Optimising natural organic matter removal from low turbidity waters by controlled pH adjustment of aluminium coagulation

Effective removal of soluble natural organic matter (NOM) from low-turbidity waters can be achieved by aluminium coagulation provided due attention is paid to micro-floc formation (coagulation) and macro-floc development (flocculation). This work provides guidelines and an explanation of how pH should be controlled to maximise soluble NOM removal while at the same time making full use of pH-influencing chemicals (acid, alum, and lime). For low-turbidity waters, essential floc nucleating sites can be provided by the lime that is used for pH and alkalinity adjustment, provided that the lime is added in sufficient quantity and at a point where it retains some of its particulate nature, i.e. immediately after coagulant addition. Adjusting the pH downwards to between 4 and 5 prior to coagulant addition allows the formation of soluble NOM-aluminium complexes that link to each other, thereby forming large insoluble bridged complexes (micro-flocs), that also act as nuclei for macro-floc development (flocculation).