1、What is Chemical Oxygen Demand（COD）？

Chemical Oxygen Demand（COD）is refers to the amount of oxygen required when the substances in waste water that can be oxidized 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. Potassium permanganate (CODMn by manganese method) and potassium dichromate (CODcr by chromium method) are commonly used in COD analysis.

Under the condition of strong acid heating, boiling and reflux, the oxidation rate of most organic matters can be increased to 85-95% when silver sulfate is used as catalyst. If the wastewater contains a high concentration of chloride ion, the chloride ion should be screened out with mercury sulfate to reduce the interference to the determination of COD.

2、What is Biochemical Oxygen Demand（BOD）？

Biochemical oxygen demand can also be used to characterize the degree of wastewater polluted by organic matter. The most commonly used biochemical oxygen demand is 5-day biochemical oxygen demand, which is expressed by BOD5, which indicates 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 BOD.

3、What is the relationship between COD and BOD?

Some organics can be biodegraded (such as glucose and ethanol), some organics can only be biodegraded partially (such as methanol), and some organics can not be biodegraded, but also have toxicity (such as ginkgolide, ginkgolic acid and some surfactants).

Therefore, we can divide the organic matter in water into two parts, that is, biodegradable organic matter and non biodegradable organic matter. It is generally believed that COD can basically represent all organic matter in water. BOD is biodegradable organic matter in water, so the difference between COD and BOD can represent the organic matter in the non biodegradable part of 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 indicate the biodegradability of wastewater. Therefore, BOD5/COD value is often used as the evaluation index of organic matter biodegradability.

5、What is wastewater pretreatment? What are the goals of pretreatment?

Pretreatment before biochemical treatment is usually called 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 also takes biochemical method as the main treatment method. However, the wastewater contains some organic substances which have inhibition and toxicity to microorganisms. Therefore, the wastewater must be pretreated before entering the biochemical tank. The purpose is to reduce or remove the substances which have inhibition and toxicity to microorganisms as far as possible, so as to ensure the normal operation of microorganisms in the biochemical tank.

There are two purposes of pretreatment: one is to reduce, remove or transform the substances that are toxic and inhibitory to microorganisms in wastewater into substances that are harmless or beneficial to microorganisms, so as to ensure the normal operation of microorganisms in biochemical tank; The other is to reduce the COD load in the pretreatment process, so as to reduce the operation burden of biochemical tank.

6、What's the use of waste water collection tank?

The function of wastewater collection tank is to collect, store and balance the quality and quantity of wastewater. Generally speaking, the quantity and quality of wastewater discharged from each workshop are not balanced. There is wastewater during production, and there is no wastewater when it is not produced. Even within one day or between shifts, there may be great changes, especially for the wastewater from fine chemical industry. If the clear and turbid wastewater is not separated, the water quality and quantity of process concentrated wastewater and light polluted wastewater will change greatly, This kind of change is very disadvantageous, even harmful, to the normal operation and treatment effect of wastewater treatment facilities and equipment.

Therefore, before the wastewater enters the main sewage treatment system, it is necessary to set up a wastewater collection tank with a certain volume to store the wastewater and make it homogeneous, so as 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 impurities, suspended solids, large particles and easily settled suspended solids with specific gravity greater than 1 in wastewater can be removed by natural sedimentation and centrifugation. For example, colloidal particles are 10-4-10-6 mm in size, which are very stable in water. Their settling speed is very slow, and it takes 200 years to settle 1m.

There are two reasons for the slow settling: 

1, colloidal particles are negatively charged, which prevents the colloidal particles from contacting with each other due to the repulsion of the same sex, and can not be bonded to each other and suspended in water. 

2，There is also a layer of molecules on the surface of colloidal particles, which also blocks and insulates the contact between colloidal particles and cannot be adhered to each other and suspended in water.

8、How to precipitate colloidal particles?

To make colloidal particles precipitate, it is necessary to make colloidal particles contact with each other to make them become large particles, that is to say, to condense and make their specific gravity greater than 1 to precipitate. There are many kinds of methods, and the common engineering technologies are: coagulation, flocculation and coagulation.

9、What is cohesion?

When coagulant with positive ions is added to wastewater, a large number of positive ions exist between colloidal particles to eliminate the electrostatic repulsion between colloidal particles, so that the particles coalesce. The process of coalescence of colloidal particles by adding positive ion electrolyte is called coagulation. Commonly used coagulants are aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc.

10、What is flocculation?

Flocculation is to add polymer coagulant into wastewater. After polymer coagulant dissolves, polymer will be formed. The structure of this kind of polymer is linear structure. One end of the line pulls a small particle, and the other end pulls another small particle. It plays the role of bonding and bridging between the two particles, making the particles gradually larger, and finally forming a large particle floc (commonly known as alum), accelerating the particle settlement. The commonly used flocculants are polyacrylamide (PAM), polyferric (PE), etc.

11、What is coagulation?

The process of coagulation and flocculation is coagulation. Coagulation is often used in experiments or engineering, such as adding ferrous sulfate in water to eliminate the electrostatic repulsion between colloidal particles, and then adding polyacrylamide (PAM) to make the particles gradually larger, forming visible alum, and finally settling.

12、What is adsorption?

Using porous solid (such as activated carbon) or floc material (such as poly iron) to adsorb toxic and harmful substances in wastewater on the surface of solid or floc or in micropores, to achieve the purpose of purifying water quality, this treatment method is called adsorption treatment. The object of adsorption can be insoluble solid matter or soluble matter. Due to its high efficiency and good effluent quality, adsorption is often used as advanced treatment of wastewater. Adsorption treatment can also be introduced into the biochemical treatment unit to improve the efficiency of biochemical treatment (for example, pact method is one of them).

13、What is biochemical treatment of wastewater?

Biochemical treatment of wastewater is one of the most important processes in wastewater treatment system. Biochemical treatment is to effectively remove the soluble organic matter and some insoluble organic matter in wastewater by using the life activity process of microorganism, so as to purify the water.

14、How do microorganisms decompose and remove organic pollutants in wastewater?

Due to the existence of carbohydrate, fat, protein and other organic substances in wastewater, these inanimate organic substances are the food of microorganisms. One part of them is degraded and synthesized into cell substances (combined metabolites), the other part is degraded and oxidized into water, carbon dioxide and other (catabolites). In this process, the organic pollutants in wastewater are degraded and removed by microorganisms.

15、What are the factors related to microorganisms?

In addition to nutrition, microorganisms also need appropriate environmental factors, such as temperature, pH value, dissolved oxygen, osmotic pressure, etc. If the environmental conditions are not normal, it will affect the life activities of microorganisms, even mutation or death.

16、What temperature range is the most suitable for the growth and reproduction of microorganisms?

In wastewater biological treatment, the most suitable temperature range for microorganisms is generally 16-30 ℃, and the highest temperature is 37-43 ℃. When the temperature is lower than 10 ℃, microorganisms will not grow. In the suitable temperature range, the metabolic rate of microorganism will increase and the removal rate of COD will increase about 10% when the temperature increases by 10 ℃; On the contrary, the removal rate of COD will decrease by 10% when the temperature decreases by 10 ℃, so the biochemical removal rate of COD in winter will be significantly lower than that in other seasons.

17、What is the optimal pH condition for microorganisms?

The life activity and material metabolism of microorganism are closely related to pH value. Most of the microorganisms adapt to pH in the range of 4.5-9, and the most suitable pH is in the range of 6.5-7.5. When the pH value is lower than 6.5, fungi begin to compete with bacteria. When the pH value is 4.5, fungi will have a complete advantage in the biochemical tank, which will seriously affect the sedimentation of sludge; When pH exceeds 9, the metabolic rate of microorganism will be hindered. Different microorganisms have different requirements for pH value.

In the aerobic biological treatment, the pH can change from 6.5 to 8.5; In anaerobic biological treatment, the requirement of microorganism pH is relatively strict, and the pH should be between 6.7 and 7.4.

18、What is dissolved oxygen? What is the relationship between do and microorganism?

Dissolved oxygen in water is called dissolved oxygen. Dissolved oxygen is the oxygen that organisms and aerobic microorganisms in water depend on to survive. Different microorganisms have different requirements for do. Aerobic microorganisms need to supply sufficient dissolved oxygen, generally speaking, dissolved oxygen should be maintained at 3 mg / L, and the minimum should not be less than 2 mg / L; The range of dissolved oxygen for facultative microorganism is 0.2-2.0mg/l; The range of dissolved oxygen required by anaerobic microorganism is less than 0.2mg/l.

19、Why does high concentration of saline wastewater have a great impact on microorganisms?

The unit structure of microorganism is cell, and the cell wall is equivalent to semi permeable membrane. When the concentration of chloride ion is less than or equal to 2000 mg/L, the osmotic pressure that the cell wall can bear is 0.5-1.0 atmospheric pressure. Even if the cell wall and plasma membrane have certain tenacity and elasticity, the osmotic pressure that the cell wall can bear will not be greater than 5-6 atmospheric pressure. However, when the concentration of chloride ion in the aqueous solution is more than 5000mg / L, the osmotic pressure will increase to about 10-30 atmospheric pressure. Under such a high osmotic pressure, a large number of water molecules in the microbial body will penetrate into the solution in vitro, causing cell dehydration and plasmolysis, and even death of microorganisms.

The engineering experience data show that when the concentration of chloride ion in wastewater is higher than 2000 mg/L, the activity of microorganism will be inhibited, and the COD removal rate will decrease significantly; When the concentration of chloride ion in wastewater is greater than 8000mg/L, the volume of sludge will expand, and a lot of bubbles will appear on the water surface, and microorganisms will die one after another.

20、What is aerobic biochemical treatment? What is facultative biochemical treatment? What is the difference between the two?

Biochemical treatment can be divided into aerobic biochemical treatment and anoxic biochemical treatment according to the different requirements of microbial growth on oxygen environment. Anoxic biochemical treatment can be divided into facultative biochemical treatment and anaerobic biochemical treatment. In the process of aerobic biochemical treatment, aerobic microorganisms must grow and reproduce in the presence of a large amount of oxygen, and reduce the organic matter in wastewater; In the process of facultative biochemical treatment, facultative microorganisms only need a small amount of oxygen to grow and reproduce, and degrade the organic matter in wastewater. If there is too much oxygen in the water, facultative microorganisms will not grow well, thus affecting its treatment efficiency of organic matter.

Facultative microorganism can adapt to the wastewater with high COD concentration, the influent COD concentration can be increased to more than 2000 mg/L, and the COD removal rate is generally 50-80%; The aerobic microorganism can only adapt to the wastewater with low COD concentration. The influent COD concentration is generally controlled below 1000-1500mg/L, and the COD removal rate is generally 50-80%. The time of facultative biochemical treatment and aerobic biochemical treatment is not too long, generally in 12-24 hours.

People make use of the differences and similarities between facultative biochemistry and aerobic biochemistry to combine facultative biochemical treatment and aerobic biochemical treatment. The wastewater with higher COD concentration is treated by facultative biochemical treatment first, and then the treated effluent of facultative tank is used as the influent of aerobic tank. Such combined treatment can reduce the volume of biochemical tank, It not only saves the environmental protection investment, but also reduces the daily operation cost. The principle and function of anaerobic biochemical treatment and facultative biochemical treatment are the same.

The difference between anaerobic biochemical treatment and facultative biochemical treatment is that no oxygen is needed in the process of anaerobic microorganism reproduction and growth and degradation of organic matter, and anaerobic microorganism can adapt to wastewater with higher COD concentration (4000-10000mg/L). The disadvantage of anaerobic biochemical treatment is that the biochemical treatment time is very long, and the residence time of wastewater in anaerobic biochemical tank generally needs more than 40 hours.

21、What are the applications of biological treatment in wastewater treatment engineering?

Biological treatment is widely used in wastewater treatment engineering. There are two kinds of most practical technologies: one is called activated sludge process, the other is called biofilm process. Activated sludge process is an aerobic wastewater treatment form based on the biochemical metabolism of suspended biological population.

In the process of growth and reproduction, microorganisms can form bacterial micelles with large surface area, which can flocculate and adsorb a large number of suspended colloidal or dissolved pollutants of wastewater, and absorb these substances into cells. With the participation of oxygen, these substances can be completely oxidized to release energy, CO2 and H2O. The sludge concentration of activated sludge process is generally 4 g/L. In the biofilm process, microorganisms attach to the surface of the filler to form a colloidal biofilm. Biofilms generally have fluffy flocculent structure, many micropores, large surface area, and strong adsorption, which is conducive to the further decomposition and utilization of these adsorbed organics by microorganisms.

During the treatment process, the surface of the biofilm is constantly contacted with water due to the flow of water and the agitation of air. The organic pollutants and dissolved oxygen in the wastewater are absorbed by the biofilm. The microorganisms on the biofilm constantly decompose these organic substances. The biofilm itself is constantly metabolized while the organic substances are oxidized and decomposed, The aged biofilm falls off and is taken out by the treated effluent from the biological treatment facility and separated from the water in the sedimentation tank. The sludge concentration of biofilm process is generally 6-8g/L.

In order to improve the sludge concentration and treatment efficiency, the activated sludge process and biofilm process can be combined, that is, adding fillers in the activated sludge pool. This kind of bioreactor with both biofilm forming microorganisms and suspended microorganisms is called composite bioreactor. It has a high sludge concentration, generally about 14g/L.

22、What are the similarities and differences between biofilm process and activated sludge process?

Biofilm process and activated sludge process are different reactors for biochemical treatment. From the appearance, the main difference is that the microorganism of biofilm process does not need packing carrier, the biological sludge is suspended, and the microorganism of activated sludge process is fixed on the packing. However, the mechanism of wastewater treatment and water purification is the same.

In addition, both of them are aerobic activated sludge, and the composition of sludge is similar. In addition, the microorganisms in the biofilm process are fixed on the filler, which can form a relatively stable ecosystem. Their living energy and energy consumption are not as large as those in the activated sludge process. Therefore, the excess sludge in the biofilm process is less than that in the activated sludge process.

23、What is activated sludge?

From the microbial point of view, sludge in biochemical tank is a biological community composed of a variety of bioactive microorganisms.

If the sludge particles are observed under the microscope, it can be seen that there are many kinds of microorganisms in the sludge - bacteria, mold, protozoa and metazoa (such as rotifers, insect larvae and worms). They form a food chain. Bacteria and mold can decompose complex organic compounds, obtain the necessary energy for their own activities and construct themselves.

Protozoa feed on bacteria and molds, and are consumed by metazoa. Metazoa can also live directly on bacteria. This kind of flocculent sludge full of microorganisms and capable of degrading organic matter is called activated sludge.

In addition to being composed of microorganisms, activated sludge also contains some inorganic substances and organic substances (i.e. metabolic residues of microorganisms) adsorbed on activated sludge that can no longer be biodegraded.

The moisture content of activated sludge is generally 98-99%. Activated sludge, like alum, has a large surface area, so it has a strong adsorption capacity and the ability to oxidize and decompose organic matter.

24、How to evaluate activated sludge in activated sludge process and biofilm process?

The discrimination and evaluation of activated sludge growth in activated sludge process and biofilm process are different. In the biofilm process, the growth of activated sludge is mainly evaluated by microscope.

In the activated sludge process, in addition to observing the biofacies directly by microscope, the commonly used evaluation indexes are: mixed liquid suspended solids (MLSS), mixed liquid volatile suspended solids (MLVSS), sludge settling ratio (SV), sludge settling index (SVI), etc.

25、What kind of microorganism directly indicates that the biochemical treatment effect is good when observing the biological phase with microscope?

The appearance of micro metazoa (such as rotifers, nematodes, etc.) indicates that the microbial community grows well, the ecosystem of activated sludge is relatively stable, and the biochemical treatment effect is the best at this time, which is just like the situation that small fish and shrimp grow well in the river where large fish can be captured frequently.

26、What is Mixed Liquid Suspended Solids（MLSS）？

Suspended solids of mixed liquid, also known as sludge concentration, refers to the weight of dry sludge contained in the mixed liquid of biochemical tank per unit volume, in mg/L, which is used to characterize the concentration of activated sludge. It includes two parts: organic and inorganic. Generally speaking, the MLSS value in SBR biochemical pool should be controlled at about 2000-4000 mg/L.

27、What is the Mixed Liquid Volatile Suspended Solid（MLVSS）？

Volatile suspended solids of mixed liquor refer to the weight of volatile substances in dry sludge contained in mixed liquor of biochemical tank per unit volume, and the unit is also mg/L. since it does not include inorganic substances in activated sludge, it can accurately represent the number of microorganisms in activated sludge.

28、What is the Sludge Settling Ratio？

Sludge settling ratio (SV) refers to the volume ratio (%) of the precipitated sludge to the mixed liquid in the aeration tank after 30 minutes of static sedimentation in a 100 ml measuring cylinder. Therefore, SV30 is sometimes used to express it. Generally speaking, SV in biochemical pool is between 20-40%. The determination of sludge settling ratio is relatively simple, and it is one of the important indexes to evaluate activated sludge. It is often used to control the discharge of excess sludge and reverse sludge bulking in time. Obviously, SV is also related to sludge concentration.

29、What is Sludge Index (SVI)?

Sludge index (SVI) is the full name of sludge volume index, which is the milliliter volume of 1G dry sludge in wet state. The calculation formula is as follows: SVI = SV * 10 / mlsssvi, excluding the influence of sludge concentration, can better reflect the coagulation and sedimentation of activated sludge. It is generally believed that when 60 < SVI < 100, the sludge sedimentation performance is good, when 100 < SVI < 200, the sludge sedimentation performance is good, Generally speaking, when 200 < SVI < 300, the sludge tends to expand. When SVI > 300, the sludge has expanded.

30、What does dissolved oxygen (do) mean?

Dissolved oxygen (do) is the dissolved amount of oxygen in water, expressed in mg/L. Different biochemical treatment methods have different requirements for do. In facultative biochemical process, do in water is generally between 0.2-2.0mg/l, while in SBR aerobic biochemical process, do in water is generally between 2.0-8.0mg/l.

Therefore, the aeration rate and aeration time of facultative tank should be small; In the SBR aerobic tank operation, the aeration rate and aeration time are much larger and longer, and we use contact oxidation, the dissolved oxygen is controlled at 2.0-4.0mg/l.

31、What factors are related to the content of dissolved oxygen in wastewater?

The concentration of dissolved oxygen in water can be expressed by Henry's Law: when the solution equilibrium is reached, C = KH * P, where C is the solubility of oxygen in water at the solution equilibrium; P is the partial pressure of oxygen in gas phase; KH is Henry coefficient, which is related to temperature; Increase aeration to make oxygen dissolution close to equilibrium, while activated sludge will consume oxygen in water. Therefore, the actual dissolved oxygen in wastewater is related to water temperature, effective water depth (affecting pressure), aeration rate, sludge concentration, salinity and other factors.

32、Why is it necessary to replenish nutrients in wastewater during biochemical process?

The main way to remove pollutants by biochemical process is to use the metabolic process of microorganisms. The cell synthesis and other life processes of microorganisms need sufficient amount and variety of nutrients (including trace elements). For chemical wastewater, due to the singleness of production products, the composition of wastewater quality is also relatively single, lacking the necessary nutrients of microorganisms.

For example, there is only carbon and nitrogen but no phosphorus in XX company's production wastewater, which can not meet the needs of microbial metabolism. Therefore, phosphorus in wastewater must be added to improve the microbial metabolism process and promote the synthesis of microbial cells. This is like eating rice, flour at the same time, but also intake enough vitamins.

33、What is the ratio of nutrients needed by microorganisms in wastewater?

Like animals and plants, microorganisms also need the necessary nutrients to grow and reproduce. The nutrients needed by microorganisms mainly refer to carbon (c), nitrogen (n), and phosphorus (P). The composition ratio of the main nutrient elements in wastewater has certain requirements. For aerobic biochemistry, it is generally C: n: P = 100:5:1 (weight ratio).

34、Why is excess sludge produced?

In the process of biochemical treatment, microorganisms in activated sludge constantly consume the organic matter in wastewater. In the consumed organic matter, some organic matter is oxidized to provide energy for microbial life activities, and the other part is used by microorganisms to synthesize new cytoplasm, so that microorganisms reproduce. While microorganisms metabolize, some old microorganisms die, resulting in excess sludge.

35、What is water eutrophication?

Water eutrophication is a natural phenomenon that occurs in fresh water, which is caused by the high content of nitrogen, phosphorus and potassium in water. The main reason of water eutrophication is that nitrogen, phosphorus, potassium and other elements are discharged into the surface water body with slow flow rate and long renewal period, which makes algae and other aquatic organisms grow and reproduce in large quantities, so that the production rate of organic matter far exceeds the consumption rate, and the accumulation of organic matter in the water body destroys the aquatic ecological balance.

    Source: MiaoZhi Hydrosphere