1、 Lack of carbon source
In the process of nitrification and denitrification, the theoretical CN ratio required for removing TN is 2.86. However, in actual operation, due to the DO carried by reflux, the CN (COD: TN) ratio is generally controlled at 4-6, and there is a lack of carbon source. This is one of the most common reasons why I have encountered many friends whose TN does not meet the standard!
terms of settlement:
Add carbon source according to CN ratio of 4-6, and adjust the amount of carbon source added based on the effluent TN! There is a carbon source calculation software in the community. If you need it, you can calculate it!
2、 Internal reflux r is too small
The full name of AO process is inverted nitrification denitrification process, and the denitrification efficiency of AO process is directly proportional to the internal reflux ratio! According to the denitrification efficiency formula η=(r+R)/(1+r+R), because R is generally fixed and the control ratio is small, the larger the internal reflux ratio r, the higher the denitrification efficiency. Some sewage treatment internal reflux pumps are partially damaged or selected too small, which can lead to low denitrification efficiency!
terms of settlement:
Increase the internal reflux ratio r to 200-400% (some denitrification processes combine internal and external reflux, and the internal and external reflux ratio should also be controlled within this range, which ensures the reflux of sludge and nitrification solution, and guarantees the denitrification efficiency)
3、 Environmental damage in denitrification tank
The sign of this situation is that the dissolved oxygen (DO) in the denitrification tank is greater than 0.5, which destroys the anaerobic environment and allows facultative heterotrophic denitrifying bacteria to preferentially use oxygen for heterotrophic metabolism instead of nitrate nitrogen, making nitrate nitrogen unable to be removed, resulting in an overall increase in TN. The anaerobic environment in the denitrification tank is also destroyed, which may often lead to excessive ammonia nitrogen. The reason is that nitrifying bacteria cannot form dominant strains, but the aeration tank is large enough, so there is still no problem!
terms of settlement:
1. If the internal reflux is too large and carries too much DO, adjust the internal reflux ratio or turn off the aeration at the internal reflux point.
2. The high DO caused by other issues, such as the high distance between the inflow and the water surface, leading to a drop and oxygenation, requires reducing the height difference.
4、 Insufficient denitrification reaction time
In the design specifications, the hydraulic retention time (HRT) of the denitrification tank is required to be between 2-10 hours, which means that the minimum HRT should be controlled at 2 hours or more. Denitrification will not be thorough below the minimum retention time, and this situation is mainly caused by the small design water volume in the early stage!
terms of settlement:
1. If the HRT is shortened due to the inflow exceeding the design standards, it is possible to consider increasing the capacity of the denitrification tank, such as building new ones or converting some excess buildings into denitrification tanks.
2. The shortened HRT caused by sludge reflux needs to be taken into account in the calculation of hydraulic retention time in practice. The larger the sludge reflux ratio, the shorter the HRT for denitrification. Excessive sludge reflux can lead to insufficient HRT for denitrification, which is rarely considered. In fact, sludge reflux does not need to be controlled too much. The larger the control, the lower the concentration of sludge reflux and the more water in the reflux! This situation can be solved by appropriately controlling the sludge reflux ratio.
5、 The denitrification efficiency is too high
In the design specifications, the denitrification efficiency is generally between 60-85%. In the problem, the first level AO denitrification efficiency has exceeded 90%, which is already very good for municipal sewage. However, in high ammonia nitrogen process wastewater, it is difficult to require TN to reach first level A for the first level AO process.
terms of settlement:
Adopting a two-stage or multi-stage AO process to solve the problem.
6、 Containing n-heterocyclic organic nitrogen
Some nitrogen-containing organic compounds cannot be broken down by ordinary biochemistry, resulting in their inability to be removed. This situation is relatively rare, mainly in a certain type of wastewater. In this case, it is mainly a problem of process selection, without considering the process of organic nitrogen ammonification (conversion of organic nitrogen into ammonia nitrogen).
terms of settlement:
1. Increase pre-treatment for hydrolysis and acidification.
2. For those that cannot be broken by hydrolysis acidification, advanced oxidation pretreatment should be added.
