The Addition of FOG to Thermal Hydrolysis: Experiences of Hampton Roads Sanitation District's Atlantic Treatment Plant

Advanced anaerobic digestion processes offer unique opportunities for process intensification, biosolids product production and resource recovery. Thermal hydrolysis is the current state of the art for digestion enhancement, commercially available, typically doubling the capacity over conventional systems. Co-digestion in conventional digesters has demonstrated that doubling biogas production is possible with the right mix and quantity. Combining THP with co-digestion has the potential to maximize the bioenergy yield from a fixed volume of digesters. Hampton Road Sanitation District (HRSD) as part of its recent upgrades to its 54 MGD Atlantic Treatment Plant added thermal hydrolysis (CAMBI) and FOG receiving to enhance energy generation and provide a much-needed outlet in the region for resource recovery from fats, oils and grease. HRSD installed two B-6-4 CambiTM thermal hydrolysis units at the plant which discharges thermally hydrolyzed sludge to its 6 mesophilic anaerobic digesters. HRSD had the option of FOG addition pre or post THP, but to preserve the Class A biosolids status the sludge pre-THP was selected as the process configuration. The introduction of FOG to THP requires that it achieve the same level of pre-treatment that the sludge entering it, screened to less the 5 mm and largely devoid of grit. Combining these technical requirements and lessons learned from site investigations of operating co-digestion programs a first of its kind FOG receiving station feeding THP was designed and commissioned. Figure 1, shows the general process flow of the FOG system. It was designed around the using the Envirocare Beast and a pair of heated storage tanks to raise the temperature of the FOG to 131°F prior to introduction to THP. Based on prior experience with FOG receiving station design and operations a coned bottom to the FOG storage tanks was incorporated to allow for the deposition of grit, removing it from the material fed to the THP units. The cone based design also allows for decanting and concentrating of FOG if needed; otherwise the storage tanks are completely mixed. The heated FOG is then introduced to the pulper of the THP system, in place of dilution water. An extensive characterization study by HRSD (see Table 1) of its FOG found that at an average of approximately 6% solids FOG would serve as good source of dilution water, while enhancing the energy value of the feedstock to the digesters. Since commissioning in the summer of 2021, the FOG station as operated reliably; however unanticipated repairs to FOG receiving facilities at other HRSD plants resulted in higher-than-planned FOG loading to AT throughout 2021 (Figure 2). As a result, the FOG facility at AT operated close to its peak week design basis (Table 1) nearly continuously through its startup and early operation period without undue mechanical or operational difficulties. Some early lessons learned during early operation include: - Rheological properties vary widely in high strength wastes. Screens and systems designed to process 6% FOG were able to manage 20% humus production waste - Introducing FOG to the pulper has proven reliable and viable means of maximizing the energy yield of the system. - Additional FOG water storage (or decanting, in some applications) may be a useful feature to allow additional capture of FOG through the dedicated receiving system in lieu of processing through the headworks are peak loading conditions. Storage, rather than material processing and biological processing, have proved to limit the capacity of the AT system at unanticipated high loading. Operations the THP and digesters has proven to be stable even with the high FOG loadings, demonstrating the viability of the unique microbial population attributed to THP pretreatment. This is critical in demonstrating the overall efficacy of THP to be integrated into a larger resource recovery and bioenergy program. Early biogas production numbers were hindered by system leakage but since correction, has been in line with the anticipated loadings. As part of the discussion of operations the acclimation of the digester to FOG and other high strength wastes will be discussed, as introducing FOG to a THP digester is not the same as a conventional digestion system which is inherently influenced by the feed sludge active biomass. Further at the time of presentation the THP system will have been shut down and restarted as part of it annual inspection and maintenance providing further information on the impacts of FOG and high strength waste loadings on the THP process. But also provide information on the recovery of the biological process and a return to full loading under a co-digestion scenario. Overall this information is important as the adoption of THP increases across North America and little information is currently available surrounding the integration of co-digestion with THP. At the time of presentation nearly a year's worth of data and lessons learned will have been collected and presented.