Reuse of aluminium-based water treatment sludge for phosphorus adsorption: Evaluating the factors affecting and correlation between adsorption and sludge properties

Equilibrium phosphorus (P) adsorption was investigated for nine aluminium-based water treatment sludges (WTS) from four water treatment plants in Victoria, Australia. Four WTS, one from each location, were characterised in-depth to unpack the P adsorption trends observed from the nine WTS. Morphology, surface area, porosity, mineralogy, and acid-digestible aluminium were measured. In a key finding resolving longstanding uncertainty on this topic, results indicate that the age of the sludge does not account for differences in P adsorption. However, there was a strong correlation between the acid-digestible Al content of the sludge, the surface area of the sludge and the P adsorption. Acid-digestible Al concentration may serve as a simple proxy to help identify WTS batches best suited to reuse for P adsorption. One of WTS was investigated in further detail to identify suitable conditions for use in adsorption. The highest P adsorption was observed at pH 4 (the lowest pH tested), with negligible release of Fe & Al at this pH. The second smallest fractions (1.18 mm) which showed the highest adsorption. We hypothesise that the smallest size fraction (0.6 mm) is composed of inert materials that do not participate in P adsorption. All WTS released organic content ranging from 1.63 mg/L to 9.7 mg/L into the solution in experiments and it was influenced by the pH and P concentration which supports that ligand exchange is the driving force. This study of provide useful information for water treatment plant operators to identify potential reuse applications for water treatment sludge.