Dodemaide Circuit Tank,12 January 2015 (Melton South)
Issue – Routine sampling at the tank resulted in detection of 1org/100mL of E.coli in the presence of <0.05mg/L free chlorine and 0.08mg/L total chlorine.
Actions – The tank was isolated from supply and spot-dosed with chlorine to return a residual of 1.2mg/L free chlorine and 1.4mg/L total chlorine. The reticulation system was flushed to then draw freshly chlorinated water from the tank via pump station. The root cause is suspected to be minor ingress of rainwater into the tank around the inspection hatch. Subsequent works were undertaken to improve the sealing of the hatch.
Resampling was conducted for three consecutive days with all results clear of E.coli. This tank will also be included in 2015/16 works to improve tank roof integrity following the trial of new technology.
“Coliforms are Gram-negative, non-spore-forming, rod-shaped bacteria that are capable of aerobic and facultative anaerobic growth in the presence of bile salts or other surface active agents with similar growth-inhibiting properties. They are found in large numbers in the faeces of humans and other warm-blooded animals, but many species also occur in the environment.
Thermotolerant coliforms are a sub-group of coliforms that are able to grow at 44.5 ± 0.2°C. E. coli is the most common thermotolerant coliform present in faeces and is regarded as the most specific indicator of recent faecal contamination because generally it is not capable of growth in the environment. In contrast, some other thermotolerant coliforms (including strains of Klebsiella, Citrobacter and Enterobacter) are able to grow in the environment and their presence is not necessarily related to faecal contamination. While tests for thermotolerant coliforms can be simpler than for E. coli, E. coli is considered a superior indicator for detecting faecal contamination…” ADWG 2011