Evaluation and Pilot Testing of Primary Filtration for Upgrade of Silicon Valley Clean Water Resource Recovery Facility

Introduction
Cloth media filters have been traditionally used for tertiary treatment at wastewater resource recovery facilities (WRRF), where effluent limits are less than 10 mg/L or 2 NTU (WEF MOP 8, 2017). When compared to more conventional granular filtration, this technology is attractive because of its lower head requirement (2.5-4 feet), lower backwash ratio (1-3%), and comparable hydraulic loading rates (3.5 gpm/ft2 to 7 gpm/ft2 respectively). Recently, the same cloth media filters have emerged as technologies suitable for primary treatment, offering far greater total suspended solids (TSS) and biochemical oxygen demand (BOD) removals than conventional primary clarification. To date, multiple pilot studies and a few installations have demonstrated TSS and BOD removals of approximately 80% and 50%, respectively (Caliskaner and al., 2020; Caliskaner and al., 2019; Caliskaner, 2022). This corresponds to an approximately 50% increase in removal, which is significant. This process can, therefore, be used to divert more carbon to the anerobic digestion process, leading to an increase biogas production in the order of 20 to 30 %, while, simultaneously, reducing the organic load to the secondary treatment, leading to a reduction in energy demand for aeration in the order of 20-25%. As the cost of electricity continues rising, it is anticipated that this process becomes more and more attractive.

Another significant advantage, when compared to conventional primary clarification, cloth disk primary filtration (CDPF) systems can remove approximately 50% more solids within approximately 20% of the footprint. This alone becomes a valuable tool for WRRFs that require additional secondary treatment capacity due to increased flows and loads, or more stringent discharge limits but have little to no room for expansion using conventional treatment methods and approaches. The later will be the case for San Francisco Bay Area's WRRFs, including Silicon Valley Clean Water's (SVCW) WRRF.

Methodology
SVCW is in Redwood City, CA. It is a Joint Powers Authority serving about 220,000 people living in the communities of Belmont, Redwood City, San Carlos, and the West Bay Sanitary District. The WRRF was designed to treat an average dry weather flow of 29 MGD, however, the current dry weather flow is 13 MGD. Considering the potential benefits of CDPF listed above, SVCW conducted a pilot project between November 2023 and April 2024 to evaluate the CDPF process at its WRRF (supplied by Aqua-Aerobic Systems, Inc.) (Figure 1).

Existing primary treatment consists of rectangular primary clarifiers (PC) that removes approximately 65% of the influent total suspended solids (TSS) concentrations and 35% of the biochemical oxygen demand (cBOD5) loading. Main goal of the project was to investigate the feasibility of replacing the primary clarifiers with CDPF as part of a major upgrade that is planned at the WRRF to meet the upcoming more stringent effluent quality requirements. Specific objectives of the CDPF pilot project were as follows:
1. Monitor the process performance for TSS, Chemical Oxygen Demand (COD) and 5-Day Biochemical Oxygen Demand (BOD5) removal at different hydraulic loading rates and compare this performance with the current plant primary clarification process side by side.
2. Determine the backwash ratio at different hydraulic and solids loading rates, and characterize the backwash water in term of TSS and verify mass balance.
3. Discuss the system operation and maintenance requirements.
4. Determine the number of required filtration units required.

Results and Discussion
Performance of the CDPF system is summarized in Table 1 and 2. The CDPF system achieved TSS removal rates above 80% despite a wide range of hydraulic and solids loading rates, while the TSS removal for the existing PC system ranged between 43 % to 68%. This shows that the CDPF not only significantly improves the primary treatment performance but also improves the consistency of the effluent quality. The pilot effluent TSS concentration ranged from 33.1 to 38.4 mg/L even if the HLR ranged from 1.5 to 5.3 gpm/ft2 and the SLR ranged from 4.6 to 12.4 lbs/ft2. This is a considerable advantage as more consistent effluent quality could lead to improvement in the secondary process. TSS and COD performance data collected during pilot testing are shown in Figure 2 through Figure 4.

Subsequent to the pilot testing, a technical evaluation study was conducted to determine the holistic impacts of implementing PF for SVCW. Based on the pilot performance results, the primary filtration process has been analyzed for its effectiveness, efficiency, and overall benefits to the existing and anticipated future processes. Key findings from this technical evaluation are listed below:
- Enhanced Treatment Efficiency: The primary filtration system improved TSS and BOD removal, reducing the loads on secondary treatment processes.
- Energy Optimization: Increased biogas production by 20-30% from enhanced solids capture in primary treatment.
- Footprint: The system requires smaller footprint compared to conventional methods, making it ideal for facilities facing land constraints.
- Wet weather Resilience: The system maintained over 80% TSS removal during extreme wet weather events, improving reliability.
- Regulatory Compliance: Project provided critical insights to meet future stringent Total Inorganic Nitrogen effluent limits.