Assessing Impacts of Thermal Hydrolysis on Mainstream Non-Biodegradable Organic Carbon and Nutrients at the Broad Run WRF

Background The Broad Run Water Reclamation Facility (BRWRF) is owned and operated by Loudoun Water (LW). The BRWRF has a design flow of 11 million gallons per day (mgd) and is an advanced treatment facility that complies with very stringent effluent standards, including monthly average effluent chemical Oxygen Demand (COD), Total Kjeldahl Nitrogen (TKN), Total Nitrogen (TN) and Total Phosphorus (TP) limits of 10 mg/L, 1.0 mg/L, 4.0 mg/L and 0.1 mg/L, respectively at current flows and loads. Currently, liquids treatment at the BRWRF employs primary clarification, 5-stage biological reactor basins and membrane filtration (MBRs) for biological nutrient removal. Granular activated carbon contactors are used as a final polishing step for organic carbon, nitrogen, and phosphorus removal. Solid stream treatment employs anaerobic digestion (AD) for stabilization of primary sludge and thickened waste activated sludge. Stabilized solids are then dewatered via centrifugation. Dewatering and thickening centrate are returned to the liquid stream process without further treatment. Figure 1 shows a schematic diagram of the treatment process at the BRWRF. Motivation As part of planning efforts to increase solids treatment expansion, sludge pre-treatment via thermal hydrolysis (TH) was identified as one of the alternatives that could help Loudoun Water meet solids treatment capacity needs at the BRWRF. TH has the potential to provide the BRWRF with a class A biosolids product while reducing digester volume requirement for solids stabilization. However, evaluating the impact of increased production of potentially non-readily biodegradable organic compounds in the combined THP-MAD process on COD, TKN, TN and TP concentrations in the mainstream as well as the plant discharge was critical, given the stringent discharge requirements. This pilot study was therefore aimed at benchmarking digester and solids process performance with upstream THP, as well as understanding the potential impacts of THP pretreatment on the effluent discharge from the BRWRF. The pilot study was performed over a 15-month period, from February 2021 through May 2022. Set-up and Methods The pilot treatment process included pre-dewatering of blended primary sludge (PS) and thickened waste activated sludge (WAS), followed by TH, AD and post-dewatering to produce the final cake solids. Pre-dewatered solids were diluted to a TS concentration of 10 12% total solids (TS) before batch treatment in a TH pilot unit provided by Cambi ® (shown in Figure 2). Digestion was performed using semi-continuous benchtop anaerobic digesters with a 5-liter working volume (Figure 3), while THP was accomplished using a pilot unit. Pre-dewatering was performed with a pilot-scale screw press, while post-dewatering was performed by a combination of centrifugation and filtration. Digester operation was performed in three phases: -Phase 1 involved digester operation without TH pretreatment, at a 23-day solids retention time (SRT). The digesters were fed with a blend of primary sludge and thickened waste activated sludge (TWAS) at a solids concentration of 4.5%, in line with current, full-scale operations. -Phase 2 involved digester operation with TH pretreatment, at a 23-day SRT. Three TH temperatures were tested in this phase, 135 ºC, 150 ºC, and 165 ºC. The digesters were gradually acclimated to TH over a period of four months. -Phase 3 involved digester operation with TH pretreatment, at a 15-day SRT. This phase mimicked projected full-scale operation with TH at the BRWRF. Similar to Phase 2, three TH temperatures were tested in this phase, 135 ºC, 150 ºC, and 165 ºC. To achieve the target retention time, digester SRT was shortened by half a day on a daily basis, over a one-month period to prevent shock changes to operating conditions. During each phase, digester operation and performance were closely monitored by analyzing pH, alkalinity, volatile fatty acid concentrations (VFA) as well as COD and nutrient species concentrations in samples of the digestate, as well as the post-dewatering centrate. Additionally, biodegradability assays were performed on the centrate to quantify the non-biodegradable fractions of COD, nitrogen, and phosphorus in the centrate recycle. These results helped inform the impacts of the centrate recycle on final effluent COD and nutrient concentrations. Summary of Results Impacts to Anaerobic Digestion Increased solids loading to the digesters (by a factor of 2 or 3 over current loading) was able to achieve same digester SRT with 50 66% lower digester volume in operation. The pilot digesters were able to acclimate to a solids feed treated using TH over a period of 3 4 months through a gradual ramping up of solids and organic loading to ensure that key indicators of digester health (such as pH, VFA, Alkalinity, gas production and solids destruction) continued to remain in optimal ranges. Overall, greater solids destruction and gas production were observed across the combined TH and AD process relative to AD without TH pretreatment. A fundamental shift was observed in digester operational parameters and digestate characteristics. The higher solids and organic loading to the digesters, coupled with increased solubilization of carbon and nutrients in the TH process led to increased concentrations of sCOD and nutrient species in the digesters. The higher solids loading also resulted in higher TS content in the digested solids, increasing from approximately 2.5% to 4.5% TS. TH pretreatment was found to result in the production of increased COD and nutrient species in the centrate, especially soluble COD, TKN and ammonia. Impacts to Centrate Characteristics Concentrations of dissolved organic nitrogen and phosphorus (DON and DOP) in the centrate were also found to be significantly higher with TH pretreatment versus without TH. In addition to an increase in DON, DOP and sCOD, the non-biodegradable fractions of the constituents (i.e., nbDON, nbDOP and nbsCOD) were also higher in the centrate when TH pretreatment was applied. Table 1 shows the difference in post-dewatering centrate characteristics with and without TH pretreatment. €ƒ Impacts on Plant Effluent and Mainstream Data from pilot testing indicated that implementing TH could potentially increase final effluent sCOD concentrations to be in the range of 8 to 12 mg/L. This would trigger a need for additional treatment to meet the monthly average permit limit for COD. Final effluent TKN was estimated to increase to between 0.50 and 0.75 mg/L, which is within 20% of the monthly TKN limit. However, in order to meet TN of 2 mg/L and lower at future flows and loads (i.e., TKN< 0.5 mg/L and TIN < 1.2 mg/L), Loudoun Water may need to implement additional treatment. Final effluent TP was estimated to increase to between 0.02 and 0.05 mg/L. This encroaches upon LW's safety buffer for the monthly TP limit. However, in order to meet effluent TP concentrations of 0.05 mg/L and lower at future flows and loads (i.e., Orthophosphate < 0.02 mg/L and DOP < 0.02 mg/L), Loudoun Water may need to implement additional treatment. Cumulatively, these results suggest that the increases in sCOD, TKN and TP that may result from implementation of TH at BRWRF may present a challenge for meeting current and future permit limits at the BRWRF. Consequently, implementation of TH at BRWRF would likely trigger the implementation of additional treatment for nutrients onsite or diversion of centrate to an offsite facility for treatment or disposal.