Experimental studies simulating potential phosphorus release from municipal sewage sludge deposits

In a laboratory experiment, artificial rainwater was passed through Fe- and Al-precipitated sludge. The release of phosphorus (P) at different temperatures and different oxygen conditions was studied up to 160 d. Under anaerobic conditions and at a temperature of 20°C, 95% of the total P in sludge was released as phosphate. In contrast, in an aerobic environment at the same temperature, only between 20 and 30% of the total phosphorus was released. Water oxygen status and flow size through the sludge were found to be the main factors controlling the P release. Phosphorus fractionations demonstrated that 75% of the total P in sludge initially was found in the NaOH extractable fraction representing P adsorbed to Fe compounds, for example. After passage of oxygenated artificial rainwater, a transformation to more inert P-compounds occurred (HCl extractable and inert P) while only 10% remained in the NaOH extractable fraction. The risk of increasing non-point pollution of P from P-accumulating sludge deposits is discussed.