It’s All Water
IT’S ALL WATER: DEMONSTRATION OF AN INNOVATIVE TREATMENT TECHNOLOGY FOR WATER BANKING IN NEVADA
Vijay Sundaram, P.E. and John P. Enloe, P.E.
ECO:LOGIC Engineering
Affordable clean water is essential to Nevada’s economy. Nevada averages only 9 inches of rainfall a year, which makes it the driest state. Water shortages forecast for the west and the possibility of extended drought pose serious challenges for Nevada. Potential water shortages can be addressed in the immediate future by insightful management of available freshwater resources, and recycling or banking “once used” water resources (e.g., reuse of municipal effluent).
Reliable Water Sources
In Nevada and other states, an under-utilized water resource is municipal effluent. Adequately treated municipal effluent, or reclaimed water, is a reliable, high quality, drought-proof water resource. A critical question is how this resource could be saved (i.e. “banked”) for future use. Two reclaimed water banking options are 1) storage in open reservoirs, and 2) storage in a local aquifer (i.e. subsurface storage in the natural groundwater aquifer). Of the two, subsurface storage is believed to be superior because: 1) water quality in open reservoirs deteriorates because of algae growth and wildlife use, 2) water is lost from open reservoirs by evaporation thereby increasing salinity, 3) costs associated with open reservoirs are dependent on land topography and availability, and 4) subsurface storage provides additional soil-aquifer treatment of the water as demonstrated in Arizona. Although current Nevada regulations allow for recharge of reclaimed water, to our knowledge, this practice has yet to be permitted and implemented within the state.
Benefits of Water Banking
Subsurface storage/banking of highly treated reclaimed water provides a unique and viable opportunity to: 1) augment traditional freshwater supplies, and 2) mitigate drought-related water supply and groundwater over-drafting problems. With subsurface storage, highly treated reclaimed water is allowed to percolate or is recharged directly into an aquifer at times when all of the water is not needed for other reuse purposes. The recharged water is later pumped from the aquifer when the demand dictates. Furthermore, many areas throughout the world and the United States have gained public and regulatory acceptance to use banked reclaimed water for potable use.
Throughout the water industry, a broad realization is developing that reclaimed water is not limited to one product or one type of use. Reclaimed water is a resource that can satisfy multiple purposes where the water quality is tailored to the specific use.
Treatment Options
Subsurface storage of reclaimed water has public health, safety, and perception concerns. To overcome these concerns, virtually all microconstituents or contaminants of emerging concern (CECs), such as endocrine disrupting chemicals (EDCs), and pharmaceuticals and personal care products (PPCPs), must be removed from the water prior to storage. Removal of some salinity may also be necessary at some point in time to prevent build-up of salinity in the overall groundwater resource.
Over the past several years, the City of Reno evaluated water banking options suitable for the greater Reno area. Soil Aquifer Treatment (SAT) can remove CECs very cost effectively under some soil and groundwater conditions as observed in Arizona. Initial hydrogeologic evaluation of soil conditions in areas north of Reno showed that SAT utilizing rapid infiltration basins will be more challenging than in Arizona. With these constraints on SAT, the other option that has been considered as the “Gold Standard” for direct recharge water banking projects (as practiced in Southern California, for example) is utilization of reverse osmosis (RO) with ancillary facilities. RO is most suitable for coastal communities, where the concentrate stream containing the salts and CECs removed by the RO process can be discharged to the ocean with little to no treatment, at least historically. An inland RO treatment process would typically include RO plus concentrate waste handling steps. Handling and disposal of concentrate waste requires immense energy and capital cost in addition to those associated with the RO process and its ancillary facilities.
Reno-Stead WRF Demonstration Project
To facilitate subsurface storage of reclaimed water without the cost of RO, the City of Reno and ECO;LOGIC Engineering investigated the feasibility of alternative treatment technologies based on the following principles:
- Refractory organics (EDCs, PPCPs, etc.) should be destroyed, not merely removed as a concentrate in need of specialized treatment and/or disposal.
- Because of RO’s high cost and energy requirements, RO should be used only for salt removal, and only when needed. (Salt removal is not required for Reno at this time because the average effluent TDS is only 350 mg/L).
- Salinity and corrosivity of the water produced for subsurface storage should be maintained at a safe level to prevent excessive leaching of subsurface constituents (e.g. arsenic) into the stored water.
- The technology should be suitable for interior communities without access to ocean outfalls.
Based on these principles, a treatment train consisting of Ozone and Biologically Active Carbon (BAC) was developed and demonstrated as a cost effective alternative to RO. The development and demonstration were accomplished over 2 years using a 10.7 gpm continuous flow pilot scale treatment process operated at the Reno-Stead Water Reclamation Facility (RSWRF) consisting of:
- Membrane Filtration (MF) or Sand Filtration to remove particulates from the wastewater.
- Ozonation (Ozone) to oxidize most refractory organics into more readily biodegradable organic compounds. This treatment step included peroxide and/or ammonia addition to control formation of bromate (a well known ozonation byproduct).
- BAC to remove the biodegradable organic byproducts of ozonation, and to remove some of the refractory organics resistant to oxidation (e.g., flame retardants).
- In full scale process, a low energy UV disinfection system would be installed to provide final disinfection. As the performance of UV disinfection is well documented, a UV system was not included in the demonstration.
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| Schematic of the Innovative Treatment Technology |
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Membrane Filtration Unit Used in the Ozone-BAC Demonstration (Supplied by WesTech) |
Ozonation Unit Used in the Ozone-BAC Demonstration (Supplied by APTWater) |
Ozonation Benefits and Its Byproducts
The ozonation process is effective in improving various critical aspects of water quality including: 1) reducing CEC concentrations and overall estrogenic activity (as measured by bioassay using human breast cells), 2) providing disinfection, 3) improving the UV transmittance (UVT) of the water, 4) increasing dissolved oxygen concentration, and 5) eliminating colorants and odor causing compounds present in the water. Formation of ozonation byproducts is a critical concern. Bromate formation is of special concern (particularly when its precursor bromide is present in concentrations in excess of 50 g/L) because bromate has a drinking water Maximum Contaminant Level (MCL) of 10 g/L. Ozone dose relative to CEC removal and bromate formation was optimized by testing the effects of three ozone dosages (3, 5, and 7 mg/L). Optimization results showed that at least 5 mg/L of ozone is required for desired CEC and estrogenic activity reductions. Bromate levels higher than the MCL were observed at ozone doses of 5 and 7 mg/L. Addition of peroxide (year-round) and ammonia (seasonally; during hot weather conditions) was found to be effective in reducing the levels of bromate to well below the MCL. The extent of CEC removal remained unchanged after peroxide and ammonia addition.
BAC and Its Benefits
BAC treatment, a type of biofiltration process, is a polishing and stabilizing step for removal of organic and inorganic contaminants, including ozonation byproducts. BAC reduces the biofilm growth potential of the water that, otherwise, can foul effluent injection well screens and gravel packs when direct recharge is practiced. BAC is an ideal polishing treatment process downstream of ozonation because it utilizes the high dissolved oxygen concentrations in the ozonated water to eliminate the need for regular regeneration of the activated carbon filter media. The BAC filter at Reno provided 30 minutes of contact time, and was backwashed once every 1-2 weeks. It took about 60 days for the BAC biomass to stabilize, and several months for the microbial population making up that biomass to stabilize. The BAC process development protocol was validated by comparing the RSWRF biomass and population results with results from a somewhat similar full-scale BAC unit in El Paso, Texas.
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Biological Activated Carbon (BAC) Unit Used in the Ozone-BAC Demonstration Project at RSWRF (Supplied by WesTech) |
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Chemicals of Emerging Concern (CEC) Sampling (by City and ECO:LOGIC Staff) |
Monitoring Program during Demonstration
A total of 490 constituents were quantified three times before and after each treatment step in the RSWRF demonstration project. This list included constituents in the California’s draft groundwater recharge regulations (CA recharge list), drinking water contaminants, priority pollutants, CECs, and pathogens. To our knowledge, Reno’s demonstration project was the first and only non-RO project to monitor all of the contaminants specified in CA recharge list. The project was in operation for 21 months.
To facilitate overall process monitoring and control, the City purchased various online sensors that were connected to the RSWRF SCADA system. Benefits of this decision included 1) reduced daily monitoring requirements, 2) generation of a comprehensive dataset that captured system performance during seasonal and diurnal variations, and 3) hands-on experience for the RSWRF plant staff in dealing with cutting edge instruments and advanced treatment technologies.
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UV Transmittance (UVT) Measured after each Treatment Step during the Ozone-BAC Demonstration Project at RSWRF |
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Emission Excitation Matrices Denoting Removal of Organic Matter after each Treatment Process (Courtesy of Southern Nevada Water Authority) |
Ozone-BAC Treatment Efficacy
Based on three sampling campaigns conducted over 10 months, Ozone-BAC appears to be equally effective as RO in the removal of CECs. The ozone step is very effective in removing a wide range of CECs except for compounds that resist oxidation (e.g., flame retardants). BAC is effective in consistently removing: 1) compounds that escaped ozone oxidation, and 2) biodegradable organic byproducts of ozonation. Mechanisms attributed to CEC removal (especially flame retardant removal) in BAC are still under investigation in collaboration with University of California at Davis and University of Nevada at Reno. Estrogenic activity was completely removed after ozone treatment. UV transmittance (UVT) of BAC effluent was consistently above 90%, which is similar to the UVT observed in RO effluent.
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Percent of Target Chemicals of Emerging Concern (CECs) Removed (at least 70% from secondary effluent) by Various Processes |
Power and Cost
For CEC removal, Ozone-BAC requires roughly one-third of inland RO capital costs, and consumes one-fourth of the inland RO energy requirements.
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| Annual Energy Cost Per MGD based on Unit Power Cost of $0.14/kWh |
Salinity Concerns
The main water quality difference between RO and Ozone-BAC is that RO removes salts, whereas Ozone-BAC does not. The average salinity of Reno reclaimed water is 350 mg/L; therefore, salt reduction and partial RO treatment are not needed at this time. Removing CECs with RO also results in substantial reduction of salinity, which makes RO water quality very corrosive. If this corrosivity is not controlled in some way, the RO water injected into groundwater increases the probability that naturally occurring metals in subsurface soils, such as arsenic, will leach into the injected effluent and the groundwater resource.
Conclusions
Subsurface storage of reclaimed water can augment existing freshwater resources, provide high quality water for people, and promote a healthy economy and a healthy environment. Implementing reclaimed water banking in Nevada must include demonstration of 1) safe and reliable water quality to the satisfaction of Nevada Division of Environmental Protection (NDEP), 2) affordability, and 3) public acceptance. To date, most subsurface storage projects involve reverse osmosis treatment. Ozone-BAC is equally effective in removing CECs when compared to RO. The synergistic role of BAC in stabilizing ozonated water is clearly demonstrated in the study. With Ozone-BAC technology, the bulk of CECs are destroyed at one-third the cost and using one-fourth the power when compared to inland RO, whereby the CECs are concentrated in a waste stream needing specialized treatment and/or disposal. Results from Reno’s cutting edge research and demonstration study indicate that water banking is more affordable than previously thought, pending public and regulatory acceptance.
Next steps
The next steps are addressing public and regulatory concerns so as to gain acceptance. Findings from the demonstration project will be presented to the NDEP. In addition to addressing comments from NDEP, the mechanisms involved in the removal of CECs in the BAC will be studied. Demonstration of a small-scale subsurface storage operation to observe the role of Mother Nature in further improving water quality (a form of SAT as an enhancement to Ozone-BAC) is being proposed.
Project Team
The City of Reno’s manager for this project was Stanley E. Shumaker, P.E. ECO:LOGIC’s principal investigator was Robert W. Emerick, Ph.D., P.E., The City’s team consisted of Michael A. Drinkwater, P.E., Stephen Long, Grade IV, and Scott Nelson, Grade IV. ECO:LOGIC’s team consisted of Vijay Sundaram, P.E., John P. Enloe, P.E., Richard E. Stowell, Ph.D., P.E., Michael J. Harrison, P.E. and Steven L. Beck, P.E. Southern Nevada Water Authority’s team consisted of Shane A. Snyder, Ph.D. and Dan Gerrity, Ph.D. Efforts regarding continuation of the demonstration are guided by Jeffry Curtis, Ph.D., Acting Executive Director, Center for Water Energy Efficiency, University of California, Davis.
Authors
Vijay Sundaram, P.E. - Research and Design Engineer
Vijay has over 9 years of research and design experience focused on advanced wastewater treatment processes. He specializes in wastewater treatment process development, process design, pilot testing, and cost-benefit analysis. Since he joined ECO:LOGIC’s team, Vijay has presented and authored several ozone related technical papers. Vijay received his MS degree in Environmental Engineering from the University of Cincinnati and BS degree in Chemical Engineering from the University of Madras, India. Vijay is a registered Civil Engineer in the States of California and Nevada.
Vijay Sundaram, P.E. sundaram@ecologic-eng.com
Research and Design Engineer
ECO:LOGIC Engineering
3875 Atherton Road
Rocklin CA 95765
Phone: (916) 773-8100
Fax: (916) 773-8448
www.ecologic-eng.com
John Enloe, P.E. - Principal Engineer
John Enloe has over 25 years of water planning, permitting, design and project implementation experience in both the public and private sector, including five years as Manager of Water Planning and Engineering for Sierra Pacific Power Company in Reno, Nevada. In 1997, John started the ECO:LOGIC Reno office and is now a working project manager and principal of the firm. For many years, John has been actively involved in regional water supply, planning and policy issues affecting Reno and the surrounding area. Recently, John was in responsible charge of the implementation of the Fish Springs Water Supply Project, an 8,000 acre- foot groundwater importation project for areas north of Reno. Currently, he is serving as the lead technical advisor to Reno, Sparks, Washoe County, SVGID and TMWA on a collaborative effort to evaluate the merits of regionalization of the areas’ reclaimed water systems.
John P. Enloe, P.E. enloe@ecologic-eng.com
Principal
ECO:LOGIC Engineering
10381 Double R Boulevard
Reno, NV 89521
Phone: (775) 827-2311
Fax: (775) 827-2316
www.ecologic-eng.com