The Transformation of Phosphorus Forms during Co-composting of Sewage Sludge and Wheat Straw.

Understanding the transformation mechanism of phosphorus is essential for assessing its impact on the environment. With sewage sludge (SS) as raw materials, wheat straw bran (WSB, particle size less than 0.5 cm) and wheat straw section (WSS, about 3cm length) as the conditioner, the experiments were carried out to investigate the transformation of phosphorus forms during Co-composting of sewage sludge and wheat straw in aerated static pile composting system and natural ventilation fermentation system. The results showed that, in 120 days’ composting period(0~35d, the first stage; 36~120d, the second stage), with the compost maturity, the total phosphorus content of Treatment SS+WSB and Treatment SS+WSS increased from 2.27 g/kg and 2.67 g/kg to 6.80 g/kg and 6.13 g/kg, respectively. The available phosphorus content of Treatment SS+WSB and Treatment SS+WSS increased from 1.45 g/kg and 1.69 g/kg to 6.24 g/kg and 5.26 g/kg, respectively. The organic P content of Treatment SS+WSB and Treatment SS+WSS increased from 0.72 g/kg and 0.83 g/kg to 2.53 g/kg and 2.16 g/kg, respectively. With the composting process in progress, a considerable part of inorganic phosphorus was converted into organic phosphorus, the ratio of organic phosphorus and total phosphorus increased, and adding small sized straw as compost conditioner contributed much to the inorganic phosphorus conversion to organic phosphorus. The transformation of inorganic phosphorus forms in the composting process was analyzed using Hedley grading method as an example. The results showed that the content of H₂O-P for Treatment SS+WSB was higher than Treatment SS+WSS throughout the composting process, but the content of Residual-P was reversed for the two treatments. All the forms of phosphorus (H₂O-P, NaHCO₃-P, NaOH-P, HCl-P and Residual-P) for the two treatments have about the same trend of increasing gradually during the composting process. Due to the "concentrating effect" and the activation of organic acids on the inorganic phosphorus, the content of various P forms has reached a peak period at the end of the first stage and tends to become stable at the second stage. The difference between total P and organic P should be less than the total content of all kinds of inorganic P.