Pretreatment of a primary and secondary sludge blend at different thermal hydrolysis temperatures: Impacts on anaerobic digestion, dewatering and filtrate characteristics

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This study evaluated the role of bulk and substrate physics on hydrolysis rates and biogas yields in anaerobic digestion (AD) pretreated by thermal hydrolysis (THP). Although THP decreases sludge viscosity, no evidence was found that bulk viscosity impacted the biogas yield or hydrolysis kinetics. In addition, no significant difference between the biogas yields for different total solids concentrations nor floc sizes was detected. However, increased mixing speeds did increase biogas yields. As a result of thermal treatment, the model protein, bovine serum albumin, was harder to degrade in terms of both overall biodegradability and hydrolysis rates when their macrostructures were changed from liquid to gel and to solid structures; the opposite was true for the model polysaccharide, amylopectin. These results demonstrated that hydrolysis in THP‐AD systems was impacted mostly by the physical properties of the substrate (gelation) rather than the bulk physical properties within the digester. Practitioner points Bulk viscosity does not significantly impact hydrolysis efficiency (biogas yield). However, mixing speed impacts hydrolysis beyond biogas holdup effect. Increasing the amount of substrate–microbe collisions through increasing biomass concentration does not have an impact on hydrolysis efficiency or biogas yield. Proteins are harder to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment. Polysaccharides are easier to degrade when macrostructure changes from liquid to gel/solid as a result of heat treatment. The time required for digesters to reach peak biogas production rates increased with decreasing specific surface available on gel and solid structures.

Section: Resultssupporting

confidence: 91%

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

Manning

Romero

et al. 2019

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“…The higher odorant production could be due to the increase in biopolymer, especially protein, in solution, as that material is captured by the polymer and deposited in the cake, and it has been shown to increase the potential for odorant production (Higgins et al., ). Overall, the odorant concentrations produced by the cakes were low in comparison with many systems (Novak et al., ), which is indicative of THP biosolids (Higgins et al., ).…”

Section: Resultsmentioning

confidence: 92%

“…The details for determining k h using the daily respirogram and COD data are described in Higgins et al. ().…”

Section: Research Approach and Methodsmentioning

confidence: 99%

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

Higgins

Beightol

DeBarbidillo

et al. 2019

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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.

“…The latter allows for efficient pumping and mixing at increased digester TS concentration and increased volatile solid (VS) loading (Barber, ; Nielsen, Thygesen, Thomsen, & Schmidt, ). For example, AD systems with THP pretreatment have been successfully operated at high loading in the range of 2.5–8.8 kg COD/m 3 ‐day with feed at 9%–12% TS (Higgins et al, ; Norli, ; Pook, Mills, Heitmann, Panter, & Walley, ; Walley, ; Wilson, Tanneru, Banjade, Murthy, & Novak, ). This is about three times higher compared to the typical operating condition of conventional digesters with OLR of typically 1.0–3.2 kg COD/m 3 ‐day with feed at 3%–6% TS (Metcalf et al, ; Hendricks, ; Higgins et al, ; Rittmann & McCarty, ; Vesilind, ).…”

Section: Introductionmentioning

confidence: 99%

“…For example, AD systems with THP pretreatment have been successfully operated at high loading in the range of 2.5–8.8 kg COD/m 3 ‐day with feed at 9%–12% TS (Higgins et al, ; Norli, ; Pook, Mills, Heitmann, Panter, & Walley, ; Walley, ; Wilson, Tanneru, Banjade, Murthy, & Novak, ). This is about three times higher compared to the typical operating condition of conventional digesters with OLR of typically 1.0–3.2 kg COD/m 3 ‐day with feed at 3%–6% TS (Metcalf et al, ; Hendricks, ; Higgins et al, ; Rittmann & McCarty, ; Vesilind, ). The higher feed TS concentration proportionally increased the concentrations of inhibitory substances such as free ammonia (NH 3 ) in the digester.…”

Section: Introductionmentioning

confidence: 99%

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

Romero

Wadhawan³

et al. 2019

This study evaluated the application of recuperative thickening (RT) to enhance anaerobic digestion (AD) performance for AD systems with thermal hydrolysis pretreatment (THP). RT was applied for two different reasons: (a) for increasing the sludge retention time (SRT) to degrade slowly hydrolyzable materials more efficiently and (b) for maintaining SRT at decreased hydraulic retention time (HRT) thus showing potential for increased AD throughput rates. A SRT increase from 15 to 30 days by RT application did not improve AD performance or hydrolysis rates significantly as 15‐day SRT was already a factor 2 higher than the estimated washout SRT. When applying RT to increase throughput rates (HRT of 7 days) while maintaining 15‐day SRT, no negative impact on biogas production or hydrolysis kinetics was observed. It was estimated that RT application on THP digesters can increase digester throughput by 100% and thus show clear potential for further AD intensification. Practitioner points Increased SRT from 15 to 30 days through recuperative thickening application did not improve biogas production. A lower required minimum SRT (6–7 days) was estimated in THP‐AD systems compared to conventional AD. Operation at decreased HRT by RT application resulted in similar AD performance under constant organic loading rates. A 100% increase in throughput rates can be applied using RT without decreasing AD performance.