Investigation of aftergrowth potential of polymers for use in drinking water distribution: Factors affecting migration of bioavailable compounds investigated by batch set-ups and continuous flow model systems

Polymeric materials are to an ever increasing extent replacing the traditionally used materials in drinking water distribution systems. The polymers are preferred since they are more flexible, less subjected to breakage and constructions cost are lower compared to the traditional materials. However, migration of bioavailable organic compounds from the materials can cause elevated bacterial numbers in the distribution systems. Although bacteria always will be present in distributions systems as planktonic cells and as a biofilm of surfaces, elevated bacterial numbers are undesirable since they can cause operational, hygienic and esthetical problems.

The purpose of the study was to enhance the understanding of how to investigate polymers aftergrowth potential, in order to gain better insight into how polymers affect bacterial levels in distribution systems.

Both an abiotic batch set-up, extracting the materials under sterile conditions and a biotic batch set-up, incubating the materials in the presence of an active biomass, were applied in the investigations. In the abiotic test, the migration of bioavailable compounds was measured as AOC_P17 in the water phase after removal of the material. In the biotic test, the migration of bioavailable compounds was measured as biomass production in the water phase and on the material surfaces determined by ATP measurements.

It was shown with both test alternatives that the migration of bioavailable organic compounds was elevated within the first weeks of use, followed by a lower but constant level over the 16 weeks investigated.

The migration of bioavailable organic compounds from the material surfaces was influenced by diffusion over the solid-liquid boundary layer under sterile conditions. This caused an inversely proportional relationship between amount of migration expressed per unit surface area of material and ratio between material surface to water volume (S/V-ratio). The thickness of the solid-liquid boundary layer was affected by gentle shaking of the water phase, which increased the migration under sterile conditions. With the presence of an active biomass, which continuously consumed migrating bioavailable compounds, the migration limitation due to diffusion over the solid-liquid boundary layer was significantly reduced. As result neither varying S/Vratios nor gentle shaking of the water phase affected the amount of migration with the materials tested.

It was shown that the characteristics of the inoculum highly influenced the amount of biomass produced during biotic investigations. Therefore demands to the inoculums’ microbial diversity are required to ensure uniform results.

No replacement of the test water versus replacement once a week or once every second week, did not appear to affect the biomass production over 16 weeks of biotic incubation.

Incubation temperatures of 10ºC and 25ºC had no significant effect on the migration of bioavailable compounds measured as biomass production during biotic incubation, though a tendency for higher biomass densities was seen at the lower temperature.

In addition to batch investigations, biofilm formation on polymers can be investigated in continuous flow model systems. Since growth will be supported both by substrate from the water phase and from the material, special requirements apply for flow model systems for use with polymers. A continues flow model system was developed, taking into account different combinations of material contact time and flow velocities. Commercial available pipe was used as exchangeable test pieces for biofilm sampling. Three materials were investigated in separate systems: PVC-C as test material, stainless steel as negative control and PVC-P as positive control.

With the materials used, there was no significant difference between biofilm densities in batch set-ups and in continuous flow model systems over 16 weeks of incubation. During 43 weeks of investigation the biofilm density continuously increased on all three materials, but no significant effect of different combinations of material contact time and flow velocity was observed. A broader range of materials with varying aftergrowth potentials may stress the effect of contact time and flow velocity.