Summary and conclusions, Miljøstyrelsen

Afgivelse af organisk stof fra polymere materialer - mikrobiel vækst

Summary and conclusions

Danish water supply is based on ground water treated without disinfectants. The treated water has in general low bacterial numbers when leaving the water works, but this does not necessarily ensure low bacterial numbers at the consumers tap. Throughout the distribution network the concentration of bacteria in the water is likely to increase, a phenomenon called bacterial regrowth. This regrowth is mainly controlled by retention time, temperature and level of biodegradable substrate. In drinking water the main part of the bacteria is heterotrophic and thus the concentration of organic carbon often will be the limiting growth factor. The bacteria can only assimilate and grow on a minor fraction of the total organic carbon, which is called Assimiable Organic Carbon (AOC). Some AOC will be present in the water when leaving the water works, but the concentration may increase in connection with leaks in the pipe system or by release from the materials of the distribution system. The use of polymeric materials (plastic) in distribution systems is still increasing, and it is expected that 75% of the pipe system will consist of PE and 19% of PVC in the near future.

The purpose of this project was to evaluate the significance of biodegradable substrate migrating from polymeric materials compared to levels of AOC in drinking water in general. This was evaluated by measuring the AOC migrating into water in contact with the materials.

A major part of the project focused on developing a suitable method with the main efforts on a two-step procedure to determinate the migration:

Extraction without bacteria
Subsequent AOC-measurement of the extract

The materials were extracted by submerging pieces of the material into water. In this way the method could be used to test other components besides pipes, e.g. fittings. By extracting the materials without bacteria, the extraction condition could be set without regarding the growth conditions for the bacteria: E.g. high/low temperature, chlorinated water, high/low pH. After the extraction the material pieces were removed and the AOC concentration in the extract was measured. In this project a slightly modified AOC measurement was used: The sterile extracts were only inoculated with P17 and incubated at 15° C. During incubation the numbers of P17 were determined by plate counts using R₂A-agar. The maximum bacterial number was converted into an AOC value by a yield constant for growth of the bacteria on acetate.

Before extraction the material pieces were rinsed by 24 hours retention in potable water followed by rinsing in running potable water for 1 hour. The material pieces were afterwards pasteurised in potable water to inactivate the bacteria on the material surfaces. The material pieces were then transferred into pasteurised test water for extraction.

While optimising the method the following parameters were evaluated:

Temperature for the extraction

Extraction time

The ratio between material surface and water volume (S/V ratio)

Shaking during extraction

Extended extraction time resulted in an increased migration. Shaking during extraction also increased the migration with approximate 50%. An increased amount of material giving an increased surface area of the material in the same volume of test water (increased S/V ratio) resulted in an increased AOC value, until a certain S/V ratio above which no further substantial increase in migration was observed. Expressing the migration as a flux (mg acetat-C/day/cm²), the flux was inversed proportional to the S/V ratio.

It was sometimes a problem to inactivate all the bacteria present on the materials surfaces. Different types of pasteurisation and chlorination were tested, but it became necessary and appropriate in each case after the extraction to test if bacteria have contaminated the extract. Measurements of adenosine triphosphate (ATP) proved to be an effective method due to its rapid analysis and high sensitivity.

Furthermore, ATP measurement could be used to measure growth curves for P17 because of the good accordance between growth curves measured by CFU and ATP.

The initial investigations lead to a procedure where materials were extracted for 15 days at 20° C with shaking. Five polymeric materials were tested: Two PE materials (black and blue) and three PCV materials (with respectively lead, zinc/calcium or organic stabilizers), which resulted in flux values in the range 3.5-5.5´ 10^-3 mg acetat-C/day/cm².

The obtained results and the observed effect of S/V ratios can theoretically be related to a release from the materials of <1 mg acetate-C/L/day in main pipes, 1-3 mg acetate-C/L/day in supply pipes and 3 mg acetate-C/L/day in service pipes/house installations. A survey involving 9 Danish water works (Jørgensen et al., 2002) showed, that the general AOC content in treated water was grouped around values of 4-6 m g acetate-C/L and 20-39 mg acetate-C/L. Migrations from polymer materials will be of less importance in water types from the last group. In water types from the first group, prolonged retentions times (e.g. supply pipes to rural districts or stagnant water in a house installation over a weekend) may more than double the waters AOC concentrations due to release from materials.