Is A Second Thermal Hydrolysis Energy-positive? A Net Energy Analysis of Dual-THP Configuration

1. Introduction Thermal hydrolysis pretreatment (THP) has been commercially applied in water resource recovery facilities (WRRFs) for over 20 years to intensify anaerobic digestion (AD) [1]. Reported advantages of THP include improved sludge digestibility and dewaterability, increased biogas production, and reduced pathogen, odor, and foaming [1-3]. Among these benefits, enhanced biogas production has received a lot attention because methane in biogas can offset energy demand and be a revenue source for WRRFs. Prior studies have reported that THP not only accelerated municipal sludge biodegradation but also increased its bioavailability during AD. Building on this established knowledge, it is our hypothesis that applying a second THP to THP-AD reactor digestate may release or create new methane potential that can be utilized by methanogens, which would otherwise remain non-bioavailable. To test this hypothesis, an on-site pilot study was conducted in collaboration with Arlington County Water Pollution Control Plant (Arlington WPCP) to investigate: (i) whether applying a second THP on the THP-AD reactor effluent can release or create additional methane potential; and (ii) the practicality of integrating a second THP into the THP-AD process from a net energy output perspective 2. Materials and Methods A blend (3:2 v/v) of primary sludge and waste activated sludge dewatered to 30% total solids (TS) was collected from Arlington WPCP, Virginia. The dewatered cake was diluted to 16% TS and then pretreated in a pilot-scale THP system (Cambi, Asker, Norway) at 165 ℃ (equivalent to 6 bars) for 30 minutes. The THP-treated sludge was then fed to three semi-continuous mesophilic AD reactors operated at 10-, 12.5-, and 15-day solids retention time. Digestate collected from the three AD reactors was first tested for their residual methane potential via a biological methane potential (BMP) test using a 16-channel anaerobic respirometer system (AER-800, Challenge Technology, Springdale, AR, USA) with a triplicate setup. At the end of the BMP test, all bioavailable organics in the THP-treated sludge were hypothesized to be experimentally depleted, and the remaining organics were regarded as non-bioavailable or non-biodegradable by AD microbes. Then, the non-bioavailable sludge from the BMP test was processed through a second THP process followed by a second BMP test to experimentally evaluate how much new methane potential can be created by the second THP from the BMP-exhausted sludge that essentially has no bioavailable organics. Based on the new BMP created by the second THP, an energy balance analysis was conducted to evaluate the impact of the second THP integration on the overall net energy output of the entire solids treatment process by following the approach used by Chen, et al. [4]. 3. Results and Discussion New BMP created by a second THP As shown in Figure 1, on top of the CH4 yields of 0.292, 0.304, and 0.321 L CH4/g volatile solids (VS) fed obtained from the THP-AD reactors operated at the 10-, 12.5-, and 15-day SRTs, the residual CH4 yields of the reactor effluent were 0.104, 0.094, and 0.082 L CH4/g VS fed, respectively. To this end, the total methane potential of the THP-treated sludge was around 0.399 L CH4/g VS fed (Figure 1). More importantly, the new BMP in terms of CH4 yield created by the secondary THP ranged from 0.099 to 0.121 L CH4/g VS fed within only 6 days of incubation time, which surprisingly marks an additional up to 30.1% increase in CH4 yield compared to the total methane potential of the sludge treated by the first THP. The creation of the new methane potential from the BMP-exhausted THP-AD reactor effluent indicates that THP can enhance sludge bioavailability by converting previously non-bioavailable organics into methanogen-accessible substrates. It should be emphasized that if the residual BMP in the THP-AD reactor effluent and the new BMP created by the second TH can both be recovered, their combined contribution would increase the total methane production by 56.2% to 77.1% relative to the CH4 yield achieved in the THP-AD reactors alone. Figure 2 shows the COD turnover in terms of soluble COD (sCOD) and particulate COD (pCOD) throughout the treatment process. The unchanged sCOD fraction throughout the first BMP test suggests that the sCOD in the AD reactor effluent, regardless of their SRTs, are completely recalcitrant under anaerobic conditions (Figure 2). Following that, the second THP hydrolyzed 18%-22% tCOD in the form of pCOD into sCOD (Figure 2). Interestingly, the BMP test of the second THP effluent (2nd BMP) only reduced sCOD and converted it to methane, leaving pCOD untouched (Figure 2), implying that the fraction of pCOD that has survived the second THP was completely recalcitrant. Process concept for dual-THP implementation There are two key findings that are valuable to potential full-scale applications for integrating a second THP process. First, the two BMP tests indicate that, if both the residual BMP in the THP-AD reactor effluent and the new BMP created by the second THP are recovered, total methane production could increase by up to 77.1% relative to the methane production achieved in the THP-AD reactors alone (Figure 1). Second, the sCOD in the THP-AD reactor effluent and the pCOD in the second THP effluent were completely recalcitrant as they did not contribute to the methane production during the two BMP tests regardless of the reactor SRTs (Figure 2). By using these three findings, a THP-AD-THP process with centrate recirculation, or a dual-THP process, that combines both the pre-AD THP and post-AD THP setups is proposed in Figure 3. Based on the two BMP tests results and COD turnover discussed above, the THP-AD reactor effluent can be post-thickened to ~16% TS, allowing only the thickened solids to undergo a second THP to convert residual hardly microbial-hydrolyzable particulate organics into methanogen-accessible soluble organics. This configuration may enable the second THP to both release the residual BMP remaining in the reactor effluent particulate fraction and create additional BMP as observed in the second BMP test (Figure 3). The post-thickening centrate, mainly comprising recalcitrant sCOD, can be discharged to mainstream treatment without energy loss. Excluding this centrate reduces the required secondary THP capacity to approximately half that of the primary THP. Following the second THP, newly generated BMP resides almost entirely in the soluble phase as supported by the results above, enabling post-dewatering separation of recalcitrant solids for disposal and recirculation of methane potential-rich centrate to the AD reactor. It should be noted that the centrate flow is sufficient to dilute the primary THP effluent without potable or reclaimed water addition, which avoids increases in digester volume (Figure 3). How much net energy output can the second THP bring? Although coupling a second THP with a conventional THP-AD process can potentially increase the methane production by up to 77.1% (Figure 1), the practical value of this intensification ultimately depends on whether the additional methane energy recovered substantially outweighs the operational energy consumption introduced by the second THP and associated unit operations. As can be seen from Figure 3, the second THP process and the additional post-thickening process are the two processes that demand extra energy input compared to a typical THP-AD configuration. The analysis results reveal that integrating a second THP yields 3,080-3,797 megajoules (MJ)/ton-TS of additional recoverable energy (heat + electricity), which substantially exceeds the added operational energy consumption (1,894-2,126 MJ/ton-TS) for the second THP and post-thickening. In terms of net energy output, as the SRT of AD reactors increases from 10 to 12.5 and 15 days, the net energy output of the entire dual-THP configuration increases from 5,168 to 5,310 and 54,04 MJ/ton-TS, respectively, marking 47.8%, 39.7%, and 28.1% increase in net energy output compared to the baseline THP-AD configuration at the corresponding SRTs. These results suggest that the dual-THP configuration brings the most energy recovery benefits for utilities constrained to short SRT operation (e.g., high-flow facilities with limited footprint). 4. Conclusions The following major remarks can be drawn from this study: 1) A second THP performed on THP-AD digestate can release 77.1% more methane potential compared to that of THP-AD reactor operated at 10-day SRT. 2) The sCOD in the THP-AD reactor effluent and the pCOD in the second THP effluent were completely recalcitrant and did not contribute to the methane production during the two BMP tests. 3) Implementing a second THP with centrate recirculation to AD can increase the net energy output as heat and electricity by 47.8% at 10-day AD SRT without increasing AD volume.