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AbstractDisinfection by chlorine is common practice in potable water treatment. The most frequently used dose control method is to inject an overdose of chlorine at the inlet to the contact tank and adjust to the desired residual chlorine level in the effluent stream. However, this method of control may not be optimal. The advent of reliable predictions of retention time distributions and the widespread use of programmable logic controllers, coupled with a greater understanding of chlorine disinfection kinetics, offers the potential for more efficient chlorine dosing algorithms. This paper describes one possible algorithm, based on theoretical models, for predictive chlorine dosing.

ABSTRACTThis paper describes a laboratory‐based study which was designed to investigate (a) the changes in selected physico‐chemical properties of a sewage‐works' final effluent after ozonation, and (b) the effect of ozone on the concentration of organic halides and effluent toxicity and mutagenicity. Different conditions were studied involving the application of ozone concentrations up to 13.5% w/w and the ozonation of the effluent under both constant and variable ozonation rates.

ABSTRACTThe performance of slow sand nitration can be; substantially improved by the application of a non‐woven synthetic fabric layer to the surface of the sand. By means of pilot‐scale experiments, using the i River Thames as the source water and pretreatment by sludge blanket clarifiers, the comparative performance of fabric‐protected slow sand filters has been evaluated over an eight‐month period. Under conditions designed to simulate poorly‐controlled pretreatment, a correctly‐specified fabric type, configuration and thickness can extend filter run times by a factor of 3–5 compared to a conventional slow sand filter. Furthermore, this can be achieved, together with the avoidance of any significant change in the hydraulic behaviour of the underlying sand, thereby avoiding the need to remove and clean sand. Fabric washing is relatively simple and efficient.

AbstractA collaborative project between Anglian Water and Imperial College has been undertaken to investigate alternative disinfection regimes for the control of trihalomethane (THM) formation in lowland surface water treatment. A laboratory experimental protocol has been developed to measure THM formation as a consequence of both pre‐ and post‐disinfection stages with chlorine dioxide, ozone and potassium permanganate, evaluated as alternative pre‐disinfectants in direct comparison with breakpoint and sub‐breakpoint pre‐chlorination. The protocol has been applied to raw water samples from eight locations within the Anglian Water region under winter and summer conditions.The results have shown that THM formation can be reduced substantially, either by the application of chlorine dioxide and ozone as substitutes to breakpoint pre‐chlorination or by reducing the prechlorination dose. The application of potassium permanganate as a pre‐disinfectant had little effect on diminishing the formation of THM compounds. A further substantial reduction in THM formation can be achieved by diminishing the contact period following post‐chlorination between free‐chlorine and the treated water (e.g. by conversion of free‐chlorine to combined‐chlorine through reaction with ammonium sulphate).