Optimisation and effects of coagulant type, pH and ozonation on clarification for automatic coagulation control

Four phases of studies were carried out at Burham Water Supply Works in Kent. Firstly jar testing was used to compare and evaluate the performance of five coagulants. The coagulants tested were Polyaluminium Chlorides (PAC) from two suppliers, Aluminium Sulphate (Alum), Polyaluminium Silicate Sulphate (Pass 100) and Alba 18. Jar tests were carried out with coagulant alone and with non-ionic (LT20), weakly cationic (LT22s), and anionic (LT25) polymers. Secondly tests were also carried out at ambient and lower pH’s, at various pre-ozone concentrations. Jar tests indicated PAC with LT20 polymer at a pre-ozone dose of 1.5 mg/1 at pH 7.4 produced the highest water quality. Alum with LT20 was next, giving good and consistent settled water qualities. Pass 100 was circumspect and dependent upon the jar test procedure, but at optimum doses gave water qualities comparable to that of PAC. Alba 18 consistently produced poor settled water qualities. For the third phase, PAC supplied by Industrial Alum Limited (INDA) and Water Treatment Solutions (WTS), Pass 100 and Alum were selected and tested on the clarifier. Alum and PAC (WTS) were tested at pH 7.5, pH 7.3 and ambient pH at pre-ozone concentrations 0.5, 1.0, 1.5 and 2.0 mg/1 with various coagulant and polymer combinations. Pass 100 and PAC (INDA) were tested as describes but at a pre-ozone concentration of 1.5 mg/1. At optimum doses PAC (INDA) was found to perform best at a pre-ozone concentration of 1.5 mg/1 at pH 7.5. PAC (WTS) was next, with similar results. Alum produced good and consistent water qualities. Pass 100 was affected by polymer addition and was out-performed by PAC and Alum coagulants. The parameters taken into account were settled turbidity, residual aluminium, and removal of TOC, chlorophyll, phaeophytin, iron, manganese, and colour. If PAC with acid dosing can be shown to work full scale annually it would be economic to do so and to dose acid to achieve a pH of 7.5 when required. Phase four involved the evaluation of an automatic coagulant control system that would automatically optimise the coagulant, acid and polyelectrolyte doses added to the water. This was evaluated using jar tests with PAC and Alum coagulants. The AC250 system indicated the potential to improve the quality of treated water whilst minimising the coagulant dose. In this case PAC would provide further coagulant savings, with consequent sludge savings.