The relationship between pipe material and biofilm formation in a laboratory model system

The aim of this study was to compare biofilm accumulation and heterotrophic bacterial diversity on three pipe materials-cast iron, medium density poly ethylene (MDPE), and unplasticised polyvinyl chloride (uPVC)- using a labor atory model system run over a short period (21 d) and a longer period (7 mo nths). Newly Modified Robbins Devices (nMRD) were run in parallel, each con taining 25 discs of one material with cold tap water flowing through the de vices at 3 ml min(-1) (Reynolds Number 9.05) for 21 d. The numbers of bacte ria on each material increased exponentially between 0 and 11 d when the bi ofilm viable count remained constant. The mean doubling times of the hetero trophic population on the materials during the exponential phase was 13.2 h for cast iron and 15.6 h for MDPE and uPVC. The same experiment was repeat ed under different environmental conditions with a lower temperature, highe r flee chlorine and lower number of organisms ml(-1) of incoming water. The exponential phase lengthened to 16 d but the steady state count remained t he same. The mean viable count after 21 d and after 7 months was on average 97% higher on cast iron than on the other materials. Very few different co lony types were isolated from each material with the largest number (nine) recovered from cast iron. The numbers of planktonic bacteria in the effluen t water leaving each of the nMRDs directly correlated with the numbers in t he biofilm phase on each of the materials. In addition the distribution and thickness of the biofilms on the MDPE and uPVC were observed using confoca l scanning laser microscopy. In conclusion, MDPE and uPVC support the lowes t numbers of bacteria in a steady state biofilm in the short term (21 d) an d over a longer term ((7 months). The diversity of heterotrophic bacteria w as greatest on cast iron.