| dc.description.abstract | Due to the increase in the amount of phosphate mining in recent years, the issue of phosphorus recovery has gradually received attention. The anaerobic digestion process in sewage treatment has high phosphorus and high nitrogen components, but it contains a lot of suspended solids and organic matter, which is easy to make recycling ineffective. Therefore, optimizing the quality of phosphorus recovery by pretreatment is a required procedure for future applications. However, when membrane filtration is used for pretreatment, it is easy to encounter the problem of fouling and scaling, resulting in poor filtration efficiency increasing costs, and reducing effectiveness.
In this study, the fouling potential and phosphorus retention rate experiments were carried out using synthetic sludge dewatering filtrate. The batch experiment and response surface methodology were used to explore the optimal condition for the concentration range of the actual wastewater treatment plant, Moreover using long-term membrane filtration experiments to compare and verify the difference in fouling characteristics between the optimal and worst conditions.
The water quality analysis of the wastewater from the actual wastewater treatment plant showed that the pH of the sludge dewatering filtrate was 7.9±0.1, the phosphate - phosphorus concentration was 128.1±8.2 mg/L, the ammonium - nitrogen concentration was 771.3±68.1 mg/L, and the total solids concentration was 1,163±80.1 mg/L and TOC was 56.2±10.3 mg/L. The pH of the anaerobic digestion supernatant was 7.6±0.6, the phosphate - phosphorus concentration was 170.7±42.0 mg/L, the ammonium - nitrogen concentration was 871.2±102.2 mg/L, the total solid concentration was 33,193±166.2 mg/L and the COD was 698.9±21.6 mg /L, compared sludge dewatering filtrate to anaerobic digestion supernatant, due to the higher suspended solids and organic matter in the anaerobic digestion supernatant, it is difficult to membrane pretreatment and phosphorus recovery, while the sludge dewatering filtrate contains less suspended solids and still has a higher phosphorus concentration. Therefore, the sludge dewatering filtrate has better recovery potential.
In the batch experiment, different pH values and calcium concentrations were used to do MFI0.2 and phosphorus retention experiments. According to the results, higher pH results in higher MFI0.2 and lower phosphorus retention. In pH 6-7, it has lower MFI0.2 and higher phosphorus retention rate, because the pH range of the precipitation reaction is about pH 7.5-12. When the pH is lower than this range, particles are not easily generated. When the pH is 7.5-9, with the increase of pH, The IAP in solution is higher than the Ksp of various phosphate compounds, such as magnesium ammonium phosphate, hydroxyapatite, etc., and the precipitate is generated. This leads to membrane scaling. At the same time, it is found that with the increase of pH and calcium concentration from low to high, MFI0.2, particle size, and possible types of precipitates all increase gradually, while phosphorus retention rate gradually decreases.
When the pH is fixed at 7.5 and analyzed by the response surface methodology under phosphate - phosphorus concentration 1.5-4.5 mmol/L (46.46-139.37 mg/L), magnesium ion 0.5-1.5 mmol/L (12.1-36.5 mg/L) and calcium ion 0.75-2.25 mmol/L (30-120 mg/L), the optimal condition is obtained when the concentration of phosphate - phosphorus is 1.5 mmol/L, the concentration of calcium is 0.75 mmol/L and the concentration of magnesium is 1.5 mmol/L, and the reaction value MFI0.2 is expected to be -0.3, the phosphorus retention rate is 100%. It means that the scaling situation is not easy to occur, and the phosphate is in a dissolved state, which is optimal for subsequent phosphorus recovery. The worst condition is that when the phosphate - phosphorus concentration is 4.5 mmol/L, the calcium concentration is 2.25 mmol/L and the magnesium concentration is 1.5 mmol/L, the expected reaction value is MFI0.2 of 3.59 and phosphorus retention rate of 94.8 %, which means membrane scaling. The precipitation reaction causes the phosphorus retention rate to decrease. This state is likely to cause the inorganic compound to precipitate out of particles during pretreatment, cause membrane scaling, and poor phosphorus retention rate. In addition, in the long-term membrane filtration, compared with the optimal conditions, the resistance of the worst conditions increased significantly. Among them, during the membrane filtration process, it is easy to cause the membrane pores to block first and then form the filter cake layer, resulting in a rapid increase in TMP. In the resistance analysis, it was found that the pore-blocking ratio of the best condition was 12%, and the pore-blocking ratio of the worst condition was 30%. Therefore, the main reason for the inorganic scaling of the dewatering filtrate was irreversible scaling. | en_US |