Effect of Primary and Secondary Sludge Blending on Recalcitrant Nitrogen Formation in Thermal Hydrolysis Pretreatment

1. Introduction Thermal hydrolysis pretreatment (THP) of sewage sludge prior to anaerobic digestion is known to be able to enhance the sludge digestibility. For that reason, higher total solids (TS) contents and shorter solids retention times (SRTs) have been commonly used for anaerobic digesters (ADs) fed with sludge processed with THP. Yet, it is also known that the sludge digestibility should decrease with the increase of TS and decrease of SRTs. Whether the application of THP can justify the sludge digestibility loss at higher TS and shorter SRTs remains to be addressed. Moreover, whether blending waste activated sludge (WAS) with primary sludge (PS) can further enhance the digestibility of the WAS following THP is another unknown. Therefore, this study is aimed to understand the effect of TS, SRTs, and PS /WAS blending on sludge digestibility with THP. 2. Materials and Methods Thermal hydrolysis pretreatment A five-gallon capacity THP system shown in Figure 1 was procured from CAMBITM . The THP unit was heated by hot steam to 165 oC for 30 min (6 bar). The treated sludge was cooled down to room temperature prior to feeding into bench-scale mesophilic ADs shown in Figure 2a. Anaerobic digester setup Six units of mechanically mixed ADs as shown in Figure 2a were operated in parallel with temperature maintained at 36.5 ± 0.3 oC using two thermal tents as illustrated in Figure 2b. Another six units of gas recirculation mixed ADs were set up and operated as described in a previous study (Zhang et al., 2020). These ADs were inoculated with fresh effluent sludge from a full-scale THP-fed mesophilic AD at DC Water. The TS feeding into these mechanically mixed ADs was 9% while the TS feeding into these gas recirculation mixed ADs was 4% (Table 1). Blended sludge (PS:WAS, 31:19, w/w dry) collected from a local wastewater treatment plant, (WWTP) was processed through THP and then fed into three mechanically mixed ADs to study the SRT effect on volatile solids reduction (VSR) at 9% TS. The same blended sludge without THP was also fed into another six gas recirculation mixed ADs to study the SRT effect on VSR at 4% TS. PS, WAS, and blended sludge (PS:WAS ,1:1, w/w dry) collected from another local WWTP were processed through THP to feed another three mechanically mixed ADs operated at a TS of 9% and SRT of 15 days to study the effect of sludge blending on the sludge digestibility improvement in terms of VSR. Detailed SRTs, TS, and PS:WAS ratios used in the study can be found in Table 1. 3. Results and Discussion VSR has been used as an indicator of sludge digestibility (Schnaars, 2012). Results in Figure 3a revealed that VSR is positively related to the SRT used in the THP-enhanced ADs. This relationship is more prominent at high TS. For example, at 9% TS, VSRs dropped by 4% as SRTs were reduced from 15 days to 10 days. In contrast, at 4% TS, VSRs dropped the same 4 % when SRTs were reduced from 50 to 15 days. It is noteworthy that, at the SRT of 15 days, VSR dropped another 4% when TS was switched from 4% to 9% even with THP (Figure 3a). The decline of VSR as TS increases is expected because of the high loading rate that was imposed on the AD system. However, the THP enhancement on the sludge digestibility is still prominent in that the loading rate has been increased for 225% when TS was increased from 4% to 9% while the VSR only dropped for 4% at a SRT of 15 days. Even at the SRT of 10 days, such a VSR drop was only 8% in sludge with THP as compared to the sludge without THP at the SRT of 15 days. As shown in Figure 3b, although the VSR of WAS was only as low as 36%, after blending it with PS at a 1:1 ratio, the overall VSR turned out to be as good as 49% which is very similar to that of the PS. This result indicates that blending WAS into PS can significantly improve the digestibility of WAS in THP enhanced AD system for 13%. 4. Conclusions The following concluding remarks can be drawn from this study: 1) The drop of VSRs as a result of SRT decrease from 50 days to 10 days can be explained by the loading rate increase as a result of the THP. 2) Blending WAS with PS can increase the digestibility of WAS by 15% at 9% TS with THP.