The total nitrogen removal rate has not been high. I also tried several methods, but the effect was not good. the external return flow increased from 100% to 130%, increasing the sludge concentration of the biochemical tank; The day before yesterday, anaerobic and hypoxia began to enter water at the same time (about half of each entered), and the effect was not good. Yesterday's microscopic examination: rotifers 2 roaming insects 3 silkworms 2, nitrate nitrogen aerobic end: 18.8, hypoxic end: 9.58. Ask the gods to reduce the carbon source while increasing the total nitrogen removal rate.
Through the calculation of nitrogen removal efficiency, the denitrification efficiency of the building owner is 80%, theoretically the effluent TN can be 8mg/L, so the final reason is still caused by the lack of carbon source, if you want to reduce the carbon source while improving the nitrogen removal efficiency, you can consider reducing the dissolved oxygen carried by the internal reflux, and then adjust the carbon source dosage according to the effluent requirements!
For the problem of low TN removal rate, there are three common situations, this article will explain in detail and propose solutions to improve the TN removal rate, such as if you have any questions about the explanation in the article, you can go to the community to communicate and interact!
1. Lack of carbon sources
For the denitrification system, the carbon source determines the depth of the denitrification efficiency, theoretically as long as the CN ratio is 2.86 in the denitrification tank, it can be completely deniitrified, and if the CN ratio is 3.70, it can be completely deniitrified if the CN ratio is 3.70 The formula is as follows:
Assuming that C is methanol, the process of methanol oxidation can be shown in equation (1),
CH3OH+1.5O2→CO2+2H2O（1）
1. When denitrifying, if it does not include the growth of microorganisms themselves, the equation is very simple, usually expressed with methanol as the carbon source.
6NO3-+5CH3OH→
3N2+5CO2+7H2O+6OH-（2）
The correspondence between methanol and oxygen (i.e., COD) can be obtained from equation (1): 1 mol of methanol corresponds to 1.5 mol of oxygen, and methanol and NO can be obtained from equation (2).3-1 mol methanol corresponds to 1.2 molNO3-, the comparison between the two can be obtained, 1molNO3--N corresponds to 1.25 molO2, that is, 14gN corresponds to 40gO2, so C/N=40/14=2.86.
2. During denitrification, if it contains microorganisms growing by themselves, as shown in equation (3).
NO3-+1.08CH3OH
→0,065C5H7NO2+0.47N2+1.68CO2+HCO3-（3）
By the same token, we can calculate C/N=3.70.
But the theory is a theory after all, and it does not take into account the oxygen carried by the internal reflux. Under normal circumstances, denitrifying bacteria only carry out denitrification after consuming the oxygen carried by the internal reflux, so this part of the oxygen also consumes the carbon source, so some manuals also give regulations, requiring the CN ratio control of the AO denitrification process to be greater than 4, and the CN (COD:TN) ratio in actual operation is generally controlled at 4~6, and the lack of carbon source is one of the reasons why I have met many friends TN is not up to standard!
Solution: According to the CN ratio of 4~6, add carbon source
2. The reflux ratio is too small
In fact, in denitrification and denitrification, the reflux ratio determines the height of denitrification efficiency, no matter how suitable the conditions are, the reflux ratio is certain, and the denitrification efficiency is also certain, just like the protons in the trisomy, the denitrification efficiency is locked in a certain range!
The formula for denitrification efficiency η=(r+R)/(1+ r+R), where R is the external reflux ratio and r is the internal reflux ratio. Formula derivation:
Before deriving this formula, we need to set some prerequisites! Assuming that the nitrate nitrogen in the influent is 0, the denitrification and denitrification are complete, and the TN (nitrate nitrogen) content in the nitrification liquid reflux is the same as the TN (nitrate nitrogen) content in the effluent, then the amount of denitrification and denitrification is the total amount of nitrogen entering the denitrification tank is (r+R) QTN out, according to the law of conservation of matter: the influent TN is equal to the sum of the effluent TN + denitrification denitrification + bacterial assimilation consumption of the nitrogen source! 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) Bring 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 backflow ratio is relatively small (30-50%), we generally 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 internal reflux! The size of the internal return determines the denitrification efficiency.
In the current denitrification process, we use pre-denitrification and variants, but no matter how large the internal reflux is, some nitrate nitrogen will go away with the water, and it cannot reach 100% nitrification liquid backflow! So we'll keep it within the right range!
Too low internal reflux ratio will lead to a decrease in nitrogen removal efficiency, effluent TN exceeds the standard, but too high internal reflux, on the one hand, will carry more DO, consume carbon sources and destroy the hypoxic environment, and lead to an increase in electricity bills, when the internal reflux ratio is greater than 600%, the improvement of internal reflux, the denitrification efficiency will not improve much, resulting in poor cost performance!
Solution:
In the case of ensuring the denitrification efficiency, combined with the relationship between DO influence and cost performance, it is generally controlled at 200~400%, some denitrification processes are combined with internal and external reflux, and the internal and external reflux ratio should also be controlled in this range, which not only ensures the reflux of sludge, but also ensures the reflux of nitrification liquid, and ensures the denitrification efficiency of denitrification!
3. The denitrification reaction time is not enough
Insufficient denitrification reaction time refers to the insufficient hydraulic residence time of denitrification, hydraulic residence time refers to the average residence time of wastewater entering the reactor in the denitrification tank, if the effective volume of the denitrification tank is V cubic meters, the actual residence time of the denitrification tank is:
HRT=V/（1+R）Q
Where:
HRT is hydraulic dwell time
V is the reactor volume
Q is the inlet water flow rate of the reactor
R is the outflow ratio
In the design specification, the hydraulic residence time (HRT) of the denitrification tank is required to be 2~10h, that is, the lowest HRT should be controlled at more than 2h, and the denitrification will not be carried out thoroughly below the minimum residence time!
Solution:
1. For shortening HRT caused by water inflow exceeding the design standard, consider increasing the volume of the denitrification tank, such as building a new one or changing some excess buildings into denitrification tanks.
2. The HRT caused by sludge reflux is shortened, in practice the calculation of hydraulic residence time needs to count the amount of sludge reflux, the greater the sludge return ratio, the shorter the HRT of denitrification, too large sludge reflux will lead to insufficient HRT of denitrification, this is rarely considered, in fact, sludge reflux does not need to be controlled very much, the greater the control, the smaller the concentration of reflux sludge, the more water in the reflux! This situation can be solved by properly controlling the sludge return ratio.
