Consultants recommend anaerobic digestion plus thermal drying as preferred biosolids option for Carlisle plant

At a March 5, 2025 workshop of the Carlisle Borough Council, consultants from Dannon Plumbing presented a study of options to replace the borough—s nearly 40-year-old lime slurry stabilization system at the Carlisle Wastewater Treatment Plant and identified anaerobic digestion with thermal drying as their preferred alternative.

The consultants said the current lime stabilization process has been reliable but creates nuisance odors, recurring equipment failures and a large hauling demand: the process requires more than 7,000,000 gallons of Class B liquid biosolids to be hauled each year—about 1,200 tanker loads. "The main goal for the study was to develop a plan that is affordable, cost effective, energy efficient and environmentally sound that will last you guys 25 years," Ben Briston, senior project manager with Dannon Plumbing, told council members.

The report evaluated four alternatives: continuing the current post-lime treatment; autothermal thermophilic aerobic digestion (ATAD); anaerobic digestion with thermal drying; and thermal drying with pyrolysis. Consultants summarized that ATAD can reduce solids by roughly half compared with current operations but is energy intensive; anaerobic digestion plus thermal drying can achieve large volume reductions (consultants cited up to roughly 90% reduction of the dewatered cake) and offers energy recovery via biogas; and pyrolysis (biochar production) also gives high volume reduction but is an emerging technology in the U.S.

Consultants presented a 20-year net-present-value comparison that placed the existing lime process as the least expensive in capital terms, alternatives two and three close to each other in lifecycle cost, and the pyrolysis option as the costliest. The presenter said prior cost estimates for the pyrolysis option previously shown to council have increased after conversations with equipment vendors.

The study team highlighted operational trade-offs. The anaerobic-digestion-plus-drying approach was recommended because it is a proven technology at other regional plants, can produce Class A biosolids, minimizes odor potential by enclosing digestion tanks and offers redundancy: "If a piece of equipment such as that dryer is out of service, a Class B product could still be produced," Briston said. The consultants also noted they would plan site connections and space so a future technology such as pyrolysis could be added later if it becomes proven and cost-effective.

Council members asked about specific concerns. One member asked whether tanks and new buildings would be vulnerable to flooding; the consultant said equipment would be housed in buildings outside the mapped floodway and at slightly higher elevations. Several members asked about staffing and operations; the consultants said they expect existing plant staff could be retrained but that the borough will need a refined estimate of manpower and maintenance needs during design. Members also asked about the facility—s ability to accept food-processing residuals and the potential for cogeneration from biogas; consultants said some industrial and food wastes can be co-digested and that biogas can provide energy recovery, potentially offsetting plant operations.

Several council members raised concerns about per- and polyfluoroalkyl substances (PFAS). The consultants said current evidence does not show a technology that reliably eliminates PFAS in all end uses and that pyrolysis biochar studies are evolving; they cautioned the borough would not want to be an early adopter until the technology is better proven and regulated.

No formal council vote on a construction contract or funding occurred at the workshop; the presentation concluded with the consultant—s recommendation and a request that staff refine design, staffing and cost estimates and return to council with project-level recommendations. The report delivered to the borough includes technical memoranda on regulatory context, market outlets and a regional market survey.

The consultants named several peer facilities visited during the study, including Hanover Wastewater Treatment Plant and Lancaster-area authorities that use anaerobic digestion and drying technologies. The presentation noted local ATAD installations at the Lower Allen and Middletown wastewater treatment plants as examples.