Forms, patterns and extractability of phosphorus retained in alum sludge used as substrate in laboratory-scale constructed wetland systems

This study examined the form, pattern and extractability of phosphorus (P) retained in alum sludge (an aluminium-containing drinking water treatment residual in dewatered cake form), which was used as substrate in laboratory scale constructed wetland systems for P-rich wastewater treatment. Used alum sludge samples from successive depth ranges in the laboratory scale vertical flow constructed wetland systems were examined for KCl extractable P. The samples were also sequentially fractionated into two main categories consisting of readily available P and P forms that are not easily decomposed. The extracted fractions included labile P, microbial P, (Fe + Al) P, humic P, (Ca + Mg) P and residual P. A major proportion of P retained in the used alum sludge was in forms that are not easily decomposed. Of the P forms that are not easily decomposed, the P associated with Ca and Mg accounted for a higher proportion of the inorganic fraction as compared to the P associated with the Fe and Al. The results also show that in most cases, the concentration of the P forms decreased with increasing depth from the topmost surface of the alum sludge in the systems.