Water quality modelling for drinking water distribution systems

A dynamic water quality model for drinking water distribution systems has been developed in this study, to include processes that occur in the bulk water, as well as those occurring in the biofilm of a distribution system. The model has been validated against water quality data obtained from extensive experimental studies conducted with biofilm reactors. Protein and carbohydrate densities in the biofilm represent biofilm biomass. This model is able to predict the disinfectant decay due to organic matter in the bulk water, as well as that due to biofilm. It simultaneously predicts the growth of biofilm in terms of carbohydrate and protein densities. While this model is complex enough to describe the water quality changes in a distribution system, it is also simple enough to be incorporated into a hydraulic model in order to describe the interaction between disinfectant and microbiological quality throughout a drinking water distribution system.

Persistence of Escherichia coli on injured iceberg lettuce in the field, overhead irrigated with contaminated water

Fresh produce is increasingly implicated in food-related illnesses. Escherichia coli can survive in soil and water and can be transferred onto plant surfaces through farm management practices such as irrigation. A trial was conducted to evaluate the impact of field conditions on E. coli persistence on iceberg lettuce irrigated with contaminated water, and the impact of plant injury on the persistence of E. coli. Lettuce heads were injured at 14, 7, 3, 2, 1, and 0 days before inoculation, with uninjured heads used as a control. All lettuce heads (including controls) were overhead irrigated with a mixture of nonpathogenic E. coli strains (10^sup 7^ CFU/ml). E. coli counts were measured on the day of inoculation and 5 days after, and E. coli was detected on all lettuce head samples. Injury immediately prior to inoculation and harvest significantly (P = 0.00067) increased persistence of E. coli on lettuce plants. Harsh environmental conditions (warm temperatures, limited rainfall) over 5 days resulted in a 2.2-log reduction in E. coli counts on uninjured lettuce plants, and lettuce plants injured more than 2 days prior to inoculation had similar results. Plants with more recent injuries (up to 2 days prior to inoculation) had significantly (P = 7.6 × 10^sup -6^) greater E. coli persistence. Therefore, growers should postpone contaminated water irrigation of lettuce crops with suspected injuries for a minimum of 2 days, or if unavoidable, use the highest microbiological quality of water available, to minimize food safety risks.

Efficient management of drinking water distribution systems through the application of water quality modeling

The quality of drinking water generally degrades when it is delivered through a distribution system due to the decay of disinfectant, which subsequently allows the re-growth of microorganisms in the distribution system. A model that describes the changes that occur in the water quality in distribution system is needed to determine whether to enhance the treatment processes or to improve the distribution system so that microbiological criteria are met. This paper describes how chlorine decay kinetics are modeled and the model output is used in finding the elements that are contributing to the consumption of chlorine at the treatment plant other than the water itself; this allows better control of chlorine dosing at the treatment plant, which in tum will reduce the formation of disinfectant by-products. In addition, the model will accurately predict the decay due to the organic/inorganic and nitrogenous compounds that are remaining in the water at any point in the distribution system, which will indicate the status of the distribution system with respect to its chlorine consumption. Further, if re-chlorination is introduced in the distribution system downstream of the treatment plant, the model will predict the chlorine decay due to the slow reacting organic and nitrogenous compounds accurately.

Rapid water quality characterization for chlorine demand and THM formation in drinking waters

The quality of drinking water generally deteriorates when it is delivered through a distribution system due to the decay of disinfectant, which subsequently allows the re-growth of microorganisms in the distribution system in addition to the formation of trihalomethane (THM). Therefore, a model which describes the changes that occur in the water quality in the distribution system is needed to determine whether to enhance the treatment processes or to improve the distribution system so that microbiological criteria are met. In this paper the chlorine decay kinetics and THM formation in treated water is modeled considering the reaction of chlorine with fast and slow reacting organic and nitrogenous compounds which are present in that water. The treated water was also passed through three types of resins to fractionate very hydrophobic acids (VHA), slightly hydrophobic acids (SHA), hydrophilic charged (CHA) and hydrophilic neutral (NEU) compounds which are present in the water. Chlorine decay tests were conducted on the effluents emerging from the resins to evaluate the chlorine demand and THM formation potential of those organic fractions. The model shows that the CHA presented in the waters has a very high THM formation potential (around 62% of the THM produced). VHA, NEU and CHA contributed to chlorine demand in the water.