Removal of coagulant aluminum from water treatment residuals by acid

• This study clarified acid washing conditions for water treatment residual sludge.
• Acid washing was effective for removal of aluminum coagulant from sludge.
• Properties of acid washed sludge were enough for the plowed soils.
• Washing at pH 3 was enough for thickened sludge to achieve 80% of aluminum removal.
• Washing at pH 1 was necessary for 80% aluminum removal from sun-dried sludge.

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.

Figure optionsDownload high-quality image (206 K)Download as PowerPoint slide