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.

08 How to precipitate colloidal particles?

To precipitate colloidal particles, it is necessary to promote colloidal particles to contact each other, so that it becomes a large particle, that is, condensed, so that its specific gravity is greater than 1 and precipitation.

There are many methods used, and the commonly used technologies in engineering are: condensation method, flocculation method and coagulation method.

09 What is condensation?

Adding a coagulation agent with positive ions in the wastewater, a large number of positive ions exist between colloidal particles to eliminate the electrostatic exclusion between colloidal particles, so that the particles coalesce, this method by adding positive ion electrolyte, the process of coalescing colloidal particles is called condensation. Coagulants commonly used are aluminum sulfate, ferrous sulfate, alum, ferric chloride and so on.

10 What is flocculation?

Flocculation is the addition of polymer coagulant to wastewater. After the polymer coagulant is dissolved, polymer will be formed. The structure of this polymer is a linear structure, one end of the line pulls a tiny particle, and the other end pulls another tiny particle, which plays the role of bonding and bridging between the two particles far apart, making the particles gradually larger, and eventually forming a large particle floc (commonly known as alum flower), accelerating the particle settlement. Commonly used flocculants are polyacrylamide (PAM), polyiron (PE) and so on.

11 What is coagulation?

Coagulation is a process in which coagulation and flocculation are combined. Coagulation is often used in experiments or engineering, such as adding ferrous sulfate and other agents in water to eliminate the electrostatic exclusion between colloidal particles, and then adding polyacrylamide (PAM), so that the particles gradually become larger, forming visible alum flowers, and finally settling.

12 What is adsorption?

The use of porous solids (such as activated carbon) or flocs (such as polyiron) to adsorb toxic and harmful substances in wastewater on the surface of solids or flocs or in micropores to achieve the purpose of purifying water quality, this treatment method is called adsorption treatment. The object of adsorption can be an insoluble solid substance or a dissolved substance. The efficiency of adsorption treatment is high, and the effluent quality is good, so it is often used as a deep treatment of wastewater. Adsorption treatment can also be introduced into the biochemical treatment unit to improve the efficiency of biochemical treatment (such as PACT method is one of them).

13 What is the biochemical treatment of wastewater?

The biochemical treatment of wastewater is one of the most important processes in the wastewater treatment system, referred to as biochemical treatment. Biochemical treatment is the use of microbial life activity process to effectively remove soluble organic matter and some insoluble organic matter in wastewater, so that the water can be purified.14 In what way do microorganisms decompose organic pollutants in wastewater?

Due to the presence of carbohydrates, fats, proteins and other organic matter in the wastewater, these inanimate organic matter is the food of microorganisms, part of the degradation, synthesis into cellular substances (combined metabolites), the other part of the degradation oxidation into water, carbon dioxide and other (catabolites), in this process, the organic pollutants in the wastewater are degraded and removed by microorganisms.

15 What factors are microbes associated with?

In addition to nutrients, microbes also need the right environmental factors, such as temperature, pH, dissolved oxygen, osmotic pressure, etc., to survive. If the environmental conditions are not normal, it will affect the life activities of microorganisms, and even mutate or die.

16 What temperature range is the most suitable for microbial growth and reproduction?

In the biological treatment of wastewater, the most suitable temperature range for microorganisms is generally 16-30 ° C, the highest temperature is 37-43 ° C, and when the temperature is below 10 ° C, microorganisms will no longer grow.

Within the appropriate temperature range, the metabolic rate of microorganisms will increase correspondingly with the increase of 10℃, and the removal rate of COD will also increase by about 10%. On the contrary, when the temperature is reduced by 10℃, the removal rate of COD will be reduced by 10%. Therefore, the biochemical removal rate of COD will be significantly lower in winter than in other seasons.

17 What is the most suitable pH condition for microorganisms?

The life activity and metabolism of microorganisms are closely related to pH value. The pH range of most microorganisms is 4.5-9, while the optimal pH range is 6.5-7.5. When the pH is lower than 6.5, fungi begin to compete with bacteria, and when the pH is 4.5, fungi will occupy a complete advantage in the biochemical tank, which will seriously affect the settling result of sludge. When pH exceeds 9, the metabolic rate of microorganisms will be hindered.

The range of pH requirements for different microorganisms is different. In aerobic biological treatment, pH can vary between 6.5 and 8.5; In anaerobic biological treatment, the pH requirements of microorganisms are relatively strict, and the pH should be between 6.7 and 7.4.