Independent thesis Advanced level (professional degree), 20 credits / 30 HE credits
Bacterial surface associated communities, so called biofilms, located on the inside of drinking water distribution pipes, are believed to be responsible for lowering the drinking water quality by releasing bacteria into the water stream. As there were some indications that a strong electric field along the pipes inner walls could deter and possibly kill bacteria trying to adhere to the surface, it was the aim of this thesis to investigate that effect and its possible application in tap water.
A closed system was set up with three cylindrical containers, made from short pieces of the pipe intended to be used in water distribution systems, with isolated electrodes on the inside. The voltage (0 – 10 kV) as well as pulse length (0 – 110 ms) and pulse repetition rate (1 Hz or 5 Hz) was varied in order to find the optimal settings. Through the system, liquid acetate enriched minimal media inoculated with
Comamonas dentitrificans 110 as model organism was being pumped. Each run lasted for a week, after which the biofilm in the containers were stained with 1 % crystal violet, and the biofilm formation analyzed.
The above system was not ideal for studying any effects of the electric field; the results were inconclusive though a trend showing biofilm deterrence could be seen, with long pulses at a high pulse repetition rate being the optimal setting.
Bacterial concentrations and ion strengths in the three container system could not be kept comparable to that in distribution pipes, so a lone container, was set up in an open system, with only tap water (adjusted to 30°C) running through it for 4 months, in order to assess the biofilm thickness and the speed at which it forms. A thin but evenly distributed biofilm was seen on the inside of the tap water container indicating that further testing could be done in tap water with a very low risk of biofilm not forming inside the control container.
To clarify how the electric field affected biofilm, small volumes of cultures containing detached pieces of biofilm were put into a 1 ml cuvette over which an electric field similar to that in the big system was applied. Electrolysis was made using similar solutions so that comparisons could be made.
Little disturbance could be observed in the structures when applying an electric field but the effects seen in these test was very subtle compared to electrolysis were almost all biofilm fragment had lost their initial structure.
Whether strong electric fields will prevent biofilm formation is unclear, it seems unlikely, but not impossible. The field will have a wider range in tap water and maybe boost the effect of chlorine, but will also go up against much tougher bacteria.