Comparison of phosphorus fractions and alkaline phosphatase activity in sludge, soils, and sediments

PURPOSE: The land application or disposal of sewage sludge generally leads to phosphorus (P) loss in aquatic environments and often results in eutrophication. The nature of P fractions plays a key role in the transport of P within the environment, and alkaline phosphatase is important for the transformation of the P fractions. The objective of this study was to assess the P fractions and alkaline phosphatase activity in sewage sludge, soils, and sediments, in order to effectively evaluate their P bioavailability and mobility. MATERIALS AND METHODS: The Standards, Measurements and Testing (SMT) harmonized procedure was adopted for the determination of the P fractions. The alkaline phosphatase activities were determined by analyzing the production of p-nitrophenol from p-nitrophenylphosphate catalyzed by phosphatase in the various samples. RESULTS AND DISCUSSION: The results demonstrated that sewage sludge possessed larger amounts of P than soils and sediments. Inorganic phosphorus (IP) was the main P fraction of all the samples investigated and it always constituted >70% of the total P (TP). However, the distribution of IP between non-apatite inorganic P (NAIP) and apatite P (AP) varied significantly according to the type of sample. NAIP accounted for >60% of IP in sediment and sewage sludge samples obtained from domestic wastewater treatment plants. However, AP was more abundant than NAIP in soil samples. For the P fractions obtained in a closed agriculture test field, NAIP content showed an obvious increase from soil to sediment samples. This suggests that NAIP is the most easily mobilizable form of P. The average alkaline phosphatase activity in sewage sludge samples is significantly higher than that in soil and sediment samples, and it may be closely attributed to the higher biomass content in sewage samples. CONCLUSIONS: The results showed that the SMT protocol could be considered a useful tool for P monitoring of sewage sludge, soils, and sediments. The NAIP is the most easily mobilizable form of P and should receive further attention. The sewage sludge samples contain higher mobilizable forms of P and alkaline phosphatase activity than soil and sediment samples. The results are not only beneficial for analyzing P in sewage, soil, and sediment samples during their transfers to watercourses but are also beneficial for evaluating the risk of eutrophication and that associated with sewage sludge treatment and application.