Many water and wastewater laboratories throughout the UK are switching their E. coli sludge testing procedures from traditional most probable number (MPN) methods to a rapid microbiological test, the Colilert-18/Quanti-Tray/2000 designed and manufactured by IDEXX.
The principle drivers behind the trend are reduced sample handling time, the ability to provide complete results in 18 hours compared to a possible 3-4 days, and the method’s recent inclusion in the UK Drinking Water Inspectorate’s the Microbiology of Sludge, Part III, Methods for the Isolation and Enumeration of Escherichia coli.
Contract laboratory ALcontrol and water company South West Water are just two of the UK laboratories which have recently adopted the new method for E. coli testing.
One of Europe’s leading analytical service organizations, ALcontrol currently carries out up to 100 sludge tests per week at its laboratory in Rotherham on behalf of regional water company Yorkshire Water Services. ALcontrol had previously employed a MPN test, which used a multiple tube fermentation process, involving various stages of dilution and as many as 15 separate test tubes per sample. The company switched to Colilert-18/Quanti-Tray 2000 as it involves less effort and gives completed results in 18 hours.
Analyzing samples
Using the new method, ALcontrol takes a suitable specimen of the sludge sample, which is then diluted to 100ml with RO water and homogenized. Colilert-18 reagent is then added to the sample and dissolved before being poured into the Quanti-Tray/2000, a multi-welled, disposable vessel that holds a 100ml sample.
The mixture is automatically distributed and sealed into 97 wells in less than 20 seconds. The sealed tray is then incubated for 18 hours and the colored wells are counted. A table converts the number of yellow wells to a coliform count and yellow/fluorescent wells to an E. coli count. The sampling system eliminates subjective colony interpretation and time-consuming confirmation.
In most instances, ALcontrol reports the results using electronic data transfer, giving E. coli count for 1g of both wet and dry samples. “The MPN test that we used previously was extremely laborious, involved multiple stages of preparation and an extensive amount of laboratory equipment,” said Clive Thompson, chief scientist at ALcontrol. “With a total hands-on time of less than one minute per sample, Quanti-Tray is faster, simpler and cost-effective for E. coli analysis.”
South West Water conducts 4,000 E. coli sludge tests per year on both wet and dry samples. The Exeter laboratory, which employs 20 microbiologists, regularly samples 24 sites in rotation over a two-week period, with five pots of sludge taken from each site.
As with ALcontrol, South West Water had previously used an MPN test but decided to conduct a validation study of the Colilert-18/Quanti-Tray method for submission. Samples were run in triplicate for digested, limed, pressed and thermally dried and composted sludge. Data indicated the new testing method compared well with traditional methods for sludge analysis.
