Pushing Feed Rates Beyond "Limits" to Accelerate THP Digestion Startup: Insights from an In-Situ Pilot Study

Introduction
Thermal hydrolysis pretreatment (THP) has been gaining popularity for intensifying the anaerobic digestion (AD) of municipal wastewater sludge by decreasing sludge viscosity, enhancing solids reduction, and boosting biogas production [1, 2]. The digested sludge also gains significant improvement in dewaterability, and the dewatered Class A biosolids are qualified for direct land application [3-5]. However, the startup phase for THP-enhanced digesters remains risky and time-intensive due to improved hydrolysis and faster acid production, as well as longer time needed to achieve shorter solids retention times (SRT). Current industry norm for digester feed rates during startup, typically ranging from 0.10 to 0.25 lb COD/lb VS/day, often lead to a prolonged timeframe to achieve full digester throughput capacity. To address this limitation, this study explored the possibility to further increase maximum feed rates to a level that are at least twice the industry norm, aiming to accelerate the startup of THP-enhanced digesters. A total of three maximum feed rates, namely 0.31, 0.33, and 0.38 lb COD feed/lb VS digester/day, were tested with the hope to investigate the upper boundary of startup pace achievable with a commonly adopted full-scale startup strategy for THP-enhanced digesters.
Materials and Methods
Samples of thickened primary sludge (TPS) and waste activated sludge (WAS) were mixed at a target dry mass ratio of 3:2 and then pre-dewatered to a TS content of 16% total solids (TS) with a cationic polymer using a pilot-scale screw press thickener. The pre-dewatered cake samples were processed through a pilot-scale THP unit (Cambi, Asker, Norway) operated at 6-bar steam pressure (equivalent to 165 C) for 30 minutes. The pretreated sludge was diluted to 9% TS and fed daily into three pilot-scale mesophilic (36.5 C) digesters with a 10 L working volume and a target SRT of 15-day.
To simulate the future startup protocol for full-scale operations, each pilot digesters was initially filled to 30% full (v/v, 3 L) which is just enough for initiating the mixing, with 20% (v/v, 2 L) filled with tap water and the other 10% (v/v, 1 L) seeded with full-scale THP-enhanced mesophilic AD sludge freshly collected from a local plant. The headspace in the digesters were purged with nitrogen gas before feeding started.
Results and Discussion
The startup plan illustrated in Figure 1a consisted of three phases. In Phase 1, the goal was to acclimate the seed to the new feed sludge produced at the Arlington WPCP. The initial feed rate on Day 1 used in all three digesters was 50% of the steady-state feed rate of the seed digester, or around 0.11 lb COD feed/lb VS digester/day. The feed rates were ramped up at 8% per day until reaching the maximum feed rates designed for this study, namely 0.31, 0.33, and 0.38 lb COD feed/lb digester VS/day (Table 1). As can be seen in Figure 2a, the actual feed rate aligned well with the plan from Day 0 to Day 17. Digestate wasting did not start until the digester volume was filled up as scheduled in Figure 1b.
During Phase 2, the three digesters maintained their respective maximum feed rates until reaching the designed daily feed volume, that is 6.7% of the digester working volume which is equivalent to a 15-day SRT. However, the actual feed rates started to be approximately 5 to 10% lower than the planned values due to a reduction in the VS content in the THP-processed sludge. This lower VS content in this batch of sludge was attributed to a lower TPS:WAS blending ratio produced by Arlington WPCP. The drop in actual feed rates between Day 59 and 76 was another example of changes in feed sludge characteristics (Figure 2a). After Day 76, the actual feed rate once again aligned well with the designed plan. As shown in Figure 2b, the digesters with designed maximum feed rates of 0.31, 0.33, and 0.38 lb COD/lb VS/day successfully achieved the 15-day SRT on Days 34, 53, and 80, respectively, without any delay. The microbial concentrations in the digesters indicated by digester VS contents showed steady buildup and well alignment with the designed plan in general (Figure 2c).
During Phase 3, all three digesters maintained the 15-day SRT until reaching steady state. The steady state was defined as the timeframe during which VSR, daily biogas production rate, and VFA concentration did not show statistical differences based on T-test results. Throughout, key digester stability indicators, including TS, VS, pH, VFA, alkalinity, biogas production rate, and biogas composition were closely monitored.
Conclusions
By increasing the maximum feed rate from 0.31 to 0.33 and 0.38 lb COD/lb VS/day, the time required to reach the target15-day SRT was significantly reduced from 80 to 53 and 34 days, respectively. Throughout the startup phases, digester stability indicators suggested that VFA accumulation has occurred during the first 16 days of the startup process; however, the digesters quickly acclimated and stabilized without any rescue or corrective measures, such as feeding pauses or alkalinity supplementation, even at the more aggressive feed rate. These findings suggest that THP-enhanced digesters can successfully withstand higher loading rates during startup, offering a practical strategy to expedite achieving full operational capacity in AD processes.