Evaluation of the Impacts of Three Advanced Primary Treatment Technologies on Performance of Water Resource Recovery Facilities

INTRODUCTION
Several filtration and micro-screen technologies have been developed to provide advanced primary treatment (APT) to replace conventional primary treatment (CPT). Primary filtration [e.g., cloth disc primary filter (CDPF)] provides improved removal of particulate material compared to CPT with a significantly smaller treatment footprint and modified effluent particle size distribution. Primary biofiltration [e.g., compressible medium biofilter (CMBF)] provides additional removal of soluble material compared to CPT. When a screening technology is used with pore size between 100 and 300 microns to replace CPT, the process application is known as micro-screening (MS). Performance data from full-, demonstration-, and pilot-scale CDPF, CMBF, and MS projects have demonstrated the feasibility of these technologies for APT. APT effluent has significantly lower concentrations of total suspended solids (TSS) and biochemical oxygen demand (BOD) compared to CPT, which results in reduced secondary treatment aeration energy requirements and increased secondary treatment capacity. APT diverts carbon from the energy-intensive secondary process towards energy-recovering anaerobic digestion.
METHODOLOGY
The results presented in this paper are from CDPF, CMBF, and MS projects since 2012, including an on-going comprehensive APT demonstration project. The evaluation of APT technologies is focused on hydraulic performance in addition to treatment performance. The hydraulic performance can be evaluated by monitoring solids loading rate (SLR), hydraulic loading rate (HLR), and the ratio of reject streams (backwash water, settled sludge, and scum) to applied wastewater. The backwash reject water (BRW) ratio, defined as the amount of backwash flow relative to the amount of wastewater filtered, is an important consideration to evaluate and optimize the performance of APT technologies. For APT systems, high HLR, low BRW ratio, and high SLR correlate to lower capital and operational costs. Treatment and hydraulic performances of APT systems are evaluated with respect to subsequent overall energy and capacity impacts for water resource recovery facilities (WRRFs).
RESULTS
CDPF. In a CPDF system (Aqua-Aerobics Systems, Inc.), cloth media (via small pore sizes of 5 µm) provides a physical barrier for filtration while floatable and settable materials are removed separately. As shown on Figure 1, the hydraulic and treatment performance across CDPF units installed at Linda WRRF (full-scale), Manteca WRRF, and Lancaster WRRF was consistent, achieving removals of 80–85% for TSS and 45–60% for organics (Table 1). The demonstration-scale filters in Lancaster and Manteca were found to have BRW ratio between 12–15%, while the Linda full-scale filter had an average BRW ratio of 10%. Figure 2 summarizes TSS removal performance for the full-scale CDPF based on 3-year operation; effluent TSS concentration remained relatively constant over a wide range of influent TSS concentrations (influent and effluent TSS averaged 300 and 55 mg/L, respectively).
CMBF. In a CMBF system (WesTech Engineering, Inc.), liquid passes through a bed of compressible filter media and creates a pore size gradient (smaller particles are captured by increasingly smaller pores). Biofiltration is promoted by allowing controlled biofilm growth within the filter media. A two-year demonstration-scale project was conducted at Linda WRRF to evaluate primary biofiltration using CMBF (Figure 3). Consistent removal of soluble material was observed. Test results for the removal of soluble BOD (sBOD), seen on Figure 4, and soluble chemical oxygen demand (sCOD) were 22 and 25%, respectively. Based on the analysis of forty samples, TSS, BOD, and COD removals were 70, 52, and 50%, respectively.
Micro-Screen. A MS (Huber, Inc.) system utilizes a horizontally installed screen basket that wastewater flows through from inside to outside. A filter carpet develops on the filter mesh that helps to retain particles that are much smaller than the nominal aperture size of the mesh. A fixed spray bar cleans the basket surface and fine screenings (primary sludge). In pilot testing of a MS (Huber Drum Screen Liquid) at Sand Island WRRF, reductions of 60–70% TSS and 25–35% BOD were achieved. These APT systems have demonstrated consistent high effluent quality across a wide range of influent raw wastewater conditions at a lower treatment footprint than CPT (Table 2). APT technologies can treat a larger volume of raw wastewater per square foot of land. For example, at Linda WRRF, the primary clarifier requires 220 m2 (2,400 ft2) to treat 11,000 m3/day (2.5 MGD), while the CDPF or MS (if scaled up to the primary clarifier) would require less than 50 m2 (500 ft2) to treat 11,000 m3/day (2.5 MGD). This significant reduction in primary treatment footprint translates to significant cost savings, particularly for WRRFs with limited land availability. DISCUSSION/CONCLUSIONS APT technologies can significantly improve the efficiency of WRRFs with respect to energy and treatment performance, capacity, and footprint. A summary of APT and CPT technologies is provided in Table 2. The following conclusions have been observed from the demonstration projects reviewed in this paper: - BOD and COD removal for APT technologies is 25–40% higher compared to CPT. - The overall WRRF energy savings possible with APT are significant and can be as high as 40–50%, depending on system specifics. Energy cost savings result from: (1) a reduction in the primary effluent organic load which, in turn, results in a 20–40% decrease in the aeration energy requirement in secondary treatment and (2) the high energy content of the captured VSS in filter BRW and the increased volume of primary sludge increases gas energy production in the anaerobic digester. - The increase in biogas energy production with APT has been estimated to be between 15 and 35%, depending on specific WRRF conditions. - APT enhances the particle size characteristics of primary effluent which allows secondary treatment processes to operate more efficiently. As shown on Figure 5, the effluent particle size distribution obtained with APT is superior compared to CPT effluent; for example, average removal for a 10-micron particle was 76, 64, and 56% for the CDPF, CMBF, and primary clarifier, respectively. - Effluent TSS concentration from APT systems remained relatively constant (Figure 2), thus providing an improved/constant mass loading to the secondary treatment process. - APT can reduce the footprint of CPT up to 80%.
ONGOING PROJECT
A technology research, demonstration, and development project is underway to further demonstrate the increased performance and economic benefits of advanced wastewater treatment technologies (Figure 6) compared to conventional treatment. The three APT technologies shown in Table 2 [CDPF (full-scale), MS (full-scale), CMBF (demonstration-scale)] are deployed at Linda WRRF. Main objectives include: (1) evaluate the performance of APT compared against CPT, (2) improve the economic feasibility and capacity of APT by reducing the BRW ratio and increasing the hydraulic performances, and (3) evaluate the performance and benefits of combined APT and advanced secondary treatment systems [supply APT effluent to a full-scale membrane aerated bioreactor and demonstration-scale aerobic granular sludge and Micro-Vi systems].