Does Thermal Hydrolysis Alter the Efficiency besides the Rate of Anaerobic Digestion?

Introduction Thermal hydrolysis pretreatment (THP) can intensify the anaerobic digestion process by applying high temperature and pressure to the municipal sludge prior to the anaerobic digestion in the water resource recovery facilities (WRRFs). Many advantages of THP have been reported in previous studies including the improvement of sludge dewaterability, digestion rate acceleration, higher anaerobic digester (AD) organic loading rate, pathogen-free in biosolids, and foaming potential reduction (Higgins et al., 2011). Many studies have been conducted to understand the mechanisms behind the enhancement of anaerobic digestion by THP (Barber, 2016). It is generally accepted that THP increases the sludge anaerobic digestion rate rather than biodegradability. A long enough solids retention time (SRT) of anaerobic digestion is believed to produce the same amount of CH4 with and without THP. In other words, it is not the accumulative but the rate of CH4 that can be presumably improved by THP. However, to our best knowledge, this hypothesis still remains to be tested. The most commonly used method to evaluate the anaerobic biodegradability of municipal sludge is the biological methane potential (BMP). Therefore, the main objective of this study is to evaluate the effect of THP on municipal sludge anaerobic biodegradability via BMP tests. Materials and Methods Experimental design A process flow chart of the experimental design is provided in Figure 1. Dewatered cake collected from a local WRRF with primary sludge and wasted activated sludge blended at a 3:2 v/v ratio was diluted from 30% total solids (TS) to 16% TS prior to feeding into THP. The treated sludge from the THP unit with TS around 9% was then used as the influent for three pilot mesophilic ADs operated at SRTs of 10d. 12.5d, and 15d, respectively. After that, BMP tests were performed on the three digestates to measure their remaining methane potential. Conventionally, the remaining biomass that can survive BMP tests are regarded as non-readily biodegradable. In this study, these biomasses were further processed in a 2nd round of THP to test the hypothesis whether THP can turn the non-readily biodegradable fraction into readily biodegradable. After that, a second round of BMP were performed on the same sludges to measure if more readily biodegradable fractions have been created by the THP. THP and anaerobic digester setup A five-gallon capacity THP system shown in Figure 2 was procured from CAMBI. The THP unit was heated by hot steam to 165 oC for 30 min. The treated sludge was cooled down to room temperature prior to feeding into bench-scale mesophilic ADs. Three units of mechanically mixed ADs with 10 L working volume were operated in parallel with temperature maintained at 36.5 ± 0.3 ËšC. These ADs were inoculated with fresh effluent sludge from a full-scale THP-fed mesophilic AD at DC Water. BMP test setup A respirometer system was used to perform BMP tests with a triplicate design for each digestate sample. The 1st BMP test was performed without inoculation because the samples tested were active digestate. After 1st BMP test, residues of all triplicate groups for each sample were combined and then treated with 2nd round of THP (160 °C, 30 min) prior to the 2nd BMP test. In the 2nd BMP test, inoculum collected from another local WRRF's full-scale mesophilic AD effluent was used as the seed. Blank bottles were prepared with only inoculum for two BMP tests. Control bottles were also prepared by adding glucose to check the inoculum activity in both BMP tests. All the experimental groups were placed in an incubator shaker with temperature controlled at 35.5 °C. During the two rounds of BMP tests, the CO2 and H2S in biogas produced from each bottle were scrubbed by an inline KOH scrubber. The residual biogas after KOH scrubbing was regarded as CH4 and quantified by a bubble counter in each respirometer channel. The CH4 production from each bottle was real-time monitored throughout the two rounds of BMP experiments until the CH4 accumulation profile reach a plateau in each BMP experiment. Results CH4 production during two rounds of BMP tests As can be seen in Figure 3a, profiles in both BMP tests reached plateaus after 40 days. It is noteworthy that the maximum specific methane production rates measured by the profile slopes in the first 2 days of the BMP tests were much greater in the 2nd BMP than that in the 1st BMP. Undoubtedly, more much readily biodegradable biomass has been created by the 2nd round of THP prior to the 2nd BMP. Moreover, the less methane was produced from the digestates collected from ADs with longer SRTs (Figure 3a), which is reasonable because more biodegradable residuals remained under shorter SRTs. The methane yields from unit VS processed in AD and two rounds of BMP tests are shown in Figure 3b. As can be seen, about 0.08 to 0.1 L g-1 VS fed remained even though 0.49 to 0.55 g-1 VS fed has been recovered from THP-AD. Moreover, the methane production in AD and 1st BMP were increased and decreased with the AD SRTs, respectively, which is expected. It is surprising to see that the even more methane was produced from the 2nd BMP test than from the 1st one as a result of the 2nd round THP (Figure 3b). Theoretically, 1st BMP should have exhausted all methane potential remained in the digestates remaining from AD. The fact that even more methane produced from the 2nd BMP after 2nd THP points to an important discovery, i.e., THP does have the capacity to turn non-readily biodegradable fraction of the sludge biomass into readily biodegradable. COD turnover The organic fraction remaining in the sludge samples can be quantified by COD, and thus their organics biodegradability can be measured by the COD removal. Since very similar trends were shown from all samples, the COD turnover in sludge digested under a 15-d SRT was selected as a representative biomass in Figure 4. Both soluble COD (sCOD) measured after 0.45 µm filtration and the particulate COD (pCOD) calculated as the difference between total COD (tCOD) and sCOD were monitored before and after each treatment. As can be seen, THP mainly turned pCOD into sCOD with minor tCOD alternation. A little bit more tCOD reduction observed in the 2nd THP can be attributed to the greater volitation of volatile fatty acids (VFAs) as a result of the VFA enrichment in samples before 2nd THP (Figure 5), which was not the case in the 1st THP when raw cake was fed. This volatile COD loss can be avoided in full-scale THP-AD when the two systems are directly connected. In line with methane production profiles in Figure 3b, majority of COD was removed in the AD. The parallel removal of sCOD and pCOD in the 1st BMP indicates that the much of the residual COD of both types in AD were readily biodegradable. In contrast, only sCOD was removed in the 2nd BMP leaving the pCOD unchanged. It should be realized that the residual sCOD after 2nd BMP is almost equal to that of the 1st BMP(Figure 4). Hence, it is a reasonable inference that all sCOD removed during the 2nd BMP might be the fraction of sCOD created in the 2nd THP from pCOD (Figure 4). Therefore, the role of THP might be turning the non-readily biodegradable portion of pCOD into readily biodegradable sCOD but leaving the biodegradability of inert sCOD unchanged. This is very plausible because one of the main mechanisms in THP is solubilization of biomass (Phuong Linh et al., 2021). For example, microbial cells grown in AD and 1st BMP are usually not biodegradable because of cell wall protection. The breakdown of cell wall by high temperature and pressure in THP can release the soluble intracellular organics thus increasing the biodegradability of the sludge (Phuong Linh et al., 2021). On the other hand, the unchanged pCOD in 2nd BMP corroborates that the non-readily biodegradable pCOD that can survive THP was indeed recalcitrant to biodegradation. These inert pCOD accounts for 13% tCOD in the original cake. Conclusions This study proved that THP not only elevated the rate but also the efficiency of AD by turning a portion of non-readily biodegradable COD into biodegradable. Specifically, it looks like that the sludge biodegradability improvement was from the sCOD created by THP from pCOD but not the portion of recalcitrant sCOD remained.