Investigation of phosphorus desorption from P-saturated alum sludge used as a substrate in constructed wetland

Phosphorus (P) desorption from P-saturated alum sludge, which was used as main substrate in a novel constructed wetland for wastewater treatment, was studied. Groups of batch experiments were designed and conducted to explore the efficiencies of P extraction using different acids (HCl, HNO3, H2SO4) and bases (NaOH, KOH). The results showed that either acid or base is efficient for P-extraction and the efficiency relied mainly on the concentrations of H+/OH-, not the type of acid or base. Considering the efficiency, price and safety of the acids and bases tested, H2SO4 was chosen as most suitable reagent for P-extraction. A Box-Behnken experimental design based on the response surface methodology (RSM) was applied to evaluate the optimum of H2SO4 extraction. The optimal condition for the mass of sludge, H2SO4 concentration and volume was 0.8 g, 0.063 M and 142 ml, respectively. At such optimal condition, the maximum P-extraction efficiency of 98.2 % was achieved. Additionally, most of the main components of the saturated sludge, such as metals (Al, Ca, Mg, Fe), TOC (total organic carbon) and nitrogen can also be extracted. Overall, the results supported that H2SO4 seems to be an efficient and cost-effective reagent among all the reagents tested for P-saturated alum sludge treatment.