A NANOFILTRATION MEMBRANE FOR THE REMOVAL OF COLOR AND DISINFECTION BYPRODUCTS FROM SURFACE WATER TO MEET STATE AND FEDERAL STANDARDS

Transcription

1 A NANOFILTRATION MEMBRANE FOR THE REMOVAL OF COLOR AND DISINFECTION BYPRODUCTS FROM SURFACE WATER TO MEET STATE AND FEDERAL STANDARDS Rich N. Franks, Hydranautics/Nitto Denko, 401 Jones Rd., Oceanside, CA, Jones Rd. rfranks@hydranautics.com, Phone : Jonathan Van Bourg - Consultant for the Inverness Public Utility District Scott McMorrow - General Manager for the Inverness Public Utility District Craig R. Bartels, Hydranautics/Nitto Denko, Oceanside, CA, Abstract The water supply in a small town in Northern California contains disinfection byproducts as well as high levels of color. The water supply comes from seven small diversions located on year round streams in the pristine watershed of three valleys above the town. The water has a low total dissolved solids (TDS) concentration of less than 100 mg/l and low hardness of less than 10 mg/l. The water supply has had periodic problems with high disinfection byproducts, specifically total trihalomethane and halo-acidic acids, (TTHM's and HAA5's). In response to the water quality issues, the town pilot tested a nanofiltration unit (NF unit) with a Sulfonated Polyethersulfone HYDRACoRe 50 membrane to remove color and TOC following microfilter and ultrafilter membrane pretreatment. The NF operated at an average feed pressure of 75 psi. Both field tests and lab results have confirmed that the NF unit achieved the intended targets. During and after periods of high color associated with prolonged rainfall events, the TOC removal and color removal have been 100% under the most stressed conditions available during the pilot test, (High color, high volume, peak pressure and minimum waste.) TTHM s were in the range of 6-12 ppb in the permeate and HAA5 s are in the range or ppb in the permeate. As expected, the NF removed little to no salts. Bicarbonate alkalinity was slightly reduced by 8.8%. Total Dissolved Solids (TDS) was reduced by only 1.2%. After five months of pilot testing and lab testing, the use of a nanofiltration process to remove color, in combination with a new ultra filtration process to remove particulates, has proven to be a cost effective, efficient and functional treatment process for removing disinfection byproduct precursors and for meeting current State and Federal drinking water standards. Based on pilot testing, the customer proceeded with the construction of a full scale NF plant while also replacing the existing, 20 year old MF units with new UF units. This paper will discuss the various alternatives that were considered for reducing DPBs and color. The paper will also present the lessons learned in pilot testing, including the effect of heavy rainfall, on membrane performance. The application of pilot testing experience to the design of the full scale NF plant, will also be discussed. The paper will present the successful startup of both the NF and new UF full scale plant 1

2 Introduction The town of Inverness, California, just north of San Francisco, has a water department that supplies water to approximately 500, predominantly residential services. The source water for Inverness comes from a heavily forested area that has been recently impacted by sudden oak death. Most of the tanbark oak in the watershed is dead or dying and this may contribute to a recent increase of color in the water. During and after periods of heavy rainfall the color of the water runs dark with humic material. There is little that the IPUD can do to impact the natural cycle of the forest. The treatment plants now shut off automatically when the turbidity of the raw water increases during heavy rainfall, which helps to moderate the problem. It might also help if IPUD were to automatically draw on the well/s during periods of heavy rainfall however highly colored water remains a problem long after the rains stop. The Inverness Public Utility District (IPUD) has had periodic problems with high disinfection byproducts in the water and has periodically been out of compliance with State and Federal maximum contaminant levels during their quarterly sampling for disinfection byproducts (DBP's), specifically total trihalomethane and halo-acidic acids (TTHM's and HAA5's). The violations have necessitated costly public notification and may eventually result in fines unless a solution to the problem is found. Feed Source and Permeate Quality Requirements The water supply for the IPUD is mostly from seven small diversions, located on year round streams in the pristine watershed of three valleys above the town. The watershed is entirely in State park, National park or Inverness Public Utility District (IPUD) land and is heavily forested with bay, bishop pine, tanbark oak and ceanothus. Transmission mains from the streams feed directly into two water treatment plants. There are also a few well sources that feed into the treatment plants and are used during the dry, summer season to augment water demand. The treatment process used a twenty-year-old, US Filter/Memcor, micro-filtration (MF) treatment technology to meet the requirements of the surface water treatment rules. Free chlorine is used for disinfection. The treatment plants feed directly into storage tanks that are strategically located throughout the hilly community. There is approximately 425,000 gallons of water storage capacity in three steel tanks and eight aging redwood tanks. The distribution system is composed mostly of PVC pipe, transite pipe, and some galvanized steel pipe. The raw water quality is low in alkalinity with a ph of so the water is mildly corrosive and what steel pipe remains is old and in poor condition. TTHM's average 78.3 ppb and peak at 242 ppb, HAA5's average 40.5 ppb and peak at 129 ppb. The actual maximum contaminant level required in the disinfection byproduct rule is an annual locational running average of 80 ppb for TTHM's and 60 ppb for HAA5's. The "theoretical" working maximum contaminant level goal for achieving a safe margin of error is the same. The peak levels of disinfection byproducts in the Inverness distribution system have a correlation with periods of heavy rainfall and are associated with high dissolved color in the water. The disinfection byproducts occur because the micro-filtration process can remove all of the particulate organic material from the water but not the dissolved organics which cause color, taste and odor. When these organics combine with free chlorine they can form harmful disinfection byproducts (DBPs) over time. Figure 1 below shows the historical TTHM and 2

3 HAAs data collected from two of the Inverness diversions over a period of almost four years. The MCL for both TTHM and HAA are graphed to show where these levels were exceeded. Figure 1. Results of historical TTHM and HAA sampling from two of the diversions in the Inverness Public Utilities District. Alternative Technologies Considered To address the problem of DBP and achieve less than 80 ppb for TTHMs and 60 ppb for HAA5s, several options were considered, including the use of activated carbon, ozone, or chloramines. Ozone was quickly ruled out because of high levels of bromine in Inverness water. Ozone combines with bromine to produce bromate which is yet another harmful disinfection byproduct. Chloramines were also considered. Chloramines would provide substantial cost savings and eliminate the need to re-chlorinate the water. However, potential problems with using chloramines, included: 1. Converting to the use of chloramines would require a large clear well or contact area at treatment plants where space was limited. 2. There was an issue with complaints from a concerned public, chloramines kill fish and can cause serious problems for dialysis patients. 3. Chloramination cannot be done in part, it would have to be done at all sources at the same time. 4. Chloramination is more corrosive and will require the use of some corrosion inhibitor. 5. Chloramines have a tendency to produce nitrosamines in the distribution system and might require much more circulation in the tanks and flushing at the end of the lines; particularly during the summer months when we can least afford the wasted water. 6. Chloramines also produce disinfection byproducts, (like NDMA) these DPB s are not currently a regulatory issue for small water districts but that may change. 3

4 7. Because chloramines involve mixing two hazardous chemicals, the use of chloramines would most likely be a safety concern and the use of chloramines would likely increase licensing requirements. 8. Chloramines are effective at lowering TTHM s but we are not convinced that they would be effective at lowering HAA5 s enough in our system. For all of the above reasons were reluctant to pilot test or recommend the use of chloramines at IPUD. Both granular activated carbon (GAC) and powder activated carbon (PAC) pilot studies were conducted. While running the GAC pilot test it was evident that spikes in total organic carbon (TOC) occurred consistently during and after rainfall events. The TOC was always associated with high color in the water and most of the color was in the form of dissolved organic carbon (DOC), which is not removed by the MF filters. The GAC was initially able to remove the DOC from the water when it was first installed, but the effectiveness dropped off rapidly over time. The GAC treatment process was helpful in improving water quality but ineffective as a sustainable solution to the DBP problem. GAC was also found to be costly, messy and labor intensive. IPUD also briefly pilot tested powdered activated carbon (PAC) to waste, but it took as long as GAC, or longer, to remove the color. PAC could not be used prior to the MF filters due to potential for membrane damage. Chosen Technology - NF Membrane Nanofiltration membranes were also considered due to relative ease of operation, small footprint, minimal chemical requirements, and effectiveness at removing DBPs. Specifically, the HYDRACoRe 50 membrane was considered due to its high rejection of color and large organics relative to its high passage of dissolved salts. The HYDRACoRe membrane consists of a sulfonated polyether sulfone polymer with a typical thickness of 0.3 μm. The surface charge of the HYDRACoRe membrane is strongly negative due to the presence of the sulfonate functional groups. Streaming potential measurements (Figure 2) show the anionic HYDRACoRe to have a constant surface zeta potential of -85 mv over a ph range of 3 to 11. In contrast, the conventional amphoteric polyamide RO membrane (CPA2) varies in charge from +10mV at ph 3 to -20mV above ph 6. The strong negative charge of the HYDRACoRe can be advantageous in that it will repel negatively charged organics present in certain waters and thus minimize membrane fouling by organic adsorption. 4

5 Figure 2. Surface Zeta potential measurement for typical polyamide membranes and the HYDRACoRe membrane. cationic RO memb.(lfc2) Surface Zeta Potential [mv] 0-50 neutral RO memb.(lfc1) amphoteric RO memb. (CPA2) anionic NF memb.( HYDRACoRe ) ph Another important characteristic of the HYDRACoRe membrane is its smooth surface relative to a typical polyamide membrane surface. Figure 3 below compares a scanning electron microscope photo of a polyamide membrane with that of the HYDRACoRe membrane. The surface roughness for the polyamide is clearly greater than that of the HYDRACoRe. The smooth surface can be advantageous in that by reducing the potential for colloidal fouling and biofouling by reducing the number of sites for the deposition of colloids or microbial cells. Figure 3. Characterization of surface roughness of a) a typical polyamide membrane b) the HYDRACoRe membrane. Smooth Surface Polyamide Membrane HydraCoRe Membrane (a) (b) Another advantage of the HYDRACoRe is its greater stability toward ph and chlorine compared to conventional polyamide membranes. Chlorine is especially harmful to polyamide membranes at concentrations above 0.01 ppm due to the hydrolysis of the polyamide which leads to an 5

6 increase in salt passage. A general rule is that the salt passage of a polyamide membrane will double after an exposure of 2000 ppm-hours of free chlorine. As a result, even low doses of free chlorine cannot be used to control or clean biogrowth on polyamide membranes. Though not as severe, chlorine can have a detrimental effect on cellulose acetate membrane as well. In contrast to the polyamide and cellulose acetate membranes, the HYDRACoRe is tolerant to chlorine. Figure 4 demonstrates the chlorine tolerance of the HYDRACoRe relative to the cellulose acetate membrane. A sample of HYDRACoRe membrane was soaked in a 1000 ppm sodium hypochlorite solution. After 50 days (1,200,000 ppm hours) the HYDRACoRe membrane maintained a stable sodium chloride rejection. In contrast, a cellulose acetate (CA) membrane was exposed to a 100 ppm sodium hypochlorite solution for 10 days (24,000 ppmhours) and showed a doubling in salt passage. Thus, the HYDRACoRe is ideally suited for low doses of chlorine to control biofouling and higher doses to enhance the removal of organic foulants. Figure 4. Chlorine tolerance of the HYDRACoRe membrane as compared to cellulose acetate membrane. Retention Rate of Rej. [%] HYDRACoRe memb. [1000ppm] CA memb. [100ppm] Immersing Period [day] NF Design and Operation To study the use of HYDRACoRe membrane for the removal of color and DBP, a pilot unit with a HydraCoRe element operated on filtrate from an MF unit. The pilot consisted of a single, 4 inch, spiral membrane operating with recirculation to simulate the high water recovery of 90%. Based on the successful completion of the pilot study, two full scale, 50 gpm, system were designed and commissioned in October, The system operates at a flux of 15 gfd and a recovery of 95%. The system is designed as two stages with a single vessel in each stage. Each vessel holds six spiral HYDRACoRe elements in series. To achieve the 95% recovery, they system operates with 10 gpm recycled from the concentrate back to the feed. The small, 2.6 gpm concentrate stream is sent to evaporative ponds. The system operates at 160 psi with 60 psi of permeate pressure to send the permeate to the distribution tanks. 6

7 During and after periods of prolonged rainfall and associated high color, the TOC removal and color removal was 100% under the most stressed conditions available during the pilot test, (High color, high volume, peak pressure and minimum waste). Figure 5 compares quality of the raw inlet with the quality of the different process streams in the system. Samples were collected following 0.5 inches of rainfall. After five months of trial, the TOC removal by the NF is 100%. TTHM & HAA5 test results remain well below MCL. TTHM s were in the range of 6.0 and 1.2 ppb and HAA5 s were in the range of 22.0 and 1.5 ppb. Quarterly compliance results are between 10.4 and 7.7 ppb for TTHM s and between 10 and 5 ppb for HAA5 s. Figure 6 compares feed and permeate TTHM and HAA results for specific feed and permeate samples collected during pilot testing. Figure 5. Quality of the raw inlet compared to quality of the different process streams in the UF + NF system. Samples were collected following 0.5 inches of rainfall. Raw 1.37 NTU 76 color units TOC = 2.16 ppm UF Filtrate 0.07 NTU 33 color units TOC = 2.12 ppm NF Permeate NTU 0.0 color units TOC = 0.0 ppm NF Concentrate 2.17 NTU 350 color units TOC = 26.8 ppm Figure 6. Feed and permeate TTHM and HAA results for specific feed and permeate samples collected during pilot testing. 7

8 Due to reduction in organics by the NF membranes, chlorine demand has dropped more than 50%. Re-chlorination in the water tanks was turned off while still maintaining a safe residual at the end of the line. Critically, the DBP s in the distribution system continue to trend lower. Bicarbonate alkalinity is only slightly reduced by the NF, (only 8.8%), Total Dissolved Solids (TDS) was reduced by only 1.2% and Langlier index was reduced by 5.8%. The Langlier Index is quite low (-3.08) which made the water more aggressive, but not significantly worse than it was before (-2.87). An ion analysis of the feed, perm, and concentrate is shown in Table 1. The low ion rejection and high organic rejection is evident from this analysis. The low ion rejection results in very little concentration of salts in the concentrate stream. This, in turn, allows the HYDRACoRe system to be run at a very high recovery. Whereas must RO system run at 75% to 85% recovery, the HYDRACoRe system was designed for 95% recovery. Table 1. Ion analysis of the feed, perm, and concentrate of the HYDRACoRe NF system during normal operation. Sample ID Na Ca Mg K Cl SO4 Color (Abs) 408nm Raw (Before Recycle) Feed (After Recycle) Concentrate Permeate Rejection (%) Conclusion The municipal water supply in a small Northern California town had intermittent problems with high concentrations of disinfection byproducts (DBPs). Specifically, total trihalomethane and halo-acidic acids, (TTHM's and HAA5's), were periodically exceeding their maximum contaminant levels of 80 ppb and 60 ppb respectively. In response to the water quality issues, several options were considered including: activated carbon, ozone, chloramines, and nanofiltration membranes. The nanofiltration membrane option was selected due to its relative ease of operation, small footprint, minimal chemical requirements, and effectiveness at removing DBPs. A six month pilot study was conducted using a Sulfonated Polyethersulfone HydraCoRe 50 membrane to remove color and organics. Based on the success of the pilot study, two 50 gpm systems, designed for 95% recovery, were installed and have been performing stably since commissioning in October, Disinfection byproduct concentrations have been consistently reduced below MCL. For example, in a specific sampling, TTHM were reduced from a 110 ppb to 20 ppb and HAA5 concentrations were reduced from 90 ppb to below 10 ppb. 8