Changes in the phosphorus availability of a chemically precipitated urban sewage sludge as a result of different dewatering processes

The objective of this study was to compare the effects of different dewater ing processes on the phosphorus (P) availability of an FeCl3-precipitated a nd lime-conditioned sludge having an initial dry matter content (DMC) of 4% . The sludge was dewatered by either drying (D), freezing/thawing (FT) or c entrifugation (C) until a DMC of about 15% was reached. Thereafter the FT a nd D sludges were gently dried to obtain a DMC increasing up to 50 to 60%. The plant-available sludge P was assessed by the change in the isotopic dil ution of P taken up by ryegrass (Lolium perenne [L]) after sludge amendment s to one soil. After the plant experiment, the soil to-solution transfer of P ions and bicarbonate-and ammonium lactate-extractable P were determined in selected soil samples. The amount uf plant-available P (L-value) and the contribution of sludge P to plant nutrition (%P-dfsl) in soil amended with non-dewatered sludge (ND) were not significantly different from the L and percentage P-dfsl values of soils amended with FT, D and C sludges. Nor did the subsequent drying of D and FT sludges significantly change the L and p ercentage P-dfsl values. Hence, the dewatering processes did not significan tly alter the initial sludge P availability. The soil-to-solution transfer of P ions and the chemically extracted P did not significantly differ betwe en the different sludge-amended soil samples. The percentage P-dfsl values can be predicted by percentage Delta E-1, since the overall average relativ e increase (%Delta E-1) in the immediate soil-to-solution transfer of P ion s did not significantly differ from the percentage P-dfsl values.