Water Science & Technology Vol 54 No 5 pp 235–242 © IWA Publishing 2006 doi:10.2166/wst.2006.567

The REAL process – a process for recycling sludge from water works

K. Stendahl^*, C. Färm^**, I. Fritzdorf^** and H. Ulmert^***

In order to produce drinking water, coagulants – such as aluminium salts – are widely used for precipitation and separation of impurities from raw water. The residual from the process is sludge, which presents a disposal problem. The REAL process is a method for recycling the aluminium from the sludge. In a first step, the aluminium hydroxide is dissolved in sulphuric acid. In a second step, an ultra filtration will separate all suspended matter and large molecules, leaving a concentrate of 15–20% dry solids. The permeate will contain the trivalent aluminium ions together with 30–50% of the organic contaminants. In a third step, by concentrating the permeate in a nano filter, the concentration of aluminium will be high enough to, in a fourth step, be precipitated with potassium sulphate to form a pure crystal: potassium aluminium sulphate. The potassium aluminium sulphate is comparable to standard aluminium sulphate. The process will give a residual in form of a concentrate from the ultra filtration, representing a few per cent of the incoming volume. This paper presents the results from a long time pilot-scale continuous test run at Västerås water works in Sweden, as well as calculations of costs for full-scale operations.

Keywords: Aluminium; filtration; precipitation; recirculation; water treatment

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