Report No FR0373

J Easton

Apr 1993



The customer will benefit from information on the potential of existing water treatment processes to reduce the concentration of Assimilable Organic Carbon (AOC). This information will indicate which processes can be optimised for improved AOC removal.


To investigate the effect of water treatment processes on the removal of AOC and to identify the most successful treatments for further study.

To optimise the identified treatment processes to improve efficiency of AOC removal.


The presence of AOC in water leaving a treatment works can cause problems as it can be one of the causes of regrowth in distribution. At present, water treatment processes are optimised more for the removal of particulate matter and bacteria rather than AOC. By investigating the effect of treatment processes on the concentration and removal of AOC it will be possible to optimise these treatments to reduce the concentration of AOC in water leaving the works and entering distribution.


Results indicate that the use of oxidants such as chlorine and ozone substantially increase the concentration of AOC in source and treated waters. Coagulation and sedimentation (clarification) achieved good removals of AOC, a maximum reduction of 57% being observed in the plants studied. Pre-chlorination, however, appeared to have an inhibitory effect on AOC removal by clarification due to the persistence of a chlorine residual through this treatment process. Pre-ozonation did not seem to inhibit reduction in AOC concentration during this process. Rapid gravity filtration using granular activated carbon (GAC) as a filtration medium achieved good AOC reductions a maximum removal of AOC of 50% being observed. A chlorine residual did not persist through this treatment. However, if sand was used as a filtration medium, a chlorine residual did persist in the plants studied, inhibiting AOC removal.


Treatment processes found to have potential for removal of AOC, such as clarification should be optimised. The effect of filtration on AOC should be further investigated and optimised as should the use of ozone.

There is a need to evaluate current methods available for the determination of biodegradable organic carbon (BDOC) and to make recommendations as to the most suitable.


This is the final report in connection with the FWR contract Measurement and Significance of Assimilable Organic Carbon. The report describes a study on the effect of existing water treatment processes on AOC concentration and removal. Section 1 outlines the background to the project and the reasons for undertaking the work. Section 2 describes the sampling regime, analytical methods used and results obtained. These results and their relevance to water treatment strategies are discussed in Section 3.

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