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

Abstract

The objective of this study was to compare the effects of different dewatering processes on the phosphorus (P) availability of an FeCl₃-precipitated and lime-conditioned sludge having an initial dry matter content (DMC) of 4%. The sludge was dewatered by either drying (D), freezing/ thawing (FT) or centrifugation (C) until a DMC of about 15% was reached. Thereafter the FT and 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 dilution of P taken up by ryegrass (Lolium perenne [L]) after sludge amendments 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 of 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 percentage P _dfsl values. Hence, the dewatering processes did not significantly alter the initial sludge P availability. The soil-to-solution transfer of P ions and the chemically extracted P did not significantly differ between the different sludge-amended soil samples. The percentage P _dfsl values can be predicted by percentage Δ E ₁, since the overall average relative increase (%Δ E ₁) in the immediate soil-to-solution transfer of P ions did not significantly differ from the percentage P _dfsl values.