[ Water treatment (drinking and sewage) use tremendous amounts of energy. Some of the statistics from this document “Water & Wastewater Utility energy research roadmap” below are:
- In 2008 municipal wastewater treatment systems (WWTP) in the United States used approximately 30.2 billion kilowatt hours (kWh) per year, or about 0.8% of total electricity used in the United States.
- These WWTPs are becoming large energy consumers and they can require approximately 23% of the public energy use of a municipality.
- About 10-40% of the total energy consumed by wastewater treatment plants is consumed for sludge handling.
- Desalination consumes 3% of annual electricity consumption in the United States Future projections estimate this percentage to double to 6% due to higher water demand and more energy intensive treatment processes
- A significant percentage of energy input to a water distribution system is lost in pipes due to friction, pressure and flow control valves, and consumer taps.
- AWWA estimates that about 20% of all potable water produced in the United States never reaches a customer water meter mostly due to loss in the distribution system. When water is lost through leakage, energy and water treatment chemicals are also lost.
- In California, agricultural groundwater and surface water pumping is responsible for approximately 60% of the total peak day electrical demand related to water supply, particularly the energy consumed within Pacific Gas and Electric’s (PG&E) controlled area. Over 500 megawatts (MW) of electrical demand for water agencies in California is used for providing water and sewer services to customers. The water related electrical consumption for the State of California is approximately 52,000 gigawatt hours (GWh). Electricity use for pumping is approximately 20,278 GWh, which is 8% of the state’s total electricity use. The remaining is consumed at the customer end side for heat, pressurize move and cool water.
This paper also looks at ways to save energy, and extraction of nutrients such as phosphorous — a good idea, since phosphate production may peak as soon as 40 years from now.
As global oil production declines and there isn’t enough energy to run civilization as we know it now, hard choices will need to be made. First in line is agriculture, which consumes about 15 to 20% of energy in the U.S. to plant, harvest, store, distribute, cook, and so on.
Clean water and sewage treatment are just as important as food. But drought threatens to increase energy requirements. “The energy intensity of desalination is at least 5 to 7 times the energy intensity of conventional treatment processes”, so even though only 3% of the population is served by desalination, 18% of electricity used in the municipal water industry is for desalination plants.
But making water systems more energy efficient is trivial compared to trying to maintain and replace our aging water infrastructure, which is falling apart.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report ]
CEC. 2016. Water and wastewater utility energy research road map. California Energy Commission. 135 pages.
ABSTRACT. Water and wastewater utilities are increasingly looking for innovative and cost effective energy management opportunities to reduce operating costs, mitigate contributions to climate change, and increase the resiliency of their operations. The Water Research Foundation, the California Energy Commission and the New York State Energy Research and Development Authority jointly funded this project to assess the current state-of-knowledge on energy management, concepts and practices at water and wastewater utilities; understand the issues, trends and challenges to implement energy projects; identify new opportunities to set a direction for future research; and develop a roadmap for energy research that includes a list of prioritized research, development, and demonstration projects on energy management for water and wastewater utilities.
EXECUTIVE SUMMARY. The water industry faces challenges associated with escalating energy costs due to increased energy consumption and higher energy unit prices. Increased energy consumption is affected by energy-intensive treatment technologies needed to meet more stringent water quality regulations, growing water demand, pumping over longer distances, and climate change. More desalinated water to augment water supply shortages and the growth of groundwater augmentation is also anticipated.
The water industry faces challenges associated with escalating energy costs due to increased energy consumption and higher energy unit prices. Increased energy consumption is affected by energy-intensive treatment technologies needed to meet more stringent water quality regulations, growing water demand, pumping over longer distances, and climate change (GWRC, 2008). Moreover, the need for desalinated water to augment water supply shortages and the growth of groundwater augmentation is also anticipated (House, 2007). The same study by the Energy Commission estimates the demand for electricity in the water industry to double in the next decade. The water sector has shown only a limited response in implementing improvements that effectively address sustainability issues due to insufficient modernization, the presence of numerous regulatory and economic hurdles, and poor integration of energy issues within the water policy decision-making process (Liner and Stacklin, 2013; Rothausen and Conway, 2011).
Energy Management Opportunities in Wastewater Treatment and Water Reuse. Currently, there are over 15,000 municipal wastewater treatment plants (WWTPs), including 6,000 publicly owned treatment works (POTWs) providing wastewater collection and treatment services to around 78% of the United States’ population (Mo and Zhang, 2013; Spellman, 2013). According to the report published by EPRI and the WRF (Arzbaecher et al., 2013) in 2008 municipal wastewater treatment systems in the United States used approximately 30.2 billion kilowatt hours (kWh) per year, or about 0.8% of total electricity used in the United States. These WWTPs are becoming large energy consumers and they can require approximately 23% of the public energy use of a municipality (Means, 2004). Typical wastewater treatment operations have a total average electrical use of 500 to 4,600 kWh per MG treated, which varies depending on the unit operations and their efficiency (Kang et al., 2010; WEF, 2009; GWRC, 2008; NYSERDA, 2008a). Treatment-process power requirements as high as 6,000 kilowatt hours per million gallons (kWh/MG) are required when membrane bioreactors are used in place of activated sludge or extended aeration (Crawford & Sandino, 2010).
Approximately 2,000 million kWh of electricity are consumed annually by wastewater treatment plants in California (Rajagopalan, 2014). Energy use by these utilities is affected by influent loadings and effluent quality goals, as well as process type, size and age (Spellman, 2013). The majority of energy use occurs in the treatment process, for aeration (44%) and pumping (7%) (WEF, 2009). In major Australian WWTPs, the pumping energy for wastewater facilities ranged from 16 to 62% of the energy used for treatment (Kenway et al., 2008). In New York, the wastewater sector uses approximately 25% more electricity on a per unit basis (1,480 kWh/MG) than the national average (1,200 kWh/MG) due to the widespread use of energy intensive activated sludge, as well as compliance with stringent New York State effluent limits, which often require tertiary or other advanced treatment. Additionally, the predominance of combined (storm water and wastewater) sewer systems at the largest facilities, coupled with significant inflow and infiltration, result in extremely large variations in influent flow rates and loading, making efficient operations difficult (Yonkin et al., 2008).
The greatest potential for net positive energy recovery occurs at larger facilities, which are only a small percentage of the treatment works nationwide, but treat a large percentage of the nation’s wastewater. By achieving energy neutrality and eventually energy positive operations at larger facilities, the energy resources in the majority of domestic wastewater can be captured. This principle guided WERF to prepare a program to conduct the research needed to assist treatment facilities over 10 million gallons per day (MGD) to become energy neutral (Cooper et al., 2011). Energy self-sufficiency has been attained at a wastewater plant in Strass, Austria, where the average power usage is approximately 1,000 kWh/million gallon (MG) treated, which is also the approximate electricity generation from the sludge (Kang et al., 2010). The design employs two stages of aerobic treatment, with innovative controls, where biosolids generated in the two stages are thickened and anaerobically digested, with gas recovery and power generation. The centrate from the dewatering operation is treated in a sequencing batch reactor using the DEamMONification (DEMON) process to reduce the recirculation of nutrients to the head of the plant.
The importance of the scale of a facility in understanding the different strategies that may be implementable for the technology or service options available is pointed out in a recent report (AWE and ACEEE, 2013). It is important that energy management best practices are defined with consideration of specific plant size or treatment process. The largest per unit users of energy are, in fact, small water and wastewater treatment plants that treat less than 1 MGD, as well as those that employ an activated sludge with or without tertiary treatment process.
Wastewater treatment facilities have significant electricity demand during periods of peak utility energy prices. An effective energy load management strategy can help wastewater utilities to significantly reduce their electricity bills. A number of electrical load management opportunities are available to wastewater utilities (Table 2.1), notably by flattening the energy demand curve, particularly during peak pricing periods and by shifting major electrical demand to lower cost tariff blocks (e.g., overnight), for intra–day operations, or from season to season where long- or short-term wastewater or sludge storage is practical (NYSERDA, 2010). Wastewater treatment facilities have the potential to benefit from electric utility demand response (DR) opportunities, programs and tariffs. Although the use of integrated energy load management systems for wastewater utilities is still in its infancy, some wastewater utilities have begun implementing strategies that provide a foundation for participation in demand response programs. Such implementations are thus far limited to control pumping in lift stations of wastewater collection systems in utilities equipped with sufficient storage (Thompson et al., 2008). Wastewater treatment processes may offer other opportunities for shifting wastewater treatment loads from peak electricity demand hours to off-peak hours, as part of Demand Management Programs (DMPs), by modulating aeration, backwash pumps, biosolids thickening, dewatering and anaerobic digestion for maximum operation during offpeak periods. Recently, wastewater utilities, such as the Camden County Municipal Utilities Authority, have developed a computerized process system that shaved the peaks by avoiding simultaneous use of energy-intensive process units, to the maximal extent possible, thereby minimizing the peak charge from the energy provider (Horne and Kricun, 2008). In addition, the East Bay Municipal Utilities District has implemented a load management strategy which stores anaerobic digester gas until it can be used for power generation during peak-demand periods. Another opportunity for shifting electrical loads from on-peak to off-peak hours is over-oxygenating stored wastewater prior to a demand response event, then turning off aerators during peak periods without compromising effluent quality (Thompson et al., 2008). For a wastewater facility to successfully implement demand response programs, advanced technologies that enhance efficiency and control equipment are needed, such as a comprehensive and real-time demand control from centralized computer control systems that can provide an automatic transfer switch to running onsite power generators during peak demand periods, in accordance with air quality requirements (Thompson et al., 2008).
An interesting opportunity for reducing energy use in municipal wastewater treatment is to improve storm water management (Lekov, 2010). The adoption of stormwater treatment only at CSO communities can reduce energy consumption for wastewater treatment systems due to reductions in volume at the treatment plant and reduction in volumes requiring pumping in the combined sewer collection system.
Wastewater utilities are actively working to reduce the energy use of their facilities by increasing efficiency. Energy efficiency is part of the process to reduce energy demand along the path to a net energy neutral wastewater treatment plant. Briefly, wastewater treatment plants can target energy efficiency by replacing or improving their core equipment, through use of variable frequency devices (VFDs), appropriately sized impellers and implementation of energy-saving automation schemes. Efficiency can also be improved at the process level, by implementing low energy treatment alternatives to an activated sludge process or improving process control.
Energy Efficient Equipment. There are numerous types of energy efficient equipment that a wastewater utility can utilize to reduce energy consumption. Common facility-wide plant improvements include upgrade of electric motors and the installation of VFDs in pumps. These modifications can result in substantial energy efficiency because at least 60% of the electrical power fed to a typical wastewater treatment plant is consumed by electric motors (Spellman, 2013). VFDs enable pumps to accommodate fluctuating demand and allow more precise control of processes. VFDs can reduce a pump’s energy use by up to 50% compared to a motor running at constant speed for the same period. Wastewater treatment facilities can also upgrade their heating, cooling, and ventilation systems (HVAC) to improve energy efficiency and reduce energy costs. The latest developments in HVAC equipment can substantially reduce cooling energy use by approximately 30 to 40% and achieve energy efficiency ratios as high as 11.5. The latest air-source heat pumps can reduce heating energy use by about 20 to 35%. Water-source heat pumps also have superior ratings, especially when outside air temperatures drop below 20 degrees Fahrenheit (°F) (15.2 energy efficiency ratio) and can use heat from treated effluent to supply space heating. The Sheboygan Wastewater Treatment Plant reduced its energy consumption by 20% from 2003 solely by implementing energy demand management strategies that targeted efficiency by equipment replacement (e.g., motors, VFDs, blowers, etc.) and scheduling of regular maintenance (Liner and Stacklin, 2013).
Wastewater treatment plants have also recently used advanced sensors and control devices to optimize energy so that what is supplied meets but does not exceed the actual demand. For example, the adoption of lower dissolved oxygen set-points in the aeration basin can still maintain microbial growth and generate energy savings of 15-20% (Kang et al., 2010). The installation of energy submeters is another important plant improvement that, however, can require high capital investments for a utility. Recent advances in lamps, luminaries, controls, and lighting design provide numerous advantages over traditional lighting systems. Since lighting accounts for 35 to 45% of the energy use of an office building, the installation of high-efficiency alternatives for nearly every plant can dramatically reduce the operational energy bill for the utility. Incentives and rebates are commonly available from electric utilities and other agencies, such as NYSERDA, to support the installation of energy-efficient fixtures and equipment that reduce energy use financial impacts
Aeration is the largest energy user in a typical wastewater treatment plant, thus the aeration process should be evaluated when implementing energy reduction programs. Installing automatic dissolved oxygen control enables continuous oxygen level monitoring in the wastewater and so that aerators can be turned off when the oxygen demand is met. Based on the aeration capacity of the wastewater treatment system and the average wastewater oxygen requirement, the automated dissolved oxygen control can be the most cost effective method to optimize aeration energy and achieve energy savings up to 25% to 40% if compared to manually controlled systems. In addition to automated control systems, the installation of smaller modular and high efficiency blowers to replace centralized blowers, the proximity of the blowers to the aeration basin to reduce energy losses from friction, and the installation of high efficiency pulsed air mixers are important efficiency measure to be considered.
About 10-40% of the total energy consumed by wastewater treatment plants is consumed for sludge handling. Most of the energy required is due to the shear force applied for dewatering, solids drying and treatment of high-strength centrate. As an example, in California centrifuge and belt filter presses consume 30,000 kWh/year/MGD and 2-6,000 kWh/year/MGD, respectively (Rajagopalan, 2014). Many studies have been conducted on understanding sludge dewatering processes and improving their efficiency. Recent studies by the Energy Commission have focused on the improvement of sludge dewatering to achieve lower energy consumption by using nanoparticulate additives. By implementing this solution at wastewater treatment plants in California, the state would be able to save an additional 10.5 million kWh per year, which includes the cost of energy, polymer and nanoadditives for sludge dewatering, and sludge disposal
Another innovation directed toward more energy efficient systems is the use of distributed systems in place of the centralized treatment systems historically favored due to their economies of scale. Centralized plants are generally located down gradient in urban areas, permitting gravity wastewater flow to the treatment plant, while the demand for reclaimed wastewater generally lies up gradient. This means higher energy demands for pumping the reclaimed wastewater back to the areas in need. These energy costs can be reduced through use of smaller distributed treatment plants located directly in water limited areas
Processes and technologies already in use at wastewater treatment plants include biogas-powered combined heat and power (CHP), thermal conversion from biosolids, renewable energy sources (e.g., systems solar arrays and wind turbines), energy recovery at the head of the wastewater treatment plant and within the treatment process.
Energy recovery from anaerobic digestion with biogas utilization and biosolids incineration with electricity generation is widespread, but there is potential for further deployment. Of the approximately 837 biogas generating facilities in the United States, only 35% generate electricity from biogas and only 9% sell electricity back to the grid (Liner and Stacklin, 2013). The low application rate is partly due to the
dominance of small wastewater systems in the United States (less than 5 MGD). It is estimated that anaerobic digestion could produce about 350 kWh of electricity for each million gallons of wastewater treated at the plant and save 628 to 4,940 million kWh annually in the United States (Stillwell et al., 2010). The electricity produced by CHPs is reliable and consistent, but the installation requires relatively high one-time capital costs. Research shows that recovery of biogas becomes cost-effective for wastewater treatment plants with treatment capacities of at least 5 MGD (Mo and Zhang, 2013; Stillwell et al., 2010). Various wastewater treatment plants, such as by the East Bay Municipal Utility District (Oakland, California) and the Strass WWTP (Austria) became a net-positive, energy-generating wastewater plant by powering low-emission gas turbines with biogas from co-digestion processes.
Biosolids incineration with electricity generation is an effective energy recovery option that uses multiple hearth and fluidized bed furnaces. Both incineration technologies require cleaning of exhaust gases to prevent emissions of odor, particulates, nitrogen oxides, acid gases, hydrocarbons, and heavy metals.
As for biogas-generating electricity, incineration can be used to power a steam cycle power plant, thus producing electricity in medium to large wastewater treatment plants where a high amount of solids is produced.
Disadvantages of incineration are high capital investments, high operating costs, difficult operations, and the need for air emissions control (Stillwell et al., 2010). Despite these disadvantages, biosolids incineration with electricity generation is an innovative approach to managing both water and energy. For example, the Hartford Water Pollution Control Facility in Hartford (Connecticut) is incorporating an energy recovery facility into their furnace upgrade project and they anticipate that biosolids incineration will generate 40% of the plant’s annual electricity consumption (Stillwell et al., 2010).
Wastewater utilities can now strategically replace incineration with advanced energy recovery technologies (MWH Global, 2014). Like incineration, gasification and pyrolysis offer the potential to minimize the waste mass for ultimate disposal from processing sewage sludge for its sludge treatment centers and also offer the prospect of greater energy recovery and/or lower operating cost than that offered by incineration (MWH Global, 2014). The range of gasification technologies available is large and at present it is believed that there are further synergies, such as recovering heat for digester and/or thermal hydrolysis process heating, that can be derived for a digestion or advanced digestion/ gasification advanced energy recovery. Pyrolysis, offers further advantages over the gasification options due to the production of a better syngas product than gasification, favoring more effective gas engine/CHP power generation.
Nutrient recovery from wastewater can offset the environmental loads associated with producing the equivalent amount of fertilizers from fossil fuels (Mo and Zhang, 2013). Various nutrient recovery methods have been applied in wastewater treatment processes and include biosolids land application, urine separation, controlled struvite crystallization and nutrient recovery through aqua-species. Biosolids land application involves spreading biosolids on the soil surface or incorporating or injecting biosolids into the soil. Urine separation involves separation of urine from other wastewater sources for recovery of nutrients. The process is promising in terms of maximizing nutrient recovery from wastewater, because around 70-80% of nitrogen and 50% of phosphorus in domestic wastewater is contained in urine (Maurer et al., 2003).
Although not widely applied, aqua-species, such as macroalgae, microalgae, duckweed, crops and wetland plants after utilizing nutrients in wastewater, can be harvested and used as fertilizers or animal feeds
While these individual resource recovery methods have been studied, there is a paucity of peer-reviewed articles focusing on the current status and sustainability of these individual methods as well as their integration at different scales
Recently, a few research programs have started investigating the potential for nutrient recovery, including carbon, nitrogen and phosphorus from wastewater treatment process. A recent report from WERF with support from the Commonwealth Scientific and Industrial Research Organization (CSIRO), Resource Recovery from Wastewater: A Research Agenda, summarized and defined the future research needs for the resource recovery opportunities in the wastewater sector (Burn et al., 2014).
WERF is developing a tool for the implementation and acceptance of resource recovery technologies at WWTPs, with a major focus on extractive nutrient (phosphorus) recovery technologies that employ greater energy efficiency and offer monetary savings (Latimer, 2014). WERF has prioritized high profile research on P concentration and recovery opportunities during wastewater treatment processes. Polyphosphate-accumulating organisms (PAO) can be responsible for P concentration in cells and direct concentration and precipitation of struvite that can be recovered for niche agricultural markets (Burn et al., 2014). This report implies that nitrogen recovery seems to be a lower priority than carbon (through biogas) or phosphorus recovery, unless combined with other recovery opportunities. N recovery is possible through the use of adsorption/ion-exchange, precipitation and stripping processes.
A $26 million ion-exchange pilot facility in New York that concentrated ammonia from recycle streams (centrate) of anaerobically digested sludge showed that the above mentioned methods are viable, however not yet as cost effective as the Haber-Bosch process (Burn et al., 2014).
Treated wastewater can be reused for various beneficial purposes to provide ecological benefits, reduce the demand of potable water and augment water supplies (Mo and Zhang, 2013). Beneficial uses include agricultural and landscape irrigation, toilet flushing, groundwater replenishing and industrial processes (EPA, 2004). Currently, around 1.7 billion gallons per day of wastewater is reused in the US, and this reuse rate is growing by 15% every year (Mo and Zhang, 2013) and Florida and California are pioneering states in the country focusing on water reuse. The level of wastewater treatment required varies depending on the regulatory standards, the technologies used and the water quality characteristics. Some of the treatment process or schemes utilized are able to save energy for the same amount of water delivered.
Although there is integrated resource recovery in practice currently, particularly at the community level, the related studies are rare. In a WWTP in Florida onsite energy generation, nutrient recycling and water reuse are combined: CHP is used to generate electricity from the digested gases, biosolids are sold for land application and part of the treated water is used for agricultural and landscape irrigation. In general, to date, very limited studies have reviewed the integrated energy-nutrient-water recovery in WWTPs, particularly on a national-scale (McCarty et al., 2011; Mo and Zhang, 2013; Verstraete et al., 2009) and there are no studies optimizing the resource recovery via multiple approaches
Energy Management Opportunities in Drinking Water and Desalination. Desalination consumes 3% of annual electricity consumption in the United States (Boulos and Bros, 2010; EPA, 2012b; Sanders and Webber, 2012; Arzbaecher et al., 2013). Future projections estimate this percentage to double to 6% due to higher water demand and more energy intensive treatment processes (Chaudhry and Shrier, 2010). Estimates indicate that approximately 90% of the electricity purchased by water utilities, or approximately $10 billion per year, is required for pumping water through the various stages of extraction, treatment, and final distribution to consumers (Bunn, 2011; Skeens et al., 2009). Despite recent energy efficiency progress in pumping systems, there has not been any notable impact on existing energy intensity values. Furthermore, the energy use in drinking water utilities, with the exclusion of energy use for water heating by residential and commercial users, contributes significantly to an increasing carbon footprint with an estimated 45 million tons of greenhouse gases (GHG) emitted annually in the UnitedStates.
In California, agricultural groundwater and surface water pumping is responsible for approximately 60% of the total peak day electrical demand related to water supply, particularly the energy consumed within Pacific Gas and Electric’s (PG&E) controlled area. Over 500 megawatts (MW) of electrical demand for water agencies in California is used for providing water and sewer services to customers (House, 2007). The water related electrical consumption for the State of California is approximately 52,000 gigawatt hours (GWh) (House, 2007). Electricity use for pumping is approximately 20,278 GWh, which is the 8% of the state’s total electricity use. The remaining is consumed at the customer end side for heat, pressurize move and cool water.
To address the challenges associated with poorer quality sources and/or reduced supply, water utilities have been exploiting new water supply options such as seawater and saline groundwater, the use of which is growing about 10% each year. The use of these new water sources require two to ten times more energy per unit of water treated than traditional water treatment technologies.
While previous studies have focused on energy requirements for water utilities, there is a lack of studies that estimate peak electric demand and peak use in the water sector (House, 2007). This lack of understanding of peak electrical demand and use is even more limited by the lack of water demand profiles that can be compared to electric use profiles in the water sector. Development of water demand profiles is very difficult and not monitored as well as electric use, due to the fact that water is billed by volume and not by time-of-use (House, 2007). Pricing water in a TOU structure is still a complicated task for water utilities, however it has the potential to offer large energy savings.
In many cases, successful water efficiency programs reduce the total revenues for water agencies under typical rate structures
Research is needed to investigate the potential for decoupling investments from revenues in water markets and other financial methods that would make conservation and efficiency programs more attractive and encourage alternative energy supplies. Better valuing of the different qualities and sources of water would also facilitate better choices of water resource applications that take the real cost/value of the supply and quality into consideration.
Energy Efficiency Estimates indicate that between 10 and 30% cost savings are readily achievable by almost all utilities implementing energy efficient programs or strategies (Leiby and Burke, 2011). In addition to cost savings, improving efficiency will result in a number of benefits, including the potential to reinvest in new infrastructure or programs, reduce the pressure on the electrical grid, achieve
Energy efficient processes and new technologies to be applied in the water treatment and desalination sector are still at the research stage or are under-development. For example, NeoTech Aqua Solutions, Inc. has developed a new ultraviolet (UV) disinfection technology (D438) that uses 1/10 of the energy compared to lamps required in similar flow conventional UV systems. The technology demands less electricity and results in a smaller electrical bill, less maintenance, and a smaller overall carbon footprint.
Estimates of energy efficiency in water supply and drinking water systems, associated economics and related guidelines are lacking.
Energy Efficient Operations and Processes
Energy efficiency can be targeted in water supply and distribution system operations as well as water treatment. Efficient pump scheduling and network optimization are significant contributors to efficiency practices
A significant percentage of energy input to a water distribution system is lost in pipes due to friction, pressure and flow control valves, and consumer taps (Innovyze, 2013).
The energy intensity (kWh per MG of water treated) of desalination is at least 5 to 7 times the energy intensity of conventional treatment processes, so even though the population served by desalination is only about 3%, we estimate that approximately 18% of the electricity used in the municipal water industry is for desalination plants. Due to the lower energy consumption, RO processes are preferred to thermal treatments for domestic water desalinization in the United States.
In an RO process, costs associated with electricity are 30% of the total cost of desalinated water. Reducing energy consumption is critical for lowering the cost of desalination and addressing environmental concerns about GHG emissions from the continued use of conventional fossil fuels as the primary energy source for seawater desalination plants.
The feed water to the RO is pressurized using a high pressure feed pump to supply the necessary pressure to force water through the membrane to exceed the osmotic pressure and overcome differential pressure losses through the system
Typically, an energy recovery device (ERD) in combination with a booster pump is used to recover the pressure from the concentrate and reduce the required size of the high pressure pump (Stover, 2007; Jacangelo et al., 2013). A theoretical minimum energy is required to exceed the osmotic pressure and produce desalinated water. As the salinity of the seawater or feed water recovery increases, the minimum energy required for desalination also increases. For example, the theoretical minimum energy for seawater desalination with 35,000 milligrams per liter (mg/L) of salt and a feed water recovery of 50% is 1.06 kilowatt hours per cubic meter (kWh/m3)(Elimelech and Philip, 2011). The actual energy consumption is larger as real plants do not operate as a reversible thermodynamic process
Typically, the total energy requirement for seawater desalination using RO (including pre- and post-treatment) is on the order of 3 – 6 kWh/m3 (Semiat, 2008; Subramani et al., 2011). More than 80% of the total power usage by desalination plants is attributed to the high pressure feed pumps
The energy consumption associated with filtration systems increases due to fouling by nanoparticles as reported in a study from the Energy Commission (Rosso and Rajagopalan, 2013). For example, flux analysis of MF 200 nanometer (nm) pore size membranes showed that particles between 100 and 2.5 nm contributed the most to the membrane fouling, more than fouling due to cake formation. Further understanding of the mechanisms of membrane fouling and of pretreatment options with coagulants will offer energy savings opportunities for water and water reclamation utilities
AWWA estimates that about 20% of all potable water produced in the United States never reaches a customer water meter mostly due to loss in the distribution system. When water is lost through leakage, energy and water treatment chemicals are also lost.
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