Optimizing Class A Biosolids: THP Startup, Troubleshooting, and Long-Term Benefits

APPLICABILITY
Hampton Roads Sanitation District (HRSD) serves 1.9 million residents across southeastern Virginia and transitioned to Class A biosolids production at the Atlantic Treatment Plant (ATP) by implementing a Cambi Mark II B6-4 Thermal Hydrolysis Process (THP) in 2020. Initially, HRSD focused on digester startup and stabilization, monitoring critical health parameters such as volatile fatty acids, pH, alkalinity, and methane production. Once the digesters achieved stability, attention shifted to ensuring consistent Class A compliance in line with HRSD's permit requirements.

The startup process involved a rigorous, phased approach. Digesters were disinfected, and then filled with 800,000 gallons of disinfected non-potable water prior to seeding with 190,000 gallons of digestate from DC Water. Feed rates of thermally hydrolyzed sludge (THS) were ramped up methodically, starting at 50% of expected loading rates to allow operators to closely monitor critical health metrics. This controlled approach allowed HRSD to respond quickly to upsets.

This presentation highlights ATP's approach to overcoming operational challenges, including troubleshooting cross-connections, addressing rapid rise events, mitigating pinhole leaks, and managing solids to enable the shutdown of incineration at the Army Base (AB) facility. These insights provide a roadmap for utilities transitioning to Class A biosolids production.

DEMONSTRATED RESULTS AND OUTCOMES

Methodology
HRSD implemented improvements and troubleshooting measures to ensure Class A compliance:

Early Startup Troubleshooting: Cross-Connection Identification and Resolution
- Cross-Connection Troubleshooting: During startup, HRSD observed intermittent fecal coliform (FC) detections that threatened Class A compliance. A detailed review of design drawings revealed three cross-connections where raw solids lines were inadvertently tied into THP cooling loops for Digesters 1, 2, and 3. Maintenance teams systematically blinded off these connections, beginning with Digesters 1 and 2. Immediate FC reductions were observed (Figure 1).

Longer-Term Lessons Learned and Benefits
- Rapid Rise Events: A rapid rise event caused by mixing loss and intermittent gas release prompted an in-depth analysis of SCADA trends (Figure 2). Indicators such as pumped flow reductions, cover rise rates, and gas spikes were identified as early warnings. Operator alerts and real-time response protocols were implemented to mitigate recurrence.
- Pinhole Leaks: After approximately 4.5 years of operation, pinhole leaks developed in the flash tank-to-pulper line. The likely cause was buildup that narrowed the pipe diameter, increasing fluid velocity and accelerating wear, particularly at bends. In response, a routine cleaning protocol was implemented to prevent future leaks and extend the asset's lifespan.
- Shutdown of AB Incinerator: ATP's capacity improvements enabled HRSD to accept thickened waste activated sludge (TWAS) from the Army Base facility. This allowed HRSD to shut down the AB incinerator in Fall of 2024, reducing costs and increasing biogas production.

#Result
Blinding the cross-connections resulted in significant FC reductions in Digesters 1 and 2 (Figure 1), while Digester 3 stabilized after an additional cross-connection was addressed. Routine monitoring was implemented to ensure the consistent production of Class A cake.

#Analysis of a rapid rise event revealed SCADA patterns that served as critical early warnings, leading to improved monitoring and response. Addressing pinhole leaks through routine cleaning successfully mitigated further issues, preventing costly failures. Finally, the shutdown of the AB incinerator validated ATP's ability to manage additional solids sustainably while enhancing biogas production and reducing environmental impacts.

Conclusions/Outcomes
HRSD's efforts resolved key operational challenges, ensuring Class A biosolids production through systematic troubleshooting and long-term improvements:
- Early Startup Fixes: Blinding cross-connections and implementing a monitoring program now ensures Class A quality of biosolids.
- Operational Improvements: Addressing rapid rise events and pinhole leaks improved monitoring, resilience, and infrastructure integrity.
- Regional Benefits: The shutdown of AB's incinerator reduced costs and increased biogas production, demonstrating ATP's capacity for sustainable biosolids management. These outcomes offer a framework for utilities transitioning to or optimizing Class A biosolids production.

RELEVANCE TO AUDIENCE

HRSD's experience at ATP delivers actionable insights for engineers, operators, and managers implementing advanced biosolids treatment. Attendees will gain knowledge on:
- Systematic Troubleshooting: Resolving contamination sources like cross-connections and improving pathogen removal.
- Proactive Maintenance: Mitigating pinhole leaks with routine cleaning protocols to extend asset life.
- Operational Resilience: Addressing rapid rise events through real-time monitoring and improved response strategies.
- Regional Optimization: Leveraging TWAS acceptance to enable incinerator shutdown while enhancing energy recovery.

This presentation provides practical solutions for achieving Class A biosolids compliance, addressing operational challenges, and improving plant performance.