Suppression of phosphorus release from sediments using water clarifier sludge as capping material.

Water clarifier sludge was assessed as a capping material for preventing release of phosphorus from eutrophic pond sediments. Sediment incubation experiments were carried out over two months using five capping materials (sludge, sludge sintered respectively at 200, 400 and 600 degrees C, and sea sand) to evaluate the effect of sintering temperature. We also examined reduction in the nitrogen eluted from the sludge. Sludge, sludge sintered at 400 and 600 degrees C, and sea sand suppressed phosphorus release from sediments into the overlying water, whereas sludge and sludge sintered at 200 degrees C increased nitrogen release relative to that without capping. Sludge sintered at 600 degrees C was the most effective in reducing total nitrogen (TN) flux. Our experiments revealed that nitrogen and organic substances must be removed completely from sludge in order to avoid their release. The phosphate removal efficiency of sludge (including sintered sludge) was expressed in terms of Freundlich adsorption isotherms. The results showed that sludge sintered at 600 degrees C was suitable for use as a capping material since (1) its organic content (ignition loss) and TN were low (1.1% and 0.014 mg g(-1) dry, respectively), (2) its phosphorus adsorption capacity was 2.2 times higher than that ofunsintered sludge, and (3) it was effective in suppressing the diffusion of phosphorus and NH4+-N from sediments when used as a capping material.