In recent years, the discharge standard of sewage treatment has become higher and higher, especially TN has been separated from the low taste of inferior class V water standard and compared with class III and IV water standards. Due to the water quality characteristics of municipal sewage with low carbon and high nitrogen, the conventional denitrification process can not meet the demand for carbon source in the anoxic denitrification stage, resulting in excessive TN, Therefore, adding carbon source is an important and only means for sewage treatment plants to solve such problems.
1. Why is sodium acetate the best carbon source?
For the selection of carbon source for denitrification process, if the price is excluded, it is generally judged from the denitrification rate and whether there is residual cod!
At present, the additional carbon sources commonly used by the sewage treatment plant to solve the low-carbon source sewage treatment are methanol, starch, glucose, sodium acetate, etc., of which methanol and sodium acetate are easily degradable substances, which do not contain nutrients (such as nitrogen and phosphorus) and do not leave any intermediate products that are difficult to degrade after decomposition. Glucose and starch are polysaccharide structures, which take a long time to hydrolyze into small molecular fatty acids, and the solubility of starch in water is poor, which is not easy to completely dissolve in water, which is easy to cause problems such as residue and too many sludge flocs. Both of them have the disadvantage of producing more sludge.
The results showed that the denitrification rate of sodium acetate as carbon source was much higher than that of methanol and starch. The main reason is that sodium acetate is a low molecular organic acid salt, which is easy to be used by microorganisms. High molecular sugars such as starch need to be transformed into the most degradable organic substances such as low molecular organic acids such as acetic acid, formic acid and propionic acid before they can be used; Although methanol is a fast and easily biodegradable organic matter, methanol must be transformed into low molecular organic acids such as acetic acid before it can be utilized by microorganisms. Therefore, the denitrification speed of using sodium acetate as carbon source is much faster than that of starch and methanol.
At the same time, methanol is a flammable and explosive dangerous goods. When methanol is used as an external carbon source, its dosing room itself has a certain fire risk. In case of fire in methanol storage tank, it is easy to lead to tank rupture or sudden boiling, liquid overflow and continuous fire and explosion, with a large range of hazards. Therefore, a certain safety distance is required for the surrounding environment in the methanol dosing room. At the same time, because its volatile steam is easy to form explosive gas mixture when mixed with air, the power devices within its scope must adopt special design.
Sodium acetate itself is not dangerous goods, which is convenient for transportation and storage. Although the price is much more expensive than other carbon sources, for some built sewage treatment plants, due to its land restrictions, when additional carbon sources are needed, using sodium acetate as additional carbon source has more advantages than methanol.
In recent years, the market share of composite carbon source has become higher and higher, mainly due to its low price and high COD equivalent, but its overall performance is still not comparable to that of methanol and sodium acetate!
2. Calculation of sodium acetate dosage
In the anoxic denitrification stage, nitrate nitrogen (NO3-N) in sewage is reduced to gaseous nitrogen (N2) under the action of denitrifying bacteria. Denitrification is a biochemical reaction completed by heterotrophic microorganisms. Under the condition of very low dissolved oxygen concentration, they use the oxygen in nitrate (NO3-N) as electron acceptor and organic matter (carbon source) as electron donor.
In practical engineering, if the BOD5 ∶ n of sewage entering the denitrification section is less than 4 ∶ 1, additional carbon source should be considered. If BOD5 / N ≥ 4, denitrification can be considered complete. When the carbon source is insufficient, the amount of carbon source added by the system can be calculated according to the amount of nitrate nitrogen removed. The calculation formula is as follows:
Dosage x = (4-cbod5 / CN) × Cn/ η
Of which:
Cbod5: BOD5 concentration of influent (in practice, it can be calculated according to the value of COD), mg / L;
Cn: TN concentration of influent, mg / L;
η： Add BOD5 equivalent of carbon source (in practice, it can be calculated according to cod equivalent).
The BOD5 equivalent of sodium acetate is 0.52 (mgbod / mg sodium acetate), so when sodium acetate is added as carbon source, the calculation formula is as follows:
Dosage x = (4-cbod5 / CN) × Cn /0.52
Example calculation:
Taking the reconstruction and expansion project of a municipal sewage treatment plant as an example, the design treatment capacity is 16000 m3 / D, and the design effluent quality meets the national class I a standard. The main indicators of inlet and outlet water quality are shown in the table:
Table water quality indexes of inlet and outlet water of sewage treatment plant
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In the project, the sewage plant was originally built with section a aeration tank. After the sewage passes through section a aeration tank, the removal rate of BOD5 is calculated as 25%. Therefore, the concentration of BOD5 in the sewage entering the new denitrification tank is 262.5% 5 mg / L, TN concentration is 70mg / L, BOD5 ∶ n = 3 75 ＜ 4, so carbon source should be added, and the dosage of sodium acetate:
X=( 4-3. 75) × 70 /0.52 = 33. 7 mg /L
Daily dosage:
X*16000=0.0337*16000=539.2kg /d
Then, according to the purity of the purchased sodium acetate, the daily dosage of sodium acetate raw materials can be calculated.