To alleviate and control the eutrophication of water body, the national sewage discharge standards set by the more and more strict, however, most of the current sewage treatment plant are widespread low carbon relatively high water quality characteristics of nitrogen and phosphorus, due to the low organic matter content, the conventional denitrification process cannot meet the needs of anoxic denitrification stage of carbon source, cause the denitrification process is blocked, and inhibition of anaerobic aerobic bacteria proliferation and make ammonia (NH3 - N) DE assimilation, greatly influence the effect of sewage treatment plant nitrogen, especially in low temperature season is more serious.
In order to solve this problem, on the one hand, the denitrification effect can be increased by increasing the volume of the anti-digestion hypoxia zone and prolonging the anti-digestion time. However, this method needs to expand the sewage treatment plant, which has high infrastructure costs and low operability. On the other hand, the rate of reverse digestion can be improved by adding external carbon source to the anoxic zone to supplement carbon source. It has been proved by practice that adding carbon source is an important means to solve such problems in sewage treatment plants.
The type of carbon source
At present, the commonly used carbon source on the market: methanol, acetic acid, sodium acetate, flour, glucose, biomass carbon source, sludge hydrolysis supernatant, beer wastewater and landfill leachate. In the process of use, it is necessary to choose the appropriate carbon source according to the actual engineering situation. Now compare various common carbon sources and analyze the advantages and disadvantages of each carbon source:
1. The methanol
It is generally believed that methanol as an external carbon source has the advantages of low operating cost and small sludge yield. When methanol is used as a carbon source, C/N >.5 can achieve better results, but there are three disadvantages:
1. As a chemical agent, the cost is relatively high;
2. The response time is slow, and methanol can not be used by all microorganisms. When methanol is added, a certain adaptation period is needed until it is fully enriched and gives full play to the effect.
3. Methanol has a certain toxic effect, and long-term use of methanol as a carbon source will also have a certain impact on tail water discharge.
2. Sodium acetate
The advantage of sodium acetate is that it responds immediately to the denitrification process and can be used in water plants for emergency disposal.
It is generally believed that the denitrification rate of ethanol is not as high as that of methanol, but because it has no toxicity and the sludge yield is similar to that of methanol, it is considered to be an alternative carbon source of methanol. When ethanol is used as carbon source and nitrate is used as electron acceptor, C/N=5, and the lack of carbon source will cause the accumulation of nitrite.
Use sodium acetate to consider the following three points:
1. Sodium acetate is mostly liquid of 20%, 25% and 30%. Due to low equivalent COD and high transportation cost, it cannot be transported for a long distance.
2. Large sludge yield, increasing the cost of sludge treatment;
3. The price is relatively high, and it is almost impossible to add sodium acetate to sewage treatment plants on a large scale.
(3) acetic acid
The advantage of sodium acetate is that it can respond immediately to the denitrification process and can be used as an emergency treatment during water plant operation. Due to the fact that sodium acetate is a small molecular organic acid, denitrifying bacteria are easy to use and the denitrification effect is good. However, due to the high price, high sludge yield, and the current sewage treatment plant sludge disposal problem is also a big public relations problem, it is almost impossible to apply sodium acetate to the large-scale addition of sewage treatment plants.
4. Sugar
Represented by glucose sweets as additional carbon source treatment effect is good, but, as a molecular compound, more easy to cause the bacteria multiply, causing sludge bulking, increase the water COD value, affect the water quality, at the same time, compared with the alcohol carbon source, sweets are more likely to produce the phenomenon of the nitrate accumulation, therefore, does not advocate outside the extensive use of glucose as carbon source.
Disadvantages:
1. It needs to be prepared into solution on site, with high labor intensity and poor dosing accuracy, which can not be used in large sewage treatment plants.
2. Industrial glucose contains many impurities and food glucose is expensive.
5. Biomass carbon source
With the improvement of sewage denitrification requirements, the emergence of new enterprises specializing in the production of carbon sources, they through the principle of bioengineering, some sugar, agricultural waste fermentation, the production of non-toxic biological products, the main components are small molecular organic acids, alcohols, sugar. It is easier to be used by microbes than a single chemical, its use cost is cheaper than a single chemical, with a very high cost performance.
Disadvantages:
The stability of the product needs to be improved, so it is necessary to test the equivalent COD of each batch of product before use.
6. Sludge hydrolyzed supernatant
Biotransformed volatile acid VFA is derived from the supernatant of sludge hydrolysis. Because the VFA produced by hydrolysis has a high denitrification rate, the carbon source can be directly provided from the sewage plant, which reduces the sludge capacity and also reduces the problems of carbon source transportation, so it is a relatively advantageous carbon source at present.
At present, there are many different conclusions on the study of sludge hydrolytic utilization as external carbon source, but it is generally considered that it is a valuable method as the carbon source of denitrification and denitrification system. For different sludge, however, different hydrolysis conditions, produced by the VFA components have bigger difference, but because of the different components, and can cause a different rate of denitrification (this is why a lot of research the cause of the inconsistent), so, how will the sludge hydrolysis product of VFA unification research and application, is still a big problem.
In addition, if hydrolyzed sludge is directly used as an external carbon source, the release of nitrogen and phosphorus in the process of sludge hydrolysis should be considered. If this part of nitrogen and phosphorus is added to sewage in the form of carbon source, it is bound to increase the load of nitrogen and phosphorus in sewage treatment plant. How to solve this problem is another big problem in the use of sludge hydrolysate.
Selection of carbon sources
In theory, all types of carbon sources can ensure that the total effluent nitrogen meets the discharge standard, but several factors should be considered:
1. The cost of adding carbon source The cost of adding carbon source is a comprehensive algorithm of equivalent COD price of carbon source + adding amount, which needs to be determined by theoretical calculation and actual operation of adding amount;
2. Sludge production rate of carbon source Adding carbon source will certainly increase the yield of sludge, and the cost of sludge treatment is very high, which is an important factor to consider when choosing carbon source.
3. Ensure the stability of sewage operation Adding carbon source is for denitrification, so when choosing carbon source, we should take into account the operation stability of sewage treatment plant, such as avoid sludge bulking, effluent COD rise, nitroso nitrogen accumulation, etc.
According to the above, the choice of carbon source is not a simple economic account, but a close combination with the actual stable operation. Scientific choice of carbon source can effectively reduce the operation cost of sewage treatment plant and the stable operation of sewage treatment plant.
