Impacts of feed dilution and lower solids retention time on performance of thermal hydrolysis/anaerobic digestion

Abstract

The goal of this study was to evaluate using feed dilution/solids retention time (SRT) control to manage potential ammonia inhibition in highly loaded anaerobic digesters after thermal hydrolysis. The study compared three digesters operated at the same target volatile solids (VS) loading rate of 5.5 kg VS/d‐m3, but at different feed concentrations resulting in SRTs of 10, 15, and 18 days. Lowering the feed concentration decreased the digester total ammonia nitrogen concentrations which averaged 1,580, 2,610, and 3,080 mg NH4+ ‐N/L for the 10‐, 15‐, and 18‐day digesters. The VS reduction and methane yields were equivalent for the 15‐ and 18‐day digesters and about 4% lower for the 10‐day digester. Ammonia inhibition of the 18‐day digester occurred early in the study, but the system acclimated over time. Feed dilution reduced the viscosity and the potential for volume expansion due to gas holdup and foaming. Practioner points Feed dilution reduces digester ammonia concentrations and inhibition potential without sacrificing digester performance at lower SRTs. Feed dilution greatly reduces digester viscosity and associated issues with digester volume expansion due to gas holdup and foaming. Operating at the lower SRT does not impact cake solids after dewatering and substantially decreases polymer demand for conditioning.

Recuperative thickening for sludge retention time and throughput management in anaerobic digestion with thermal hydrolysis pretreatment

“…So, despite the THP pretreatment, hydrolysis rates within the AD system were still driven by the PS/WAS ratio fed into the THP system. Overall, the reported first‐order hydrolysis rates in this study (0.5–0.65 day −1 ) were consistent with the previously reported hydrolysis rates for THP digesters under similar PS/WAS sludge feeding (Higgins et al, ) and based on literature can be defined as relatively fast hydrolysis rates (Vavilin et al, ). This could be favored by the THP process, and the observed fast hydrolysis rates could be linked to the readily biodegradable substrates present in the THP feed sludge.…”

“…Based on the simulation exercise performed, the minimum SRT to avoid performance loss is 6–7 days, independent of organic loading. The latter thus seemed to be a lower estimated washout SRT than other studies suggested (Higgins et al, ). With a more extensive model calibration using a larger range of data sets than only this study, in combination with better understanding of the impact of diluted feed on AD kinetics, the washout SRT estimate might be further improved.…”

“…Ngwenya et al () showed no AD performance difference for THP‐AD systems operating at SRT of 10 days versus 18 days at constant organic loading rate. In contrast, Higgins et al () showed decreased performance at 10‐day SRT versus 18‐day SRT. Based on the simulation exercise performed, the minimum SRT to avoid performance loss is 6–7 days, independent of organic loading.…”

“…Other than ammonia toxicity, other inhibitors such as light metal ions (Na, K, Mg, Ca, and Al), heavy metals (Fe, Zn, Ni, and Co), and some organic compounds (alkylbenzenes, halogenated benzenes, nitrobenzenes, phenol, and alkylphenols) were also reported in the literature (Barber, ; Mosey et al, ; Zayed & Winter, ). Recuperative thickening could offer a method for intensification without application of increased TS concentrations and thus might avoid inhibition impacts; Higgins et al () evaluated feed dilution as an approach to lowering the potentially inhibitory concentration of NH 3 . Decreased AD performance was observed when an SRT and HRT at 10 days were applied using diluted feed at similar OLR than 15‐ to 20‐day SRT reactors.…”

Exploring the impact of bulk and substrate physics on hydrolysis rates and biogas yields of anaerobic digesters pretreated with thermal hydrolysis

“…Although the AD microbial community used in this study degraded THP-pretreated feed sludge, it showed similar kinetics compared to conventional AD sludge used for previous studies. Observed k H values for THP-AD systems have been reported in the range of 0.5-0.6/day (Higgins et al, 2019;unpublished data) and were thus close to the gel BSA feed condition k H values reported in this study (0.44 -0.49/day, Table 2).…”