Cross-Connections in Potable Water Distribution Systems: The Fate of Wastewater Microbes Introduced into a Distribution System Simulator
The U.S. EPA's drinking water Distribution System Simulator (DSS) was used to assess a long-term wastewater cross-connection's effect on the biofilm of a simulated water distribution system. Initial experimentation determined that wastewater injected into the DSS at 0.6% wastewater to system volume over a 24-hour period with 0.1% wastewater to system volume injected every eight hours resulted in continual detection of microorganisms with only slight measurable aesthetic problems (turbidity) within the DSS water column. The DSS was operated with a direct cross-connection of 0.3% wastewater to system volume per day for 90 days. During the cross-connection, tap water, wastewater, and system discharge water were monitored to ensure that the source of indicator organisms was only the wastewater cross-connection and that indicator organisms were present in the water column of the DSS. Following elimination of the cross-connection, samples showed that indicator organisms were removed from the water column within 24 hours, which was the hydraulic retention time of the system. Higher numbers of heterotrophic organisms were detected in the pipe discharge following the cross-connection. Similar results were obtained for the system biofilm. The effectiveness of free chlorine levels of 0, 1, 2, and 5 mg/L to inactivate microbes in biofilms growing on PVC and cast iron sample coupons following a wastewater cross-connection was determined. A significant reduction in all indicator organisms (total coliforms, Escherichia coli, Enterococcus spp., Salmonella spp., Aeromonas spp., aerobic endospores, and total culturable heterotrophic organisms) occurred in the biofilm after 12 hours for both non-chlorinated PVC and cast iron coupons. The indicator organisms were inactivated both in the biofilm and in the water column. Water column data showed that biofilm sloughing resulted in release of culturable heterotrophs and endospores into the previously water column throughout the evaluation period of 72 hours at all chlorine levels evaluated. Experimentation was conducted to determine if an existing biofilm could act as an inhibitor to introduced microorganisms, preventing them from being incorporated into the existing biofilm or forming a biofilm themselves. Coupons were challenged by a solution of a single indicator organism where one group had a biofilm in existence and a second group of sterile coupons had no biofilm. Effects of this challenge on sterile coupons and those that contained a pre-existing biofilm were addressed. The indicator organisms used separately in the solutions were Enterococcus faecium, Escherichia coli, Aeromonas hydrophilia, and a sterile solution that contained no organisms as a control. Glucose was added to separate groups to assess the lack of carbon within the system and its effect on biofilm development. Conclusions reached by this study were that existing biofilm acted as a barrier to indicator organisms regardless of the addition of glucose suggesting that an inhibition relationship exists between the existing biofilm and separately introduced indicator organisms. This suggests that a distribution system would be most vulnerable to inclusion of pathogenic biofilm during biofilm formation.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:biofilm cross connection
Date of Publication:01/01/2002