Study On Characteristics And Removal Efficiency Of Two-sludge SBR-BAF Hybrid Process

With the deterioration and eutrophication of water environment, it is essential to treat and reuse the municipal wastewater due to the increasingly severe water resource shortage in China. There are several limitations with the suspended growth and attached growth single-sludge system respectively. Moreover, the effluent quality can't meet the wastewater reuse standard by means of gravity sedimentation in suspended growth sludge system. Under this background, an innovative two-sludge SBR-BAF process, combining the advantages of suspended growth and attached growth process, was developed to provide new brainstorm for the upgrade of built-up WWTPs (wastewater treatment plant) and the design of new WWTPs.In order to investigate the operation characteristics and removal efficiency of the SBR-BAF process, a series of bench-scale and batch experiments were carried out with synthetic municipal wastewater as influent. The mechanism of phosphorus and nitrogen removal was explored. Meanwhile, the operation and maintenance parameters were optimized by analyzing the wastewater sample quality and online hydrochemical readings such as EC (electric conductivity), ORP (oxidation reduction potential), DO (dissolved oxygen), and pH value.Firstly, the SBR-BAF system was start up by seeding sludge from a municipal WWTP. The SBR-BAF system consists of an anaerobic/aerobic SBR (sequencing batch reactor) and an anoxic/aerobic BAF (biological aerated filter). The nitrobacteria were elutriated from PAOs (phosphate accumulating organisms) and proliferated within the BAF because of the short 8 days'SRT (sludge retention time) of SBR. On the 21st day, the removal efficiency of NH₄⁺-N, TN, and TP increased to 83%, 71%, and 80% respectively, which meant the accomplishment of sludge cultivation and acclimation.Secondly, the SBR-BAF system was optimized. The characteristics and mechanism of the SBR-BAF process were discussed. The results of batch experiments show that: the variation of EC and PO₄^3- is identical; the peak values of pH in aerobic and anaerobic stage indicate the end of nitrification and denitrification respectively; ORP remains constant after phosphate having been released from PAOs. Based on these results, the anaerobic duration was shortened to 60 minutes, so did the aerobic duration to 150 minutes. The P-release rate is smaller when using the glucose as carbon source than using acetate, however, there's no difference as for the final TP removal efficiency. Other optimal operation parameters of SBR are as flows: the influent COD/TP ratio more than 30; the pH value ranging from 7.0 to 7.5; the SRT ranging from 8 days to 10 days.The results of continuous experiments with BAF show that: the short nitrification occurs when the Qair/Qinfluent ratio is 4:1 with NO₂^--N accounting for 72% of NOx--N; the suitable recirculation rate is 200%; the ratio of external acetate dose (as COD) to NH4+-N concentration is 2:1 attributing to the short nitrification; BAF needs being backwashed when the headloss increases to 6 cm every 17²0 days; in the water reuse context, 98.7% water yield can be obtained.Lastly, the long-term operation of SBR-BAF system indicates it can treat the wastewater efficiently. The sludge settling characteristic is satisfying with SV varying from 22 to 30 and SVI varying from 58 to 80ml/g.The sludge bulking problem has been avoided successfully. The excess sludge can be thickened to 3%-4 %( as solid mass proportion). The alkalinity remains equilibrious between the nitrification and denitrification in SBR-BAF system. The COD and TP loading of SBR are 2.08kgCOD/kgMLSS·d and 41.68gTP/kgMLSS·d, respectively. The NH4+-N loading of BAF is 1.16kgNH₄⁺-N/m³.d. The total HRT (hydraulic retention time) is about 10 hours which is only about half of suspended growth process for simultaneous phosphorous and nitrogen removal. Under the above mentioned high loadings and short HRT, the mean removal efficiency of COD, TP, NH4+-N, TN is 96%, 98%, 93%, and 84%, respectively. The mean effluent concentration of COD, TP, NH4+-N, TN, SS is 20mg/L, 0.23 mg/L, 3.24 mg/L, 7.68 mg/L, 5 mg/L respectively, which means every index can meet the first class (A) of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant.