Renovation of Wastewater Reduction Process for Ammonia Nitrogen Concentration

The AAO activated sludge method mainly converts the complex organic matter in the wastewater into simple substances through the catabolism of various microorganisms. The advanced treatment adopts the Fenton catalytic oxidation method. The process flow is shown in Figure 1.

Problems in biochemical treatment

The designed ammonia nitrogen concentration of the phenol cyanide wastewater from the coking plant of Beiying Company after biochemical treatment is ≤15mg/L, but the average concentration of ammonia nitrogen in the secondary sedimentation tank effluent from May to August 2016 was as high as 41.93mg/L, as shown in Table 1, which affected Fenton’s catalysis The effect of advanced oxidation treatment also brings problems such as corrosion and blockage to the slag flushing equipment in iron works.

The problems existing in the biochemical treatment of phenol and cyanide wastewater from the coking plant of Beiying Company mainly include three aspects: excessive ammonia distillation wastewater index, poor pretreatment effect and poor treatment effect of AAO system.

1.1 The indicators of ammonia distillation wastewater exceed the standard

The designed influent water quality indicators of the phenol cyanide wastewater treatment system are COD concentration ≤3500mg/L, ammonia nitrogen concentration ≤200mg/L, oil concentration ≤30mg/L, but the actual influent water quality often exceeds the standard. The main reason is that the index of the remaining ammonia water entering the ammonia distillation tower fluctuates greatly, and the untimely adjustment of the amount of alkali added leads to the excessive index of wastewater after ammonia distillation. In addition, the remaining ammonia tank and ceramic membrane filter have less oil discharge times, resulting in high oil content in the wastewater after ammonia distillation, which affects the normal operation of the AAO system.

1.2 The preprocessing effect is not good

The main reason for the poor pretreatment effect is the blockage of the dissolved air water nozzle in the flotation cell. Because the dissolved air in the flotation cell is mixed with the air after the advanced treatment of Fenton catalytic oxidation, it is easy to block the nozzle, resulting in poor oil removal effect, and the high oil content of the wastewater can easily poison the microorganisms and affect the treatment effect of the AAO system.

1.3 The processing effect of AAO system is not good

The phenolic cyanide wastewater is pretreated to remove oil, and the quantity and quality of the wastewater are adjusted before entering the AAO system.

1) The anaerobic biological nutrients are not balanced, resulting in poor degradation effect of the anaerobic tank.

The combined filler is hung in the anaerobic tank, and the anaerobic organisms on the filler unzip the polycyclic aromatic compounds and hydrolyze the cyanide and thiocyanide, so that the refractory aerobic or facultative biodegradable substances become easy to be degraded. Degradable substances, improve the biodegradability of wastewater. The ratio of nutrients required by anaerobic organisms is BOD5:N:P≈100:2.5:0.5, but the coking wastewater basically does not contain phosphorus, so the degradation effect of the anaerobic tank is not good.

2) The effect of denitrification reaction is not good.

The effluent of the anaerobic tank and the return water of the secondary sedimentation tank are lifted by the pump and sent to the anoxic tank. In the anoxic tank, the denitrification reaction of facultative bacteria is used to reduce the nitrate nitrogen in the sewage to nitrogen and escape from the wastewater to achieve the purpose of removing ammonia nitrogen. The suitable pH value of facultative bacteria is 6.5-7.5, and the pH value of incoming water is high, which affects the denitrification reaction of the anoxic tank; the sludge on the filler grows slowly and falls off seriously, which also affects the denitrification reaction of the anoxic tank; , the secondary sedimentation tank has a large amount of backflow sewage, which leads to a short residence time of denitrifying bacteria and does not reach the generation renewal time of denitrifying bacteria.

3) The nitrification reaction effect is not good.

The effluent from the anoxic tank flows into the aerobic tank. In the aerobic tank, the phenol, cyanide and other harmful substances in the wastewater are degraded by aerobic microorganisms, and the ammonia nitrogen in the wastewater is oxidized to nitrate nitrogen through the nitrification reaction. The suitable growth temperature of nitrifying bacteria is 25-30 °C, and the growth needs of nitrifying bacteria cannot be met when the temperature is low in winter; the generation time of nitrifying bacteria is relatively long, but the amount of defoaming water in the aerobic tank is large, and the sludge return flow is also large. , resulting in a short residence time of nitrifying bacteria, which cannot reach the minimum generation update time of nitrifying bacteria; the ratio of nutrients required by aerobic microorganisms is BOD5:N:P≈100:5:1, while the phosphorus salts actually put in The amount is too large, which affects the effect of the nitrification reaction.

Improvement measures

1) In response to the situation that the indicators of ammonia distillation wastewater exceed the standard, return the excess ammonia distillation wastewater to the mechanized ammonia water clarification tank; increase the number of oil discharges of the remaining ammonia water tank and ceramic membrane filter, and the number of oil discharges is changed from once a week to the remaining ammonia water. The tank drains oil once a day, and the ceramic membrane filter drains oil 2 to 3 times a week.

2) Aiming at the problem of clogging the nozzles of the flotation tank, using fire water mixed with air to dilute and remove oil from the raw water can improve the hydrolysis and acidification effects of anaerobic bacteria.

3) In view of the current situation of low phosphorus content in coking wastewater and too much phosphorus salt in aerobic pools, according to the experimental data, the phosphorus salt dosing in aerobic pools was changed from 100kg/d to anaerobic pools and aerobic pools, respectively. Add 50kg/d.

4) In view of the high pH value of the anoxic tank, reduce the dosage of liquid caustic soda in the ammonia distillation tower, the dosage of liquid caustic soda is reduced from 4t/d to 3t/d, and the pH value of ammonia distillation wastewater is reduced from 8 to 9 to 7 to 7.5.

x5) In view of the slow growth and serious shedding of the sludge on the filling of the anoxic tank, the compressed air is passed through the water distribution pipe of the anoxic tank for 30 minutes each time, which can blow off the dead sludge in time and promote the growth of the sludge.

6) In view of the short residence time of denitrifying bacteria, reduce the amount of backflow sewage in the anoxic tank, from 55-60m3/h to 38-40m3/h.

7) In view of the problem of slow growth of nitrifying bacteria due to low temperature in the aerobic pool in winter, the defoaming water in the aerobic pool is heated with medium pressure steam in winter.

8) In view of the short residence time of nitrifying bacteria, reduce the amount of defoaming water and return sludge in the aerobic tank, and reduce the amount of defoaming water in the aerobic tank from about 17m3/h to 6-10m3/h (about 6m3/h in summer, About 10m3/h in winter), reduce the amount of return sludge from 35-40m3/h to 20-25m3/h.

After the improvement, the ammonia nitrogen concentration in the effluent of the secondary sedimentation tank reached the design value, and the average value from January to April in 2017 reached 13.27 mg/L, see Table 2.

Conclusion

After detailed analysis, the reason for the high ammonia nitrogen concentration in the effluent of the biochemical system was determined, and targeted measures were taken, and good results were achieved.

1) By monitoring the incoming water index, the unqualified ammonia steaming wastewater is re-distilled for ammonia, and the oil discharge times of the remaining ammonia water tank and ceramic membrane filter are increased, which stabilizes and reduces the incoming water index.

2) The fire water is mixed with air and added to the flotation cell, which solves the problem of clogging of the nozzle of the flotation cell and is beneficial to the operation of the AAO system.

3) By analyzing the growth conditions and reaction conditions of anaerobic microorganisms, facultative bacteria and aerobic microorganisms, a series of improvement measures were formulated to make the AAO system work well.