04

2021

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06

What is the COD equivalent of carbon source


HYDROKING TECH

1. What is the COD equivalent of carbon source?

Some time ago, a small partner asked what exactly is the COD equivalent? After consulting a lot of data, I found that there is no official definition of the COD equivalent of carbon sources, so a summary definition is made based on actual usage habits. The COD equivalent of carbon sources can be understood as all carbon sources per unit volume or unit mass. After being oxidized, the number of milligrams of oxygen required, in mg/L, mg/g or mg/kg.

Carbon sources are divided into two types: dry agent and liquid. For dry agent carbon sources, we generally calculate by unit mass, usually in grams or kilograms. For example, the COD equivalent of 1Kg dry powder glucose is 1Kg, which is converted into common The unit of mg is 1 million mg/Kg. For liquid carbon sources, the industry uses liters (L) as the unit. The COD equivalent of liquid carbon sources claimed by many carbon source manufacturers is hundreds of thousands, which is the amount of COD of 1L of liquid carbon sources. We feel very powerful. In fact, it is not as high as 98% glucose COD equivalent (density is calculated at 1kg/L)!

Of course, the COD equivalent of carbon source is not the only evaluation indicator. Utilization, cost performance, denitrification efficiency, carbon source residues, etc. must be considered (if I have time, I will write these indicators later), so, Low COD equivalent is not necessarily a good carbon source! For example, sodium acetate is recognized as a good carbon source, but the COD equivalent of its dry agent is only 780,000 mg/Kg, and the liquid sodium acetate is only about 300,000 mg/L. The COD equivalent is not comparable to glucose, and the price is still several times that of glucose. , But they are indeed a better carbon source than glucose!

2. Summary of ideas for calculation of carbon source dosage

The idea of ​​carbon source dosing calculation needs to start from the deepest point to see the essence. As long as you understand this article, you don't need to read the so-called carbon source dosing calculations in the future! (This idea of ​​carbon source investment was written last year, but there are still many small partners who don't know how to ask for help. Therefore, in order to make up the word, please send it out again today. If you have any questions, you can go to Effective Topia CommunityCommunication and interaction!)

1. Judgment of process

Many small partners are still at the beginner stage in their understanding of carbon source dosing. They only know that the CNP ratio is 100:5:1 and the CN ratio is controlled at 4-6. However, when are these ratios used? What craft is it used for? It may not be clear! Therefore, the carbon source must first be clearly distinguished what kind of process it is! Carbon removal? Denitrification? Phosphorus removal? Or denitrification and phosphorus removal?

How to distinguish?

It's easy! Remember these points of judgment: carbon removal process is pure aeration (such as simple aeration tank, simple MBR, contact oxidation, classic SBR, etc.); nitrogen removal is the alternation of hypoxia and aerobic experience (such as AO in-band recirculation, oxidation ditch, AAO, etc.); phosphorus removal is the alternation of anaerobic and aerobic (AO without internal recirculation, AAO, oxidation ditch, etc.); nitrogen and phosphorus removal is experienced anaerobic and hypoxia , Alternation of aerobic environment (AAO, oxidation ditch, etc.).

The carbon source for the denitrification process must be added to the inlet of the anoxic tank, and the carbon source for the dephosphorization process must be added to the inlet of the anaerobic tank! The nitrogen and phosphorus removal process can be distributed and added!

Why add a carbon source to the carbon removal process?

There must be a few words here. If you don't wait until you understand the calculations, you will have questions. The carbon removal process not only removes COD, but also synergistically removes nitrogen and phosphorus, just like although the author looks very handsome, in fact, the soul is also very beautiful! Therefore, in the carbon removal process, you only need to be responsible for matching these nutrient ratios. This article is carbon source dosing. The setting is under the condition of sufficient N and TP, but under normal circumstances, TP is often too much. , In fact, the value of TP will not be used to balance, this point should be paid attention to!

2. Choice of nutritional ratio

After distinguishing what kind of craft you are, you must choose the nutritional ratio!

Carbon removal process: CNP ratio 100:5:1

Denitrification process: CN ratio is 4-6, the middle value is 5

Phosphorus removal process: CP ratio 15:1

3. Selection of carbon value

Many colleagues are confused about whether to use COD or BOD for carbon source calculations. My personal idea is to use COD calculations in the project, so that there is a buffer of margins, so that the carbon source will not be excessively added. Since everything is for actual services, It is impossible to choose COD for calculation under any circumstances!

So, choose COD or BOD?

Then COD!

4. Selection of nitrogen value

For the choice of nitrogen value, most of the friends are confused and often overlook this point!

Remember a little!

Select TKN (Kjeldahl nitrogen, ammonia nitrogen + organic nitrogen value) for the carbon removal process, but for municipal sewage, if there is no industrial wastewater mixed, there is little organic nitrogen, you can directly use ammonia nitrogen, anyway, you can do it yourself You know whether the water has organic nitrogen or not, and judge for yourself!

The denitrification process chooses TN (total nitrogen, ammonia nitrogen + nitrate nitrogen + organic nitrogen value), why the carbon removal process does not have nitrate nitrogen, let me make it clear here, everyone can remember after understanding, because it is simple In the carbon removal process, microorganisms cannot use nitrate nitrogen metabolism (synthesis + decomposition) and can only use ammonia nitrogen, and nitrate nitrogen is precisely the necessary electron acceptor (hydrogen acceptor) for the denitrification stage of the denitrification process!

5. Selection of phosphorus value

There is nothing to say, the value is as much as it is! However, as mentioned earlier, TP is generally excessive, this value is not necessary!

6. Unit conversion

For the calculation of carbon source addition, I have always emphasized that it is actually the conversion of units. In this step, many small partners will make mistakes. This test is the high school physics knowledge.

However, the author writes down the conversion process, remembering this ratio, there will be no mistakes in the future

1PPM=1mg/L=1g/m^3=0.001kg/m^3

7. General formula

The usual carbon source dosing formulas are not detailed and inconsistent. This article will unify everyone:

1. Carbon removal process:

X=Water intake*(20*N difference 1-C difference)/COD equivalent of carbon source

Where:

X——Carbon source dosage for carbon removal process

N difference 1-influent ammonia nitrogen (or TKN)-ammonia nitrogen required for discharge

C difference-influent COD- effluent COD

2. Denitrification process:

Y=Water intake*(5*N difference 2-C difference)/COD equivalent of carbon source

Where:

Y——Carbon source dosage for denitrification process

N difference 2-influent TN-discharge required TN

C difference-influent COD- effluent COD

3. Phosphorus removal process:

Z=Water intake*(15*TP difference-C difference)/COD equivalent of carbon source

Where:

Z——Carbon source dosage for phosphorus removal process

TP difference-influent TP-TP required for discharge

C difference-influent COD- effluent COD

4. Denitrification and phosphorus removal process:

W=Water intake*(5*N difference 2+15*TP difference-C difference)/carbon source COD equivalent

Where:

W——Dosing amount of carbon source for nitrogen and phosphorus removal process

N difference 2-influent TN-discharge required TN

TP difference-influent TP-TP required for discharge

C difference-influent COD- effluent COD