The wastewater reuse segment of the water industry has experienced both rapid growth and tremendous change over the last several years. The global demand for increased water supplies has fueled the development of alternative water sources, including the use of reclaimed wastewater. A 2012 study by the National Science Foundation states that U.S. coastal cities could increase their water supplies by 27 percent with treated wastewater. The demand for water supply in the future will be even greater, due to population growth and other demographic factors.
The interest in the beneficial reuse of wastewater of all types, both in the U.S. and abroad, has seen a significant increase. In the U.S., the historical leaders in the water reuse market include the States of California, Florida, Arizona, and Texas. For example, the current volume of wastewater reused in Florida is estimated to exceed 750 MGD. Recent cycles of drought in both California and Texas have resulted in multiple pilot and demonstration projects that involve potential Indirect and Direct Potable reuse schemes. In addition, state and national regulatory bodies are developing guidelines and regulations to better aid in the approval and licensing of these facilities.
In 2019, the City of Los Angeles announced an initiative to reuse 100 percent of the wastewater from the Hyperion Wastewater Reclamation Facility by the year 2035. The Facility can currently process 450 MGD daily and 800 MGD during storm events and is currently reclaiming about 200 MGD. There is also a bill in the California State Legislature that would mandate a 95 percent reduction in ocean discharges by wastewater plants in the State by the year 2040.
The largest users of water reuse in the municipal sector are the landscape and agricultural irrigation areas. For example, roughly 80 percent of municipal wastewater that is reclaimed is used for agricultural irrigation. Much of this reclaimed water in metropolitan areas is used for greenbelt and golf course irrigation.
The second largest use for reclaimed water is for groundwater recharge, whether as aquifer replenishment in Wichita, KS and many other locations, or in a barrier well system against saltwater intrusion in Southern California. Lastly, reclaimed water is being used for such things as cooling tower makeup, boiler feedwater, internal plant process water, and production both in the industrial and commercial sectors. Water reuse in the industrial market is rapidly becoming a part of many company’s core initiatives. Through the development of sustainability goals, these companies are being viewed as more environmentally friendly and socially conscious.
Wastewater can be treated and reused for a wide variety of purposes, however, the end use dictates the type of unit processes and final water quality required. The processes utilized at a minimum are tertiary filtration and progress to various levels of required treatment. One utility in California coined the expression “designer water” that covered the various levels of treatment required to meet the effluent objectives. For example, cooling tower make-up or boiler feedwater requires advanced biological treatment and reverse osmosis for boiler process water.
A small plant in Northern California treats wastewater from a lagoon with severe algae issues. A pilot test demonstrated the efficiency of using pretreatment along with tertiary filtration in order to meet Title 22 effluent standards. This innovative approach proved that a process of SAF (suspended air flotation) was able to increase the size of the small algae particles so the compressed media filtration system could easily produce Title 22 effluent required. This is just another example of how wastewater reuse can be utilized in both small and large facilities.
Most operating facilities use some type of filtration process in the treatment of secondary wastewater, however, there are other process trains that are being tested. This is especially true in the area of indirect and direct potable reuse. Other potential processes utilized may involve granular activated carbon coupled with some type of oxidation process, which could include ozone, UV, or a UV/ peroxide combination in some applications.
Filtration is a key unit operation in water reclamation, providing a separation of suspended and colloidal particles, including microorganisms, from water. Depth filtration was the most common method used for the filtration of wastewater effluent. In addition to providing supplemental removal of suspended solids, depth filtration is used as a conditioning step for effective disinfection. At larger reuse facilities mono- and dual-media filters were the most commonly used systems in the past for wastewater filtration with gravity or pressure as the driving force.
More recently, surface filtration has been used for either tertiary filtration or as pretreatment for membrane filtration or UV disinfection. In surface filtration, particulate matter is removed by mechanical means by passing water through media that includes thin filter material that can be composed of cloth fabrics, woven metal fabrics, or compressible media of synthetic materials. The advantage of the newer processes is more flexibility and more consistent operation at higher flux rates than possible with depth filtration, which translates to less space required for the filtration process.
In summary, the reuse of wastewater for all applications will continue to increase in the future. While acceptance in the public arena still lags behind the technical capability in the wastewater treatment market, the leaders in the reuse market are creating new tools to educate the general public. The expansion of wastewater reuse will continue to increase across the U.S. and abroad as critical stakeholders understand the potential and benefits to all involved.