Report No DWI0768

IMPACT OF HEAD SPACE ON TEST FOR MICROBIOLOGICAL GROWTH [BS6920 Section2.4]

DWI0768

Jul 1994

SUMMARY

A recent study carried out for the American Water Works Association recommended some changes to Section 2.4 of BS6920 to improve the accuracy of the test. Briefly these changes were to increase the head space used in the test vessel, and to standardise the inoculum used in the test. To assess the relevance of one of these changes to the testing of materials in the UK the Department of the Environment, as part of Research Contract PECD 7/7/370 managed by the Drinking Water Inspectorate, asked WRc to investigate the influence of head space on the results obtained from the test method.

In the WRc study the impact of a range of head spaces, which included the current BS6920 value of 15% and the AWWA suggestion of 35%, were investigated. Several materials, which would normally be expected to give pass, fail and borderline responses, were used in the study. The effect of changes to the inoculum will be studied in another project.

The results of the study did not agree with the recommendations of the AWWA study, and produced the following conclusions:

    1. Increase in head space reduces the sensitivity of the MDOD test.
    2. No evidence that reproducibility is reduced at 15% head space.
    3. Different acceptance criteria required for different head spaces.
    4. Not all materials produce a plateau of DOD, therefore concern over a fixed period for MDOD determination.
    5. Water change period affects MDOD result.

This study has demonstrated that the MDOD test in BS6920 Section 2.4 would not be improved by adopting the recommendations of the AWWA study.

The WRc study, whilst not confirming the results of the AWWA study, has suggested several ways in which the current MDOD method may be modified to improve its sensitivity. Methods being developed in Holland and Germany already have greater sensitivity, and if a more stringent acceptance criteria was to be agreed for the European Standard, then the current BS6920 Section 2.4 MDOD test would no longer be appropriate. Research is therefore required to improve the sensitivity of BS6920 so that it remains the standard method.

This study has suggested three ways in which sensitivity could be improved, which are as follows:

    1. Reduction of the head space
    2. Increase in the time between water changes
    3. Increase in the ratio of surface area of material to volume of test water

It is recommended that these three options are investigated so that the UK is in a stronger position to promote the MDOD test procedure.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.