WATER QUALITY DETERIORATION IN POTABLE WATER RESERVOIRS RELATIVE TO CHLORINE DECAY
921/1/01
October 2001

EXECUTIVE SUMMARY

1. 1. Abstract

The proper understanding, characterisation and prediction of water quality behaviour in drinking water distribution systems are critical in meeting regulatory requirements and ensuring customer oriented expectations. Quality control through the modelling of water quality in distribution systems storage facilities is a relatively new field. Realising that the water quality after treatment deteriorates with time, the distributor cannot always guarantee the quality at the end of the distribution network. For this reason, it is necessary to monitor water quality throughout the distribution network. Reservoirs are an important part of the distribution network, as they represent the final point at which the water quality can be modified before it reaches the consumer. Therefore special attention must be paid to the quality deterioration in and around reservoirs. It is becoming increasingly important to design and manage reservoirs carefully so as to ensure safe drinking water right up to the consumer's tap.

Since chlorine content is one of the most important variables used to indicate water quality, special attention should be paid to its decay and factors influencing its decay in reservoirs. As chlorine concentrations deplete within the reservoir, the overall quality of the water deteriorates as a result of increasing microbial flora. This study investigates the factors contributing to the loss of chlorine residuals in water storage facilities.

The starting point in resolving these problems is the analysis of the decay rate of chlorine in water. Factors that influence the chlorine decay rate are:

• The reaction between chlorine and compounds in the water. This reaction is independent of the storage medium.
• The reaction between the chlorine and the reservoir's walls and base is a function of the area/volume relationship of the reservoir, the reservoir material, the presence and nature of biofilm on the walls and the presence, nature and thickness of the organic layer on the reservoir floor.
• The diffusion of chlorine from the water surface into the atmosphere. This diffusion is dependent on temperature, the turbulence in the reservoir and the concentration gradient.