Optimisation of the coagulation step in a coagulation/direct filtration process in which filtration takes place in a lightweight expanded clay aggregates (Filtralite) filter is investigated. The water is high in NOM but low in turbidity, a situation frequently found in Norwegian lake water. Experimental studies in a pilot-scale direct filtration plant compare filtration efficiencies for different coagulants and coagulation conditions. Type of coagulant and coagulant dose are optimised as well as the pH of coagulation for four different coagulants. It is demonstrated that the biopolymer chitosan is a very interesting alternative coagulant in this particular process situation. Also poly-aluminium chloride (PAX) is superior to the traditional metal-salts, aluminium sulphate (ALG) and ferric chloride sulphate (JKL) in terms of turbidity and residual metal. Coagulation with ALG and JKL results in a certain floc-particle (turbidity) ‘leakage’ from the clay aggregate filter, while such ‘leakage’ is not equally evident when PAX and chitosan are used for coagulation.

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