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New effluent bacteria level testing method pays off for Sausalito-Marin City Sanitary District
The Sausalito-Marin City Sanitary District (SMCSD) is a publicly owned treatment works in California, responsible for collecting, treating and discharging domestic wastewater into San Francisco Bay.
The SMCSD, like most wastewater treatment facilities in the U.S., is regulated at the state level through the National Pollution Discharge Elimination System (NPDES). Permits issued through NPDES specify discharge limits for certain bacteriological indicators of water quality.
Under the direction of the U.S. EPA, many state agencies, including California’s Regional Water Quality Control Board (RWQCB), have begun to change their requirements for bacteriological testing. Many wastewater plants are switching from fecal coliforms to monitoring for E. coli and Enterococcus, because the EPA has identified these bacteria as the best indicators of fecal contamination in water. Testing for these bacteria in wastewater helps treatment facilities determine the effectiveness of their treatment and disinfection processes.
Options to explore
In the 2000 revision of the SMCSD’s NPDES permit, the plant was given the option to explore other testing alternatives to determine bacteria levels in their effluent.
Previously, the SMCSD enumerated total coliform bacteria as its indicator of water quality. The SMCSD decided to take advantage of the flexibility in its NPDES permit and evaluate the option of testing for Enterococcus instead of total coliforms.
Because Enterococcus is a more specific indicator of fecal contamination in water, Laboratory Manager Omar Arias-Montez determined the bacterium would be a more specific indicator of the effectiveness of the SMCSD’s treatment works.
By comparing plant compliance under the Enterococcus requirement (30-day geometric mean of 35 MPN/100mL) to plant compliance under the total coliform requirement (5-day mean of 240 MPN/100mL), the SMCSD could evaluate potential reductions in chemical disinfection costs.
As part of this option’s ultimate approval, the SMCSD would have to demonstrate to the RWQCB that a change from total coliforms to Enterococcus would not affect public health or adversely impact San Francisco Bay.
To determine if a switch should be considered, the SMCSD conducted a six-month study from August 2002 to January 2003, testing treated effluent and receiving waters in San Francisco Bay for Enterococcus. Two Enterococcus testing alternatives were examined, the first being membrane filtration.
Membrane filtration relies on filtering 100mL of water through a membrane and then placing the membrane on a growth medium. The medium is incubated for 24 hours at 41ºC and a direct bacterial count is made on the membrane. Bacteria then need to be confirmed biochemically in the quality control phase of the test.
The second option was the Enterolert test manufactured by IDEXX Laboratories. The test is run in a multi-well Quanti-Tray and enumerates Enterococcus in a 100mL sample in 24 hours without the need for additional biochemical confirmations. The two key objectives for the SMCSD study included:
Results prove positive
The SMCSD determined through the six-month period from August to January 2002 to 2003 that 31,324 gal. of hypochlorite and 31,811 gal. of bisulfite were used to comply with the total coliform requirement.
For the same six-month period in 2002 to 2003, during the Enterococcus study, the district used 64% less hypochlorite and 72% less bisulfite in meeting the Enterococcus requirements.
Extrapolated over 12 months, the SMCSD’s expected savings could total $45,968 per year. In conjunction with the U.S. EPA’s endorsement of Enterococcus as a better indicator of water quality, the significant cost savings justified switching indicators; the SMCSD could save money and improve public health protection simultaneously.
To compare the membrane filtration method to the rapid Enterolert method, SMCSD’s Arias-Montez evaluated the total cost of testing using each.
Arias-Montez estimated that using membrane filtration to run SMCSD’s daily compliance testing would require about 40 minutes per day. In addition, hands-on time was calculated for steps associated with the 24-hour incubation period as well as the reading and recording of results. Beyond routine sampling and testing, quality control and verification procedures are required and take an additional two to four hours to perform.
Arias-Montez found that Enterolert provided measurable time-savings when compared to the membrane filtration method. “The Enterolert test takes significantly less time to perform due to fewer steps,” he said.
The total number of steps were five for Enterolert, compared to almost 40 for membrane filtration, according to Arias-Montez. Using the testing method, only 20 minutes of an analyst’s daily schedule was needed to run compliance Enterococcus testing, compared to 40 minutes using membrane filtration. More importantly, according to Arias-Montez, “the quality control procedure for Enterolert required only 25 to 30 minutes per lot, compared to two to four hours for the membrane filtration method.”
Arias-Montez concluded that the testing system brought the SMCSD significant time and labor savings.
He determined that the material cost associated with membrane filtration totaled approximately $11.75 per test compared to less than $6 per test for Enterolert. In addition, it required annual labor costs of $18,039 compared to $45,037 for membrane filtration, a 72% reduction in labor costs for the SMCSD.
In summary, a number of important issues surfaced during the SMCSD’s Enterococcus monitoring study. The SMCSD’s NPDES permit provided valuable flexibility—the option of an alternative method for bacterial monitoring.
Because Arias-Montez was able to evaluate Enterococcus monitoring against total coliform monitoring, significant cost savings were realized due to reduced chemical consumption in their treatment processes.
“Choosing Enterolert over membrane filtration would not only be more cost-effective, but would make monitoring for permit compliance easier,” Arias-Montez said.
In addition to cost-savings and compliance, a switch to Enterococcus monitoring for the SMCSD means more efficient measure of water treatment and better protection of public health.