Enumeration of faecal indicator bacteria in large water volumes using on site membrane filtration to assess water treatment efficiency

Abstract

Sample volumes as tested in routine microbiological methods for determining the presence and absence of faecal indicator bacteria in water are too small to assess the actual concentration in the last stages of a water treatment. Consequently no accurate information can be obtained about the removal efficiency of a water treatment for micro-organisms. Therefore a method for on site isolation of faecal indicator bacteria from large volumes (100 litre or more) of treated water using membrane filtration (MF-sampling) was developed and tested. The procedures for culturing the isolated microorganisms were similar to those applied in the routine methods for small volumes using membrane filtration (mf-method). The recovery efficiency of MF-sampling for E. coli, S. faecalis and spores of sulphite-reducing clostridia ranged from 74.6 to 100% and only for E. coli a slight decrease with increasing sample volume was found. Field studies revealed that MF-sampling can easily be implemented in (routine) laboratory practice for an accurate determination of the concentration of faecal indicator bacteria in treated water after various treatment stages. From these data the treatment efficiency of the involved processes and the overall treatment for those micro-organisms and the fluctuation in micro-organism removal were determined. Such data can be used to improve water treatment regarding the removal of micro-organisms and for quantitative microbial risk assessment. Validation of the use of faecal indicator bacteria as a surrogate parameter for the assessment of the effects of treatment processes on pathogenic micro-organisms needs further investigation.