Phosphorus Removal And Recovery From Anaerobic Digestion Supernatant Of Sewage Sludge Using Dolomitic Lime

Phosphorus was not only a key factor of all living systems, but also a kind of limited resource in the earth, which could not naturally regenerate and was going to be exhausted. Meanwhile, it could cause the eutrophication of water bodies. Therefore, it is the inevitable choice to remove and recover phosphorus from wastewater. The phosphorus concentration of anaerobic digester in sewage treatment plants could achieve dozens or even hundreds of milligrams per liter. And it was suitable for recycling phosphorus in magnesium ammonium phosphate (MAP) and calcium phosphate (CP) form. At present, most of the methods to recover phosphorus was adding calcium salt and magnesium salt directly. Using these methods could increased the cost of treatment. Dolomitic lime (D-lime) with low price was utilized in this research. It could release calcium and magnesium in the solution, at the same time improve the pH values. In addition, D-lime could also provide seed for the crystallization reactions. As a result, it could effectively remove and recover phosphorus from anaerobic digestion supernatant of sewage sludge. The following results were gained by batch experiment:1. Different dosages, initial pH values, reaction temperature, stirring speeds, particle sizes of materials and reaction time on the effects of phosphorus removal and recovery were studied. Then the ranges of optimum conditions were confirmed, the removal efficiencies of ammonia were carried on the corresponding research, because ammonia nitrogen could be removed by MAP precipitation. The results showed that 300mg/L dolomitic lime, pH 8.50-9.50,25.0℃,150 r/min stirring,180-200 mesh and 24 h were optimum conditions.2. The linear, quadratic and interaction effects of dolomitic lime dosage, initial reacting pH values and temperature on the removal efficiencies were investigated using response surface methodology. An empirical mathematical model and the influence factors were presented. Results demonstrated that regression equation fit well with experimental data (R2 equals to 0.9680 and 0.9922, respectively). Considering simplicity and efficiency of operation, the optimum conditions were selected:300 mg/L of dolomitic lime, initial pH 9.00 and 25.0℃. Based on these conditions, Visual MINTEQ was used to build a chemical equilibrium model, which could fit well with experimental data.3. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were performed to characterize the composition of the recovered products. It was shown that the recovered products consisted of MAP and amorphous calcium phosphate (ACP), as well as impurity, (e.g. calcium and carbonate) MAP could be a kind of slow-release fertilizer while ACP were used as raw materials of phosphate fertilizer. This research indicated that phosphorus could be economically and efficiently recovered from anaerobic digestion supernatant of sewage sludge by using dolomitic lime.4. There was a high concentration of carbonate in anaerobic digestion supernatant of sewage sludge. Therefore, the effects of impurities (e.g. calcium and carbonate) on phosphorus recovery by MAP method were discussed. The results showed that the presence of calcium ions was beneficial to the removal rate of phosphorus. However, calcium phosphate reaction could restrain the formation of MAP crystal to become dominant reaction. And products were mainly composed of calcium phosphate. In addition, the presence of carbonate could extend the induction time, reduce crystallization rate and the removal efficiency of phosphorus.5. Although the dosage of dolomitic lime needed was greater than that of othermaterials, like pure calcium oxide, magnesium oxide, and magnesium chloride, it has considerable economic advantages.