A Low-Tech Treatment Strategy for Treating and Reusing Wastewater in Arid Lands

Many cities in arid regions with limited renewable surface or ground water supplies will need to consider reusing treated municipal wastewater for long-term sustainability. On the U.S. Mexican border, a lowtech treatment train composed of aerated lagoons, constructed treatment wetlands, and a soil aquifer treatment system was evaluated as a method to treat and store wastewater for later use. Literature review and field and lab experiments show that such a system could provide water with low nitrate and DOC that would probably require only disinfection prior to municipal use. Results were used to propose design and operational guidelines for a low-cost, low-tech treatment system. The potential impact of recharging water, which is now being discharged directly into the Santa Cruz River at Nogales, on the flow of the river was also evaluated. TH E NEED TO REUSE W ASTEWATER: WATER SH O RTAGES ALONG THE U.S. MEXICAN BORDER Potable water is a scarce commodity in many parts of the world, particularly in arid regions. The problem of water scarcity involves both the quantity and quality of supply. Water quantity is often measured in terms of scarcity at < 1000 m/year per person and water stress at < 1,700 m/year. These values include requirements for household, agricultural, and industrial use, including energy production. Using these criteria, 28 countries with a total population of 335 million experienced water scarcity or stress in 1990. By 2025, 46 to 52 countries with a total population of 2.8 to 3.3 billion people will experience water stress or scarcity. Increasing population, rising per capita consumption, and declining water availability will exacerbate the problem in the future (Postel 1997). Poor quality of what little water is available further amplifies the problem of obtaining potable water in arid lands. W orldwide, about 20 million children die of waterborne diseases each year (Tchobanoglous and Schroeder 1985). Diarrheal diseases are a major cause of infant mortality in the Mexican border region; rates of hepatitis A are seven times higher in Nogales, Arizona, than in the general population (Sanchez 1995; Varady and Mack 1995). Limited water supply and poor water quality are key concerns along the U.S. Mexican border because the region is arid, the population is growing rapidly, and the area is economically depressed. The problem of providing an adequate supply of potable water has generated a growing interest in the reuse of municipal wastewater effluent. Treated wastewater can be reused either directly or indirectly. The most common direct use of treated wastewater is irrigation of nonfood crops or turfgrass (e.g., parks and golf courses) or industrial applications (e.g., cooling water). Treated wastewater is rarely used directly as municipal water supply. Indirect reuse generally means that the water is stored for some period of time prior to reuse. An increasingly common practice in the arid southwestern United States is the storage of reclaimed wastewater in underground aquifers. In most ongoing or planned projects in the United States, wastewater is treated at a fairly high level prior to reuse. In larger cities in the United States, treatment prior to recharge would typically consist of conventional secondary treatment followed by nitrification and denitrification (NDN) or advanced secondary treatment with simultaneous NDN. Further treatment may include sand filtration to reduce suspended solids, thereby reducing the potential for clogging during recharge; reverse osmosis to reduce total dissolved solids (salts); and chlorination and/or UV disinfection to kill pathogens prior to recharge. These treatment systems generally have high capital costs, extensive high-tech maintenance requirements, and high energy requirements. They are therefore not well suited for use in small municipalities in the United States or in less developed countries. The goal of this project, which was funded by the Southwest Center for Environmental Research and Policy (SCERP), was to evaluate the technical feasibility of a low-tech, low-cost system to treat and reuse wastewater on the U.S. Mexican border. A low-tech approach was considered for the following reasons: The U.S. Mexican Border Environment