Recovery of aluminum from water treatment sludge for phosphorus removal by combined calcination and extraction

[Display omitted] •Combined thermal treatment and extraction was applied for Al recovery and P removal.•Calcined Al sludge had high Al and low COD content.•Acid and alkali extracts removed P effectively.•Precipitate formed by Al extract consists of AlPO4 and Al(OH)3. Combined calcination (700 °C) and extraction was applied for aluminum recovery from alum sludge (AS) for phosphorus removal. Calcined AS had higher aluminum and lower COD content than the intact and dried AS. Both acid and alkali extraction recovered more than 90% of the aluminum in the intact and the pretreated AS. Alkali extract of the AS had less heavy metals and COD contents than the acid extract. Response surface methodology (RSM) of Box–Behnken Design (BBD) was applied to find the optimal condition for phosphorus removal with the recovered aluminum solutions. The aluminum solutions showed that the highest phosphorus removal efficiency was more than 98% at the optimal condition (pH 6.4 and Al/P molar ratio higher than 2) with the extract from the calcined AS. FTIR and XRD analyses confirmed that the recovered precipitate contained AlPO4 and Al(OH)3.