EVALUATION OF THE MICROBIOLOGICAL PERFORMANCE OF FULL SCALE
TREATMENT PROCESSES
Report No FR0328

Dec 1992

SUMMARY

I BENEFITS

The studies will provide the client with assurance on the microbiological efficiency of current treatment practices and indicate the conditions when processes are most vulnerable to microbiological breakthrough. Current indicators of treatment efficiency and monitoring will be assessed and alternatives suggested if necessary.

II OBJECTIVES

To provide assurance about the efficiency of water treatment processes against microbiological agents and identify where further improvements should be made.

III REASONS

Treatment of water to meet the Water Supply (Water Quality) Regulations, 1989 has to ensure the removal of organisms of public health significance. However, breakthrough of micro-organisms from works can occur and it is important to assess the importance of this, identify the reasons for breakthrough and if necessary try to identify feasible solutions.

IV CONCLUSIONS

The results of the studies carried out so far suggest that where prechlorination is used there would appear to be only a low risk of microbial breakthrough. Micro-organisms surviving the initial impact of prechlorination were removed efficiently by the combined effects of chlorine continuing to act and the subsequent treatment processes used.

It would appear that GAC filtration encourages the survival or even proliferation of micro-organisms including those of sanitary significance such as members of the coliform group. The implications of this finding require urgent investigation.

Final disinfection would appear in most cases to be acting as an effective final barrier and there is no evidence to suggest that, even in the case of GAC effluents, this is being unduly challenged because of any inadequacies of previous treatment stages.

V RECOMMENDATIONS

These conclusions are based on preliminary observations, and recommendations will be made when the studies are complete.

VI RESUME OF CONTENTS

This report describes progress on the monitoring of full scale water treatment processes undertaken in connection with the FWR project `Microbial Efficiency of Water Treatment F-1401'. The removal of selected micro-organisms by individual processes at four treatment works has been monitored. The results indicate that the use of prechlorination reduces the microbial loading of source waters very effectively and that subsequent processes seem to be providing effective additional barriers to micro-organisms. A survey has been carried out to compare the performance of sand and GAC filtration systems. In contrast to the effluent from the sand filter, water which had been filtered by GAC did contain thermotolerant coliform organisms. The sanitary significance of these organisms is being considered further.

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