UC Irvine

Water shortages in the western United States have made alternative sources of water for landscape irrigation increasingly important. Tertiary treated wastewater is used to irrigate greenbelt areas, school grounds in certain communities and many parks in Southern California. However, the disinfection (or the microbial removal efficiencies) are directed toward removal of state standards that require the removal of coliform bacteria. Other bacterial removal that can have a negative effect on the quality of the water are not routinely monitored for and necessarily controlled. As a result, inefficient disinfection, caused by inadequancies or inconsistencies in filtration and disinfection processes has become a routine phenomenon.

Occurrence of bacteria in the recycled distribution system can relate to biofouling of pipes and blocking of emitters in the irrigation system, increasing maintenance costs and performance failures that can require landscaping replacements. Furthermore, if ammonia oxidizing bacteria enter the distribution system and become established in recycled water reservoirs, disinfection residuals (chloramines) can be degraded and disinfection residual lost. For these reasons this study applied quantitative polymerase chain reaction to quantify removal efficiencies of bacteria and compare the numbers of total bacterial numbers as well as the ammonia oxidizing bacteria (AOB) from four different stages of a tertiary wastewater treatment plant secondary flow (equalization basin, before, after filtration and from the P11flow equalization basin, after chlorination respectively) located in Southern California. The results showed sampling machine/pump used to extract the water at any stage must be cleansed regularly. This finding is based on the first 5 weeks of our samples that showed unexpectedly high CFU values, while the sampling was being carried out with a sampling machine, as opposed to 9 later samplings when we used a sampling stick. Moreover, the plate counts and qPCR analysis across 18 weeks of samples indicated that the bacteria numbers as well as the log removal through the four stages varied largely from week to week, with the tertiary treatment log removal ranging from 1-5 logs. Also, the log removals across consecutive treatment stages, such as EQ-before filtration, before-after filtration, after filtration-after disinfection and before filtration-after disinfection were in the range of 0-3 logs, 1-2 logs, 1-7 logs and 1-5 logs respectively.