Energy Considerations in Membrane Treatment and Brine Disposal

Energy Considerations in Membrane Treatment and Brine Disposal Energy Requirements Membrane treatment systems require significant energy inputs. Therefore, energy consumption is one of the major cost considerations that must be addressed in determining the feasibility of membrane treatment for municipal raw water supplies. Table AJ.1 contains hypothetical information about energy requirements for a two stage reverse osmosis (RO) membrane treatment process followed by brine concentration. Stage I of the RO treatment process is conventional, low pressure membrane treatment for freshwater, with a total dissolved solids (TDS) concentration of 700 milligrams per liter (mg/l), which happens to be the approximate TDS concentration of the South Platte River in the lower portion of Segment 15 near Brighton, Colorado. Stage II of the treatment is aimed at recovering a low TDS permeate from the stage I brine concentrate stream. The membranes used in the second stage are designed for treating brackish water, which is similar in TDS concentration to stage I RO brine concentrate streams. This treatment train represents one example of a relatively new approach to municipal RO system design that may be feasible in cases where there are significant constraints on brine discharges, because of a lack of assimilative capacity in the receiving stream. Table AJ.1. Energy Requirements for RO Treatment and Brine Concentration Treatment System Plant Capacity (MGD) Treatment Stages Design Parameters 1 3 6 12 25 Raw Water [TDS] mg/l 700 700 700 700 700 Stage I RO Raw Water Pump rate (gpm) 694 2083 4167 8333 17361 Permeate Recovery rate 0.85 0.85 0.85 0.85 0.85 Permeate Produced (MGD) 0.85 2.55 5.1 10.2 21.25 Permeate [TDS] mg/l 35 35 35 35 35 Brine Produced (MGD) 0.15 0.45 0.90 1.80 3.75 Brine [TDS] mg/l 4865 4865 4865 4865 4865 Energy Req'd (kwh/day) 1106 3318 6636 13272 24885 Stage II RO Pumping rate (gpm) 104 313 625 1250 2604 Recovery rate 0.8 0.8 0.8 0.8 0.8 Permeate Produced (MGD) 0.12 0.36 0.72 1.44 3.0 Permeate [TDS] mg/l 243 243 243 243 243 Brine Produced (MGD) 0.03 0.09 0.18 0.36 0.75 Brine [TDS] mg/l 30000 30000 30000 30000 30000 Energy Req'd (kwh/day) 7687 23060 46120 92240 192168 Stage III Brine Concentration Flow rate (gpm) 21 63 125 250 521 Distillate Recovery Rate 0.95 0.95 0.95 0.95 0.95 Distillate Produced (MGD) 0.029 0.086 0.171 0.342 0.713 Distillate [TDS] (mg/l) 10 10 10 10 10 Brine Concentrate (gal/day) 1500 4500 9000 18000 37500 Concentrator Capacity (gpm) 21 63 125 250 521 Energy Req'd (kwh/day) 2700 8100 16200 32400 67500 Totals Total Energy Req'd mwh 11.5 34 69 138 285 Total Water Recovered (MGD) 0.999 2.996 5.991 11.982 24.963 J-1

The information in Table AJ.1 is provided to illustrate the relative energy requirements for RO treatment and brine concentration for different sized systems aimed at substantially achieving zero liquid discharge (ZLD). As Dr. Phil Brandhuber of HDR Consultants has stated, with RO systems moving toward zero liquid discharge, it is not a problem of treatment, it is rather, a problem of energy. In the examples set forth in Table 1, stage I of the RO treatment process is conventional, low pressure membrane treatment for raw source water, with a TDS concentration of 700 mg/l. The pressure pumps for these membranes require approximately 1.6 kilowatt hour (kwh) per 1,000 gallons (gal) of water treated. Energy requirements may be estimated using the following empirical formula: 1.58 kwh/1,000 gal/1,000 mg/l TDS. The freshwater membranes are operated with a permeate recovery rate of 85% (i.e., 15% of the treated raw water is rejected as concentrate residual). Stage II of the treatment is aimed at recovering a low TDS permeate from the stage I brine stream. The membranes used in the second stage are designed for treating brackish water, or stage I RO brine streams, as the case may be. Accordingly, they require higher pressures and more energy than the stage I membranes, up to 7.7 kwh/1,000 gallons treated (2.0 kwh/m 3 ). Stage III of the treatment process involves brine concentration. Brine concentrators have been used historically to convert highly saturated industrial wastewaters into distilled water for reuse as boiler makeup, NOx control, cooling tower makeup, and plant processes requiring pure water. Presently, brine concentrators are viewed as key components in ZLD treatment strategies for public water systems relying upon RO treatment. Brine concentrators, such as the GE Infrastructure Water and Process Technologies (Ionics) System, are seeded-slurry, falling-film evaporators that are typically operated by mechanical vapor recompression. They can also run on plant steam or low-pressure turbine exhaust steam, which has importance in the context of renewable energy solutions for the brine concentration process. Commercially available brine concentrators are capable of treating up to 1,000 gallons per minute (gpm) per minute or as little as 5 gpm of highly concentrated brine. In the vapor compression configuration, a brine concentrator uses 25 to 37 BTU per pound of wastewater feed. This converts to 60 to 90 kwh per 1,000 gallons of feed, which is 30 times more efficient than conventional single effect steam-driven evaporators. The two stage membrane treatment system dramatically reduces the amount of brine concentrate that must be treated by a brine concentrator for a system of any given size. As shown in Table AJ.1, even a 25 mgd two-stage RO system would generate only 0.75 MGD of brine concentrate. This is equivalent to 520 gpm, which could be treated readily by standard off-the-shelf brine concentration equipment. With a typical brine concentrator, 95% to 99% of brine wastewater can be recovered as high purity distillate (distilled water with <10 mg/l TDS). The remaining 1% to 5% concentrated slurry can then be sent to an evaporation pond prior to removal to a monofill, or other solid waste facility. In some cases (i.e., markets), it may be economically feasible to recover mineral resources directly from the brine concentrate, through the use of a crystallizer. As shown in Table AJ.1, a 1 MGD facility would generate only 1,500 gpd of concentrated slurry for evaporation, while a 25 MGD RO facility would generate 37,500 gpd for evaporation and subsequent transport. In the area surrounding Denver, it is estimated that J-2

one acre of evaporation pond area is required per gpm of concentrated slurry for complete passive evaporation. The raw water supply that serves as the case example has a relatively low TDS concentration as is found in the South Platte River, just downstream of Denver. Water providers in this area could consider employing membrane treatment to remove specific pollutants, such as nitrate, in addition to lowering the TDS content of the raw water supply. The different power requirements shown for different sized systems are based on a linear relationship keyed to treatment capacity. Energy requirements for membrane treatment systems are often presented in terms of kwh/1,000 gallons of water treated. Reduction in energy requirements for membrane treatment and brine concentration would provide a means for making membrane treatment more cost-competitive with conventional water treatment technologies. Energy Recovery Systems According to Murray Thomson of the Centre for Renewable Energy Systems (CREST), designers of medium to large seawater RO systems have led the way in developing energy recovery approaches for RO systems. Energy recovery systems take advantage of the residual pressure in the brine waste stream to reduce the total energy needed to power the high pressure RO pumps required for desalinization. Existing energy recovery systems can be divided into two groups: pressure exchangers that transfer the brine pressure directly to feed; and devices that transfer brine pressure to mechanical power, such as pelton turbines and back running pumps. The main difference between groups is the flow pumped by the high pressure pump. In the pelton turbine group the high pressure pump pumps the entire feed flow, where as in the pressure exchangers group the high pressure pumps pump only part of the feed flow equal to the product flow. Name brand examples of pressure exchangers include DWEER or dual work exchanger energy recovery systems, which were developed in Switzerland, and American made ERI systems. The efficiency of energy recovery in the pressure exchangers group is around 96%. The energy transfer in the pelton group is indirect with the brine jet hitting the turbine buckets. The efficiency of the pelton turbine itself is around 87%. The equipment and maintenance costs in the pressure exchanger group are higher than in the pelton group. The decision to implement one or another energy recovery system depends on several factors such as energy cost, project lifetime and interest rates. The Pelton turbine, DWEER and the ERI devices, and their similar competitors, help to achieve very low energy consumptions, sometimes approaching 7.6 kwh per 1,000 gallons (2 kwh/m3) for medium to large sized seawater RO systems. This is comparable to the amount of energy required for brackish water RO systems without energy recovery technology. According to Dr. Boris Liberman of IDE Technologies Ltd., in a paper entitled The importance of energy recovery devices in RO desalination even though freshwater and brackish water RO systems require less energy to operate than seawater RO systems, it is likely that energy recovery systems will be incorporated into these systems increasingly, as energy prices continue to climb. Presently, many small systems are built without incorporating any energy recovery technology. Renewable Energy in Membrane Treatment Applications Renewable energy sources have been used and will continue to be used either directly or indirectly in water and wastewater treatment. Solar energy is the simplest technology for J-3

desalination of high TDS waters and for water disinfection. Solar energy can be converted into steam (or hot water) to produce mechanical energy or into electricity which can be used to power pumps, ultraviolet systems, RO and conventional surface water treatment systems. According to the report entitled Renewable Energy in Water and Wastewater Treatment Applications prepared by the National Renewable Energy Laboratory (NREL), renewable energy sources, unlike conventional fossil fuel-based power sources, are mostly used for small to medium water utility applications because of their high initial investment costs. The power needed to treat a small municipal supply is relatively modest. For this reason, renewable energy power sources are widely used by water utilities in many developing countries. Recent engineering advances in the development of concentrating solar power systems have made solar energy a more viable source of energy for RO treatment. Skyfuel is a new company that is involved in the development of municipal utility scale concentrating solarthermal and solar-electric power plants. This company is developing and deploying linear power tower technology for stand alone solar power plants as well as fuelsaver installations at existing power plants. Fossil fuel hybridization is easily incorporated into parabolic trough and power towers. Dr. Arnold Leitner, president of Skyfuel, has indicated that the most efficient way water utilities employing RO technology can utilize solar energy is by way of direct thermal energy, as opposed to solar electric configurations. According to Leitner, concentrating solar systems can already produce thermal energy considerably cheaper than fossil fuel powered systems. In simple terms, the concentrating solar system produces heat which is used to produce hot water or steam which can then be used to power pumps and other devices required for membrane treatment processes, directly. The infrastructure for concentrating solar power systems is not particularly costly. For example, according to Dr. Leitner, an array of linear Fresnel reflectors that would fill up a 1.5 acre site and produce approximately 1 megawatt of thermal power is estimated to cost about $300,000, excluding the cost of the required land area. A linear Fresnel reflector power plant uses a series of carefully angled plane mirrors to focus light onto a linear absorber. These systems may offer lower overall costs because they permit the heat absorbing element to be shared between several mirrors. The mirrors can therefore be smaller and do not require complex pivoting couplings for the fluid flowing from the absorber. The design can also permit mirrors to be placed closer together, allowing for a more efficient use of land area. According to a 2002 NREL report entitled Fuel from the Sky: solar power s potential for western energy supply, another approach for concentrating solar power involves dish designs. A dish system uses a large reflective parabolic dish, similar in shape to satellite television dish. It focuses all the sunlight that strikes the dish up onto a single point above the dish where a thermal collector is used to capture the heat and transform it into a useful form. Dish systems, like power towers can achieve much higher temperatures due to the higher concentration of light which they receive. Typically, the dish is coupled with a stirling engine. That combination is known as a dish/stirling system. Sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator. Of these concentrating solar technologies the solar dish/stirling has the highest energy efficiency. The current record is a conversion efficiency of 40.7% of incident solar energy. Renewable energy offers a means of stabilizing energy costs after the initial capital investment in solar collectors is made. Properly sized concentrating solar applications J-4

should produce energy at relatively level costs over the long-term. This is a significant advantage at a time when costs are rising for petroleum-based fuels, while their carbonbased emissions are becoming increasingly problematic. Solar technology is reliable and essentially emission free over very long operating periods. Equipment installations are designed to remain operational for a period of 40-years, or longer. The German Aerospace Center (DLR), in a 2005 repot entitled Concentrating Solar Power for the Mediterranean Region, determined that the southwestern portion of the United States has some of the best solar resources in the world. This is due to a combination of factors including the latitude, low cloud cover and humidity, and the high altitude of the Colorado plateau. Large areas of the west receive average sunshine of between 6 and 7.5 kilowatt hours per square meter per day. Premium solar resource areas are found in six of the western states, including Colorado. Nevertheless, the sun does not shine year around with the same intensity and during its path across the sky, the sun s intensity changes. In addition, weather conditions such as clouds or haze can change the level of direct normal solar radiation received by the collectors of a solar power plant. The amount of solar energy that a solar power plant can convert to electricity depends on the technology. As noted above, dish/stirling systems produce more energy per acre than power tower plants. Both wind and solar energy production require relatively large land areas. Sunshine, like wind, is an intermittent resource. No solar radiation is available at night and cloud cover, smog or haze can further limit generation from a solar plant. The arrival of night in the western states causes solar radiation to go to zero within an hour across the entire region. Thermal solar generating technologies can provide electricity even when the sun does not shine because unlike photovoltaic cells which convert sunlight directly to electricity, thermal solar technologies first convert the light into heat and then use a thermal dynamic cycle to produce electricity. For the power cycle, however, it does not matter whether the heat comes directly from sun from heat energy storage or even a fossil fuel fired boiler. Currently only parabolic trough plants and power towers allow for off-sun generation by using either heat storage or fossil fuel hybridization. This is a distinct advantage, especially compared with other intermittent renewable energies such as photovoltaic and wind, which require batteries for electrical storage. Solar power plants with heat storage collect thermal energy during the day by increasing the temperature of a large heat reservoir. In near term future applications, the heat reservoir will likely by a large vessel of molten salt. NREL has observed that molten salt heat storage is market-ready, safe, and the most economic of all thermal energy storage technologies. It allows thermal energy to be collected during the day and to be saved for use at night or it can be used to keep the plant at full output when clouds pass over the plant location. The effectiveness of heat storage increases with the operating temperature of the thermal solar power plant. The high temperatures of the power cycle in power towers make this technology particularly attractive for heat energy storage. Additional flexibility in the operation of a thermal solar plant with storage comes from over-sizing the solar field so that the collectors generate more heat than required by the steam turbine (or other solar powered device) while the additional energy goes into storage for later use. J-5