TN exceeds the standard, we found the specific reason! It can help you solve 80% of the total nitrogen exceeding the standard problem

For TN exceeding the standard, excluding the reason of ammonia nitrogen exceeding the standard, these four reasons account for more than 80% of the reasons for total nitrogen exceeding the standard. According to the statistics of Wutopia Community, the lack of carbon source is the most common reason, followed by insufficient reflux ratio. This article will specifically analyze the reasons for TN exceeding the standard and the solutions. If you have any questions about the explanations in the article, please contact us at sales@zoomri.com.cn .

1. Lack of carbon source

For the denitrification system, the carbon source determines the depth of denitrification efficiency. In theory, in the denitrification pool, as long as the CN ratio is 2.86, complete denitrification can be achieved. If the growth of microorganisms is added, the CN ratio can be completely denitrified when it is 3.70. The formula is as follows:

Assuming that C is methanol, the process of methanol oxidation can be shown by formula (1),

CH3OH+1.5O2→CO2+2H2O（1）

During denitrification, if the growth of microorganisms themselves is not included, the equation is very simple and is usually expressed using methanol as the carbon source.

6NO3-+5CH3OH→

3N2+5CO2+7H2O+6OH-（2）

From formula (1), we can get the corresponding relationship between methanol and oxygen (i.e. COD): 1 mol methanol corresponds to 1.5 mol oxygen. From formula (2), we can get the corresponding relationship between methanol and NO3-, 1 mol methanol corresponds to 1.2 mol NO3-. Comparing the two, we can get that 1 mol NO3--N corresponds to 1.25 mol O2, that is, 14g N corresponds to 40g O2, so C/N=40/14=2.86.

During denitrification, if the growth of microorganisms themselves is included, as shown in formula (3).

NO3-+1.08CH3OH

→0,065C5H7NO2+0.47N2+1.68CO2+HCO3-（3）

By the same token, we can calculate C/N=3.70.

But theory is theory after all, and it does not take into account the oxygen carried by the internal recirculation. Under normal circumstances, denitrifying bacteria will only denitrify after consuming the oxygen carried by the internal recirculation. Therefore, this part of oxygen also consumes the carbon source. Therefore, some manuals also provide regulations requiring the CN ratio of the AO denitrification process to be controlled to be greater than 4. In actual operation, the CN (COD: TN) ratio is generally controlled at 4~6. Lack of carbon source is one of the most common reasons why many friends I have met do not meet the TN standard!

Solution

Add carbon source according to the CN ratio of 4~6, and adjust the amount of carbon source added according to the effluent TN! Zoomri has a method for calculating carbon source. If you need it, please contact us at sales@zoomri.com.cn!

2. The reflow ratio is too small

In fact, in denitrification, the reflow ratio determines the efficiency of denitrification. No matter how suitable the conditions are, the reflow ratio is certain and the denitrification efficiency is also certain, just like the protons in the three-body, locking the denitrification efficiency within a certain range!

The formula for denitrification efficiency is η=(r+R)/(1+ r+R), where R is the external reflow ratio and r is the internal reflow ratio. Formula derivation:

Before deriving this formula, we need to set some prerequisites! Assuming that the nitrate nitrogen in the influent is 0, denitrification is complete, and the TN (nitrate nitrogen) content in the nitrification liquid reflow is the same as the TN (nitrate nitrogen) content in the effluent, then the amount of denitrification denitrification is the total amount of nitrogen entering the denitrification tank (r+R)QTN out. According to the law of conservation of matter: influent TN is equal to the sum of effluent TN + denitrification denitrification + nitrogen source consumed by bacterial assimilation! The formula is as follows:

QTN in = QTN out + (r + R) QTN out + TN assimilation

We ignore the nitrogen source consumed by bacterial assimilation! Then the formula becomes:

QTN in = QTN out + (r + R) QTN out ↓ TN out / TN in = 1 / (1 + r + R) ~ ①

Substitute formula ① into the denitrification efficiency formula:

η=（TN in-TN out）/TN in

↓η=1-TN out/TN in↓η=［（1+r+R）-1］/（1+r+R）↓η=（r+R）/（1+r+R）

Because the external reflux ratio is controlled to be relatively small (30-50%), we usually omit it as η=r/(1+ r)!

According to the formula, when the carbon source is sufficient, the denitrification efficiency of denitrification is only related to the internal reflow! The size of the internal reflow determines the denitrification efficiency.

For the current denitrification process, we use pre-denitrification and its variants, but no matter how large the internal reflow is, some nitrate nitrogen will flow away with the water, and it cannot reach 100% nitrification liquid reflow! So we will control it within a suitable range!

Too low internal reflow ratio will lead to a decrease in denitrification efficiency and excessive TN of the effluent, but too high internal reflow will carry more DO, consume carbon sources and destroy the anoxic environment, and lead to an increase in electricity bills. When the internal reflow ratio is greater than 600%, the increase in internal reflow will not improve the denitrification efficiency much, resulting in a poor cost performance!

Solution:

Under the premise of ensuring denitrification efficiency, combined with the impact of DO and the relationship between cost performance, it is generally controlled at 200~400%. Some denitrification processes combine internal and external reflows, and the internal and external reflow ratio must also be controlled within this range. This range ensures the reflow of sludge and nitrification liquid, and ensures the denitrification efficiency of denitrification!

3.The denitrification reaction time is not enough

Insufficient denitrification reaction time means that the hydraulic retention time of denitrification is insufficient. The hydraulic retention time refers to the average retention time of the wastewater entering the reactor in the denitrification tank. If the effective volume of the denitrification tank is V cubic meters, the actual retention time of the denitrification tank is:

HRT=V/（1+R）Q

In the formula:

HRT is the hydraulic retention time

V is the reactor volume

Q is the reactor inlet flow rate

R is the external reflux ratio

In the design specifications, the hydraulic retention time (HRT) of the denitrification tank is required to be between 2 and 10 hours, that is, the minimum HRT must be controlled above 2 hours. If the retention time is lower than the minimum, denitrification will not be thorough!

Solution:

1. If the HRT is shortened due to the inflow exceeding the design standard, you can consider increasing the volume of the denitrification tank, such as building a new one or converting some redundant buildings into a denitrification tank.

2. If the HRT is shortened due to sludge return, in practice, the calculation of hydraulic retention time needs to take into account the amount of sludge return. The larger the sludge return ratio, the shorter the denitrification HRT. Excessive sludge return will lead to insufficient denitrification HRT. Few people consider this. In fact, sludge return does not need to be controlled very much. The larger the control, the lower the return sludge concentration and the more water in the return! This situation can be solved by properly controlling the sludge return ratio.

4. Environmental damage in denitrification pool

The sign of this situation is that the DO of the denitrification pool is greater than 0.5, which destroys the anoxic environment and makes the facultative heterotrophic denitrifying bacteria preferentially use oxygen for heterotrophic metabolism instead of nitrate nitrogen, making it impossible to remove nitrate nitrogen, resulting in an overall increase in TN. The anoxic environment of the denitrification pool is damaged, which may often lead to excessive ammonia nitrogen. The reason is that nitrifying bacteria cannot form dominant strains. However, if the aeration pool is large enough, there is no problem!

Solution:

1. If the internal reflow is too large, resulting in excessive DO, reduce the internal reflow ratio or turn down the aeration at the internal reflow.

2. High DO caused by other problems, such as the distance between the inlet and the water surface is too high, resulting in falling oxygenation, and reducing the height difference.