Studies On Wastewater Quality Of Xi'an No.4 Wastewater Treatment Plant And Step-feeding A/O Process

Engineering design and operational management of municipal wastewater treatment plants have close relationships with their inflow qualities, sedimentation efficiencies, kinetic parameters and treatment process. Yet, statistical determination of related data is still not standardized and needs to be improved in China. This paper, aimed at the Xi'an No. 4 Wastewater Treatment Plant to be constructed, discusses methodologies for determination of design inflow wastewater qualities, sedimentation efficiencies to remove the nutrients N and P, biological kinetic parameters, and performance of step-feeding A/O process for biological wastewater treatment.To determine the inflow wastewater qualities, the concept of assurance rate is put forward. Based upon investigations of the raw wastewater flowing into Xi'an Dengjiacun Wastewater Treatment Plant and Beishiqiao Wastewater Purification Center, this paper presents 85 percentages of assurance rate to determine the inflow quality as the design basis. This methodology is applied to determine the influent quality of Xi'an No. 4 Wastewater Treatment Plant and the result is shown as 380mg/L, 190mg/L, 260mg/L, 34mg/L, 45mg/L and 4.2mg/L for COD, BOD₅ , SS , NH₃-N , TN and TP respectively.Sedimentation experiment is conducted on the influent pollutants into the primary tanks of the projected Xi'an No. 4 Wastewater Treatment Plant. The primary sedimentation can not only remove SS and COD, but also nutrients N and P. At effective water depth of 3 meters and detention time of 2 hours, the average removal efficiency can reach 51.3%, 20.8%, 7.0% and 8.1% for SS ,COD,TN and TP respectively, and that the volatile solid content of sediments is averaged at 57.7%.Four sets of bench-scale fully-mixed bioreactors with sludge return are equipped in parallel to investigate the biological kinetics of the influent wastewater from Xi'an No 4 Wastewater Treatment Plant. The result shows that the sludge yield coefficient a is 0.4573kgVSS/kgBOD₅, the decay coefficient b 0.0125d^-1 the oxygen coefficient a'0.6266kgO2/kgBOD5, the endogenous respiration coefficient b' 0.0924kgO2/kgVSS.d.Pilot-scale experiment has been conducted for 2 years on the step-feeding A/O nitrogen removal process. The removal efficiencies and the related factors are studied under different working conditions. Raw wastewater enters the step-feeding A/O nitrogen removal process at four stages. The flow ratio for the four stages Qj:Q2."Q3:Q4 is 40%:30%:20%:10%. The volume ratio of the anoxic section to the oxic section is 1:3. Hydraulic retention time is 7.5-9 hours. Sludge retention time is 10d-15d. Sludge recycling is 50%. The result shows that, although inflow wastewater fluctuates significantly, the effluent keeps stabilizing relatively. Removal efficiencies for TCOD, SCOD, SS ,NH3-N , TN and TP are higher than 90%, 84.5%, 90%, 90%, 71% and 75% respectively. The experiment confirmed that the step-feeding A/O nitrogen removal process is stable and strongly resistant to flow shock.Batch experiments are also conducted to determine specific oxygen uptake rate, the maximum specific ammonium uptake rate, and the maximum specific denitrification rate. The result shows that the activated sludge bears high activity in the mixed liquid.Compared to the single-stage A/O nitrogen removal process, the step-feeding A/O process does not apply internal mixed liquid return and has its sludge return only at the rate of 50%, its operation is thus cost effective. As the sludge is returned directly to the first anoxic tank, a high gradient of sludge concentration is formed within the reactor, which will benefit definitely biochemical reaction taking place. When the solid load does not change in the mixed liquid before flowing into the secondary settling tank, the step-feeding A/O process bears a mixed liquid concentration 30% higher than that of the single staged one. Therefore, under the same sludge load, the volume of the reactor can be reduced by 30 percent.