Disinfection Byproducts Formation in Mammoth Title 22 Recycled Water and their Fate in Landscape Irrigation

Abstract

The Mammoth Community Water District (MCWD) operates a 1.5-mgd tertiary wastewater treatment plant without nitrification and disposes the disinfected plant effluent in a 70-acre percolation pond. They intend to produce Title 22 recycled water to irrigate two golf courses, Sierra Star and Snowcreek, as a partial solution to reducing groundwater demands on the local aquifer. However, there are concerns about protecting the basin groundwater quality from disinfection byproducts (DBPs) and other pollutants present in the recycled water. This study was conducted to (1) examine the existing levels of DBPs in the existing effluent and understand the fate of DBPs in the present effluent disposal in the percolation pond, (2) determine DBPs formation from disinfection of recycle water including total trihalomethanes (TTHM) and haloacetic acids (HAA5), and (3) predict, using a mass balance model, the fate and transport of TTHM and HAA5 to groundwater and the impacts on groundwater quality from using recycled water on the golf courses.In the present treatment plant chlorinated effluent, concentrations of TTHM and HAA5 went from 20 and 58 μg/L, respectively to non-detectable in the groundwater wells around the percolation pond. Available literature suggests that dilution, bioconversion and volatilization are major factors transforming and/or reducing TTHM and HAA5. Bioconversion and dilution were concluded to be the major processes responsible for the reduction of DBPs in the percolation pond system. Drum-sized batch chlorination tests and bench-scale tests performed to predict the concentration of DBPs in the recycled water showed average TTHM and HAA5 concentrations of 3.1 µg/L and 41.7 µg/L, respectively. These values are significantly lower than the DBPS levels found in the present plant effluent. The higher HAA5 concentrations compared to TTHM indicate that chloramines are preferentially formed when chlorinating un-nitrified effluent, but rapid halogenic reactions forming HAA5 are difficult to control and only slightly reduced. All THM compounds that are in chloroform form are likely to be completely volatilized during spray irrigation. The estimated chloroform concentration in air was calculated to be less than the chronic inhalation Reference Exposure Level (REL) of 300 μg/m 3 for chloroform (CalEPA). The mass balance model, along with other study results, led to the conclusion that DBPs released to the golf course with the use of recycled water would be reduced biologically in the vadose zone to non-detectable levels in the groundwater and that recycled water use in the golf courses is not likely to have an adverse effect on basin groundwater resulting from DBP contamination.