Report No DWI0108

Oct 1984


The objectives of the work were

  1. To investigate the application of GAC adsorption at two types of water treatment works (slow sand and two-stage sedimentation/ filtration works).

  2. To study the effect of pre-ozonisation on the performance of organic removal processes and on water quality.

  3. To survey the application of biological systems in water treatment with a view to their potential for application in trace organics control.

The work on this project was undertaken on site at two types of water treatment plant. These were:

  1. Slow sand filtration works - Walton, Thames Water

  2. Two-stage chemical coagulation) - Clapham, Anglian Water sedimentation/filtration works)

This report summarises the main findings of the three operational runs carried out jointly with Thames Water and Anglian Water.

Detailed results from these runs have been presented previously in WRC reports 110-S, 215-S and 247-S(1,2&3).

Main Conclusions

a) Slow Sand Filtration Works (Walton, Thames Water)

Pre-ozonation at >3 mg 03/l provided both a quality improvement and an operational benefit by increasing the TOC removal by the slow sand filters from 16 to 35% and by doubling the slow sand filter run length. The increased TOC reduction was attributed to biological removal due to the enhancement of the biodegradability of the organics present by the ozone treatment. The increase in length of filter runs was attributed to reduced algal activity above the pre-ozonised slow sand filter. The most effective treatment in terms of TOC removal was the combination of pre-ozonation/slow sand filtration/GAC treatment. However, with the exception of the initial 50 days, the effect of GAC on TOC removal was comparatively small. Ozonation itself did not significantly reduce the concentration of the organics present but changed their nature as indicated by the reduction in absorbance at wavelengths of 254 and 400 nm and by the increase in ATP value. Absorbance is therefore not a suitable parameter for estimating the amount of organics present or removed when oxidants are being used in the treatment.

b) Chemical Coagulation Works (Clapham, Anglian Water)

Pre-ozonation at 3 mg 03/l enhanced only marginally the TOC removal by GAC treatment (empty bed contact time (EBCT) 14 minutes). However, doubling the EBCT and eliminating pre-chlorination increased the benefit derived from pre-ozonation.

GAC treatment was effective at removing only some of the specific compounds identified by GCMS. Pre-ozonation provided little additional benefit.

Because of the effort involved in analysing GCMS data and the limitations of using this tool in establishing concentrations, GCMS should be used only for the identification of the organics present. Specific GC analysis should be employed if a quantitative analysis for individual organic compounds is required.

c) Biodegradability

A survey of methods available for estimating the amount of biodegradable organics present in water was carried out which indicated that the adenosine triphosphate (ATP) technique was the most suitable method. Evaluation of the ATP technique has shown that it is a useful method for establishing the amount of biodegradable material present in the water. However, indications are that the method is not sufficiently sensitive, to be used for selecting the optimum ozone dose. A good correlation was found between ATP value and viable plate count. The result of the ATP test was not significantly effected by the choice of inoculum.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.