Challenges in the operation of coking wastewater treatment

Principle of the A-A/O process: Nitrogen in wastewater exists mainly in the form of organic nitrogen or ammonia nitrogen. Organic nitrogen can be converted into ammonia nitrogen through bacterial decomposition and hydrolysis. The basic principle of biological nitrogen removal is to first oxidize ammonia nitrogen to nitrate nitrogen (NO3--N) through nitrification, and then reduce nitrate nitrogen to nitrogen gas (N2) through denitrification, allowing it to escape from the water. Biological nitrification includes ； There are two steps: in the first step, ammonia nitrogen is oxidized to nitrite nitrogen (NO2--N) through the action of nitrite bacteria; in the second step, nitrite nitrogen is further oxidized to nitrate nitrogen through the action of nitrate bacteria. The two types of bacteria responsible for nitrification are both Gram-negative aminotrophs, and they are strictly aerobic, obligate chemosynthetic organisms. The reaction is as follows: wherein C5H7O2N represents the cells of nitrite-oxidizing bacteria and nitrate-oxidizing bacteria. If the proliferation of nitrifying bacteria during the nitrification process is not taken into account, the nitrification process can be expressed by the following formula. As can be calculated from the above reaction equation, 4.57 g of oxygen is required to oxidize 1 g of ammonia nitrogen to nitrate nitrogen, and 7.14 g of alkalinity (expressed in terms of CaCO3) is consumed. Additionally, the nitrification process generates acidity; therefore, for wastewater with low alkalinity and high ammonia nitrogen concentrations, alkali must be added to maintain a pH level suitable for nitrification. The optimal pH range for nitrification is 8.0–8.4. Biological denitrification is the process by which denitrifying bacteria use organic carbon as a carbon source to reduce nitrate nitrogen to nitrogen gas, which is then released into the air. Denitrifying bacteria are facultative heteroaerobes. The reaction equation is as follows: Based on the calculations from the above reaction equation, it can be seen that the reduction of 1 g of nitric nitrogen provides 3.74 g of alkalinity (expressed in terms of CaCO3). Additionally, 5 organic carbons must be provided to remove 4 nitrate nitrogens. 2 oxygen atoms are required to oxidize 1 carbon atom into carbon dioxide; 5 carbon atoms correspond to a BOD value of 160 (32×5=160). Therefore, theoretically, the BOD/TN ratio in the denitrification tank must be greater than 2.86 in order to meet the carbon source requirements of denitrifying bacteria. Denitrification occurs under anaerobic conditions, with an optimal pH of neutral or slightly alkaline. If the organic matter present in the wastewater can be used for denitrification, no additional organic matter is needed; otherwise, organic matter must be added externally, usually methanol. In this case, the denitrification reaction can be expressed as follows: The A-A/O process consists of three stages of biological treatment. Depending on the form in which microorganisms exist, the A-A/O process includes both the activated sludge method and the biofilm method. This process subjects the pretreated wastewater to three sequential treatment stages: anaerobic, anoxic, and aerobic. Its feature is the addition of an anaerobic stage on top of the conventional anoxic/aerobic process (A/O). The anaerobic stage can effectively hydrolyze and acidify wastewater, thereby improving the efficiency of the anoxic/ aerobic treatment process (hydrolysis and acidification enhance the biodegradability of coking wastewater). At present, this process is one of the more advanced biological nitrogen removal processes for treating coking wastewater in China. Figure 1 Process flow diagram of the phenocyanide treatment station