Advanced Treatment Of Electroplating Tail Wastewater Through Two-staged Anoxic-oxic (A/O) Process

Electroplating wastewater is one of the three major pollutions in the world for its complexity,difficulty to control and strong toxicity.After being treated by traditional oxidative complex-breaking and the alkali precipitation to remove most of the heavy metal ions,large amounts of organic substances,ammonia nitrogen and total nitrogen still remain in the electroplating tail wastewater.In the previous process,a large amount of NaCl,NaClO and HCl were added during the removal of cyanide,the breakage of heavy metal ions and adjustment.Therefore,the conductivity of the waste water is usually up to 10 000?s/cm.Traditional physico-chemical methods?such as advanced oxidation?are not only costly,but also would bring about secondary pollution and large amounts of sludge.Most of all,efficient nitrogen removal could not be achieved by any means of physico-chemical methods.Thus,biological treatment might be one comptetive alternative to remove COD,ammonia nitrogen,TN and even phosphate from electroplating tail wastewater simultaneously.Based on the two-staged A/O process,hydrolysis acidification reactor was added to increase the biochemical properties of the electroplating tail wastewater.The first step of the experiment was to acclimatize and start the combined reactor.The main tasks of this study were to optimize the operating parameters of the combined reactors,to study the removal efficiency of the reactor to treat the actual electroplating tail wastewater under the optimum operating conditions.The composition of the microbial community in the stable operation period combination system was analyzed,and the effects of different NaCl impact on the treatment performance and activated sludge of the combination system were discussed.The main results were shown as follows:?1?The sludge acclimation and start-up based on the two-staged A/O process were divided into two stages:simulated wastewater stage and actual wastewater stage.After 50 days of operating,the combined reactor had a stable removal effect on nitrogen and carbon in the actual electroplating tail wastewater,and the effluent quality can meet the national discharge standard A,class 1 according to GB21900-2008.?2?After the combined reactor was domesticated and started,the influence on the performance of the combined reactor was explored mainly from the key factors such as the HRT and the reflux ratio of the nitrified liquid to optimize the operating parameters of the reactor.When the HRT increased from 30 to 120 h,the removal rate of COD and ammonia nitrogen in the combined reactor showed a trend of first rising and then decreasing,while the removal rate of TN showed a different increase.Considering the control of energy consumption and pollutant removal rate,HRT was suitable for 60 hours.When the reflux ratio of nitrifying liquid increased from 100%to 250%,the combined system had little effect on the removal rate of COD and ammonia nitrogen,and the TN removal showed a trend of first rising and then decreasing.When the reflux ratio of nitrifying liquid was 200%,the removal rate of TN reached the highest,which can be regarded as the best reflux ratio.?3?The actual electroplating tail wastewater with high fluctuation of water quality was treated by the combined reactor under the optimum operating conditions.During the whole operation period,the average removal rate of COD was 83%,with most of the effluent COD below 50 mg/L;the average removal rate of NH₄⁺-N was as high as 97%,and the average concentration of NH₄⁺-N was as high as 97%;the average removal rate of nitrogen was 65%,the average concentration of effluent TN was 9.25 mg/L.The effluent meet the discharge standard A class 1 of GB21900-2008,with alkanes and heterocyclic hydrocarbons being the main components of influent COD.The removal rate of COD,TN and NH₄⁺-N in the first stage of A/O process was better than that in the second stage of A/O process.Three-dimensional fluorescence spectra also showed that the fluorescence intensity of the tryptophan fluorescence peaks in the second stage of A/O process was not obvious.However,there were some other substances continue to be degraded in the second stage of A/O process.Apparently,the first stage of A/O process played an important role for COD and nitrogen removal during the whole process,while second stage of A/O process could also be a crucial complement for the betterment of NH₄⁺-N,TN and COD removal.?4?The impact of different conductivity on the combined reactor was simulated by adding NaCl in the influent.When the NaCl concentration is 0⁶ g/L in the influent,the combined reactor still had good removal effect on nitrogen and carbon,and the effluent quality could reach the discharge standard.When the concentration of NaCl increased to 6 g/L in the influent,the removal rate of ammonia nitrogen and TN decreased and the effluent of COD and TN could not meet the discharge standard.When the concentration of NaCl in the influent continued to increase to 8 g/L,the removal rate of NH₄⁺-N and TN decreased sharply,as the salinity caused irreversible damage to activated sludge and biofilm in the combined reactor.With the increase of salinity from 0 g/L to 8 g/L,the specific oxygen uptake rate?SOUR?of inoculated sludge,AE1 and AE2 increased at the beginning and then they decreased.SOUR of inoculated sludge,AE1 and AE2 began to decrease when the concentration of NaCl reached 2 g/L,6 g/L and 8g/L,respectively.Compared with inoculated sludge,the change of SOUR in AE1 and AE2 were relatively slight,indicating that the microorganisms in the comnined reactor had better resistance to the chloride ions.In addition,the composition and structure of EPS in activated sludge were affected by the increase of salinity.When the concentration of NaCl in influent reached 4 g/L,the value of PN/PS was lower.When the influent NaCl concentration was 0⁶g/L,the combined reactor could still maintain a better good removal and resistance to chloride ions.However,when the concentration of NaCl exceeded 8 g/L,it will cause irreversible damage to the composite system.?5?The microbial communities in ANA,AE1 and AE2 were analyzed by high throughput sequencing on the 60th day under the optimum operating conditions.The dominant phylum in ANA,AE1,and AE2 was Proteobacteria.The dominant calss in ANA were mostly related to hydrolytic acidification bacteria,such as Anaerolineae?14.47%?;Alphaproteobacteria was the dominant calss of AE1?25.1%?and AE2?21.8%?.In AE1 and AE2,Nitrosomonas?4.81%,4.71%?and Nitrospira?4.94%,4.93%?were dominant bacteria in AOB and NOB,respectively.Denitrifying bacteria such as Denitratisoma?AE1:1.28%,AE2:1.66%?also existed in the two-staged A/O process,confirming the simultaneous nitrification and denitrification in the aerobic reactor.Moreover,both salt-tolerant bacteria?Bacillus?Marinobacter?Hyphomicrobium,etc.?and halophili bacteria?Halomonas?Pseudomonas?Sphingopyxis,etc.?were also found in the combined process.Therefore,microbial community within the two-staged A/O process could be acclimated to high electrical conductivity,and adapted for electroplating tail wastewater treatment as well.