1. What is COD (chemical oxygen demand)?

Chemical oxygen demand (COD) refers to the amount of oxygen required by the oxidizable substances in the wastewater when they are oxidized by chemical oxidants, in mg/L of oxygen. It is the most commonly used method to determine the content of organic matter in wastewater at present. The commonly used oxidants in COD analysis include potassium permanganate (CODMn by manganese method) and potassium dichromate (CODCr by chromium method), and now the potassium dichromate method is commonly used. Wastewater oxidizes organics under the condition of strong acid heating boiling reflux. When silver sulfate is used as catalyst, the oxidation rate of most organics can be increased to 85-95%. If the wastewater contains high concentration of chloride ion, the chloride ion should be shielded with mercury sulfate to reduce the interference to COD determination.

2. What is BOD5 (biochemical oxygen demand)?

Biochemical oxygen demand can also represent the degree of pollution of wastewater by organic substances. The most commonly used is the 5-day biochemical oxygen demand, expressed as BOD5, which represents the amount of oxygen required for biochemical degradation of wastewater in the presence of microorganisms within 5 days. In the future, we will often use 5-day biochemical oxygen demand.

3. What is the relationship between COD and BOD5?

Some organic substances can be degraded by biological oxidation (such as glucose and ethanol), some organic substances can only be partially degraded by biological oxidation (such as methanol), and some organic substances can not be degraded by biological oxidation and also have toxicity (such as ginkgol, ginkgolic acid, and some surface active agents). Therefore, we can divide the organic matter in water into two parts, namely, biodegradable organic matter and non-biodegradable organic matter.

It is generally believed that COD can basically represent all organic substances in water. BOD is the biodegradable organic matter in water, so the difference between COD and BOD can represent the biodegradable organic matter in the wastewater.

4. What is B/C? What does B/C mean?

B/C is the abbreviation of the ratio of BOD5 to COD, which can represent the biodegradability of wastewater. Therefore, BOD5/COD value is often used as an evalsuation index of biodegradability of organic substances.

BOD5/COD

zero point five

Easily biodegradable

BOD5/COD

zero point four

Biodegradable

BOD5/COD

zero point three

Difficult biodegradation

BOD5/COD

zero point two

It is difficult to biodegrade

5. What is wastewater pretreatment? What are the purposes of pretreatment?

The treatment before biochemical treatment is usually called pretreatment. Because the treatment cost of biochemical method is relatively low and the operation is relatively stable, the general industrial wastewater is treated by biochemical method, and the biochemical method is also the main treatment method for wastewater treatment. However, the wastewater contains some organic substances that inhibit and poison microorganisms. Therefore, the wastewater must undergo necessary pretreatment before entering the biochemical tank to reduce or remove the substances that inhibit and poison microorganisms in the wastewater as much as possible to ensure the normal operation of microorganisms in the biochemical tank.

The purpose of pre-treatment is to reduce, remove or transform the substances that inhibit, poison and inhibit microorganisms in the wastewater as much as possible into substances that are harmless or beneficial to microorganisms, so as to ensure the normal operation of microorganisms in the biochemical tank; The second is to reduce the COD load in the pretreatment process to reduce the operating burden of the biochemical tank.

6. What is the purpose of the waste water sump?

The function of waste water collecting pool is to collect, store and balance the quality and quantity of waste water.

The quantity and quality of production wastewater discharged from each workshop are generally unbalanced. There is wastewater in production, and there is no wastewater in production. Even in one day or between shifts, there may be great changes, especially in the fine chemical industry. If the clear and turbid wastewater is not separated, the quality and quantity of process concentrated wastewater and light pollution wastewater will vary greatly, This change is very harmful to the normal operation and treatment effect of wastewater treatment facilities and equipment, even harmful. Therefore, before wastewater enters the main wastewater treatment system, a wastewater collection tank with a certain volume should be set up to store and homogenize the wastewater to ensure the normal operation of wastewater treatment equipment and facilities.

7. Why is it difficult for colloidal particles in wastewater to settle naturally?

Many impurity suspended solids, large particles and easily settled suspended solids with a specific gravity greater than 1 in the wastewater can be removed by natural sedimentation, centrifugation and other methods.

However, suspended particles with a specific gravity of less than 1, small and even invisible to the naked eye are difficult to settle naturally. For example, colloidal particles are particles with the size of 10-4~10-6 mm, which are very stable in water. Their settling speed is extremely slow, and it takes 200 years to settle 1m.

There are two reasons for slow sedimentation: first, colloidal particles have negative charges, which prevent the contact between colloidal particles due to the reason of mutual repulsion. They cannot be bonded to each other and suspended in water. 2、 The surface of colloidal particles is also tightly surrounded by a layer of molecules. This hydration layer also blocks and insulates the contact between colloidal particles and cannot be bonded to each other and suspended in water.

8. How to precipitate colloidal particles?

In order to make colloidal particles precipitate, it is necessary to make colloidal particles contact with each other to make them become large particles, that is, agglomerate them to make their specific gravity greater than 1 and precipitate.

There are many methods used, and the commonly used technologies in engineering include coagulation, flocculation and coagulation.

9. What is condensation?

The coagulant with positive ions is added to the waste water, and a large number of positive ions exist between the colloidal particles to eliminate the electrostatic repulsion between the colloidal particles, so as to make the particles coalesce. This process of making the colloidal particles coalesce with each other by adding positive ion electrolytes is called coagulation. Commonly used coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc.

10. What is flocculation?

Flocculation is the addition of macromolecular coagulant in wastewater, which will form macromolecular polymer after dissolution. The structure of this polymer is linear. One end of the line pulls a small particle and the other end pulls another small particle. It plays a role of bonding and bridging between the two particles far away, making the particles gradually larger, and finally forming a large particle floccule (commonly known as alum flower) to accelerate particle sedimentation. Commonly used flocculants include polyacrylamide (PAM), polyiron (PE), etc.