In water treatment processes, the individual unit operations are complex, highly non-linear and poorly understood. Whilst many models have been developed to improve process understanding, these are rarely in a form easily exploited by the control engineer. Attempts to improve the performance of water treatment works through the application of improved control and measurement have had variable success. This paper discusses investigations into the application of feedback control on the clarification process of a large-scale pilot plant using a streaming current detector (SCD). The application is aimed towards maximising the efficiency of the chemical coagulation process. To achieve this, a simple model of the interactions of process operating conditions on the SCD measurements must be made.