Basic knowledge of sewage treatment

01 What is COD (Chemical Oxygen Demand)?

Chemical oxygen demand (COD) refers to the amount of oxygen required when substances that can be oxidized in wastewater are oxidized by chemical oxidants, in mg/l of oxygen. It is one of the most commonly used methods to determine the content of organic matter in wastewater. The oxidants commonly used in COD analysis are potassium permanganate (manganese method CODMn) and potassium dichromate (chromium method CODCr), and potassium dichromate method is now commonly used.

The oxidation rate of most organic compounds can be increased to 85-95% when silver sulfate is used as catalyst. If the wastewater contains a high concentration of chloride ions, the chloride ions should be shielded with mercury sulfate to reduce the interference to the determination of COD.

What is BOD5 (Biochemical Oxygen Demand)?

Biochemical oxygen demand can also characterize the degree of pollution of wastewater by organic matter, the most commonly used is 5-day biochemical oxygen demand, expressed by BOD5, which indicates the amount of oxygen required for biochemical degradation of wastewater in the presence of microorganisms for 5 days. In the future, we will often use 5-day BOD.

What is BOD5 (Biochemical Oxygen Demand)?

Biochemical oxygen demand can also characterize the degree of pollution of wastewater by organic matter, the most commonly used is 5-day biochemical oxygen demand, expressed by BOD5, which indicates the amount of oxygen required for biochemical degradation of wastewater in the presence of microorganisms for 5 days. In the future, we will often use 5-day BOD.

04 What is B/C? What does B/C mean?

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

05 What is the pretreatment of wastewater? What are the goals of pretreatment?

The treatment before biochemical treatment is commonly referred to as pretreatment. Because the cost of biochemical treatment is relatively low and the operation is relatively stable, the general industrial wastewater is treated by biochemical method, and the treatment of wastewater is also taken by biochemical method as the main treatment means.

However, the wastewater contains some organic substances that are inhibited and toxic to microorganisms, so the necessary pretreatment must be carried out before the wastewater enters the biochemical tank. The purpose is to reduce or remove the substances that are inhibited and toxic to microorganisms in the wastewater as much as possible to ensure that the microorganisms in the biochemical tank can operate normally.

There are two purposes of pretreatment: one is to reduce and remove or transform the substances in the wastewater that are toxic and inhibiting to microorganisms as much as possible into substances that are harmless or favorable to microorganisms, so as to ensure the normal operation of microorganisms in the biochemical pool; The second is to reduce the COD load in the pretreatment process to reduce the operating burden of the biochemical tank.

06 What is the waste water sump used for?

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

The production wastewater of each workshop, the amount of wastewater discharged and the quality of water is generally unbalanced, there is no waste water when the production is not produced, and there may be great changes even within a day or between classes, especially the wastewater of the fine chemical industry, if the clear wastewater is not shunted, the water quality and water quantity of the process concentrated wastewater and light pollution wastewater change greatly. This change is very unfavorable to the normal operation and treatment effect of wastewater treatment facilities and equipment, and even harmful.

Therefore, before the wastewater enters the main sewage treatment system, a wastewater catchment tank with a certain volume should be set up, and the wastewater should be stored and homogenized to ensure the normal operation of wastewater treatment equipment and facilities.

07 Why colloidal particles in wastewater are not easy to settle naturally?

Many impurities in wastewater with a specific gravity greater than 1 suspended matter, large particles, easy to settle suspended matter can be removed by natural sedimentation, centrifugation and other methods.However, the proportion of less than 1, small or even invisible suspended particles are difficult to settle naturally, such as colloidal particles are 10-4 ~ 10-6mm particles in size, very stable in water, its settling speed is very slow, settling 1m requires 200 years of farming.

There are two reasons for the slow settlement: first, colloidal particles are negatively charged, due to the reason of same-sex repulsion, thereby preventing the contact between colloidal particles, can not be bonded to each other, suspended in water. Second, the surface of colloidal particles is tightly surrounded by a layer of molecules, this layer of hydration layer also impedes and insulates the contact between colloidal particles, can not be bonded to each other, suspended in water.