Seeding, Startup, and Commissioning of THP Systems at Three WRRFs

Implementation of thermal hydrolysis process (THP) systems is becoming increasingly common at water resource recovery facilities (WRRFs). These systems reduce required anaerobic digester volume (for both rehabilitation and new construction), increase biogas production, improve dewaterability, and can produce Class A biosolids. Because THP systems involve coordination amongst several unit processes, startup and commissioning of these systems is complex and requires significant coordination between the owner, engineer, and contractor. CDM Smith designed and participated in the startup and commissioning of three THP facilities ranging in size from 16 to 189 mgd or 11 to 200 dtpd. All facilities installed Cambi THP systems, including the B-2, B-6 and B-12 skids as well as pre-dewatering, anaerobic digesters, and all required ancillary systems. This presentation will provide guidance and lessons learned on the initial digester seeding, sequential startup and operation of the system, and troubleshooting and operational data analysis.

-Digester Seeding-
Digester seeding can be achieved by using onsite sludge and slowly cultivating the correct microorganisms or by using seed sludge from an existing facility. Since THP was new to the US when the first of these systems was commissioned, there were no options for THP seed and limited options for quality seed sludge, all of which required hauling large amounts of sludge long distances. An innovative solution was developed to utilize liquid class A pasteurized sludge at the first plant then dewatered Class A THP digested cake, rather than liquid sludge, as the seed source at the other plants. By using dewatered cake hauling costs were reduced and rapid startup of the digesters was achieved. Figure 1 shows a schematic of a typical seeding flow diagram using this method. A series of frac tanks were provided for dilution and mixing of the seed cake prior to feeding to the digesters. Numerous points were included for alkalinity adjustment. The seed sludge was heated to mesophilic temperatures using hot water in the heat exchangers (which typically operate for cooling instead of heating). Lessons learned and considerations for digester seeding will be discussed, including regulations for seed material, transportation, alkalinity adjustment, and on-site handling procedures.

-Sequential Startup-
While digesters are being seeded, equipment and subsystems must be started to provide feed and maintain the digesters. These subsystems include pre-dewatering, THP, digester cooling, boilers, and digester gas conditioning. Overall, solids processing at each facility was carefully sequenced with the equipment startup to ensure appropriate timing of THP commissioning with upstream processes. This allowed for processing solids through the system and ultimately to the digesters without the need for temporary piping, bypassing equipment, or shutdowns. Figure 2 provides an example flow schematic from one facility and outlines seven distinct modules for commissioning: 1) plant effluent water pumping (not shown), 2) WAS storage facilities, 3) pre-dewatering centrifuges, 4) Digester 1 and digester seeding, 5) THP and biogas utilization equipment, 6) Digester 2, and 7) post-dewatering equipment. The three facilities' startup sequencing and solids handling will be compared to illustrate the general approach as well as the site-specific complexities for startup and commissioning.

-Troubleshooting and Operational Data-
It is typical for commissioning teams to have to overcome numerous challenges during the initial months of operations, especially on complex systems. Foremost among these challenges during THP commissioning is maintaining digester stability while also troubleshooting equipment startup. Any issues which are not resolved quickly may result in a digester upset delaying full capacity by weeks or months. It is imperative that digester operating data is monitored and trended continuously to ensure digester health and inform the commissioning team for next steps. A detailed data analysis of key digester operating parameters for the three plants will be presented and a simulated experience of the day-to-day decision-making process required to startup a THP and digester system will be provided. As an example, Figure 3 shows a comparison of digester feeding (kg VS fed/kg TS) and VSR in the digesters during the first 120 days after startup. Additional data, including biogas production, VFAs, alkalinity, pH, etc. will also be presented. Other issues resolved during the startup and commissioning phase which will be discussed include sludge viscosity differences between design and reality, potential digester foaming, and feed sludge quality.

-Conclusions-
As more facilities in the US implement THP systems, lessons learned and shared experiences from the startup and commissioning of these facilities can provide valuable insight for designers, constructors, and operators of similar biosolids processing facilities. Digester seeding methodology is an important first step for THP system startup as it directly impacts the commissioning duration and the ability to meet Class A requirements. Sequential startup and operation of new THP system equipment to simplify solids handling during startup and maintain digester stability is key. Operational issues are inevitable to any startup; however, monitoring digester operating data can help troubleshoot issues and develop quick resolutions.