Direct and Indirect Recycled Water Use. Water Recycling 2030. Recycled Water Task Force Final Report (chapter 2, page 7). (June, 2003). California's Recycled Water Task Force. Retrieved November 20, 2008 from http://www.owue.water.ca.gov
In 2000, an estimated 16,225 Publicly Owned Treatment Works (POTWs) were in operation in the United States, treating over 40 billion gallons of wastewater daily. In 1996, 98% were municipally owned and provided wastewater collection, treatment and disposal service to 190 million people Ð 73% of the 1996 U.S. population. Of the enormous amount of water being treated, only 1 billion gallons per day, or 2.5% of treated wastewater, is reclaimed to meet nonpotable water needs such as irrigation of golf courses and public parks (University of Michigan, 2008). This is a small percentage compared to other countries. Israel, for example, recycles 73% of its treated municipal sewage (Tal, 2006).
In 2000, California, Texas and Florida accounted for one-quarter of all water withdrawals. The largest surface water withdrawals were by California, which used a great proportion of the water for irrigation and thermoelectric power, and Texas, which made large withdrawals for thermoelectric power as well. The largest groundwater withdrawals were made by California, Texas and Nebraska, all of which used much of the withdrawn water for irrigation. If more of this water came from reclaimed wastewater, the groundwater and surface water sources would not be depleted at the enormous present rate. It has been estimated that reclaimed water could free up enough freshwater to meet the household water demands of 30 to 50% of the 17 million additional Californians expected to reside there in 2030. Reclaimed water can be used for landscape irrigation, toilet flushing, and recharging many groundwater basins in much of western North America (Asano, 2007).
Reclaimed water can solve some of the problems with water depletion both indirectly through reuse and directly through recharge. While the use of reclaimed water in the southwest is much greater than the national percentage, it has great potential for growth.
Types of Recycled Water Use in California. Water Recycling 2030. Recycled Water Task Force Final Report (chapter 2, page 3). (June, 2003). California's Recycled Water Task Force. Retrieved November 20, 2008 from http://www.owue.water.ca.gov/
In California, it is estimated that about 5 million acre-feet per year of treated municipal wastewater is produced, with recycled water use currently at a level of about 500 thousand acre-feet per year. With population projected to increase, it is estimated that by 2030, the amount of wastewater available for water recycling projects will increase to about 6.5 million acre-feet per year (California's Recycled Water Task Force, 2003).
In Los Angeles County, eleven wastewater treatment facilities operate, ten of which are water reclamation plants (WRPs). These facilities serve ~5 million people in areas within Los Angeles County and produced an average of 486.43 million gallons per day (MGD), or 545,067 acre-feet per year (AFY) of effluent in 2006-7. Effluent quality from these WRPs ranges from undisinfected secondary to coagulated, filtered, disinfected tertiary. Out of the total effluent produced, 175.10 MGD (196,211 AFY) was recycled water suitable for reuse and the amount recycled is 36.0% of the total amount of effluent produced. Of the recycled water produced, 48.5% is reused. The figure below shows the distribution of recycled water usage in Los Angeles in 2006-7 (Sanitation Districts of Los Angeles County).
In Los Angeles, recycled water costs are significantly lower than those of potable water. Table 1 shows a rate comparison in a number of WRPs in Los Angeles County.
Water reuse summary for fiscal year 2006-2007. Retrieved November 20, 2008, from Sanitation Districts of Los Angeles County Web site: http://www.lacsd.org
Distribution of Recycled Water Usage Fiscal Year 2006-07. Water reuse summary for fiscal year 2006-2007. Sanitation Districts of Los Angeles County. Retrieved November 20, 2008 from http://www.lacsd.org
Tucson Water provides drinking water to over 690,000 people, or about 80% of the metropolitan population. In 2004, the utility delivered approximately 110,000 acre-feet of potable water and 12,000 acre-feet of reclaimed water. In the Tucson region, the combined annual municipal, agricultural, and groundwater pumpage is almost three and a half times greater than the rate of aquifer replenishment (Dotson). Tucson Water created the Reclaimed Water System to utilize reclaimed water, thereby offsetting a portion of customer demand as well as reducing groundwater mining (Tucson Water Department, 2007).
Approximately 12,000 AF/YR of the City's effluent is reused as reclaimed water. In 2004, 11,900 acre-feet of reclaimed water was delivered to over 750 customers: 62% to 14 golf courses, 18% to parks, 10% to schools, 2.8% to single family, 2.6 % to agriculture, and 1.8 % to commercial (Dotson).
The annual average demand on the Reclaimed Water System is currently about 12 MGD. By the year 2030, the total projected volume of effluent that will be produced by the Tucson Metropolitan area is 118,900 AF/YR and the City of Tucson's effluent entitlement is projected to grow to ~ 62,000 AF/YR (Tucson Water Department, 2007).
A plan that has been recommended in Tucson is to replace the large Roger Road Plant with a smaller treatment plant (32 MGD down from 41 MGD) and transferring excess flows to the another plant, the Ina Road Plant (increasing the treatment capacity from 37.5 MGD to 50 MGD). It is expected that such a plan would significantly improve the effluent water quality produced at each facility (Tucson Water Department, 2007).
Denver Water is building a non-potable recycled water systemthat will treat and deliver up to 17,000 acre feet of water per year for industrial and outdoor irrigation use. That amount of recycled water can free up enough drinking water resources to serve 36,000 households (Denver Water).
In order to combat the shortage of potable water supplies in Western North America, water reclamation programs across the southwest must be expanded to ensure that all or most of water treated in urban areas is recycled, whether for potable (direct or indirect) or nonpotable use. Every drop of water that is reused is a drop of freshwater saved for drinking for the next 100 years.
The following outlines possible uses of reclaimed water and the requirements for feasibility.
The various agricultural crops which can be irrigated with reclaimed water are listed below (table 1). Appropriate water reclamation processes must be adopted depending on the crop type and irrigation method. Additionally, regulations or guidelines for reclaimed water irrigation for food crops differ between states (see table below) (Asano, 2007).
Water quality and treatment requirements for food crops (Asano, 2007)
Water quality and treatment requirements for nonfood crops (Asano, 2007)
In 2002, 46% of total reclaimed water produced in California, or 220 Mgal/d, was used for agricultural irrigation. This contributed 0.7 percent of the overall irrigation water use in the state.
In some regions, reclaimed water is a vital source of irrigation water. Florida is the largest user of reclaimed water in the United States. In 2003, 95 Mgal/day of reclaimed water was used for agricultural irrigation. Agricultural irrigation comprises only 16% of the state's total reclaimed water use.
Suspended solids in reclaimed water can potentially clog the irrigation water distribution lines and emitters. A TSS concentration of less than 30 mg/L is generally considered suitable for most irrigation systems, although other factors (temperature, sunlight, emitter types, and flowrate) also affect clogging potential. Typically, secondary effluent contains 5-25 mg/L of total suspended solids (TSS). In tertiary effluent, TSS level is typically <10mg/L (Asano, 2007).
Ions present in reclaimed water may be harmless or even beneficial at low concentrations, but can harm the crop at high concentrations. The three major chemicals that may cause specific ion toxicity from reclaimed water irrigation are sodium, chloride, and boron. Trace levels of metals, inorganic compounds and organic compounds, and also pH and temperature can affect the feasibility of using reclaimed water for irrigation. It is therefore imperative to monitor these factors when using reclaimed water for irrigation (Asano, 2007).
Reclaimed water can be advantageous to crops due to the nutrients that it contains. Nitrogen, phosphorus, and potassium are the main macronutrients found in reclaimed water and are considered to be beneficial to irrigation at certain levels. If the level of nutrients in reclaimed water is higher than the desired level, reclaimed water quality can be altered through nutrient removal, or by blending it with water from other sources. The feasibility of water reuse for agricultural irrigation using existing facilities will depend on the location of the facilities in relation to the crops, the type of crop, the quality and quantity of the reclaimed water, and the cost of providing the reclaimed water. The existing facilities may have to be upgraded in order for the reclaimed water to be used for irrigation (Asano, 2007).
The following information is required in order to assess the feasibility of using reclaimed water for agricultural irrigation:
Drip and subsurface irrigation are the preferred types of irrigation systems when using reclaimed water, as they minimize the effect of salinity and almost all crops can be grown with very little reduction in yield. These irrigation systems are preferred from the public health standpoint as well, as they pose less potential risks to field workers, neighbors, and workers handling crops. There is low to negligible human exposure when using drip or subsurface irrigation (Asano, 2007).
It is important to reiterate that drip irrigation systems and other systems with low water velocities are prone to clogging by biological growth and chemical precipitation. The following table shows the clogging potential in irrigation systems based on water quality (Asano, 2007).
There are different methods of drainage disposal, but it is also possible to reuse drainage. If the salinity of the reclaimed water is low enough to allow irrigation of salt-sensitive crops, the drainage water may be reused to irrigate crops with higher salt tolerance. Irrigating crops that are increasingly tolerant of salt in a series of irrigated land is an effective way to use reclaimed water and reuse drainage water. Eventually the salt accumulated in the drainage water must be disposed of appropriately (Asano, 2007).
The Monterey Wastewater Reclamation Study for Agriculture (MWRSA) was the first large scale study investigating the risks and effects of irrigation with reclaimed water on food crops, including raw-eaten vegetables. The study began in 1976 and was reported in 1987. When the results of the study were analyzed, it was reported that no viruses were found on samples of crops from experimental plots irrigated with reclaimed water. Additionally, the levels of naturally occurring bacteria were not significantly different between well-water-irrigated crops and reclaimed-water-irrigated crops. There were also no adverse health effects to farmers detected from exposure of reclaimed water constituents. In fact, the quality, yield, and longevity of all the crops irrigated with reclaimed water were equal to or better than those of the crops grown with well water (Asano, 2007).
Table 2 shows the estimated costs of reclaimed water for various tertiary treatment processes. The cost is based on a flow of 114x103 m3/day where 28x106 m3/day of reclaimed water will be delivered for irrigation (Asano, 2007). A detailed calculation for the levelized cost of water from a generic sewage treatment plant is can be found here.
Landscape irrigation with reclaimed water is another viable option to reduce potable water demand, as well as to reduce or eliminate wastewater discharge to aquatic environment. An evaluation of the water billing records in Orlando, Florida, showed that the average residential irrigation demand was approximately 506 gpd (1.9 m3/d), compared to 350 gpd (1.3 m3/d) water for in-house use. The EPA used this data to indicate that a 59 percent reduction in residential potable water demand could be accomplished if a dual distribution system were to provide irrigation service (EPA, 2004).
The Irvine Ranch Water District in southern California estimates that over 70% of their total water use is for landscape irrigation. California is the second largest user of reclaimed water for landscape irrigation in the U.S. In 2004, 1.4x103 m3/day of reclaimed water was used for landscape irrigation in California (Asano, 2007).
When planning to reuse wastewater for landscape irrigation, public health must be taken into consideration, as irrigating with reclaimed water may result in subsequent contamination of potable water systems and human exposure to reclaimed water and its constituents. The following table shows various state water quality and treatment requirements for unrestricted urban use. In restricted access area, exposure to reclaimed water can be controlled so quality and treatment requirements are typically less stringent (Asano, 2007).
Typically, plants with low water requirements and high salt tolerance are preferred for use in landscape areas that are irrigated with reclaimed water. Microsprinklers, drip systems, and subsurface irrigation systems are recommended for use with reclaimed water due to high irrigation efficiency and low risk of human exposure (Asano, 2007).
The following table shows the different reclaimed water uses for landscape irrigation, the irrigation methods, and treatment requirements in California (Asano, 2007).
As with agricultural irrigation with reclaimed water, water quality must be monitored to prevent the irrigation system from clogging and the plants from being harmed. Please see the above sections.
Golf course irrigation uses the largest amount of reclaimed water among landscape irrigation uses. The suitability of reclaimed water for irrigation must be evaluated based on the salt tolerance of the turf. Most turf is not affected by soil water salinity that is less than 3 dS/m (Asano, 2007).
Where reclaimed water is used to for irrigating residential landscapes, dual distribution and plumbing systems need to be provided. In El Dorado Hills, California, reclaimed water has been used for irrigating residential landscapes through a dual plumbing system since 1990 (Asano, 2007).
Reclaimed water for industrial reuse refers to applications such as cooling water, boiler feedwater, and manufacturing processes. Reclaimed water quality must be monitored to prevent pipe corrosion in utilities.
In utilities where cooling water systems are being used, the use of reclaimed water can be applied to reduce potable water use (Asano, 2007). There are two basic types of cooling water systems that use reclaimed water: once-through and recirculating evaporative. The recirculating evaporative cooling water system is the most common reclaimed water system due to its hefty water use and consumption by evaporation (EPA, 2004). The main obstacle to using reclaimed water in industrial processes is corrosion. The best way to prevent corrosion in the cooling water systems is to monitor the quality of the reclaimed water and to select the system material according to the level of quality of the reclaimed water. This has already been implemented in Denver, Colorado where a thermal power generation plant is located adjacent to the water reclamation plant (Asano, 2007). For more information on using reclaimed water in cooling systems, refer to the energy section.
In manufacturing, the suitability of reclaimed water for use in industrial processes depends on the specific use. For example, the electronics industry requires water of almost distilled quality for washing circuit boards and other electronic components, while the tanning industry can use relatively low-quality water. Requirements for textiles, pulp and paper, and metal fabricating are site specific. Thus, when investigating the feasibility of industrial reuse with reclaimed water, potential users must be contacted to determine the specific requirements for their process water (EPA, 2004).
The EPA reported that toilet flushing can account for up to 45 percent of indoor residential water demand (EPA, 2004). The use of reclaimed water for toilet flushing in commercial and residential buildings can reduce potable water demand and do not require potable water quality. The feasibility depends primarily on plumbing and related infrastructure cost. In California, tertiary treated reclaimed water is deemed safe for toilet flushing. Dual plumbing for potable and reclaimed water is necessary for the use of reclaimed water in buildings (Asano, 2007).
The Irvine Ranch Water District in California researched the economic feasibility of expanding its urban dual distribution system to provide reclaimed water to high-rise buildings for toilet and urinal flushing. The study concluded that the use of reclaimed water was feasible for flushing toilets in buildings of 6 stories and higher. Following this study, an ordinance was enacted requiring all new buildings over 55 feet (17 meters) high to install a dual distribution system for flushing in areas where reclaimed water is available (EPA, 2004). In 1991, the Irvine Ranch Water District began using reclaimed water for toilet flushing in high-rise office buildings. Potable water demands in these buildings have decreased by as much as 75 percent due to the reclaimed water use (EPA, 2004).
While the need for an additional water source in arid and semi-arid areas has been the main motivation for many water reclamation and reuse programs, many programs in the U.S. were started in response to rigorous and costly requirements to remove nitrogen and phosphorus for effluent discharge to surface waters. By eliminating effluent discharges for all or even a portion of the year through water reuse, a municipality can reduce the need for the costly nutrient removal treatment processes. For example, the South Bay Water Recycling Project in San Jose, California, provides reclaimed water to 1.3 million residents. By reusing water rather than releasing it to the San Francisco Bay, San Jose has avoided devastating impacts to its economy (EPA, 2004).