BENEFITS OF SOIL AQUIFER TREATMENT AND NANOFILTRATION FOR POTABLE REUSE IN TUCSON, AZ Michael Hwang, CH2M HILL, 1501 W. Fountainhead Parkway, Suite 401, Tempe, AZ 85282 Email: mhwang@ch2m.com Phone: 480-377-6296 Larry Schimmoller, CH2M HILL, Denver, Colorado Jeff Biggs, Tucson Water, Tucson, Arizona Jim Lozier, CH2M HILL, Tempe, Arizona Introduction To meet the needs of the greater Tucson Metropolitan area, the City of Tucson and Tucson Water are exploring the concept of potable reuse as a means to diversify and expand their water portfolio. Preliminary studies indicate that the use of a dual-membrane treatment approach, which includes including microfiltration (MF), reverse osmosis (RO), and ultraviolet advanced oxidation (UVAOP), is a cost-effective method to attain reuse goals. However, implementation of RO-based projects at inland locations such as Tucson can be challenging because of the environmental difficulty and high cost of disposing of the waste stream generated by the RO process. Consequently, as Tucson and other communities look to expand future water supplies through reuse, an opportunity exists to explore the efficacy of alternative methods of treatment that may be more cost-effective and sustainable than RO-based approaches. Alternative treatment approaches for potable reuse must provide multiple treatment barriers for pathogens and organics, and adequately remove or dilute salts when needed. In addition, future potable reuse projects should strive to utilize energy more efficiently and effectively mitigate concentrate disposal cost. To this end, a six-month pilot test of an innovative and sustainable potable reuse treatment scheme was conducted by CH2M HILL, Tucson Water and the University of Arizona under a tailored collaboration project with the WateReuse Research Foundation (WRRF-13-09). The treatment scheme piloted included short term soil aquifer treatment (SAT), slip-stream nanofiltration (NF), ozone oxidation, and biologically activated carbon (BAC) filtration. These processes were selected to meet and address the critical treated water quality considerations for a successful potable reuse program in Tucson which includes the following: (1) partial reduction of total dissolved solids (TDS), multiple barriers for contaminants of emerging concern (CECs) and pathogens, and reduction of total organic carbon (TOC) to control the formation of disinfection byproducts (DBPs) during distribution. The results from the pilot testing showed that the tested treatment train warrants consideration as an alternative potable reuse scheme as the final treated water quality produced during the six months of testing was excellent. This paper will focus specifically on the performance of SAT and NF and the benefits they provide in the alternative potable reuse scheme Overview of Proposed Treatment Scheme A simplified process flow diagram for the proposed potable reuse treatment scheme is presented in Figure 1. All of the treatment processes are mature and proven in providing various degrees of pathogen and trace organics removal. These benefits are briefly described in this section 1
Figure 1. Process flow diagram for proposed alternative potable reuse treatment train Soil Aquifer Treatment Soil Aquifer Treatment (SAT) has been shown to provide excellent removal of organics, pathogens, and nitrogen compounds, thereby providing a robust first barrier in the potable reuse treatment scheme (Fox, 2006). In some cases where water travel time through SAT is significant and TDS in the recharge water are sufficiently low, such as the Montebello Forebay spreading basins, additional treatment downstream of SAT is not necessary nor provided prior to potable use. However, at other locations where potable reuse is considered, less SAT travel time and higher TDS in the recharge water may dictate additional treatment for removal of TDS, trace organics, and pathogens. At these locations, SAT will provide excellent pretreatment prior to a downstream salt removal process (e.g., reverse osmosis) because of its excellent removal of solids and turbidity. In fact, SAT may provide enough solids removal to eliminate the need for MF that is typically necessary on secondary effluent to provide suitable feed water quality for the downstream membrane desalination process. MF treatment can be very costly, therefore, elimination of this process from the overall potable reuse treatment scheme can provide substantial capital and annual operating cost savings. Historically, SAT with long travel times in the aquifer prior to withdrawal (months to years) has been utilized. In fact, California s groundwater recharge regulations using reclaimed water grant 1-log virus reduction for each month of travel time in the aquifer, which typically results in a required travel time of six to twelve months to meet log reduction requirements for IPR projects. This long-term SAT approach has provided excellent pathogen removal, but in many locations its application is infeasible due to site limitations. Short-term SAT, which can be achieved through engineered systems such as riverbank filtration and shallow aquifer extraction wells, has shown excellent treatment in recent years and is easier to implement at full-scale than long-term SAT. The pilot system studied in this project used short-term SAT through use of water extracted by a monitoring well adjacent to Tucson Water s Sweetwater Recharge Basins, where treated wastewater has been polished for non-potable reuse over the past 20 years. The extraction well, which is described in more detail later, results in a water travel time of approximately 14 days. Although the short-term SAT provided by this process provides good removal of many constituents, preliminary water quality data indicated that additional treatment for the removal of some constituents was required prior to potable use. 2
Nanofiltration RO has traditionally been used in potable reuse schemes for the removal of organics and pathogens; however it is energy intensive and produces a high-salinity concentrate stream that is difficult and expensive to treat and or dispose of. Nanofiltration (NF), which has been used extensively in Florida for softening and organics removal, can be used to remove pathogens, organics, and divalent ions. Therefore, depending on the ion profile of the secondary effluent (mix of monovalent and divalent ions), NF could be used to meet specific TDS goals at significantly lower energy consumption compared to RO while producing a lower salinity concentrate. In addition, because monovalent ions are poorly rejected by NF, the potential to use the divalent-enriched concentrate for beneficial purposes, such as irrigation of salt tolerant crops, increases substantially. This approach could eliminate or significantly reduce costly concentrate treatment schemes including mechanical evaporation and evaporation ponds that would be necessary for RO-based potable reuse plants implemented at inland locations. Nanofiltration will also provide substantial reduction of pathogens and organics, although this is not its primary objective since it only treats a portion of the flow and could, at most, only achieve approximately 50% reduction (0.3-log) of pathogens and organics. Removal of hormones and antibiotics by full-stream NF has been demonstrated by Koyuncu et al. (2007) in which 95% rejection was achieved for chemicals with molecular weights of 300 Da or more. Ozone and Biological Activated Carbon Filtration The use of ozone for oxidation of trace organics and inactivation of pathogens in secondary effluent has been extensively studied by the University of Arizona. Recent research has shown excellent oxidation of trace organic contaminants in secondary effluent with the use of ozone, as well as transformation of organics to more biologically assimilable fractions (Snyder, 2012). Biological activated carbon (BAC) filtration provided downstream of the ozone will assimilate and remove a significant portion of the transformed organics. In addition, for conditions where the total organic carbon (TOC) concentration is low, such as post-sat applications, the granular activated carbon (GAC) media will concurrently remove trace organics through adsorption. As stated, this paper will focus on the treatment performance of SAT and NF and not on the results for the ozone and BAC systems Pilot Overview To evaluate this process, a pilot was designed and built in 2013 as part of a collaboration between Tucson Water, University of Arizona and CH2M HILL. The pilot facility was constructed at Tucson Water s Sweetwater Recharge Facility (SRF), which receive secondary effluent from Pima County s Agua Nueva Water Reclamation Facility (ANWRF). The SRF comprise eleven recharge basins (four on the west bank of the Santa Cruz River and seven on the east bank), twelve monitoring wells used for compliance sampling, eighteen piezometers monitoring water levels, and seven high-capacity extraction wells used to withdraw water for non-potable use. Secondary effluent from Pima County s ANWRF is delivered directly to the recharge basins. Currently, recharge operations take place twelve months of the year, with periodic amounts of time reserved for routine recharge basin maintenance needed to maximize infiltration rates. 3
The pilot site is located centrally to eight of the eleven SRF recharge basins. These eight basins have a total recharge area of 27.4 acres. The pilot site is adjacent to Tucson Water groundwater monitoring well WR-069B, which is between recharge basins 1, 2, and 3. Monitoring well WR- 069B was chosen as the water supply well for this project due to its proximity to the recharge basins, its shallow construction that results in a short travel time, and the open area capable of accommodating all pilot equipment. Well WR-069B was drilled in 1991 and constructed with 6- inch diameter, low-carbon steel casing to 200 feet below land surface. A dedicated pump/motor assembly was installed utilizing a 2 inch, galvanized steel riser pipe. A map of the site is shown in Figure 2. Figure 1. Pilot Site Location at Sweetwater Recharge Basins in Tucson Three equipment skids were provided by the University of Arizona to be used for testing of NF, ozone and GAC/BAC. An existing pilot trailer containing a RO skid was retrofitted for this project and used to house and operate the NF system during this study. The pilot trailer was located adjacent to Tucson Water s Well WR-069B and supplied 11.6 gpm of recovered effluent during the pilot. To achieve the treatment-bypass ratio required to meet Tucson Water s TDS goal, 60% of the flow was directed to a 500-gallon (NF feed) tank for subsequent treatment by the NF train, while the remaining 40% of the flow was bypassed and blended with the NF permeate in a downstream blend tank. A hybrid design utilizing Dow Filmtec NF 270-2540 elements in Stage 1 and Dow Filmtec NF 90-2540 elements in Stage 2 (in a 2:1 hydraulic taper) was used to provide a TDS in the NF permeate sufficient to meet the TDS target of 500 mg/l or less in the blended NF permeate while minimizing the salinity of the NF concentrate. The basic design criteria for the NF is presented in Table 1. 4
Table 1 Nanofiltration Design Criteria Parameter Unit Value Number of Stages # 2 Pressure Vessel Array 2:1 Number of Pressure Vessels # 6 Elements per Vessel # 3 Total Elements # 18 Stage 1 Element Dow NF 270-2540 Stage 2 Element Dow NF 90-2540 Design Recovery % 82.4% Average Design Flux gfd 13.4 Bypass Flow Percentage % 40% Total Feed Flow gpm 8.8 Bypass Flow gpm 3.1 NF Feed Flow gpm 5.7 NF Permeate Flow gpm 4.7 NF Concentrate Flow gpm 1.0 Feed TDS mg/l 750 Permeate TDS mg/l 312 Combined Permeate Bypass TDS mg/l 487 Antiscalant Product Avista Vitec 4000 Antiscalant Dose mg/l 2 5 The blended water will be treated by an ozone pilot skid (Wedeco) which includes an onsite ozone generator. A target dose of 1.0 mg/l was initially selected based on preliminary laboratory tests conducted by UA then later reduced to 0.5 mg/l to mitigate bromate formation. The ozone effluent will then be treated using a four column GAC skid. During phase 1 of the pilot, two activated carbons will be tested over a three-month period during which the GAC will become biologically active (BAC). In phase 2 of the pilot the two BAC columns were tested in parallel with two new filter columns filled with virgin GAC. Performance Monitoring During the pilot, both operating parameters and water quality were monitored. Water quality samples were collected on a weekly basis at ten locations in the process train and analyzed for metals, salts, nutrients, trace organics, nitrosamines, bromate, pathogens and microorganisms. Performance monitoring of the NF included flow and conductivity measurements of the feed, product and concentrate flow, and pressure readings. Membrane performance was measured using two primary parameters: A value (water transport coefficient) and B value (salt transport coefficient). Differential pressure was also monitored as an indicator of particulate and biofouling, as well as salt passage and rejection. Flux and recovery were also monitored as primary operating parameters. Summary of Results TOC was reduced from a 50th percentile concentration of 8.7 mg/l in the secondary effluent to consistently below the detection limit (< 0.25 mg/l) after BAC/GAC, a reduction of more than 97%. Short-term SAT provided most of the removal (91%), reducing TOC to about 0.75 mg/l. 5
The TOC in the nanofiltration permeate was consistently less than 0.25 mg/l resulting in a TOC of 0.4 mg/l after blending. Forty-two CECs were monitored during the pilot test, with 33 consistently present in the secondary effluent. Results for the CECs showed that after SAT, only 18 of the 42 target CECs could be detected in the Well WR-069B samples. Among these compounds, SAT removed more than 95% of acesulfame, DEET, diclofenac, and sulfamethoxazole, and more than 85% of iopamidol, meprobamate, and tris (1-chloro-2-propyl) phosphate (TCPP). Among the CECs present in Well WR-069B water, iopamidol, meprobamate, primidone, sulfamethoxazole, and sucralose were most effectively removed by NF process (> 90%), followed by carbamazepine (83%), TCEP (77%), TCPP (77%), simazine (69%), and acesulfame (67%). Benzotriazole was not significantly removed by NF (< 1%). Viruses, protozoa, and bacteria were measured at various locations in the pilot process to assess pathogen removal. No protozoa were detected at any location in the treatment process. Due to the limited presence of protozoa in the secondary effluent, log reduction values could not be calculated for protozoa for any treatment process. However, higher concentrations of viruses present in the secondary effluent allowed log reduction calculations for viruses. Adenovirus, pepper mild mottle virus (PMMoV), and aichi virus were detected in the secondary effluent at levels up to 4.6, 6.3, and 4.5 Log10/L, respectively. After the SAT process, no PMMoV or aichi virus were detected, and Adenovirus was only detected in one sample at a concentration of 0.38 copies/l, indicating greater than 4-log removal of viruses by the short-term SAT process alone. The finished water goal for TDS was set at 500 mg/l to match EPA s secondary maximum contaminant level. The TDS concentration in the secondary effluent averaged 754 mg/l, which was reduced to 327 mg/l in the NF permeate; this resulted in a blended TDS concentration in the finished water of 486 mg/l, meeting the TDS goal of less than 500 mg/l. The combination of short-term SAT, cartridge filtration, and antiscalant dosing provided excellent pretreatment for NF. Silt density index (SDI) values were consistently below 3 in the cartridge filter effluent, which indicates low colloidal and particulate fouling potential. NF permeability data, quantified through the A-value, showed consistent and stable operation in vessels 1&2 and 3&4 (Stage 1), indicating little if any fouling in this stage. Some permeability decline was observed in vessels 5&6 (Stage 2), especially during Phase 2 operation, that was attributed to organic fouling. Salt passage data for the NF, quantified through the B-value, showed little decline over the course of the pilot. The B-values for V1+2 and V3+4 (Stage 1) were much higher than B values for V5 and V6 (Stage 2), consistent with the lower salt rejection of the NF270 versus NF90 elements. B-values were stable for Stage 1, indicating that little, if any, fouling was experienced by the NF270 elements, consistent with the stable A-values. B- values were also stable in Stage 2, indicating that whatever fouling caused a decline in water transport (A-value) had little, if any, effect on salt transport (B-value). The stability of B value for V6 indicates that mineral precipitation was unlikely. A graph of the A-values is presented in Figure 2, while graphs of the B-values are presented in Figures 3 and 4. 6
Figure 2. Water passage coefficient (A) during the pilot study 8.0 V1&2 V3&4 7.0 B (10-6 m/s) 6.0 5.0 4.0 3.0 2.0 1.0 Phase 1 Phase 2 0.0 1-Oct-14 15-Oct-14 29-Oct-14 12-Nov-14 26-Nov-14 10-Dec-14 24-Dec-14 7-Jan-15 21-Jan-15 4-Feb-15 18-Feb-15 4-Mar-15 18-Mar-15 1-Apr-15 15-Apr-15 29-Apr-15 13-May-15 Figure 3. Membrane salt coefficient (B) for Dow NF 270-2540 elements in Stage 1 0.60 V5 V6 0.50 B (10-6 m/s) 0.40 0.30 0.20 0.10 0.00 Phase 1 Phase 2 1-Oct-14 15-Oct-14 29-Oct-14 12-Nov-14 26-Nov-14 10-Dec-14 24-Dec-14 7-Jan-15 21-Jan-15 4-Feb-15 18-Feb-15 4-Mar-15 18-Mar-15 1-Apr-15 15-Apr-15 29-Apr-15 13-May-15 Figure 4. Membrane salt coefficient (B) for Dow NF 90-2540 elements in Stage 2 7
Membrane autopsy results suggested little inorganic deposition in the lead tail elements of the system, thereby confirming that the decline in permeability (A-Value) in Stage 2 was likely due to organic fouling. Chemical cleaning, which was not conducted during the six months of operation, would likely have restored the Stage 2 permeability. Consequently, the pretreatment utilized in this testing (SAT) appears adequate for stable NF operation. Conclusions The collective results, showing excellent removal of bulk and trace organics by SAT and also stable performance of the NF over the 6-month pilot (after SAT), demonstrate that conventional pretreatment methods typically used for RO-based potable reuse applications (MF and chloramination) can be replaced with short-term SAT, which provides both excellent membrane fouling control and significant water quality benefits. In addition, the use of NF can provide substantial TDS reduction, adequate to meet Tucson Water s goals. The use of SAT and NF can significantly reduce capital and annual operating costs. Given these significant performance and cost benefits, these processes in the alternative treatment scheme warrant serious consideration for future potable reuse projects, especially those located at inland locations where disposal of RO concentrate is costly and environmentally challenging. References 1. Drewes, J.E., Dickenson, E., Snyder, S., (2011), WRRF 05-004: Development of Surrogates to Determine Efficacy of Groundwater Recharge Systems for the Removal of Trace Organic Chemicals, WateReuse Research Foundation. 2. Fox et al, (2006), Advances in Soil Aquifer Treatment Research for Sustainable Water Reuse, Water Research Foundation. 3. Schimmoller, L., Angelotti, B. (2011), Achieving Indirect Potable Reuse without Reverse Osmosis: Using GAC-Based Treatment to Improve Sustainability and Reduce Cost, WateReuse Research Foundation Potable Reuse Conference. 4. Snyder et al, (2012), Use of Ozone in Water Reclamation for Contaminant Oxidation, WateReuse Research Foundation. 5. U.S. Environmental Protection Agency (2012), Guidelines for Water Reuse; EPA/600/R-12/618, Office of Water. 8