As described above, water reuse for agricultural crop production has the potential to utilize large volumes of irrigation water sourced from treated municipal wastewater. However, expanded water reuse for irrigation is limited currently by a number of logistical challenges. Several of these challenges are summarized in Table 3
. Two key considerations that impact the implementation of agricultural reuse are declining water quality and competing uses for recycled water.
7.1. Declining Water Quality
The concentration of salts and other dissolved solids in municipal water supply has a direct impact on the dissolved solids in the effluent from wastewater treatment. Increasing salt concentrations in surface water and groundwater supplies, due in part to overdrafting of aquifers and climatic factors, can have a negative impact on the reuse of wastewater effluent for crop irrigation. Further, reductions in indoor water use also increase the dissolved solids concentration of wastewater effluent.
Impact from source water quality.
A strong correlation exists between the quality of the source water used for municipal water supply and the characteristics of the corresponding recycled water produced. In southern California, water received from the Colorado River Aqueduct has a total dissolved solids (TDS) concentration which has varied from around 300 mg/L to nearly 700 mg/L over the last decade [19
]. More extreme climate conditions are expected to impact the salinity of many water supply sources due to increased evapotranspiration, discharge of effluent with increased TDS (due in part to water conservation), and less rainfall to replenish reservoirs and groundwater.
Impacts from water conservation and reuse.
In areas prone to drought, as is the case in much of California and the western United States, there is a tendency over time for incremental reductions in indoor water usage associated with mandated water conservation measures, and long-term changes in behavior and installed fixtures/appliances. Indoor water use has declined by about 25% over the last decade in southern California, with a net effect of increasing wastewater effluent TDS content by about 50 mg/L on average [19
]. The increasing TDS makes the water less suitable for irrigation of sensitive crops. More extreme flow reductions occur under acute dry conditions, such as during the California drought period 2015–2016 when wastewater influent flows were reduced by 10 to 50 percent [20
]. Future voluntary and mandated reductions in indoor water use will continue to increase effluent salinity.
The changes in indoor water usage also reduce the flow velocity in wastewater collection systems and reduce the ultimate water available for reuse during the peak irrigation demand. Recent action to divert organics from landfills has also resulted in more co-digestion projects that impart salts and nutrients into wastewater treatment facilities. The net effect of reduced indoor water use and imported organics for co-digestion is less recycled water available during the irrigation season and with increasing salt concentrations.
7.2. Competing Uses for Recycled Water
Water reuse by irrigation is among the earliest and the most extensively practiced forms of water reuse, but as the value of water increases and technologies for purification of water become more effective and feasible, there will likely be a shift away from irrigation in some regions.
The centralized/regional model of wastewater collection and treatment makes widespread agricultural water reuse a significant challenge. In large metropolitan regions, much of the urban population is located in coastal areas. Water reuse was not considered when these areas were being planned and developed. Therefore, wastewater effluents are drained to coastal zones for treatment and discharge, far away from agricultural regions. Unfortunately, the proximity is too great between most key agricultural regions and population centers that are producing effluent for reuse to make this type of water reuse feasible.
While early practices may have benefitted from nutrients embodied in urban sewage, modern recycled water, by comparison, is relatively low in nutrients following treatment for nutrient reduction, which is required to meet inland surface water discharge requirements. Agricultural reuse typically does not have stringent nutrient standards compared with surface water discharge, but there is a general trend towards the implementation of biological nutrient removal and tertiary/advanced treatment systems. Because of the discharge standards in the United States, the level of water quality achieved with modern wastewater system designs may exceed that required for irrigation. Therefore, as an alternative water supply, recycled water may be considered for a wider range of potential applications including industrial reuse, groundwater recharge, reservoir augmentation, and potable reuse. Given that the technology and public acceptance of potable reuse has matured, it is expected that more potable reuse projects will be expanding in arid regions in the USA.
The seasonality of irrigation reuse must also be considered when looking to expand and optimize water reuse systems. Applications that have a stable demand throughout the year, such as industrial and potable reuse, will increase the total amount of water that can be reused. Competition with alternative and new types of reuse applications will need to be considered when evaluating the viability of any recycled water projects.
As shown on Figure 1
in California there was a significant increase in agricultural reuse in the 1970s until about 2009. Since 2009, there has been growth in urban and potable reuse (groundwater recharge) applications. Nearly all of the large reuse projects that are in development are focused on potable reuse applications that reduce consumption of potable water use in an effort to adapt to ongoing and worsening water scarcity.
One example of this competitive trend in coastal areas is the indirect potable reuse project for Soquel Creek Water District, which primarily provides water service to municipal clients in the central coast area of California. In an effort to stop seawater intrusion effects on its groundwater production wells, Soquel Creek is implementing advanced treatment of regional wastewater for direct injection of recharge water into aquifers. The advanced treatment includes advanced oxidation and reverse osmosis of tertiary effluent. The high value of water for municipal uses combined with the advanced treatment technology now available out-competes potential agricultural uses of the recycled water.
While there are competing interests for the recycled water, there is a significant opportunity developing for the reuse of agricultural nutrients recovered from urban wastewater. The recovery of nitrogen and phosphate from digestate and other concentrated side-streams can provide a source of locally sourced agricultural fertilizers. As the development of main-stream nutrient recovery processes progresses, the practice of agricultural reuse will grow to include nutrients sourced from wastewater. Thus, while it may not be feasible to transport water from coastal wastewater facilities to agricultural regions, there is potential to export fertilizers from these urban areas.
7.3. Water Availability for Inland Areas
Although recycled water for agriculture use faces increased challenges in coastal areas, the economics can still be favorable in arid inland areas. The City of Lodi in central California continues to irrigate row crops and forage crops on farmland surrounding its wastewater treatment plant. The distribution and treatment costs are low, enabling costs of irrigation reuse that compare favorably with tertiary treatment and river discharge.
The Sacramento Regional County Sanitation District is undertaking a new project to extend pipelines for supplying recycled water to farms south of the regional plant. The project has received supplemental state funding because it helps reestablish sustainable groundwater conditions in an area of historic groundwater overdraft, providing benefits to both agriculture and riparian habitat.
The inland city of Fresno, California continues to direct secondary effluent to percolation ponds that recharge groundwater. The groundwater is then subsequently extracted and discharged into a canal for agricultural use during the growing season. The soil-aquifer system provides economical advanced treatment and seasonal storage to the benefit of local agriculture.