Report No FR0464

APRIL 1994



The studies will provide assurance on the microbiological efficiency of current water treatment practices and indicate the conditions when processes are most vulnerable. The outcome of the investigation of AOC will indicate the potential for removal by conventional processes. Also, as part of the overall study on water treatment practices an assessment will be made of the microbial hazards associated with the off-site disposal of waterworks sludges.


To provide assurance about the microbiological efficiency of water treatment processes when operated at extremes of hydraulic and microbial loading and during filter backwashing.

To establish the impact of conventional treatment facilities to remove AOC and BDOC when operating under extremes of hydraulic loading.

To assess if water works sludges intended for disposal off-site present a microbial hazard.


The microbiological safety of drinking water must be ensured to protect public health. To achieve this objective several barriers to micro-organisms must be used in the treatment of drinking water so that failure of any one process cannot jeopardise the quality of water entering distribution. It has been demonstrated that both rapid sand filtration following clarification and slow sand filtration are effective barriers to microbiological challenges under normal loading conditions, however, there are doubts whether rapid gravity filters may be compromised by extreme hydraulic or microbial loading.

It is now well recognised that improvements to the microbial quality of water in distribution can be achieved through the production of more biostable water. However, such a strategy requires the optimisation of existing water treatment practices to remove biodegradable organic matter.

Increasing awareness amongst waste disposal authorities has raised concerns regarding the disposal of water works sludges off site. Little information is available on which to produce an assessment of the potential microbial loading of these products.


  1. Operation of clarification and rapid filtration under optimum hydraulic loading gave good removal of thermotolerant coliforms but were less effective at reducing the numbers of heterotrophic plate count bacteria.

  2. Reduced hydraulic loading to the clarifier improved its removal efficiency for all groups of bacteria especially the HPC bacteria.

  3. Increased hydraulic loading to the rapid gravity filters did not impair microbial removal efficiencies.

  4. The microbial removal efficiency of the filters was reduced under increased turbidity of the influent water.

  5. The quality of the filtrate from the rapid gravity filters was similar for either sand or GAC.

  6. There was no increase in thermotolerant coliforms in the filtrate from the GAC filter.

  7. The microbial quality of the filtered water was reduced for a short period immediately the filters were brought back on line after backwashing.

  8. Clarification showed wide fluctuations in the removal of AOC.

  9. Better removal efficiencies for AOC by clarification were found during the winter months.

  10. The BDOC technique failed to produce consistent and reliable data.

  11. The microbial loading of waterworks sludges varied between source although the number and frequency of occurrence of specific pathogenic micro-organisms tended to be low.


This is a final report produced for the FWR contract Microbiological Efficiency of Water Treatment (Project Ref F-1401). It describes studies on the microbiological efficiency of water treatment processes at pilot scale under different regimes of hydraulic loading and during filter backwashing. The removal efficiency of AOC by clarification was also examined and compared to the equivalent BDOC measurement. An assessment of waterworks sludges was undertaken to determine if a microbial hazard existed when disposed off-site.

Copies of the Report are available from FWR, price 15.00 less 20% to FWR Members