The $100 million project at the East Central Region Water Reclamation Facility in West Palm Beach, Florida, featured conversion from aerobic to anaerobic digestion of biosolids. The upgrade also included a FOG receiving station, centrifuge dewatering and odor control for the biosolids process. Unison was a partner in this project, providing biogas conditioning equipment to treat gas for use in the boilers at the plant.
Logistical challenges
Before the upgrade, the plant’s solids process faced challenges. The infrastructure, including the belt filter presses and the aerobic digester, was aging. Feeding oxygen to the aerobic digestion process was becoming costly.
“When they decided to upgrade some things in the plant, they felt that anaerobic digestion might be a good way to save on energy costs,” says Sanders.
The plant upgrade included the addition of a new aeration basin with anoxic and aerated zones. The aerobic digester was converted to the new aeration basin, and two of the decanting tanks were converted to a centrate tank and a waste tank.
To help mitigate permit exceedances, the plant team brought in Synagro, which provided mobile centrifuges to process sludge that was sent to NEFCO, a contractor for the Palm Beach County Solid Waste Authority.
The contractor had to process a million gallons a day just to allow Sanders to run his operation without backups. “We couldn’t have any breakdowns or stoppages,” he says. “Of course, during the whole construction phase, we did have those issues. But that just illustrates how tight it had to be.”
This stress on the system revealed the inefficiency of the dewatering operation. The old gravity belt thickeners had been taken out of operation 20 years earlier.
Data showed that, even on a good day, the plant wasting process was removing only about 1% of the solids. The old aerobic digester was achieving only 10-11% volatile solids destruction in a stream with about 60% volatile solids. The East Central Region plant staff had to find a way to remove far more solids from the waste stream, to relieve pressure on the downsized storage capacity and decrease the energy required for solids processing.
The team decided to rehabilitate and reactivate the gravity thickeners to increase the percent solids ahead of the digesters. That move has increased total solids removal to 5%, helping the digesters to achieve 60% volatile acid destruction.
Doubling down
Another efficiency increase came from replacing the dewatering belt presses with decanter centrifuges (Andritz) to dewater the material entering from the six digesters. The Hazen and Sawyer engineering firm designed a thermophilic/mesophilic digestion process in which digesters operate at temperatures between 92 to 98 degrees F (mesophilic) and 131 to 140 degrees F (thermophilic). Thermophilic digestion yields greater biogas production, pathogen destruction and substrate degradation.
However, during the final stages of construction and the six- to seven-month ramp-up to full operating efficiency, the digesters operated in the mesophilic mode (95 to 99 degrees F), a range that is easier range to maintain, requires less energy and is more stable. Although the thermophilic system will need more energy for heating, Sanders expected to see meaningful cost savings once the digester reached peak performance.
Another addition to the plant was a dedicated collection tank where pumpers offload fats, oil and grease. This helps keep FOG out of the main waste stream while providing a much-needed service for area contractors, who generally dump 6,000 gallons at a time, totaling 150,000 gpd.
The removal of FOG from the main wastewater treatment process helps keep buildups and blockages out of the system while increasing biogas production. The FOG is heated and transferred to the anaerobic digesters, which are maintained at their respective temperatures through three biogas-fueled boilers (Preferred Utilities).
The boilers are fueled by Unison Solutions gas compressors, which dry and compress biogas from the anaerobic digesters. Removing the water from the biogas before usage helps prevent the formation of acids in the boilers. The boilers can be operated on diesel or the produced biogas, the latter being preferred.
Future opportunity
Biogas supplements the diesel fuel that feeds the digester-heating boilers; it offers a potential fuel for future cogeneration, although for the time being the excess is flared. As of last spring, engineers were currently considering options using biogas once the biosolids facility is completely online.
“There is the potential of selling the biogas to the Solid Waste Authority because that’s where the landfill is, and there is a need for a sustainable amount of gas,” Sanders says. “I can see that cogeneration may be in the future of the plant. We’re looking at different ways of saving energy because that’s a strong initiative from the city.”
Sanders concludes, “I owe all the successes of the plant during this upgrade to the dynamic, diligent team that works with me. Our operators, mechanics and electricians truly make magic happen here.”