UC COOPERATIVE EXTENSION CENTENNIAL: Can irrigation with municipal wastewater conserve energy?
California Agriculture 68(3):73-73.
Published online July 01, 2014
Water conservation and energy costs were concerns 35 years ago, just as they are today. This study looked at whether reuse of wastewater on farmland would require less energy than discharging it to the ocean. If so, would it require more or less energy than importing fresh water for irrigation? In 1977, the energy costs came out about even. Would today's energy costs and irrigation/wastewater technologies yield a different result?
“Approximately 80 percent of the potential for reclamation in California is in basins where wastewater is being discharged to brackish or saline water — mainly the Pacific Ocean.
“One of the expected benefits of wastewater reuse is energy savings in those situations where reuse is an alternative to importation of fresh water‥… Two important questions, then, are: (1) Would reuse of wastewater on farmland require less energy than discharge to the ocean? (2) If so, would it require more or less energy than importation of fresh water for irrigation?
“Municipal wastewater discharged to the Pacific Ocean requires considerable energy for secondary treatment (biological oxidation and assimilation of organic matter) and pumping through a long ocean outfall. Since wastewater reused for irrigation of fodder, fiber, and seed crops requires only primary treatment (screening and settling processes), each acre-foot reused could save about 200 KWH in direct energy requirements — compared to ocean disposal — by eliminating the secondary treatment and ocean outfall pumping.
“Under current health regulations wastewater reused for pasture irrigation and surface irrigation of food crops requires secondary treatment. Therefore reuse instead of ocean disposal would save only the approximately 50 KWH otherwise required for outfall pumping. Wastewater reused for sprinkler irrigation of food crops requires secondary treatment plus chemical coagulation and filtration. Such reuse would require slightly more direct energy — possibly 10 KWH/AF — than ocean disposal of the wastewater.
“When only these direct energy requirements are considered, it appears that irrigation with wastewater could save very large amounts of energy compared with importing fresh water. However, elevation and quality differences tend to offset the benefits.”