Optimization of Advanced Primary Treatment Technologies for Carbon Diversion and Management at Water Resource Recovery Facilities

Introduction Advanced Primary Treatment (APT) is a process in which filtration or micro-screening technology is used to replace or supplement Conventional Primary Treatment (CPT). In this project, the Cloth Disc Primary Filter (CDPF) and Proteus Primary Filter (Proteus PF) have been demonstrated for 3 and 1.5 years, respectively. Significantly higher treatment performance values (i.e., 40 to 50 % higher) were observed with both primary filtration (PF) technologies compared to CPT. PF increases capacities of primary and secondary treatment systems minimizing (or sometimes even eliminating) expansion requirements for existing installations or reduce the required footprint for new installations. Primary filtration is also the most effective carbon diversion strategy at Water Resource Recovery Facilities (WRRFs). Carbon diversion involves reducing the organic load on secondary systems by redirecting more biochemical oxygen demand towards the solids line at the primary treatment stage, which reduces the aeration requirements for aerobic biological processes downstream. Resultant benefits of PF are: (1) savings in aeration energy costs, (2) increase in digester gas energy production, and (3) reduction in primary and secondary treatment footprint and volume requirements. Significant CAPEX and OPEX savings are realized because of these benefits. One important design and operational challenge for PF is the high primary sludge volume produced during backwash operation. Sludge produced from PF backwash is also thinner (i.e., 0.2 to 0.4 %) compared to CPT sludge. Thickening is an essential process component of PF systems and its design conditions are quite different due to high sludge volumes, low solids concentrations, and significant differences in sludge characteristics. Optimization of PF systems to minimize backwash sludge volumes and still achieving high treatment efficiencies is one of the main objectives of this on-going technology research and demonstration project. Methodology Funded by California Energy Commission, the focus of this project is to evaluate the treatment and hydraulic performance of several PF technologies at Linda County Water District's (Linda) WRRF located in Olivehurst, CA (Fig. 1). General information for the Proteus PF and CDPF systems are provided in Table 1. The Proteus PF and CDPF units are shown in Figs. 2 and 3, respectively. CDPF unit has been in operation since January 2022 and Proteus PF has been in operation since July 2023. The specific objectives of the project include: (1) Evaluate the performance of PF technologies; (2) Improve the treatment and hydraulic performances; (3) Optimize the design & operational criteria including sludge treatment system, and (4) Evaluate impacts on downstream secondary process. Inline turbidity/TSS sensors and flow meters have been installed at each PF system and sensor data are synchronized to provide real-time monitoring of treatment performance. Inline sensor data are complemented by lab analysis of 24-h composite samples from influent and effluent channels of each unit. Overall, the evaluation is based on hydraulic and treatment performances. The hydraulic performance was assessed using solids loading rate (SLR), hydraulic loading rate (HLR), and backwash reject ratios (BRR) of both PF systems. High HLR, low BRR, and high SLR typically correlate to lower capital and operational costs. The key parameters impacting BRR include: HLR, SLR, pressure build-up rate, backwash flowrate, and backwash frequency and duration. The treatment performance was evaluated based on total COD (tCOD), soluble COD (sCOD), total BOD (tBOD), soluble BOD (sBOD), TSS and total Kjeldahl nitrogen (TKN) removal. Treatment Performance of APT systems The performance of the Proteus Filter and Cloth Disk Primary Filter are reported and compared to conventional primary sedimentation. Proteus Filter: The TSS and COD removal performances of the Proteus Filter are shown in Fig. 4 and Fig. 5, respectively. Results were obtained based on 24-hr composite samples collected between July 2023 and September 2024. More than 100 samples were used to evaluate the removal performances. As shown in Fig. 4, influent and effluent TSS concentrations averaged at 471 and 100 mg/L, respectively, when the unit was operated between its average and peak flow design conditions. Influent and effluent COD, as reported in Fig. 5, averaged 981 and 419 mg/L, respectively. Cloth Disk Primary Filter: The TSS and COD removal performances of the full-scale CDPF system are shown in Fig. 6 and 7, respectively. Results were obtained based on 24-hr composite samples collected between May 2022 and September 2024. More than 150 samples were used to evaluate the removal performances. As shown in Fig. 6, influent and effluent TSS concentrations averaged at 421 and 56 mg/L, respectively. It should be noted that even though influent wastewater TSS concentrations varied significantly, a relatively stable effluent TSS between 50 to 70 mg/L was observed. Influent and effluent COD, as reported in Fig. 7, averaged 805 and 335 mg/L, respectively. Overall, the CDPF performance has been observed to be similar to the results obtained from previous demonstrations where 75-85% and 50-60% removal for TSS and COD were observed, respectively (Caliskaner et al., 2018, 2019, 2020, 2021, 2022). Performance Comparison versus Primary Sedimentation: Both PF systems were operated in parallel with the existing primary clarifiers at the Linda WRRF, and performance comparisons were conducted using samples taken during the same 24-hr sampling intervals. As seen in Fig. 8, primary clarifier (PC) achieved average removals of 38% for COD and 66% for TSS. As seen in Fig. 8, the Proteus PF unit achieved average removals of 65% for COD and 85% for TSS when operated at 2 gpm/ft2, which are 40 to 50 % higher compared to PC removal efficiencies. Similar high treatment performances were also obtained with the CDPF system when compared to primary clarifier. Average removals of 56% for COD and 85% for TSS were achieved with the CDPF system. Hydraulic Performance of APT systems The optimum design and operational conditions are investigated in this project considering HLR, SLR, headloss development (across the filter media), and BRR ratios. Summary of hydraulic values observed for both PF systems are presented in Table 2. PF backwash is triggered by preset filtration time or maximum headloss development , whichever comes first. Headloss development is measured by pressure transducers. Headloss development rates were observed to change with changes in HLRs. Proteus PF: As illustrated in Fig. 9, headloss development increases as the HLR increases resulting in higher BRRs. The daily average headloss development under HLR of 2 gpm/ft2 and 6 gpm/ft2 were 5.2 and 6.2 psig, respectively. The increased flowrate and HLR caused increased accumulation of particulate material within the filter media resulting in larger headloss values and hence higher BRR values. The HLR and BRR values with respect to time are summarized in Fig. 10. The Proteus PF system has been operated between 2 and 9 gpm/ft2 most of the demonstration resulting in an average BRR of 9.4%. Daily average HLR and SLR for the Proteus PF System during the demonstration is shown on Fig. 11. Based on an analysis of HLR and BRR values, HLR ranging from 2 to 2.5 gpm/ft2 are considered for optimum average flow design conditions and 5 to 6 gpm/ft2 are considered for peak flow design conditions. Cloth Disk Primary Filter: The volumes of filtered effluent, backwash reject, and sludge wasting are all monitored along with HLR and SLR values. Recorded HLR and BRR values are shown in Fig. 12. Similar to Proteus PF system, headloss development and BRR increase as the HLR increases. Average daily BRR values were observed to be approximately 10 percent under average design flow/loading conditions. Based on an analysis of HLR and BRR values, HLR value of approximately 2 gpm/ft2 is considered for optimum average flow design conditions and 4 to 4.5 gpm/ft2 are considered for peak flow design conditions. Impact of Primary Filtration Systems on Energy Consumption and Recovery One major project objective is to demonstrate the energy and capacity impacts of PF on WRRFs. Specifically, impacts of PF on MLE process and three Advanced Secondary Treatment technologies including Microvi, AGS and MABR were evaluated. Energy savings and footprint reductions are measured directly in addition to the process modeling studies to further investigate the impacts on BNR kinetics. The impacts of PF on secondary treatment and anaerobic digestion were predicted with SUMO process modeling using the obtained PF demonstration data. PF results in 25% in aeration energy savings and 35% increase in digester gas production (Fig. 13).