In conventional treatment of variable quality mountain water, an inability to immediate adjust the coagulant dose to variations in water quality, causes overdose or underdose of coagulant in relation to its optimal value. The results of the research showed that the reuse of post-coagulation sludge was an effective method to maintain high and stable coagulation effectiveness both under polyaluminium chloride (PACl) overdose or underdose conditions. Recycled sludge contains a large portion of insoluble aluminium hydroxides that could be utilized in underdose PACl conditions. Post-coagulation sludge recycled to a flocculation tank enabled to reduce a dose of low basicity PACl by 15% compared to a dose required in conventional coagulation. In periods of coagulant overdosing sludge addition prevented from an increase in the number of fine particles in an outflow from sedimentation tanks which could not be retained in a filter bed.
- coagulant dose control
- particle counter
- pre-hydrolyzed coagulant
- recycled sludge
- First received 24 February 2016.
- Accepted in revised form 28 April 2016.
- © IWA Publishing 2016