Control charts for the toxicity of finished water--modeling the structure of toxicity.

The aim of this study was to construct control charts for the data that were obtained from the daily toxicological analysis of finished water (that is the water which has already been treated for human consumption) from the treated-water tanks of the Water Supply & Sewerage Corporation of Athens. The basic idea of the control charts is to test the hypothesis that there are only common causes of variability versus the alternative that there are special causes. The control charts are designed and evaluated under the assumption that the observations from the process are independent and identically distributed (iid) normally. However, the independence assumption is often violated in practice. Time or serial dependence (autocorrelation) may be present in many chemical procedures, and may have a significant effect on the properties of the control charts. The fact that a serious amount of autocorrelation was present in the data for the toxicity of the fished water had a serious impact on the typical control charts. The problem of the autocorrelation was overcome by the usage of a more sophisticated method based on time-series ARIMA models.

Correlation between bacterial indicators and bacteriophages in sewage and sludge.

The use of bacteriophages as potential indicators of faecal pollution has recently been studied. The correlation of the number of bacterial indicators and the presence of three groups of bacteriophages, namely somatic coliphages (SOMCPH), F-RNA-specific phages (FRNAPH) and phages of Bacteroides fragilis (BFRPH), in raw and treated wastewater and sludge is presented in this study. Raw and treated wastewater and sewage sludge samples from two wastewater treatment plants in Athens were collected on a monthly basis, over a 2-year period, and analysed for total coliforms, Escherichia coli, intestinal enterococci and the three groups of bacteriophages. A clear correlation between the number of bacterial indicators and the presence of bacteriophages was observed. SOMCPH may be used as additional indicators, because of their high densities and resistance to various treatment steps.