The effect of domestic ion-exchange water softeners on the microbiological quality of drinking water

Abstract

This paper covers the testing and evaluation of the impact an ion-exchange water softener has on the microbiological quality of water. Water quality was assessed in two situations, firstly in normal domestic use and secondly under microbial shock loading conditions in the laboratories at Cranfield University. This study was undertaken to determine whether the passage of water through an ion-exchange softener would lead to a significant change in the total bacterial count of the softened water. Samples taken from the outlet of the water softener had on average higher total viable count (TVCs) than samples taken from the inlet. The average inlet TVC was 300 CFU/ml whilst the average outlet TVC was 1330 CFU/ml, a less than 1 log increase in CFU/ml between inlet and outlet. This trend was the same for samples incubated at both 22 and 37°C. There was no evidence for proliferation of coliform and/or Pseudomonas species in the water softener. Tests were undertaken to investigate what effect periods of stagnation and regeneration had on the TVC. These tests showed no significant increase in colony counts after 20 days’ stagnation. As expected, regeneration led to decreased colony counts in the softened water. To further test whether the water softener could act as a growth site for micro-organisms, a series of microbial shock loading experiments were undertaken using a 90 litre batch solution containing 1.90×10 5 Escherica coli/ml. Tests showed that after day 1 there was no E. coli in the softened water although there was a steady increase in the total viable count.