Phosphorus removal from return flows of a wastewater treatment plant
Introduction: The paper reviews an issue associated with negative consequences resulting from discharge of treated wastewater with residual phosphorus concentrations. The known methods of phosphorus removal are briefly assessed. The purpose of the study was to identify places of wastewater phosphorization at Novosibirsk wastewater treatment plants (WWTPs) and suggest a simple method for its reduction to obtain nitrogen and phosphorus-containing sludge suitable for reuse. Methods: The studies were carried out using natural wastewater. Basic physical and chemical wastewater quality indicators were defined with the help of modern laboratory equipment. Results: The waste liquid entering the WWTPs has several sources of phosphorus saturation: wastewater from residential buildings and industrial enterprises, sludge water from thickeners of excess activated sludge, sludge water from thickeners for joint thickening of raw sludge and excess activated sludge, sludge water from thickeners of washed sludge used with methane tanks and vacuum filters, as well as sludge water of sludge beds, centrifuge centrate and filtrate of filter presses. It has been found that discharge of such wastewater into the WWTP inlet chamber for retreatment leads to an increase in the concentrations of suspended substances, organic substances, and nutrients (nitrogen and phosphorus) in the primary wastewater, depending on the facilities structure and season. As a result, their concentrations in treated wastewater, discharged into a water reservoir, increase as well. Our studies involved phosphorus removal using sodium hydroxide only. They were conducted in a chemical laboratory at the Department of Water Supply and Disposal of the Novosibirsk State University of Architecture and Civil Engineering (Sibstrin). The centrate and sludge water from thickeners were analyzed. Data on the volume of nutrients in return flows of the sludge processing units and workshop of mechanical sludge dewatering were received. The paper shows results of studies on phosphorus removal from the wastewater of the on-site sewage by means of calcium and magnesium ions present in the waste liquid in an alkaline medium. It has been founded that at phosphorus concentrations less than 30 mg/ dm3 in the primary stream, it is possible to reduce phosphorus at pH = 11 to 1 mg/dm3, and at higher values, the phosphorus concentration can reach 10-20 mg/dm3 at the same pH values. Conclusion: An easy-to-use process scheme for phosphorus removal from the wastewater of the on-site sewage has been developed. It allows reducing phosphorus in treated wastewater, discharged into a water reservoir, to 0.5-0.7 mg/dm3 at phosphorus concentrations up to 30 mg/dm3 using only sodium hydroxide. At high phosphorus concentrations, it is possible to introduce lime with the ratio P:Ca2+ = (1-1.5) and sodium hydroxide, required to raise the pH level to 10.5-11. Based on the obtained results, a scheme for phosphorus removal and sludge obtaining has been developed and proposed for implementation. After stabilization and neutralization, the sludge can be used as an organic-mineral fertilizer in agriculture.