[Bacterial regrowth in drinking water. III. Reasons for regrowth with oligocarbotolerant bacteria]

Investigations have been undertaken into the bacterial regrowth in a Zurich drinking water plant and over a distance of 12 km along the drinking water distribution system. This required installation of eleven chromium steel dead-end water pipes. Counts of oligocarbotolerant bacteria were carried out in 7 or 8 repetitions in fresh water and water left to stagnate for 7 and 14 days respectively (7,8). Additionally ten different biological, chemical and physical parameters were determined in the fresh water samples. Multiple linear regression analysis was used to determine what influence the different parameters had on the bacterial regrowth during stagnation. Two regression models were evaluated, one for the data obtained during treatment, and the other for the data obtained along the distribution system. In both models the content of DOC (Dissolved Organic Carbon) and the content of phosphate was correlated with the growth of oligocarbotolerant bacteria. Further, a relationship between the contents of organic matter and bacterial regrowth was discovered during two measuring series conducted in different seasons. The model of data obtained during treatment generated two additional parameters correlated with aftergrowth: Firstly the initial colony content, which probably resulted because the stagnation period was only half that of the model. Secondly, the oxygen content which resulted because of ozonization. Under the given test conditions the following parameters did not appear in the regression model (Cp-statistics according to Daniel and Wood, 5): AOC (Assimilatible Organic Carbon) and UV-absorption used for measuring organic matter, nitrate content, content of chlorine/chlorine dioxide, electrical conductivity, pH-values and the room temperature during the 14 days stagnation in the distribution system model.