Report No DWI0715



May 1995


The potential of organic substances in materials in contact with drinking water to promote the growth of micro-organisms can cause a deterioration in water quality. This deterioration may be as a result of growth on nutrients leaching from the material or from surface growth on insoluble organic compounds present in the material. To ensure that only suitable materials are used in the construction of water distribution systems and fittings they must comply with BS6920. Section 2.4 of this standard addresses the influences due to microbial growth.

A recent study carried out for the American Water Works Association (AWWA) recommended some changes to BS6920 Section 2.4 to improve the accuracy of the test, one of which was to increase the head space used in the test procedure. The Department of the Environment funded a study at WRc (PECD 7/7/370) which addressed the impact of head space on the materials testing procedure (WRc report DoE 3696/1 July 1994). The AWWA study suggested that increase of the head space improved reproducibility, this was not confirmed by the WRc study. Furthermore, an increase in head space was shown to reduce the sensitivity of the test. The WRc study did suggest ways in which the sensitivity of the procedure may be improved.

To assess whether improvements could be made to this section of 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 options to improve the sensitivity of the test method.

In the study the impact of reduction of the head space, increase in the surface to volume (S/V) ratio of the test material to the test water and changes to the period between water change were investigated. Several materials, which would normally be expected to give pass, fail and borderline responses, were used in the study. The research programme involved two phases. In the first phase the three options for improving sensitivity were investigated independently, so that an assessment of their individual impact on sensitivity could be made. In the second phase the most promising combinations of these three options were investigated in an attempt to produce an optimum of sensitivity.

The results from the first phase indicated that sensitivity was improved by both a reduction in head space and also by increase of the S/V ratio, there was a less marked improvement by the increase in the period between water change. The results from the second phase, where combinations of reduced head space and increased S/V ratios were studied, indicated that there was an additive effect of benefit. The greatest improvement in sensitivity was achieved with the combination of lowest head space and highest S/V ratio.

The study produced the following conclusions:

    1. Reduction in head space increases the sensitivity of the MDOD test.
    2. Increase in surface to volume ratio increases the sensitivity of the MDOD test.
    3. Period between water change has a limited effect on the sensitivity of the MDOD test.
    4. A combination of increased surface to volume ratio and reduction of head space produced the optimum increase in the sensitivity of the MDOD test.
    5. A new acceptance criteria will need to be defined for the new higher sensitivity test conditions.

The conclusion from this study is that the sensitivity of BS6920 Section 2.4 can be increased by minor modifications to the test procedure. The increased sensitivity is the result of an increased MDOD from the test material which effectively improves the limit of detection. As the MDOD of any material tested under the higher sensitivity conditions would be higher than that obtained with the current standard conditions, it will be necessary to make an upward adjustment of the acceptance criteria currently applied in this procedure. Further work would be required to define the new criteria.

To aid in the definition of a new criteria level it may be possible to produce a reference material. This could be based on the epoxy resin used in these trials, but formulated to contain a defined amount of benzyl alcohol which would produce an MDOD of 2.4 mg/l using the existing standard procedure. This material when tested under the conditions recommended in this report should produce an MDOD value at the required new acceptance criteria level. By defining the error on producing this result it should be possible to define the range where a retest would be required. This reference material may also be used as an MDOD standard which would be of value in determining whether test laboratories were producing comparable results.

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