Study On Nitrogen Removal Of Low Carbon-nitrogen Ratio Domestic Sewage With Waste Activated Sludge As Supplement Carbon Source

In most cities of our country, the widespread problem of domestic sewage being low carbon and nitrogen ratio gradually becomes the bottleneck of domestic sewage to attain the treatment standard. Meanwhile, as the most widely used technology in sewage wastewater treatment, activated sludge technique has produced lots of waste activated sludge (WAS), while the expenses of WAS disposal take a more larger proportion in sewage plant operation costs. This topic for denitrification of low carbon and nitrogen ratio sewage and disposal of WAS has been studied from two aspects. In process, a new combined process—hydrolysis acidification/anoxic moving bed biofilm reactor /aerobic process (H/AMBBR/O process for short) was employed to improve the content of biodegradable carbon source in sewage wastewater, and to optimize the present wastewater treatment process. Laboratory pilot study of H/AMBBR/O and its feasiblility to remove nitrogen, as well as to reduce WAS were also explored. In carbon source, WAS was digested under alkaline condition to prepare high carbon source supernatant, and which was used to remove more nitrogen. Further study was to investigate the denitrification rate of supernatant, then a new method to determine recycling dosage was proposed according to the specific stage denitrification rate. Besides, supernatant was recycled to the real A/O process to test its denitrification rate and its influent to the A/O process. Finally, the optimum operation conditions of H/AMBBR/O and A/O process with supernatant were compared in treatment efficiency and cost of investment (for estimate).①Results of H/AMBBR/O process:Firstly,denitrification performance of both suspended biofilm with two phase and pure anoxic sludge were compared through preliminary experiments under 10.9~13℃, results showed that, removal efficiency of COD, ammonia notrogen, nitrate and total nitrogen (TN) in two phase sludge system were 74.37%, 10.48%, 60.86% and 21.42%, respectively, higher than those in suspended sludge system of 58.89%, 3.71%, 41.58% and 13.75%, respectively, and the activity of biofilm was better. So adding suspended fillers to the anoxic reactor increased sludge quantity and improved sludge activity, as well as enhanced low-temperature resistance.Secondly, the start-up of the combined process was studied. Sludge inoculation and domestication by signal reactor was adopted for hydrolysis acidification reactor, AMBBR and aerobic reactor at the same time. The hydrolysis acidification reactor was started within 15 days by increasing hydrolic loading from small amount influent, while the AMBBR was better to started-up by aerobic means and operated under anoxic condition, and the biofilm grew well within 20 days. As the combined process was started-up, pollutants removal efficiency got stable in two weeks.Thirdly, through Single factor experiment, the optimum running condition of H/AMBBR/O was screened. Results showed that, 1) the best hydraulic retention time of hydrolysis acidification reactor was 2.5h, and longer HTR would consume more carbon source, 2) the longer the HRT of AMBBR, the better for the denitrification, so the optimum ratio was 3h, 3)the larger of nitrifying liquid reflux ratio the better for denitrification, while larger reflux ratio means more power consumption, and the optimum ratio was 300%, 4) while the water temperature was below 18℃, incomplete nitrification resulted in poor TN removal efficiency, so adding fillers to aerobic reactor was proposed to enhance the effect of nitrification under low temperature, especially in wastewater treatment plant with good economic conditions. So when the flow was 50L/h, HRT of aerobic reactor was 6.0h and HRT of secondary sedimentation was 1.2h, and filler packing rate was 30%, the optimum running condition for the combind process was: when the HRT of acidification reactor and AMBBR were 2.5h and 3.0h, respectively, as well as nitrifying liquid reflux ratio was 300% and T≥20.0℃, and the optimum treatment efficiency was obtained: the average removal efficiency of COD, ammonia nitrogen and TN were 90.35%,98.24% and 71.92%, respectively, and the concentrations of these parameters in the outflow were 22.6mg/L,0.89mg/L and 16.35 mg/L, respectively.Finally, comparison of pretreatment performance for hydrolysis acidification reactor between pure wastewater and the mixture of wastewater and sludge was done. And results showed that when WAS from the second sedimentation reactor was returned back to the hydrolysis acidification reactor, the carbon source can be improved and increased to provide advantage to the following denitrification, and, meanwhile, realize the resources reuse and sludge reduction, which reached 56%.②Results of alkaline hydrolysis sludge supernatant reused in A/O process as carbon source:Firstly, the optimum condition of WAS alkaline hydrolysis and sludge reduction were studied throgh experiments. pH value and mixing condition were first determined, and the optimum SRT of 9 days was determine through parameter analysis from static continuous tests, denitrification rate and electron micrograph of cracked sludge. Besides, the sludge reduction rate attained 56.3% during WAS alkaline hydrolysis.Secondly, denitrification efficiency and denitrification dynamics of three kinds of carbon source such as alkaline hydrolysis supernatant, acetic acid sodium and sewage were investigated and compared. Results showed that alkaline hydrolysis supernatant could be carbon source for denitrification.Denitrification rate of alkaline supernatant of WAS as carbon sources for denitrification were further investigated, hereby the recycling dosage was determined preliminarily. First, WAS was digested under alkaline conditions, and batch tests were conducted under different VFA/N ratio, to examine the change of nitrate concentration and choose the optimum ratio. Then the chosen ratio was applied in pure sewage wastewater compared with the pure sewage wastewater, to study its feasibility in recycle and to determine the recycling dosage. Results showed that, the denitrification rate increased obviously with the increasing VFA/N ratio, and the higher ratio the longer time needed for nitrite peak, besides, the degradation of nitrate and change of nitrite were consistent with the pH curve. Further research showed that when the supernatant was added to the sewage wastewater to certain degree, denitrification rate increased significantly, and the nitrate removal amount of two parallel tests attained 47.02 and 33.95mg/L, respectively, which was twice to triple to that of pure sewage wastewater. In addition, the definition of specific phase denitrification rate was proposed to define different reaction period, and the initial VFA/N, as well as pH value were used to judged supernertant dosage, which had instructive meanings to practical operation.Finally, the method of how to determine supernatant recycling dosage was applied to real A/O process. Two tests both in winter and spring under different temperature have been studied. During winter test, there was not so much WAS, so the recycling dosage was about 50ml/min, which is below theoretical recycling dosage, while during spring test the amount was about 85ml/min, which equal to the theoretical recycling dosage. The removal efficiency of COD, ammonia nitrogen and TN before and after recycling in both tests were analysed, as well as the influence of nitrogen and phosphrus to the system. Results showeed that: 1)during winter test, real TN removal amount was 42mg/L, 14mg/L more than the calculated value, 2) during spring test, the calculated value according to the specific phase denitrification rate was 50.4mg/L, close to the real value of 55mg/L, and the effluent TN concentration met standard A of GB18918-2002 level. 3) during the two recycling tests, the ratio between import TN and TN of raw wastewater were 10.55% and 21.27%, respectively, considering TN removal efficiency, the effect was ignored; while the import total phosphorus ratio was 20.86% and 79.60%, respectively, and the corresponding effluent concentration of TP was 2.84 and 3.52mg/L, as there was no special phosphorus removal reactor, phosphorus would accumulated to affect the normal operation, further research should try to recover phosphate through precipitation by adding metal salt.③Comparison of the best conditions between the H/AMBBR/O process and supernatant recycling A/O process Results of pollutants removal efficiency under their optimum condition show that,1) both of the two processes has good COD and ammonia nitrogen removal efficiencies, and the effluent COD and ammonia concentration met the standard A of GB18918-2002 level. 2) effluent TN concentration of supernatant recycling A/O process was below 15mg/L, which met the standard A of GB18918-2002 level, while in H/AMBBR/O process, the effluent TN concentration could only met the standard B of GB18918-2002 level. 3) the A/O process with recycling supernatant had stronger denitrifying ability and flexibility, while H/AMBBR/O process can achieve integration of wastewater and sewage sludge treatment, as well as carbon sources recycling, so its operation and management were more easily.The operational cost prediction between two processes showed that, the difference of operational costs between the two processes were suspended filler in AMBBR and acid-base reagents used to adjust pH of supenertant. The longer service life of suspended filler (more than 10 yeas) and its characteristic as one-time investment material made the filler more cheaper than that of acid-base reagents, and its cost were 15% of acid-base reagents. So in practical, both wastewater treatment requirements and local economic level should be considerd when the processes were chosen.