Effect of coagulation conditions on the dewatering properties of sludges produced in drinking water treatment

The coagulation and flocculation processes in conventional drinking water treatment generate aggregates which settle to form a sludge waste. This sludge can be dewatered further by thickening, centrifugation and filtration operations in order to recover water and minimise the volume of the waste stream.A range of water treatment sludges generated in the laboratory were characterised according to a phenomenological method that is valid from the dilute free-settling regime to the concentrated cake compression stages. These were compared with plant samples.Experimental results show that raw water natural organic matter (NOM), coagulant dose and coagulation pH affected both the rate and potential extent of dewatering. Similar effects were observed for both aluminium sulfate and ferric chloride. These results suggest that increasing dose or pH leads to an increase in the proportion of rapidly precipitated material in the sludge or flocs, which form looser aggregates and hence exhibit inferior dewatering properties.