Phosphorus in Urban Sewage Sludges as Assessed by Isotopic Exchange

Sewage sludges have been used for years as fertilizers but little information exists on the availability of P in these complex materials. Isotopically exchangeable P was measured in 12 urban sewage sludges using the isotopic exchange kinetics method. The results obtained with this method were then interpreted according to the multicompartmental model recently proposed. The two activated sludges and the composted sludge had the highest quantities of P isotopically exchangeable within 1 min ( E 1min higher than 0.77 g P kg ‐1 sludge). The flocculated and heat‐treated sludges contained low quantities of rapidly exchangeable P but high quantities of slowly exchangeable P ( E >12wk higher than 9 g P kg ‐1 sludge). The quantity of P exchangeable within 1 d was positively correlated to the sum of water‐ and bicarbonate‐extractable P while slowly exchangeable P was positively correlated to the quantity of P present in precipitated forms deduced from a sequential extraction (i.e., HCl plus NaOH‐extractable P for FeSO 4 ‐flocculated sludges and HCl‐extractable P for the other sludges). Results of this study demonstrate that the isotopic exchange kinetics method can give valuable information on inorganic P forms and exchangeability in urban sewage sludges.