Report No FR0219

Sept 1991



To review the available literature on the removal of bacteria, viruses and cysts by water treatment processes. To identify circumstances in which breakthrough of different groups of micro-organisms could occur and the subsequent treatments which could be used to protect public health.


The microbiological safety of drinking waters must be ensured to protect public health. Investigations in 90/91 on the assessment of drinking water disinfectants (F-0501A) have demonstrated that "natural" populations of micro-organisms may be more resistant to treatment than is predicted by studies with laboratory cultures. Although slow sand filtration has been demonstrated to be an effective barrier to microbiological challenges there are doubts whether rapid gravity filters are equally effective and the use of granular activated carbon is known to pose some potential microbiological problems. Recent outbreaks of Cryptosporidiosis have demonstrated the vulnerability of certain aspects of some treatment strategies.


The literature reviewed quotes high micro-organism removal efficiencies by water treatment processes. However, outbreaks of waterborne disease do occur which demonstrate that conventional water treatment cannot guarantee the safety of drinking water sup plies at all times. The report highlights certain water treatment practices that are susceptible to the passage of micro-organisms into supply.

The report highlights the failure of current bacteriological monitoring to ensure that water supplies are free of viruses and protozoan cysts. The use of alternate indicators of water treatment efficiency such as coliphages and clostridia are proposed by several authors.


The objectives of future work can be summarised as follows :

  1. Methods will be developed to examine 10-100 litre samples for the following organisms - Thermotolerant coliforms, faecal streptococci, Clostridium perfringens, coliphages and enteric viruses.

  2. Two treatment works will be routinely monitored, one a two stage filtration plant and one coagulation followed by rapid sand filtration plant. Samples will be taken throughout the treatment works to monitor the efficiency of the unit processes at removing micro-organisms. This study will examine sample volumes of up to 100 litres for a range of micro-organisms including viruses, coliphages and clostridia. If current bacteriological monitoring (ie the absence of coliforms in 100 ml of water) is shown to be inadequate, it is hoped to recommend an alternative indicator of water quality.

  3. A pilot plant will be constructed at Medmenham to evaluate the efficiency of water treatment processes at removing micro-organsisms. The pilot plant will be used to optimise unit processes for the removal of micro-organisms and provide recommendations for treatment plant operation.


This review discusses waterborne pathogens and their removal by conventional water treatment processes. Outbreaks of waterborne disease and their possible causes are reported. Water treatment practices that could lead to the passage of pathogens into supply are highlighted.

Copies of the Report are available from FWR, price 25.00 less 20% to FWR Members