BIOLOGICAL NUTRIENT REMOVAL AS MEMBRANE PRETREATMENT IN DIRECT POTABLE REUSE: PILOT STUDY CONSIDERATIONS AND RESULTS. Introduction

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1 BIOLOGICAL NUTRIENT REMOVAL AS MEMBRANE PRETREATMENT IN DIRECT POTABLE REUSE: PILOT STUDY CONSIDERATIONS AND RESULTS Christopher C. Boyd, Alan Plummer Associates, Inc., 1320 S. University Drive, Suite 300, Fort Worth, TX 76107, Ph: Alan E. Davis, Alan Plummer Associates, Inc., Fort Worth, TX Dexter F. May, Alan Plummer Associates, Inc., Fort Worth, TX Tymn Combest, City of San Angelo, San Angelo, TX David A. Gudal, Alan Plummer Associates, Inc., Fort Worth, TX Phillip A. Lintereur, Alan Plummer Associates, Inc., Fort Worth, TX Introduction The City of San Angelo (City) performed a direct potable reuse (DPR) pilot study at the City s Water Reclamation Facility (WRF) in San Angelo, Texas. The DPR project, if implemented, would repurpose the municipal WRF effluent for potable reuse by upgrading the WRF to include biological nutrient removal (BNR) and treating BNR effluent with advanced water treatment processes. In the full-scale DPR treatment concept, municipal wastewater influent would be treated by BNR for removal of organic carbon, nitrogen, and phosphorus. BNR effluent would then be treated by an advanced treatment system that includes ultrafiltration (UF), reverse osmosis (RO), and an ultraviolet advanced oxidation process (UV/AOP). The combination of UF, RO, and UV/AOP processes is commonly referred to as full advanced treatment. Municipal wastewater effluent is a challenging feed water for membrane systems, and the BNR process provides pretreatment benefits by reducing the concentration of nutrients in the advanced treatment source water. This paper discusses operational considerations for a BNR system in a DPR application and investigates BNR as pretreatment for UF and RO processes. The paper also evaluates cloth media disk filtration as a pretreatment to pressure UF systems. Pilot Study Process Flow Overview Figure 1 presents a simplified liquid process flow diagram for the pilot study. Screened and degritted influent was pumped to the BNR pilot from the full-scale WRF headworks. The BNR pilot consisted of multiple treatment zones with subsequent clarification followed by chlorine disinfection. Chlorinated effluent was pumped to the advanced treatment pilot building, dosed with additional chemicals, and split between parallel filtration schemes. In the first scheme, a cloth media filter provided pretreatment for three pressure UF membranes. The second scheme featured a submerged UF pilot. UF filtrate blended upstream of the RO pilot system. Liquid streams from the advanced treatment pilot building, including filtrate, permeate, backwash waste, cleaning waste, and concentrate, were collected and returned to the San Angelo Water Reclamation Facility (WRF) headworks via drain. 1

2 Figure 1: DPR Pilot Study Process Flow Diagram Pilot Equipment Descriptions This section describes the pilot-scale equipment evaluated during testing. BNR Pilot System The City s WRF includes a chlorine contact basin (CCB) that is not currently in use. A portion of the existing CCB structure was converted into a BNR pilot system with a University of Cape Town (UCT) process configuration. The UCT process consisted of a suspended activated sludge system composed of different sub-reactors/zones arranged in series with multiple recycle streams. Raw influent entered the BNR pilot in the anaerobic zone. The anaerobic zone provided an environment with negligible dissolved oxygen and nitrate. From the anaerobic zone, wastewater flowed into the anoxic zone where denitrification occurred. Flow from the anoxic zone entered the aerobic zone where the mixed liquor was aerated to promote organic carbon removal, nitrification, and phosphorus uptake. Solids separation of the mixed liquor occurred in a secondary clarifier, and a chlorine contract zone downstream of the clarifier provided contact time for chlorine disinfection. 2

3 The BNR pilot system was a custom design that treated approximately 120 gallons per minute and included a raw grinder pump, anoxic and anaerobic zone mixers, recycle pumps, fine bubble diffusers, a return activated sludge pump, and associated piping and valves. Since the BNR pilot was a temporary installation, the system did not include features such as online water quality monitoring, flow metering, or mechanical sludge collection that are typical of full-scale installations. Process flows were periodically checked with a portable flow meter. A photo of the BNR pilot system is provided in Figure 2. Figure 2: BNR Pilot System Cloth Media Disk Filter Pilot System The pilot study evaluated the MD-3 Aqua MiniDisk cloth media filter manufactured by Aqua- Aerobic Systems, Inc. The cloth media disk filter treated BNR effluent using two cloth media disks and provided pretreatment for the pressure UF pilot systems. Figure 3 presents an image of the cloth media disk filter pilot installed at the pilot site. 3

4 Figure 3: Cloth Media Disk Filter Pilot Ultrafiltration Pilot Systems Three pressure UF membrane products (Dow SFD 2880, Evoqua Memcor L40N, and Toray HFU-2020N) and one submerged UF membrane product (Evoqua Memcor S10N) were evaluated during the pilot study. The Dow SFD-2880 and Toray HFU-2020N were equipped on a single, dual train pilot system; whereas, the two Evoqua membrane products were loaded into separate pilot systems designed for pressure and submerged membrane applications. Table 1 summarizes the UF vendor information. Photos of the UF pilot systems are provided in Figure 4. Table 1: UF Vendor Summary UF Membrane UF Type System Integrator System Model Dow SFD-2880 Pressure Wigen Water Technologies, Inc. UFDT-1 Evoqua Memcor L40N Pressure Evoqua Water Technologies, CPII LLC Evoqua Memcor S10N Submerged Evoqua Water Technologies, XS LLC Toray HFU-2020N Pressure Wigen Water Technologies, Inc. UFDT-1 4

5 UFDT-1 CPII XS Figure 4: UF Pilot Systems 5

6 Reverse Osmosis Pilot System The pilot study evaluated the Toray TMG20D-400 RO membrane product in a Harn R/O Systems, Inc. pilot skid. The RO skid was designed to operate in either a 2-stage or 3-stage configuration and was equipped with a feed pump as well as an interstage booster pump between the 1 st and 2 nd stages. An independent CIP system was also provided. Figure 5 presents a photo of the RO pilot system. Figure 5: RO Pilot System BNR Performance Overview The primary objective of the BNR process was to provide biological treatment for carbon, nitrogen, and phosphorus. Biological phosphorus removal is advantageous for reuse applications because it limits the need to feed significant coagulant doses to remove phosphorus chemically. Table 2 and Table 3 provide water quality data summaries for common wastewater process parameters after BNR pilot acclimation, and Figure 6 presents treatment efficiencies for ammonia, biochemical oxygen demand (BOD), and phosphorus. The BNR pilot provided effective nutrient treatment; however, the pilot system did not achieve the phosphorus removal efficiency typical of full-scale BNR installations as a result of pilot equipment and instrumentation limitations. Accordingly, aluminum sulfate (alum) was dosed at the entrance to the BNR pilot clarifier to further reduce the effluent phosphorus concentration. Figure 7 presents the results of an analysis comparing the fraction of phosphorus removed biologically versus the 6

7 fraction removed chemically during the pilot study. On an average basis, the BNR pilot system treated approximately 75 percent of influent phosphorus biologically and 25 percent chemically using alum. These phosphorus removal results were derived from BNR effluent data collected with and without an active alum feed. Table 2: BNR Influent Data Parameter Min. Max. Ave. Standard Total Deviation Samples Ammonia, Total (mg/l as N) Biochemical Oxygen Demand (mg/l) Chemical Oxygen Demand (mg/l) Dissolved Oxygen (mg/l) Orthophosphate (mg/l as P) ph Phosphorus, Total (mg/l as P) Temperature ( C) Total Suspended Solids (mg/l) Table 3: BNR Effluent Data Parameter Min. Max. Ave. Standard Total Deviation Samples Ammonia, Total (mg/l as N) < Biochemical Oxygen Demand (mg/l) < Dissolved Oxygen (mg/l) < Orthophosphate (mg/l as P) ph * Phosphorus, Total (mg/l as P) Temperature ( C) Total Suspended Solids (mg/l) * Reported as median value 7

8 Figure 6: Nutrient Treatment Efficiency Figure 7: Pilot BNR Phosphorus Removal Mechanisms 8

9 Feed Turbidity (NTU) Membrane Performance Results The following section provides select results from the pilot study with a focus on interactions between the BNR pilot system and downstream UF and RO processes. Cloth Media Disk Filters as UF Pretreatment Pressure UF membranes are less capable of handling solids than submerged UF system due, in part, to the potential for solids accumulation within the pressure module housing. The cloth media disk filter was evaluated as pretreatment for the pressure UF membranes to reduce solids loading and provide protection against solids carryover events from the BNR. Figure 8 presents a comparison of feed turbidity data for the Evoqua XS UF pilot (submerged), which treated unfiltered BNR effluent, and the Evoqua CPII UF pilot (pressure), which received filtered effluent from the cloth media filter. The average feed water turbidity values for the CPII and XS pilots were 2.2 and 8.9 NTU, respectively. The Evoqua CPII pilot consistently received a lower turbidity feed water supply than the Evoqua XS pilot indicating the disk filter provided beneficial solids pretreatment. Additionally, the disk filter limited the effects of high solids carryover events from the BNR pilot system. Multiple solids carryover events increased XS feed turbidity above the maximum feed turbidity meter limit of 100 nephelometric turbidity units (NTU) during the first two months of UF testing. XS feed turbidity improved in May 2016 once pilot clarifier operations were optimized Evoqua XS UF Pilot (Submerged) Evoqua CPII UF Pilot (Pressure) /9/16 4/8/16 5/8/16 6/7/16 7/7/16 Date Figure 8: UF Feed Turbidity Data 9

10 Effects of Alum Addition on UF Performance UF membrane fouling was an issue during the first month of operation prompting the UF vendors to vary system setpoints in an attempt to achieve stable operation. Membrane performance generally declined in response to a challenging water quality and periodic secondary clarifier upsets that resulted in substantial solids carryover. An alum feed was implemented at the end of March 2016 for phosphorus polishing as part of the RO pretreatment strategy. Originally, the alum feed was started upstream of the cloth media disk filter; however, the alum rapidly fouled the filter resulting in unsustainable operation. The project team moved the alum feed to the BNR pilot clarifier and incrementally increased the alum dose over a one week period to achieve the effluent orthophosphate goal. In addition to lowering the effluent phosphorus concentration, the alum feed also allowed the vendors to stabilize membrane performance. Figure 9a presents UF specific flux trends for the pilot study. As shown in the figure, specific flux values improved within approximately one week of adding the alum feed in the clarifier. The UF membranes did not respond uniformly at a given alum dose. As shown in Figure 9b, the Toray HFU-2020N membrane demonstrated a notable improvement at an alum dose of 34 mg/l; whereas, the Evoqua L40N and S10N membranes required a dose of 46 mg/l before appreciable improvement was observed. Importantly, alum feed alone does not explain the UF performance improvements. Vendor setpoint optimization and cleaning protocol modifications were also required. The Dow module provides an example of the importance of vendor optimization, because module performance improved after further UF optimization rather than immediately following alum addition. The project team further investigated the effects of alum on UF performance by discontinuing the alum feed at the conclusion of testing. At the time of alum feed discontinuation, the membranes had operated for 49 days without a clean-in-place (CIP) procedure. Figure 9c presents temperature corrected transmembrane pressure (TMP) trends for the four UF membranes before and after the alum feed was discontinued. As shown in the figure, TMP trends were stable prior to discontinuing the alum feed and began to rise thereafter. The TMP results suggest that alum was an important factor in stabilizing membrane performance. The study did not investigate the mechanism by which alum improved performance; however, possible explanations include improved removal of colloids, particulate matter, and dissolved organic carbon. 10

11 Figure 9: Effects of Alum Addition on UF Performance 11

12 RO Performance Overview Permeability trends for the RO pilot are provided in Figure 10. Pilot testing began in April 2016 with a 3-stage system configuration at 88-percent recovery using the setpoints listed in Table 4. After observing third stage scaling following approximately one month of operation, the project team determined that 88-percent recovery was not sustainable using a conventional, 3-stage RO system. Accordingly, the pilot was converted to a 2-stage system at 85-percent recovery using the setpoints defined in Table 5. In response to first stage fouling, a mini-cip was performed in June 2016 consisting of a high ph caustic soak of the first stage. A low ph, citric acid mini-cip of the first and second stage was performed approximately one week later. The caustic mini-cip recovered first stage permeability to approximately baseline conditions; however, the citric acid mini-cip had a negligible effect on either the first or second stage. A CIP was performed at the end of June 2016 and recovered the first and second stage permeability; however, second stage permeability rapidly declined following restart of the pilot system. Table 4: 3-Stage RO Pilot Setpoints Parameter Overall 1 st Stage 2 nd Stage 3 rd Stage Elements per Pressure Vessel Total Elements Scale Inhibitor Vitec 1400* Nominal Flux (gfd) Recovery Cumulative (%) * The Vitec 1400 scale inhibitor product is manufactured by Avista Technologies, Inc. Table 5: 2-Stage RO Pilot Setpoints Parameter Overall 1 st Stage 2 nd Stage Elements per Pressure Vessel Total Elements Scale Inhibitor Vitec 1400 Nominal Flux (gfd) Recovery Cumulative (%)

13 Figure 10: RO Pilot Permeability Trends RO Element Fouling Three RO elements were sent to Avista Technologies, Inc. (San Marcos, California) for autopsy during the pilot study. The elements included a tail-end third stage element, a tail-end second stage element prior to the CIP, and a tail-end second stage element after the CIP that was collected at the conclusion of testing. A membrane autopsy is a destructive testing process that examines the membrane element for visual defects, foulant composition, and cleaning effectiveness. Aluminum silicate was the primary foulant; however, silica, calcium, and microorganisms were also identified on the three RO elements in addition to trace amounts of other constituents. Silica scale is an operational concern for RO systems containing elevated feed water silica concentrations and is a documented issue in municipal wastewater reuse applications (Moreno et al., 2013; Knoell et al., 2015). Malki & Abbas (2011) reported that silica scale can form in the presence of precipitated calcium phosphate; however, calcium phosphate was not identified during the three RO element autopsies. Separate studies have demonstrated that dissolved aluminum in water with elevated silica concentrations can lower silica solubility and reduce scale inhibitor effectiveness (Sheikholeslami & Bright, 2002; Wen-Yi Shih et al., 2006). While the dissolved aluminum concentrations measured in the RO feed during the pilot study were lower than the value used in the scale inhibitor projections, the authors hypothesize that second and third stage biofouling acted as a seed layer that promoted aluminum silicate scale formation. 13

14 Accordingly, a lower recovery would be required using the scale inhibitor evaluated during testing. Alternative Coagulant Evaluation Coagulant addition assisted with controlling calcium phosphate scale in the RO system but may have promoted aluminum silicate formation. Accordingly, minimizing residual aluminum in the BNR effluent is an important operational consideration for the reuse system. The BNR pilot did not include an efficient system for rapidly mixing alum into the process flow. Alum was fed into a turbulent area at the clarifier inlet, and the alum dose was selected using a trial and error procedure that involved adjusting the alum feed rate and measuring effluent phosphorus. Inadequate mixing likely resulted in the need to overfeed alum. An improved mixing approach would allow for optimization of the alum dose with the goal of minimizing residual aluminum in the BNR effluent. In addition to optimizing mixing, an alternative coagulant could be evaluated with the goals of reducing both total aluminum added during chemical phosphorus polishing and the fraction of residual aluminum remaining in the effluent. The project team conducted an initial coagulant evaluation to assess the performance of acid alum 1, aluminum chlorohydrate (ACH), and polyaluminum chloride (PACl) for phosphorus polishing. Jar testing results for the three coagulants are provided in Figure 11. On average, approximately 0.5 mg/l (as P) of orthophosphate needed to be removed chemically to meet RO phosphorus pretreatment requirements. As shown in the figure, ACH and PACl achieved 0.5 mg/l (as P) of phosphorus removal at lower doses than alum or acid alum. Accordingly, ACH or PACl could be viable alternatives for managing residual aluminum concentrations and warrant further testing. 1 Acid alum contains aluminum sulfate and free sulfuric acid. 14

15 Figure 11: Alternative Coagulant Jar Testing Results Conclusions The pilot-scale evaluation of BNR as UF and RO pretreatment provided valuable design information for the project. The following conclusions are provided based on the results: The cloth media disk filter provided effective pretreatment for the pressure UF systems. Specifically, the cloth media disk filter protected the pressure UF pilots from secondary clarifier solids upsets and allowed the pressure UF membranes to operate within a narrow range of feed water turbidities. Alum addition immediately upstream of the cloth media disk filter resulted in rapid cloth media fouling. While the alum solids were effectively removed from the cloth, filter backwashes occurred too frequently to allow for sustainable disk filter operation. The submerged UF system effectively recovered from secondary clarifier solids upsets and demonstrated an ability to treat BNR effluent directly without cloth media disk filter pretreatment. While the frequency and severity of clarifier upsets may have been an issue with the pilot clarifier arrangement and operating protocol, the ability to handle solids loading is important for downstream filtration processes since biological process upsets do occur. The BNR pilot provided effective nutrient treatment; however, the pilot system did not achieve the phosphorus removal efficiency typical of full-scale BNR installations as a result of pilot equipment and instrumentation limitations. Optimization of a full-scale 15

16 BNR process is anticipated to reduce or eliminate the need for chemical phosphorus polishing. Residual aluminum from the alum coagulant likely contributed to aluminum silicate scaling. The issue may be managed by optimizing biological phosphorus removal, lowering RO recovery, increasing cleaning frequency, and/or optimizing the coagulant feed. A scale inhibitor study could also be implemented to identify a scale inhibitor product capable of achieving stable operation at 85-percent recovery. Alum, in combination with vendor equipment optimization, resulted in stable UF membrane performance. Accordingly, a future coagulant optimization study should consider coagulant dosing effects on both UF and RO process performance. References Sheikholeslami, R. and J. Bright, J., 2002, Silica and metal removal by pretreatment to prevent fouling of reverse osmosis membranes, Desalination, 143, Wen-Yi, S., Gao, J., Rahardianto, A., Glater, J., Cohen,Y. and Gabelich, C. J., 2006, Ranking of antiscalant performance for gypsum scale suppression in the presence of residual aluminum, Desalination, 196, Malki, M. and Abbas, V., 2011, A Relationship Between Phosphate Scales and Silica Fouling in Wastewater RO Membrane Systems, IDA World Congress, Perth, Western Australia, September 4-9. Moreno J., Monclús, H., Stefani, M., Cortada, E., Aumatell, J., Adroer, N., De Lamo-Castellví, S., Comas, J., 2013, Characterisation of RO fouling in an integrated MBR/RO System for Wastewater Reuse, Water Science & Technology, 67(4), Knoell, T., Dunivin, W., Gonzalez, R., Patel, M., 2015, Optimizing Reverse Osmosis Pretreatment Chemicals in Orange County s Groundwater Replenishment System, AMTA Solutions, Summer. Acknowledgements The authors are grateful for the contributions of the City of San Angelo, including Abel Morales, Allison Strube, Bill Riley, Carl van der Sterre, Elena Velez-Reyes, the Water Quality Laboratory team, and the Water Reclamation Facility staff. Their tireless efforts were instrumental in the completion of the pilot study. The authors also recognize and appreciate the efforts of project team members Scott Hibbs, Jordan Hibbs, Joshua Berryhill, Justin Lane, and Dave Baker with Enprotec / Hibbs & Todd, Inc and Andrew Aldridge with Alan Plummer Associates, Inc. 16