Sediment sludge during coagulation and sedimentation in drinking water treatment is
called “water treatment residuals (WTR)”. Polyaluminum chloride (PAC) is mainly used as a
coagulant in Japan. The recycling of WTR has been desired; one method for its reuse is as
plowed soil. However, WTR reuse in this way is inhibited by the aluminum from the added
PAC, because of its high adsorption capacity for phosphate and other fertilizer components.
The removal of such aluminum from WTR would therefore be advantageous for its
reuse as plowed soil; this research clarified the effect of acid washing on aluminum
removal from WTR and on plant growth in the treated soil. The percentage of aluminum
removal from raw WTR by sulphuric acid solution was around 90% at pH 3, the percentage
decreasing to 40% in the case of a sun-dried sample. The maximum phosphate adsorption
capacity was decreased and the available phosphorus was increased by acid washing, with
90% of aluminum removal. The enhancement of Japanese mustard spinach growth and the
increased in plant uptake of phosphates following acid washing were observed.