| According to serious nitrogen pollution in waterbodies and low nitrogen removal efficiency in the conventional wastewater treatment plants(WWTPs),the main objective of this paper was to improve the TN removal efficiency in biological nitrogen removal processes.The SND phenomenon,the key factors effecting SND and the SND mechanism were investigated in both full-scale and lab-scale reactors,and a kinetic model for SND was established.The removal of nitrogen from penicillin wastewater in a full-scale facility was studied.The original CASS tank was reconstructed to simultaneous nitrification and denitrification(SND)system.A good performance was achieved during stable operation.When the influent concentrations of COD,TKN and NH3-N were 3089±453,251.4±26.5 and 124.5±26.8 mg·L-1,the removal efficiency of COD,TN and NH3-N were 89.7%,88.7%and 99.5%,respectively.Nitrification and denitrification occurred at different spaces,i.e.,71.4%of TN removal occurred in the first 40%of the aeration tank,while 68.8%of the TKN removal occurred in 40-100%of the aeration tank.The denitrification rate under aeration conditions was 10.7 mgN·gVSS-1·h-1 when easily biodegradable organics(EBo)were sufficient in the front of aeration tank.When EBO were completely degraded,denitrification was limited due to insufficient of carbon sources,while the nitrification rate increased.In order to study the effect of carbon sources on biological nitrogen removal via SND,three parallel reactors with methanol,potassium biphthalate and phenol as sole carbon sources were carried out.Under the condition of identical COD loading(5.0 kgCOD·m-3·d-1)and COD/TN(5.0),the highest TN removal efficiency obtained by methanol,with an average value at 80.4%,followed by potassium biphthalate at 70.5%and phenol at 66.0%.The reason for different TN removal by carbon source was that the competition capacity of denitrifier on organics was was different from one another.Among three carbon sources above,denitrifier showed the strongest competition capacity on methanol.The sludge activity and the denitrification rate were 4.35 and 3.94 mgTN·gVSS-1·h-1,and the ratio of COD consumed by denitrification was 76.6%.Therefore,in order to improve the TN removal via SND,EBO with rapid denitrification rate should be chosen.The experiments were carried out to investigate the effect of areation rate on biological nitrogen removal via SND.The results show that,the removal rate of COD and NH3-N increased with the increasement of areation rate,while the TN removal efficiency decreased.This is because the carbonaceous oxidation rate increased and more COD was consumed by carbonaceous oxidation,leading to the deterioration of TN removal efficiency.Therefore,in order to improve TN removal efficiency,the areation rate must be reduced as much as possible on the premise of complete nitrificaiton and well mixed in the areation tank.In addition,there was a competition on oxygen between carbonaceous oxidation bacteria and nitrifier.When the microorganism in activated sludge were constant,the rate competition between carbonaceous oxidation and nitnfication on oxygen under the conditon of various areation rate was close to the ratio between the carbonaceous oxidation activity and nitrification activity,that's rCO/rNH?rCOM/rNHM.According to the findings in this paper,SND mechanism could be summarized:SND is essentially that nitrification and denitrification occurred in different spaces of the aeration tank,that is.nitrification occurred in the front of the aeration tank,while nitrification primarily occurred in the rear.Sufficient EBO in the front of the aeration tank were key to the occurrence of SND.There were two types of competition in SND:competition for organics between denitrification and carbonaceous oxidation and competition for oxygen between nitrification and carbonaceous oxidation.According to the rate competition in SND,Monod equations were simpled,model hypothesis were proposed and SND kinetics models were established.This model contains four curves:the variations in COD,TN,NH3-N and NO3-N.Base on the SND model,nitrogen removal proeesses were simulated using operational data from four groups of batch tests.The results of simulation could exactly reflect the actual nitrogen removal process,with the correlation coefficients of above 0.99 between modeling curves and experimental data.The results provide directive significance for improving the nitrogen removal efficiency of the exsiting WWTPs,and eould support and promote the futher investigate of SND mechanism. |
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