The effect of water treatment processes on the corrosion and biofilm growth promoting properties of water within distribution systems

Concern over the levels of pesticides and halogenated organics in potable water have led to increased treatment processes. Removal of these micropollutants can be achieved using oxidation with ozone gas. A further effect of this treatment is the increase in the biodegradability of the organic matter of the water. This may result in an increase in the biological activity within distribution systems and associated problems. Increased biological activity may cause increased corrosion, objectionable tastes and odours, interference with the testing for faecal contamination, the possibility of protection of pathogenic bacteria and the occurrence of bacterial metabolic products of public health significance such as nitrite. The aim of this research is to examine the affect of ozonation on the biodegradability of the water via a model distribution pipe rig and monitoring water quality in an actual water distribution system. Water treatment plants in practise follow ozone treatment by granular activated carbon (GAC) adsorption. This reduces the subsequent organic loading to the water distribution system. The contact time, age of the carbon and backwash regime will affect the mode and degree of organic carbon removal. The research showed that inclusion of ozonation stages increased the amount of assimilable organic carbon (AOC) in the treatment plant effluent water despite a final GAC stage. This resulted in increased growth of biofilm on the model distribution pipe rig. These results were backed up by the results of the actual water distribution system water quality monitoring. It was seen that ozonation resulted in an increased suspended bacterial population and an increased total organic carbon removal rate within the distribution system. Further research into the disinfection of biofilm showed monochloramine disinfection to be more effective than free chlorination on biofilm growth on surfaces with a high inorganic content e.g. cast iron. The reverse was true for corrosion free surfaces e.g. polyethylene. It was further seen that the growth of biofilm is closely linked to the extent and structure of corrosion deposits. Particles eroded from cast iron pipe surface were seen to deposit onto polyethylene pipes and substantially increase the measured biofilm