those exclusively composed of waste activated sludge. TH was shown to be beneficial in mono-digestion of waste activated sludge biosolids, more than doubling MMP and increasing MDMPR by more almost seven times. However, TH does not intensify anaerobic digestion when applied to co-substrates of biosolids and GIW. This study found that TH is suitable for application to biosolids at the current state of practice. Incorporation of TH GIW showed no consistent benefit over addition of untreated GIW. The application of TH to GIW does not always lead to increased methane yield or improved digestion kinetics: many factors, including the type of biosolids (WAS vs. primary sludge; thickened vs. unthickened) affect the methane yield.