| Coking wastewater is complicated in compositions and is characterized by a high NH4+ concentration and slowly degradable organics such as aromatic compounds, heterocyclic and polycyclic aromatic hydrocarbons (PAHs) which are very slow in degradation. Pollution control of coking wastewater is a tough topic in the pollution control of industrial wastewater in China. A popular problem faced in most of coking plants is that effluent COD and NH4+ concentrations from biological treatment processes cannot meet the needs of the General Wastewater Discharge Standards I (GB9878-1996:COD≤100 mg/L, NH4+≤15 mg N/L), ifinfluent is not diluted by tap water.Coking wastewater is usually treated by activated sludge processes such as A/O and/or A2/O, however, which are difficult to realize simultaneous removals for both COD and NH4+ to the standards mentioned above. The reasons are analyzed for: (1) depressed nitrifying bacteria by high influent COD concentrations and bacteria-inhibited organics in influent; (2) a small part of degradable COD, a short HRT and less carbon source in anoxic reactor. As the capacity of COD removal by denitrification is not fully utilized, the effect of denitrification is poor so that the A/O and/or A2/O processes cannot make use of the whole process for COD removal.In this thesis, a pilot-scale biofilm A2/O2 (anaerobic/anoxic/oxic/oxic) process was studied for its process performance and results, based on the coking wastewater from an air-flotation equipment at a coking plant of Tongshida Co. Ltd. Linfen,, Shanxi. Ceramic pellets were packed in the anaerobic and anoxic reactors with an upflow-filter mode. Hollow plastic balls were packed in the first aerobic reactor, and ceramic pellets were packed in the second aerobic reactor. During the trial of the biofilm A2/O2 process, the influent COD varied between 1000 and 2200 mg/L, and the influent NH4+ concentration fluctuated between 200 and 400 mg N/L.The key points about every reactor in the A2/O2 biofilm system are concluded as follows:1. Hydrolysis-acidification (anaerobic) reactorThe bacteria responsible for hydrolysis-acidification were poor in adherence and difficult to grow on the surface of packing materials, so that the hydrolysis- acidification reactor took longer time to start up. Violent changes on the influent COD and NH4+ concentrations seriously affected the start-up and operation of the hydrolysis-acidification reactor. The hydrolysis-acidification reactor with ceramic pellets packed took half a year from the start-up to the normal operation with ripe biofilm. Hydrolysis-acidifcation was used for improving the degradability of coking wastewater. During hydrolysis-acidification, much NH4+ was released due to a high ratio of nitrogen-containing organics. Therefore, it was very easy to judge the status of biofilm growth and the operational performance by the change of in fluent-effluent BOD/COD ratio and NH4+ concentration. Both COD and BOD could be removed in the hydrolysis-acidification reactor to a certain extent. Under the conditions of the on-site experiment, 20 hrs (HRT) was found to be optimal for hydrolysis-acidification. At HRT=20hrs, CODload=1.61～2.65kgCOD/(m3·d) and BOD/CODin=0.05～0.17, BOD/COD(eff) was calculated at 0.16～0.48, which meant a great improved degradability of organics with an average increase of 175% and/or a top increase of 336.4% on BOD/COD.Due to fluctuating characteristics of coking wastewater, HRT is proposed as a parameter of process design for hydrolysis-acidification. The biomass concentration in the hydrolysis-acidification reactor was measured up to 8960 mg/L on the basis of SS (VSS=7420 mg/L). For this reason, the hydrolysis-acidification reactor had a good adaptability to fluctuating pH, temperature and wastewater quality. The dominant bacteria in the hydrolysis-acidification reactor were facultative bacteria, such as Bacillus sp., Aeromonas sp., Flavobacterium sp. and Paracoccus sp. Further, the sludge production in the hydrolytic acidification reactor was low. The hydrolysis-acidification reactor had a long SRT and resulted in a low sludge production. Although backwashing was not done during the two-year's experiment, the performance of the reactor was not affected at all.2. Anoxic reactorDuring the start-up, biofilm was under growth and denitrification capacity was poor so that a low recirculation ratio should be applied. The optimal recirculation ratio for the anoxic reactor was 300%, at which an average NO3- -N reduction efficiency was maintained at＞90%. Normal denitrification in the anoxic reactor occurred above 20℃. After hydrolysis-acidification, pH in the influent to the anoxic reactor was between 6 and 8 and could meet the needs of the anoxic reactor. Denitrification played a very important role in removing COD from coking wastewater. Denitrifying bacteria could use some slowly degradable organics for denitrification, which had not been degraded by aerobic and anaerobic microorganisms. Therefore, nitrate in the anoxic reactor was helpful for sturdy COD removal, which could remove COD up to 40% in the anoxic rector and further increased the total COD removal efficiency of the system. So, the reasonable design for anoxic reactor in the A2/O2 biofilm process is critical to assure the effluent COD standard. For denitrification of coking wastewater, carbon source was relatively enough and no external carbon was needed if denitrifying bacteria could be fully used to degrade slowly degradable organics in the anoxic reactor. An influent C/N ratio above 5 was enough of carbon needed for denitrifying in the anoxic reactor. Because denitrifying bacteria in the A2/O2 biofilm process used not only degradable COD bu also slowly degradable COD and even endogenous carbon, the denitrifying rate of coking wastewater was lower than that of municipal wastewater. The volumetric load of NO3- -N in the anoxic reactor was relatively low, which was lower than 0.24 kgNO3- -N/m3·d under the stable operation. HRT in the anoxic reactor should be longer than 24 hrs. The biomass amount (SS) decreased from the bottom to the top, with an average SS of 4160 mg/L and an average VSS of 3240 mg/L. When the size of ceramic pellet packed in the anoxic reactor fell in 3～6mm, the upflow velocity was so high due to a high recirculation ratio that release of dinitrogen gas produced in the denitrifying process could naturally occur and that a recirculation for releasing dinitrogen gas and backwashing for the reactor was not needed. The dominant bacteria in the anoxic reactor were Alcaligenes sp., Pseudomonas stutzeri UP1, Flavobacterium, etc. Though the DO level in the second aerobic reactor was high (4～5 mg/L), denitrifying in the anoxic reactor was not disturbed as the oxygen introduced by the recirculation flow from the aerobic reactor was consumed by denitrifying bacteria at the bottom of the anoxic reactor.3. Aerobic reactorThe start-up of the second aerated biological filter reactor was good in the summer, which could shorten the time of biofilm growth. During the period of biofilm growth, the influent NH4+ concentration should be limited below 60 mg N/L in order to avoid the effect on young biofilm. A high dilution ratio was proposed for a low toxicity of organics from coking wastewater during biofilm growth. A low gas-liquid ratio was also proposed to avoid washing out young biofilm during biofilm growth. The COD removal efficiency in the first aerobic reactor was good and was maintained above 80% if the COD volumetric load was lower than 2.79 kg COD/(m3·d). The nitrifying process in the second aerobic reactor was good at CODin＜200mg/L and became poor at CODin＞200mg/L. A good performance of nitrifcation could be realized at CODVL＜0.67 kgCOD/(m3·d) and NH4+VL＜0.49 kg N/(m3·d). Without any dilution, both the effluent COD and NH4+ of the system could meet the needs of the General Wastewater Discharge Standards I, at HRT=20, 24, 48 and 48 hrs respectively in the anaerobic, anoxic, first aerobic and second aerobic reactors, and at DO=5～6mg/L and 4～5mg/L, CODVL=0.40 and 0.07 kg COD/m3·d and NH4+VL=0.128 and 0.022 kg N/m3·d in the first aerobic and second aerobic reactors respectively. Aerobic organic conversion rate and denitrifying rate of coking wastewater were lower than those of municipal wastewater, as COD and NH4+ concentrations of coking wastewater were quite high and there were a lot of slowly degradable and detrimental organics. Therefore, low volumetric loads and long HRTs were proposed for both the first aerobic and second reactors to reach the discharge standards (GB8978-1996). The amounts of biomass were 7440 and 3870 mg/L (SS) in the first and second aerobic reactors respectively, which were much higher than that in conventional nitrifying activated sludge process (about 2000 mg N/L). For this reason, a biofilm process is proposed for single nitrifying process. Heterotrophic bacteria were the dominant microorganisms in the first aerobic reactor, such as Bacillus sp., Zoogloea sp., Flavobacterium, Nocardia, Alkaligenes, etc.; nitrifying bacteria were dominant microorganisms in the second aerobic reactor, such as Nitrobacter, Nitrosococcus sp. Environmental factors such as organic concentration, DO, temperature, pH had effects on the nitrifying process in the second aerobic reactor and the optimal condition was at DO=5 mg/L, T=25℃, pH=7.0～7.8 and ALKeff≥150 mg/L. The aerobic filters were also helpful to remove SS, and low levels of SS were measured in the effluents of the aerobic filters after the operation for 5 months, which served the function of lowering effluent biomass COD. Two-stage aerobic reactors were proposed for coking wastewater treatment: the first aerobic reactor for COD removal and the second for ammonium removal. Because of different operational conditions, process optimization should be taken for each rector. A single nitrifying reactor was helpful to have more nitrifying bacteria as high and detrimental COD concentrations were decreased in the first aerobic reactorThe experimental research showed that the effluent of the A2/O2 biofilm system could reach the discharge standards: (COD≤100 mg/L, NH4+≤15 mg N/L (GB8978-1996) with CODin=1000～2000 mg/L and NH4+in=200～400mg N/L and without any dilution, at HRT=20, 24, 48 and 48 hrs respectively in the anaerobic, anoxic, first aerobic and second aerobic reactors and at a nitrifying recirculation ratio of 3.Creative results obtained in this research were as follows:The A2/O2 biofilm process was proposed for coking water treatment. Ceramic pellets were used in the upflow anaerobic and anoxic filters and in the second upflow aerated filter; hollow plastic balls were employed in the first upflow aerated filter. The pilot-scale experiment of the A2/O2 biofilm process obtained some useful technical parameters for process design.The A2/O2 biofilm process could meet the needs of the general wastewater discharge standards I (GB8978-1996: COD≤100mg/L, NH4+≤15mg/L) without any dilution, after coking wastewater was pretreated for oil removal by isolating and flotating.COD removal during the anoxic denitrifying process was enhanced. A reasonable process design for the anoxic reactor was critical for a low COD effluent. Carbon source was relatively enough and no external carbon was needed if denitrifying bacteria could be fully used to degrade slowly degradable organics in the anoxic reactor. An influent C/N ratio above 5 was enough of carbon needed for denitrifying in the anoxic reactor.
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