The formation of potentially harmful trihalomethanes (THM) when using chlorine as a disinfectant in potable water supplies has led to tighter regulatory controls and hence a need for better models for THM management. The prediction of THM concentration is difficult due to the complex and changing hydrodynamic and chemical regimes found in water treatment works and distribution systems. A simple and cost-effective model is described to model chlorine decay and total THM concentration through water treatment works and within distribution systems. With knowledge of the bulk chlorine decay characteristics and the THM productivity of the water, this model offers a simple and straightforward tool which can be readily applied to water treatment works and distribution systems alike to provide an initial assessment of the risks of total THM formation at different sites, and to identify sites and systems at risk of compliance failure.

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Orbal multi-channel oxidation ditches have received relatively little attention and thus knowledge of their characteristics is not as highly advanced as for other oxidation ditch systems. Dye tracer and dissolved oxygen measurements have been undertaken to elucidate the mixing characteristics of a three-channel Orbal system treating 80 Ml/d. The dissolved oxygen concentrations showed a complex variation in space due to the input, approximating a continuous line source, non-uniform distribution of turbulence and secondary flows caused by channel configuration. Consideration of the DO measurements and the dye tracing results suggest that the flow could be treated as 2 CSTRs in the outer channel and a single CSTR in each of the two inner channels.