Enhancing Biosolids Management with THP: From Startup to Optimization and Troubleshooting at HRSD's Atlantic Treatment Plant

APPLICABILITY Hampton Roads Sanitation District (HRSD) serves 1.7 million residents across southeastern Virginia. In 2020, HRSD installed a Cambi Mark II B6-4 Thermal Hydrolysis Process (THP) at the Atlantic Treatment Plant (ATP), transitioning from Class B to Class A biosolids. This upgrade has increased digester capacity, improved dewatering efficiency, and produced a marketable biosolids product. By combining THP with aerobic curing, ATP has improved biosolids quality while reducing environmental impacts. This presentation will explore operational lessons, including startup and commissioning, maintenance strategies, and solutions to challenges such as preventing rapid rise events. ATP's experience provides a roadmap for utilities to enhance biosolids management, reduce costs, and meet regulatory requirements. DEMONSTRATED RESULTS AND OUTCOMES Methodology: The THP system was installed at ATP to optimize biosolids management, focusing on increasing digester capacity, volatile solids reduction (VSR), gas production, and dewaterability. The startup involved seeding digesters with THP digestate from DC Water, followed by a gradual ramp-up of feed rates while closely monitoring health parameters. Early challenges, such as rising hydrogen sulfide (H2S) concentrations, were resolved with feed rate adjustments and liquid soda ash. Startup and Commissioning: Startup and commissioning of the THP system at ATP presented several unique challenges and opportunities for learning. To ensure a successful transition, the startup process began with seeding the first of three THP digesters using digestate trucked in from DC Water. Over 190,000 gallons of seed material were combined with approximately 800,000 gallons of filtered and disinfected non potable water. The goal was to establish a stable microbial population capable of processing thermally hydrolyzed sludge (THS). Initial seeding targeted a total solids (TS) concentration of 1% before introducing the THS feed, but practical limitations led to starting at 0.7% TS. The goal of the startup process was to begin at 50% of DC Water's loading rate and gradually ramping up over a 2-3 week period. During this time, key digester health parameters, such as volatile fatty acids (VFA), pH, alkalinity, and methane content, were closely monitored to identify any signs of process instability. Figures 1 and 2 show the trends in these parameters over the first 70 and 120 days of operation, respectively. Early challenges included an unanticipated rise in H2S concentrations in the digester headspace, attributed to residual sulfur compounds from a bioscrubber upgrade. This caused a temporary inhibition of methanogens, leading to increased VFA levels, reduced gas production, and lower pH starting around day 75, as observed in Figure 2. Through a combination of feed rate adjustments and the addition of liquid soda ash, digester health was restored. The experience highlighted the importance of routine, real-time measurements to identify and address upsets early in the process. By day 150, the digester had reached steady state, demonstrating resilience to variations in feed rates and loading (Figure 3). Retrospective Operational Analysis: Multiple retrospective analyses evaluated the performance of the THP system, focusing on gas production and VSR. A cyclical VSR pattern emerged, influenced by the primary sludge (PS) to waste activated sludge (WAS) ratio. Higher VSR rates were observed in winter due to a higher PS/WAS ratio in the feed, while lower VSR rates in summer were associated with reduced volatile solids content in WAS (Figure 4). This variation is driven by the digester's extended retention time, which allows it to consistently reach the same ending VS content in the finished digestate. Consequently, the VSR achieved is directly influenced by the initial VS content in the feed. Troubleshooting and Optimization: The integration of the THP system at ATP was not without its challenges. One of the most significant issues was a rapid rise event in one of the three THP digesters, primarily due to the loss of mixing (digesters are typically mixed using 3 perimeter and one central draft tube mixer). Figure 5 illustrates a plant SCADA trend leading up to and during the rapid rise event, which began on 6/4/24 and concluded on 6/7/24. Although digestate overflow did not occur until 6/7/24, a post-hoc evaluation revealed that the conditions leading to the overflow started as early as 6/4/24. The first indication of the event was the rate of rise in the digester cover level. As shown in Figure 5, the green line represents the cover level, and after the event began, the rate of rise increased despite no significant change in feed rates. Figure 6 further highlights this, showing the rate of cover rise exceeding 2 inches per minute once the rapid rise event started. On 6/5, pumped recirculation flows (purple line in Figure 5) began to intermittently drop despite constant pump speed, without any clear cause. The noise in the purple line from 6/1 to 6/3 is attributed to bubbles from foul gas condensate being added to the digester. However, during the flow drops on 6/5, no condensate was added to the digester. Additionally, these flow drops were immediately followed by a spike in digester gas flow rates (yellow line), suggesting that both issues were linked to intermittent gas release prior to the overflow event. This post-hoc analysis identified the need for a new alarm to monitor the rate of cover rise as an early warning for operators, potentially preventing future overflow events. Further optimization of the THP system focused on improving the efficiency of aerobic curing, where solids are formed into windrows and periodically turned with a windrow turner to produce a cured product that is drier and less odorous. By conducting a series of curing pilots, ATP determined that adding 25% cured product by volume accelerated the stabilization process, reducing curing time by one week. This approach not only improved the overall quality of the final product but also reduced the intensity of odors emanating from the curing piles. Figure 7 presents the seasonal variation in volatile solids reduction (VSR) during aerobic curing, with significant improvements observed in spring and summer. Operational Achievements and Financial Impacts: One of ATP's most significant achievements has been performing a rapid three-day annual THP O&M turnaround, eliminating the need for raw cake loadout, all without have a standby pulper or flash tank included in the design. Traditionally, THP systems require a one-week shutdown, leading to raw cake loadout and neighborhood odor complaints. ATP successfully completed maintenance without needing to haul unstabilized biosolids, saving the plant hundreds of thousands of dollars in hauling fees and preventing odor complaints. RELEVANCE TO AUDIENCE HRSD's experience at the Atlantic Treatment Plant offers critical insights for operators, design engineers, and managers looking to improve biosolids management practices through advanced technologies. The strategic combination of thermal hydrolysis, dewatering optimization, and innovative aerobic curing has resulted in sustainable, cost-effective operations that reduce environmental impact while meeting regulatory requirements. This presentation will highlight key takeaways such as the ability to rapidly perform a three-day O&M turnaround without raw cake loadout, providing both operational and financial benefits. Another key takeaway is the importance of monitoring and interpreting data to identify issues during start-up; the extensive monitoring has proven extremely useful in fine-tuning the THP digestion process. Attendees will gain practical knowledge on leveraging continuous monitoring, optimizing gas production, and enhancing volatile solids reduction, which can be applied to other facilities. The lessons learned at ATP present a valuable roadmap for utilities seeking to modernize their biosolids management practices, improve efficiency, and address the challenges of future regulatory demands.