The Impact of Solids Retention Time Reduction on the Thermal Hydrolysis Pretreatment-Enhanced Mesophilic Anaerobic Digestion

INTRODUCTION Thermal hydrolysis pretreatment (THP) holds promise to intensify the solid handling capacity of mesophilic anaerobic digesters (AD) by improving the sludge biodegradability, pumpability, dewaterability, and biosafety. Typically, the digester feedstock total solids concentration (TS) can be doubled from 5 to 10%, and the solids retention time (SRT) can be halved from 30 to 15 days after THP is installed, which directly enables existing AD to quadruple the loading rate capacity (Higgins et al., 2017 and Walley, 2007). Although the feedstock TS is hard to further increase due to its viscosity limit and the potential to cause free ammonia inhibition, the digester SRT has been believed to have room for further intensification (Zhang et al., 2022 and Xue et al., 2015). However, to what extent the SRTs can be further shortened without deteriorating the expected digester performance needs to be explored. To this end, a pilot study was conducted in Arlington Water Pollution Control Plant (AWPCP) to understand the impact of further reducing SRT from 15 to 12.5 days and then to 10 days on AD. This study is anticipated to provide engineering guidance for other utilities that consider pursuing process intensification of THP-AD systems. METHODS AND MATERIALS As shown in the process flow diagram in Figure 1, a pilot-scale CAMBITM THP unit shown in Figure 2 was operated at a steam pressure of 6 Barr (equivalent to 165 oC) for 30 minutes to pretreat municipal sludge in preparation for subsequent ADs. Three 10-liter, mechanically mixed ADs shown in Figure 3a were operated in parallel with temperature maintained at 36.5 ± 0.3 oC and SRTs controlled at 10, 12.5, and 15 days, respectively, by manipulating the daily sludge exchange volume. The system temperature control configuration is illustrated in Figure 3b. All ADs were inoculated with the effluent from a full-scale THP-AD system in DCWater. AWPCP cake consists of primary sludge (PS) and wasted activated sludge (WAS) blended at a dry mass ratio of 3:2. The cake was diluted with distilled water or THP steam in each step of the flow diagram according to the TS levels shown in Figure 1. This AD system was continuously operated for 273 days from 6/3/2021 to 3/2/2022. RESULTS AND DISCUSSION AD Startup All three ADs were started with the same SRTs of 30 days (Figure 4). Their SRTs were then step-wise decreased by 0.5 days per day until reaching a 16-day SRT at which all three reactors were stabilized for 1.5 months to prevent reactor failure due to high volatile fatty acid (VFA) concentration. After that, the SRTs were further reduced to 15 days and stabilized for another 1.5 months. In the end, the SRTs of two ADs were finally reduced to 12.5 and 10 days, respectively, at a pace of 0.5 days every other day, and all three ADs were operated without any SRT change thereafter for the remaining 152 days of operation. Effect of SRTs on solids reduction and methane yield As shown in Figure 5a, the steady-state volatile solids reductions (VSRs) were measured to be almost linearly correlated to the SRTs, indicating that shortening SRTs played a major impact on the sludge hydrolysis rate. This is because shortening SRTs from 15 days to 12.5 days and then to 10 days has reduced VSR from 60.7% to 57.5% and then to 53.6%, respectively. In contrast, the methane yield did not show a clear dependence on the SRTs even though the AD with 15-day SRT produced 7% more methane than those with 10-day and 12.5-day SRTs (Figure 5b). This asynchronization between VSR and methane yield suggests that it was the methanogenesis but not hydrolysis limiting the methane production (Figure 5a and 5b) probably due to the THP enhancement on hydrolysis. This can be seen from the substantial reduction of sludge particle size before and after THP as well as AD as shown in Figure 5c. It is noteworthy that SRTs did not cause any significant impact on the particle size distribution of digested sludge (Figure 5c). Effect of SRTs on COD, nitrogen, and phosphorus turnover The total chemical oxygen demand (tCOD) was slightly decreased after THP probably due to the loss of some volatile organics during sludge transfer to AD (Figure 6a). A linear SRT effect on tCOD can be seen in Figure 6a, which is consistent with the trend of VSR in Figure 4a. In contrast, total kjeldahl nitrogen (TKN) in Figure 6b remained unchanged during THP but slightly decreased in ADs with increasing SRT probably due to the volatilization of ammonia nitrogen at a higher bulk concentration as shown in Figure 6d regarding the total ammonia nitrogen (TAN). The soluble COD (sCOD), on the other hand, was significantly increased after THP, evidencing the effectiveness of thermal hydrolysis (Figure 6c). Figure 6c showed that about 60% of sCOD was removed in the form of methane in ADs. The levels of residual sCOD, especially the 1.5 µm filtered, appeared to be inversely related to the SRTs, corroborating the rate-limiting step is methanogenesis inferred from Figure 5a (Figure 6c). It is interesting to see that the orthophosphate (OP) released into the bulk solution was mainly affected by THP but not by AD SRTs probably due to the convolution between hydrolysis and precipitation in ADs (Figure 6e). Effect of SRTs on sludge rheology and dewaterability The sludge rheology was mainly influenced by the THP instead of SRTs. This can be seen from the dramatic drop of apparent viscosity from 3.6*105 millipascal-second (mPas) to 2.8*105 mPas measured at a shear rate of 0.021 s-1 after THP, while only slight changes were observed at different SRTs (Figure 7a). Nevertheless, all of the sludge showed non-Newtonian, shear thinning behavior which is in consensus with the literature (Higgins et al., 2017). The sludge dewaterability as indicated by the capillary suction time (CST) showed that the sludge dewaterability became worse after AD which is expected based on the literature (Novak et al., 2003). However, SRTs did not show any significant impact on the CST, suggesting that the sludge dewaterability should remain the same when SRTs were reduced from 15 days to 10 days. CONCLUSIONS This study revealed that shortening SRTs from 15 to 10 days reduced VSR from 60.7% to 53.6%; however, other key AD performance parameters such as methane yield and sludge properties in terms of rheology and dewaterability largely remained unchanged.