Essence: Classic Q&A on Wastewater Treatment - Read this and you won't be stumped by clients!

1. Why are COD and BOD frequently used as pollution indicators in wastewater analysis? Wastewater contains numerous organic substances; it's common to encounter wastewater with tens, dozens, or even hundreds of different organic substances. Performing qualitative and quantitative analyses of each organic substance in wastewater is time-consuming and requires many reagents. Is it possible to use a single pollution indicator to represent all organic substances and their quantities in wastewater? Environmental scientists have found that all organic substances share two common characteristics: firstly, they are all composed of at least carbon and hydrogen; secondly, the vast majority of organic substances can be chemically oxidized or oxidized by microorganisms, with their carbon and hydrogen forming harmless carbon dioxide and water, respectively. The oxidation of organic substances in wastewater, whether chemical or biological, consumes oxygen. The more organic substances present, the more oxygen is consumed; these two factors are directly proportional. Therefore, environmental scientists call the amount of oxygen consumed when wastewater is oxidized with chemical reagents the Chemical Oxygen Demand (COD), and the amount of oxygen consumed by microbial oxidation the Biochemical Oxygen Demand (BOD). Because COD and BOD can comprehensively reflect the quantity of all organic substances in wastewater and are relatively simple to analyze, they are widely used in wastewater analysis and environmental engineering. In fact, COD doesn't solely represent organic substances in water; it also represents inorganic substances with reducing properties, such as sulfides, ferrous ions, sodium sulfite, and even chloride ions. For example, if ferrous ions in the effluent of an iron-carbon pool are not completely removed in the neutralization pool, the presence of ferrous ions in the effluent of the biological treatment may cause the COD to exceed the standard.

2. What is COD (Chemical Oxygen Demand)? Chemical Oxygen Demand (COD) refers to the amount of oxygen required to oxidize oxidizable substances in wastewater using a chemical oxidant, expressed in milligrams of oxygen per liter (mg/L). It is currently the most common method for determining the organic matter content in wastewater. Commonly used oxidants in COD analysis include potassium permanganate (CODMn, permanganate method) and potassium dichromate (CODCr, dichromate method), with potassium dichromate being more commonly used now. Under conditions of strong acid heating, boiling, and reflux, wastewater oxidizes organic matter. Using silver sulfate as a catalyst can increase the oxidation rate of most organic matter to 85-95%. If the wastewater contains a high concentration of chloride ions, mercuric sulfate should be used to mask the chloride ions to reduce interference with COD determination.

3. What is BOD5 (5-day Biochemical Oxygen Demand)? Biochemical Oxygen Demand (BOD) can also characterize the degree of organic pollution in wastewater. The most commonly used is the 5-day Biochemical Oxygen Demand, denoted as BOD5, which represents the amount of oxygen required for the biochemical degradation of wastewater in the presence of microorganisms over five days. We will frequently use the 5-day Biochemical Oxygen Demand in the future.

4. What is the relationship between COD and BOD5? Some organic substances can be biodegraded (such as glucose and ethanol), some can only be partially biodegraded (such as methanol), and some are non-biodegradable and even toxic (such as ginkgo phenol, ginkgo acid, and some surfactants). Therefore, we can divide organic matter in water into two parts: biodegradable and non-biodegradable organic matter. COD generally represents all organic matter in water, while BOD represents biodegradable organic matter in water. Therefore, the difference between COD and BOD can represent the non-biodegradable organic matter in wastewater.

5. What is B/C? What does B/C represent? B/C is the abbreviation for the ratio of BOD5 to COD. This ratio can represent the biodegradability of wastewater. If CODNB represents the non-biodegradable portion of COD, then the proportion of organic matter in wastewater that is not biodegradable by microorganisms can be expressed as CODNB/COD.
The relationship between BOD5/COD and CODNB/COD is shown in the table below:

CODNB/COD 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
BOD5/COD 0.52 0.46 0.41 0.35 0.29 0.23 0.17 0.12

When BOD5/COD ≥ 0.45, non-biodegradable organic matter accounts for less than 20% of the total organic matter. When BOD5/COD ≤ 0.2, non-biodegradable organic matter accounts for more than 60% of the total organic matter. Therefore, the BOD5/COD value is often used as an evaluation indicator of the biodegradability of organic matter. BOD5/COD 0.45 Easily biodegradable
BOD5/COD 0.30 Biodegradable
BOD5/COD 0.30 Less easily biodegradable
BOD5/COD 0.20 Difficult to biodegrade
B/C has very important and practical significance in environmental engineering.

6. What is pH? pH is a way of expressing the acidity or alkalinity of an aqueous solution. We usually use percentage concentration to express the acidity or alkalinity of an aqueous solution, such as a 1% sulfuric acid solution or a 1% alkaline solution. However, when the acidity or alkalinity of an aqueous solution is very small, using percentage concentration becomes cumbersome. In such cases, pH is used. The pH range is 0-14. When pH = 7, water is neutral; when pH < 7, water is acidic, and the smaller the pH, the stronger the acidity; when pH > 7, water is alkaline, and the larger the pH, the stronger the alkalinity. All living organisms in the world depend on water, but the pH range suitable for the survival of organisms is often very narrow. Therefore, the National Environmental Protection Agency strictly regulates the pH value of treated effluent to be between 6 and 9. pH in water is often measured using pH paper, or with instruments such as a pH meter.

7. Why is mg/L (milligrams per liter) frequently used as a concentration unit in wastewater analysis? Generally, the content of organic and inorganic substances in wastewater is very small. Using percentage concentration or other concentration units would be too cumbersome and inconvenient. For example, one ton of wastewater often contains only a few grams, tens of grams, hundreds of grams, or even thousands of grams of pollutants. The unit is grams/ton (g/T). If tons are converted to liters, the unit becomes milligrams/liter (mg/L).

Refer to the following table for conversion calculations:

1 milligram/liter = one part per million
1000 milligrams/liter = one part per thousand
10000 milligrams/liter = one percent

8. What is wastewater pretreatment? What are the purposes of pretreatment? Treatment before biological treatment is generally called pretreatment. Because biological treatment is relatively inexpensive and stable, most industrial wastewater uses biological treatment. ***Company's wastewater treatment also uses biological treatment as the main method. However, ***Company's wastewater contains certain organic substances that inhibit or are toxic to microorganisms. Therefore, before the wastewater enters the bioreactor, necessary pretreatment must be carried out to reduce or remove substances that inhibit or are toxic to microorganisms, ensuring the normal operation of the microorganisms in the bioreactor. Pretreatment has two purposes: firstly, to reduce or remove substances that inhibit or are toxic to microorganisms, or to convert them into harmless or beneficial substances for microorganisms, ensuring the normal operation of the microorganisms in the bioreactor; secondly, to reduce the COD load during pretreatment to reduce the operating burden on the bioreactor. ***Company's pretreatment process is iron-carbon micro-electrolysis and Fe2+/Fe3+ reduction-oxidation, forming countless tiny iron-carbon micro-batteries that facilitate redox reactions, destroying and removing toxic and harmful substances in the wastewater. During the neutralization and precipitation process, the active flocs formed by ferrous and ferric iron under alkaline conditions can also adsorb organic matter in the wastewater to reduce the COD load, ensuring the normal operation of the subsequent biological treatment system.

9. What is the purpose of a wastewater collection pool? The purpose of a wastewater collection pool is to collect, store, and equalize the water quality and quantity of wastewater. The wastewater discharge from various workshops is generally unbalanced in terms of quantity and quality. Wastewater is produced during production, but not when production stops. There may be significant variations even within a day or between shifts, especially for fine chemical wastewater. If clear and turbid wastewater is not separated, the variation in water quality and quantity between concentrated process wastewater and lightly polluted wastewater will be very large. This variation is very unfavorable, even harmful, to the normal operation and treatment effect of wastewater treatment facilities and equipment. Therefore, before entering the main wastewater treatment system, a wastewater collection pool with a certain capacity must be set up to store the wastewater and make it homogeneous in quality and quantity, ensuring the normal operation of wastewater treatment equipment and facilities.

10. Why are colloidal particles in wastewater not easily naturally settled? Many impurities with a specific gravity greater than 1, large particles, and easily settleable suspended solids in wastewater can be removed by natural sedimentation, centrifugation, etc. However, suspended particles with a specific gravity less than 1, tiny or even invisible to the naked eye, are difficult to naturally settle. For example, colloidal particles are micro-particles of 10-4-10-6 mm in size, very stable in water, and their sedimentation rate is extremely slow; it takes 200 years to settle 1 m. There are two reasons for the slow sedimentation: (1) Generally, colloidal particles carry a negative charge. Due to the mutual repulsion of the same charges, the contact between colloidal particles is prevented, and they cannot be bonded to each other, remaining suspended in water. (2) There is a layer of molecules tightly surrounding the surface of the colloidal particles. This hydration layer also hinders and isolates the contact between colloidal particles, preventing them from bonding to each other and remaining suspended in water.

11. How to make colloidal particles precipitate? To make colloidal particles precipitate, it is necessary to promote contact between colloidal particles to form larger particles, that is, to coagulate them, so that their specific gravity is greater than 1 and they precipitate. Many methods are used, and commonly used engineering techniques include coagulation, flocculation, and mixing.

12. What is coagulation? Adding coagulants with positive ions to wastewater, a large number of positive ions exist between colloidal particles to eliminate the electrostatic repulsion between colloidal particles, thus causing the particles to aggregate. This process of making colloidal particles aggregate by adding positive ion electrolytes is called coagulation. Commonly used coagulants include aluminum sulfate, ferrous sulfate, alum, and ferric chloride.

13. What is flocculation? Flocculation is the addition of high-molecular coagulants to wastewater. After dissolution, high-molecular coagulants form high-molecular polymers. The structure of this polymer is linear, with one end of the line pulling one tiny particle and the other end pulling another tiny particle, playing a bridging role between two distant particles, causing the particles to gradually become larger and eventually forming large floccules (commonly known as alum flowers), accelerating particle sedimentation. Commonly used flocculants include polyacrylamide (PAM) and polyferric (PE).

14. Why use polyferric for flocculation and adsorption pretreatment of wastewater? During coagulation, polyferric forms ferric hydroxide flocs with a good ability to adsorb organic matter in wastewater. Experimental data show that after flocculation and adsorption with polyferric, 10%-20% of COD in wastewater can be removed, which can greatly reduce the operating burden of the bioreactor and is conducive to the discharge of treated wastewater that meets standards. In addition, coagulation pretreatment with polyferric can remove trace substances in wastewater that are toxic or inhibitory to microorganisms, ensuring the normal operation of microorganisms in the bioreactor. Among many coagulants, polyferric is relatively inexpensive (25-300 yuan/ton), so the treatment cost is relatively low, making it suitable for the pretreatment of process wastewater. Polyferric is an acidic substance with strong corrosiveness, so the treatment equipment should be well protected against corrosion.

15. What is mixing? The process of combining coagulation and flocculation is called mixing. Mixing is often used in experiments or engineering. For example, ferrous sulfate is first added to water to eliminate the electrostatic repulsion between colloidal particles, and then polyacrylamide (PAM) is added to make the particles gradually larger, forming visible alum flowers, and finally producing sedimentation.

16. What is adsorption? Adsorption treatment utilizes porous solids (such as activated carbon) or flocculent substances (such as polyferric) to adsorb toxic and harmful substances in wastewater onto the surface or micropores of the solid or flocculent, achieving the purpose of water purification. The adsorption target can be insoluble solid substances or soluble substances. Adsorption treatment is highly efficient and produces high-quality effluent, so it is often used for advanced wastewater treatment. Adsorption treatment can also be introduced into biological treatment units to improve the efficiency of biological treatment (e.g., the PACT method is one of them).

17. What is the iron-carbon treatment method? The iron-carbon treatment method, also known as the iron-carbon micro-electrolysis method or iron-carbon internal electrolysis method, is an application of metal iron wastewater treatment technology. Using the iron-carbon method as a pretreatment technology to treat toxic and harmful wastewater with high COD has a unique effect. The treatment mechanism of the iron-carbon method is not yet fully understood. A currently accepted explanation is that under acidic conditions, countless micro-current reaction cells are formed between iron and carbon, and organic matter is reduced and oxidized under the action of micro-currents. The iron-carbon effluent is then neutralized with lime or lime milk, and the generated Fe(OH)2 colloidal flocculent has a strong flocculation and adsorption capacity for organic matter. Therefore, the iron-carbon method comprehensively utilizes the reducing properties of iron, the electrochemical properties of iron-carbon, and the flocculation and adsorption of iron ions. It is the combined effect of these three properties that makes the iron-carbon method highly effective. The disadvantages of the iron-carbon method are: (1) Iron filings are prone to agglomeration and blockage after prolonged immersion in an acidic medium, causing channeling and making operation difficult and reducing treatment efficiency; (2) The amount of iron dissolved in acidic conditions is large, and the amount of sludge produced after alkali neutralization is also large.

18. Why does iron-carbon effluent need to be neutralized with lime powder? After wastewater adjusted to pH 2 with sulfuric acid undergoes iron-carbon treatment, the sulfuric acid becomes ferrous sulfate, and the pH of the wastewater increases from 2 to 5-6. So why does the iron-carbon effluent still need to be neutralized with lime powder? Or can less lime powder be added during neutralization? The iron-carbon effluent contains a large amount of ferrous sulfate, which, if not removed, will affect the growth and reproduction of microorganisms in the subsequent bioreactor. Therefore, we must use lime to increase the pH of the wastewater from 5-6 to above 9, converting the water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate, and then removing them through coagulation sedimentation to ensure that the wastewater entering the bioreactor does not contain ferrous sulfate. Can less lime powder be added during neutralization? We can conduct a comparative experiment in the laboratory. Take the same amount of iron-carbon influent (pH around 2) and iron-carbon effluent (pH 5-6) and place them in two beakers, then add lime powder for neutralization and coagulation in a metered manner. When the pH of the wastewater in both beakers is adjusted to 9, we can find that the amount of lime powder added to the two beakers is the same. This is because iron is not a neutralizing agent, and the ferrous sulfate converted from sulfuric acid is still an acidic substance. The lime powder consumed when ferrous sulfate is converted into ferrous hydroxide and calcium sulfate during neutralization cannot be reduced. Therefore, lime powder cannot be reduced during the neutralization of iron-carbon effluent.

19. How to estimate the production of chemical sludge? Sludge produced through chemical reactions (such as neutralization) and physicochemical treatment (such as chemical coagulation) is generally called chemical sludge. The sludge formed after neutralization and coagulation of iron-carbon effluent is mainly composed of ferrous hydroxide and calcium sulfate. The amount of sludge produced can be calculated based on the amount of sulfuric acid and lime powder added. In engineering, it can also be estimated using experience. Generally speaking, if the pH of the iron-carbon influent is around 2, the amount of chemical sludge produced per ton of wastewater after neutralization and coagulation (water content 80%) is around 50 kg.

20. What is biological treatment of wastewater? Biological treatment of wastewater is one of the most important processes in wastewater treatment systems, also known as biotreatment. Biotreatment utilizes the life activities of microorganisms to effectively remove soluble and some insoluble organic matter in wastewater, purifying the water. In fact, we are not unfamiliar with biotreatment. A food chain exists in natural water bodies: big fish eat small fish, small fish eat shrimp, shrimp eat insects, insects eat microorganisms, and microorganisms eat wastewater. Without this food chain, nature would be in chaos. In natural rivers, there are a large number of microorganisms that rely on organic matter for survival. They work day and night to oxidize or reduce the organic matter discharged into rivers by humans (such as industrial wastewater, pesticides, fertilizers, feces, etc.), ultimately converting them into inorganic matter. Without microorganisms, the rivers around us would become foul-smelling rivers in a few months or a year or two. It's just that microorganisms are too small and dispersed to be seen with the naked eye. Wastewater biotreatment engineering is an intensification of this process under artificial conditions. Countless microorganisms are concentrated in a tank, creating an environment very suitable for the reproduction and growth of microorganisms (such as temperature, pH, oxygen, nitrogen, phosphorus, and other nutrients), allowing microorganisms to proliferate to increase the speed and efficiency of organic matter decomposition. Then wastewater is pumped into the tank, and the organic matter in the wastewater is oxidized and degraded during the life activities of microorganisms, purifying and treating the wastewater. Compared with other treatment methods, the biotreatment method has the advantages of low energy consumption, no chemical addition, good treatment effect, and low treatment cost.

21. How do microorganisms decompose and remove organic pollutants in wastewater? Due to the presence of carbohydrates, fats, and proteins in wastewater, these lifeless organic substances serve as food for microorganisms. Part of them are degraded and synthesized into cell substances (anabolic products), while another part is degraded and oxidized into water and carbon dioxide (catabolic products). In this process, organic pollutants in wastewater are degraded and removed by microorganisms.

22. What factors are related to microorganisms? In addition to nutrients, microorganisms also require suitable environmental factors such as temperature, pH, dissolved oxygen, and osmotic pressure to survive. Abnormal environmental conditions can affect the life activities of microorganisms, even causing mutations or death.

23. What is the optimal temperature range for microbial growth and reproduction? In wastewater biological treatment, the optimal temperature range for microorganisms is generally 16-30℃, with a maximum temperature of 37-43℃. When the temperature falls below 10℃, microorganisms will cease to grow. Within the suitable temperature range, for every 10℃ increase in temperature, the metabolic rate of microorganisms will increase accordingly, and the COD removal rate will also increase by about 10%; conversely, for every 10℃ decrease in temperature, the COD removal rate will decrease by 10%, therefore, the biochemical removal rate of COD in winter will be significantly lower than in other seasons.

24. What is the optimal pH range for microorganisms? The life activities and material metabolism of microorganisms are closely related to pH value. Most microorganisms adapt to a pH range of 4.5-9, while the most suitable pH range is 6.5-7.5. When the pH is below 6.5, fungi begin to compete with bacteria; when the pH reaches 4.5, fungi will completely dominate in the bioreactor, resulting in serious effects on sludge settlement. When the pH exceeds 9, the metabolic rate of microorganisms will be hindered. Different microorganisms have different requirements for the suitable pH range. In aerobic biological treatment, the pH can vary between 6.5-8.5; in anaerobic biological treatment, microorganisms have stricter pH requirements, and the pH should be between 6.7-7.4.

25. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms? Oxygen dissolved in water is called dissolved oxygen. Aquatic organisms and aerobic microorganisms rely on dissolved oxygen for survival. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require sufficient dissolved oxygen; generally, dissolved oxygen should be maintained at 3mg/L, and should not be lower than 2mg/L; facultative anaerobic microorganisms require dissolved oxygen in the range of 0.2-2.0mg/L; while anaerobic microorganisms require dissolved oxygen below 0.2mg/L.

26. Why do high concentrations of saline wastewater have a particularly significant impact on microorganisms? Let's describe an experiment on osmotic pressure: Two salt solutions with different concentrations are separated by a semi-permeable membrane. Water molecules from the low-concentration salt solution will pass through the semi-permeable membrane into the high-concentration salt solution, and water molecules from the high-concentration salt solution will also pass through the semi-permeable membrane into the low-concentration salt solution, but in smaller quantities. Therefore, the liquid level on the high-concentration salt solution side will rise. When the height difference between the two liquid levels produces enough pressure to prevent further water flow, osmosis will stop. At this time, the pressure generated by the height difference between the two liquid levels is the osmotic pressure. Generally speaking, the higher the salt concentration, the greater the osmotic pressure. The situation of microorganisms in salt water solution is similar to the osmotic pressure experiment. The unit structure of microorganisms is the cell, and the cell wall is equivalent to a semi-permeable membrane. When the chloride ion concentration is less than or equal to 2000mg/L, the osmotic pressure that the cell wall can withstand is 0.5-1.0 atmospheres. Even with the certain toughness and elasticity of the cell wall and cell membrane, the osmotic pressure that the cell wall can withstand will not exceed 5-6 atmospheres. However, when the chloride ion concentration in the aqueous solution is above 5000mg/L, the osmotic pressure will increase to about 10-30 atmospheres. Under such high osmotic pressure, water molecules inside the microorganisms will permeate into the external solution in large quantities, causing cell dehydration and plasmolysis, and severe cases will lead to microbial death. In daily life, people use table salt (sodium chloride) to pickle vegetables and fish and meat for sterilization and preservation, which is based on this principle. Engineering experience data shows that when the chloride ion concentration in wastewater is greater than 2000mg/L, the activity of microorganisms will be inhibited, and the COD removal rate will decrease significantly; when the chloride ion concentration in wastewater is greater than 8000mg/L, it will cause sludge volume expansion, a large amount of foam will appear on the water surface, and microorganisms will die successively. However, after long-term domestication, microorganisms will gradually adapt to growth and reproduction in high-concentration salt water. Currently, some people have domesticated microorganisms that can adapt to chloride or sulfate concentrations above 10000mg/L. However, the principle of osmotic pressure tells us that microorganisms that have adapted to growth and reproduction in high-concentration salt water have a very high salt concentration in their cell fluid. Once the salt concentration in the wastewater is low or very low, water molecules in the wastewater will enter the microorganisms in large quantities, causing the microbial cells to swell, and severe cases will lead to rupture and death. Therefore, microorganisms that have been domesticated for a long time and can gradually adapt to growth and reproduction in high-concentration salt water always require the salt concentration in the biochemical influent to remain at a relatively high level, which cannot fluctuate greatly, otherwise, a large number of microorganisms will die.

27. What is aerobic biochemical treatment? What is facultative anaerobic biochemical treatment? What are the differences between the two? According to the different oxygen requirements of microorganisms for growth, biochemical treatment can be divided into two categories: aerobic biochemical treatment and anoxic biochemical treatment. Anoxic biochemical treatment can be further divided into facultative anaerobic biochemical treatment and anaerobic biochemical treatment. In the aerobic biochemical treatment process, aerobic microorganisms must grow and reproduce in the presence of a large amount of oxygen to reduce organic matter in wastewater; while in the facultative anaerobic biochemical treatment process, facultative anaerobic microorganisms only need a small amount of oxygen to grow and reproduce and degrade organic matter in wastewater. If there is too much oxygen in the water, facultative anaerobic microorganisms will not grow well, thus affecting their efficiency in treating organic matter. Facultative anaerobic microorganisms can adapt to wastewater with higher COD concentrations, and the influent COD concentration can be increased to more than 2000mg/L, with a COD removal rate generally ranging from 50-80%; while aerobic microorganisms can only adapt to wastewater with lower COD concentrations, and the influent COD concentration is generally controlled below 1000-1500mg/L, with a COD removal rate generally ranging from 50-80%. The time for facultative anaerobic biochemical treatment and aerobic biochemical treatment is not long, generally 12-24 hours. People utilize the differences and advantages between facultative anaerobic biochemical treatment and aerobic biochemical treatment, combining facultative anaerobic biochemical treatment and aerobic biochemical treatment, allowing wastewater with higher COD concentration to undergo facultative anaerobic biochemical treatment first, and then using the effluent from the facultative anaerobic pool as the influent of the aerobic pool. This combined treatment can reduce the volume of the bioreactor, saving both environmental investment and daily operating costs. The principle and function of anaerobic biochemical treatment are the same as those of facultative anaerobic biochemical treatment. The difference between anaerobic biochemical treatment and facultative anaerobic biochemical treatment is that anaerobic microorganisms do not require any oxygen in the process of reproduction, growth, and degradation of organic matter, and anaerobic microorganisms can adapt to wastewater with higher COD concentrations (4000-10000mg/L). The disadvantage of anaerobic biochemical treatment is that the biochemical treatment time is long, and the residence time of wastewater in the anaerobic biochemical pool generally needs more than 40 hours.

28. What are the applications of biological treatment in wastewater treatment engineering? Two major types of biological treatment are most widely and practically used in wastewater treatment engineering: activated sludge process and biofilm process. The activated sludge process is an aerobic wastewater treatment method that utilizes the biochemical metabolism of suspended microbial populations. During growth and reproduction, microorganisms can form flocs with a large surface area, which can flocculate and adsorb a large amount of suspended colloidal or dissolved pollutants in wastewater, and absorb these substances into the cells. With the participation of oxygen, these substances are completely oxidized to release energy, CO2, and H2O. The sludge concentration in the activated sludge process is generally 4 g/L. In the biofilm process, microorganisms attach to the surface of the filler to form a biofilm connected by a gel-like substance. The biofilm generally has a fluffy flocculent structure with many micropores and a large surface area, which has a strong adsorption capacity, facilitating further decomposition and utilization of the adsorbed organic matter by microorganisms. During the treatment process, the flow of water and the stirring of air ensure continuous contact between the biofilm surface and water. Organic pollutants and dissolved oxygen in the wastewater are adsorbed by the biofilm, and the microorganisms on the biofilm continuously decompose these organic substances. While oxidizing and decomposing organic substances, the biofilm itself also undergoes continuous metabolism, and the aging biofilm sloughs off, being carried out of the biological treatment facility with the treated water and separated from the water in the sedimentation tank. The sludge concentration in the biofilm process is generally 6-8 g/L. To increase the sludge concentration and thus improve the treatment efficiency, the activated sludge process and the biofilm process can be combined, i.e., adding fillers to the activated sludge tank. This type of bioreactor, which contains both attached-growth and suspended-growth microorganisms, is called a composite bioreactor, which has a high sludge concentration, generally around 14 g/L.

29. What are the similarities and differences between the biofilm process and the activated sludge process? The biofilm process and the activated sludge process are different reactor forms for biochemical treatment. The main difference in appearance is that the former does not require a filler carrier for microorganisms, and the biological sludge is suspended, while the latter's microorganisms are fixed on the filler. However, their mechanisms for treating wastewater and purifying water quality are the same. In addition, the biological sludge in both processes is aerobic activated sludge, and the composition of the sludge is also similar. Furthermore, the microorganisms in the biofilm process, because they are fixed on the filler, can form a relatively stable ecosystem, and their energy consumption is not as high as that of microorganisms in the activated sludge process. Therefore, the excess sludge in the biofilm process is less than that in the activated sludge process. Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd.'s contact oxidation pool uses the biofilm process, while the SBR bioreactor uses the activated sludge process.

30. What is activated sludge? From a microbiological perspective, the sludge in the bioreactor is a biological community composed of various microorganisms with biological activity. If the sludge particles are observed under a microscope, various microorganisms can be seen—bacteria, fungi, protozoa, and metazoa (such as rotifers, insect larvae, and worms). They form a food chain. Bacteria and fungi can decompose complex organic compounds, obtaining the energy needed for their own activities and constructing themselves. Protozoa feed on bacteria and fungi, and are consumed by metazoa, which can also directly rely on bacteria for survival. This flocculent particle, full of microorganisms and capable of degrading organic matter, is called activated sludge. In addition to microorganisms, activated sludge also contains some inorganic substances and organic substances adsorbed on the activated sludge that cannot be further biodegraded (i.e., metabolic residues of microorganisms). The water content of activated sludge is generally 98-99%. Activated sludge, like alum floc, has a large surface area and therefore has a strong adsorption capacity and the ability to oxidize and decompose organic matter.

31. How to evaluate the activated sludge in the activated sludge process and the biofilm process? The judgment and evaluation of the activated sludge growth in the activated sludge process and the biofilm process are different. In the biofilm process, the evaluation of activated sludge growth mainly uses direct microscopic observation of the biological community. In the activated sludge process, in addition to direct microscopic observation of the biological community, commonly used evaluation indicators include mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), sludge settling ratio (SV), and sludge volume index (SVI).

32. When observing the biological community under a microscope, which type of microorganism directly indicates good biochemical treatment effect? The appearance of microscopic metazoa (such as rotifers and nematodes) indicates that the microbial community is growing well and the activated sludge ecosystem is relatively stable. At this time, the biochemical treatment effect is optimal, which is similar to the situation where small fish and shrimp grow well in rivers where large fish can often be caught. 38. What is mixed liquor suspended solids (MLSS)? Mixed liquor suspended solids (MLSS), also known as sludge concentration, refers to the weight of dry sludge contained in a unit volume of bioreactor mixed liquor, with units of mg/L, used to characterize the activated sludge concentration. It includes both organic and inorganic matter. Generally, the MLSS value in the SBR bioreactor should be controlled at around 2000-4000 mg/L.

33. What is mixed liquor volatile suspended solids (MLVSS)? Mixed liquor volatile suspended solids (MLVSS) refers to the weight of volatile substances in the dry sludge contained in a unit volume of bioreactor mixed liquor, also with units of mg/L. Since it does not include inorganic matter in the activated sludge, it can more accurately represent the number of microorganisms in the activated sludge.

34. Sludge settling ratio (SV)? The sludge settling ratio (SV) is the ratio (%) of the volume of settled sludge to the volume of mixed liquor after 30 minutes of static sedimentation in a 100 ml cylinder in the aeration tank. Therefore, it is sometimes expressed as SV30. Generally, the SV in the bioreactor is between 20-40%. The determination of sludge settling ratio is relatively simple and is one of the important indicators for evaluating activated sludge. It is often used to control the discharge of excess sludge and timely respond to abnormal phenomena such as sludge bulking. Obviously, SV is also related to sludge concentration.

35. Sludge volume index (SVI)? The sludge volume index (SVI), also known as the sludge volume index, is the number of milliliters occupied by 1 gram of dry sludge in the wet state. The calculation formula is as follows: SVI = SV * 10 / MLSS. SVI eliminates the influence of sludge concentration factors and can better reflect the cohesion and settling properties of activated sludge. Generally, it is considered that: When 60 < SVI < 100, the sludge settling performance is good
When 100 < SVI < 200, the sludge settling performance is generally good
When 200 < SVI < 300, the sludge tends to swell
When SVI > 300, the sludge has swelled

36. What does dissolved oxygen (DO) represent? Dissolved oxygen (DO) represents the amount of oxygen dissolved in water, with the unit expressed as mg/L. Different biochemical treatment methods have different requirements for dissolved oxygen. In the anoxic biochemical process, the dissolved oxygen in water is generally between 0.2-2.0 mg/L, while in the SBR aerobic biochemical process, the dissolved oxygen in water is generally between 2.0-8.0 mg/L. Therefore, the aeration volume should be small and the aeration time should be short during the operation of the anoxic pool; while in the SBR aerobic pool operation, the aeration volume and aeration time should be much larger and longer. We use contact oxidation, and the dissolved oxygen is controlled at 2.0-4.0 mg/L.

37. 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 dissolution equilibrium is reached: C = KH * P where: C is the solubility of oxygen in water at dissolution equilibrium; P is the partial pressure of oxygen in the gas phase; KH is the Henry coefficient, which is related to temperature; increasing aeration efforts makes the oxygen dissolution close to equilibrium, while at the same time, activated sludge will also consume the oxygen in the water. Therefore, the actual dissolved oxygen in wastewater is related to water temperature, effective water depth (affecting pressure), aeration volume, sludge concentration, salinity, etc.

38. Who provides the oxygen needed by microorganisms in the biochemical process? The oxygen needed by microorganisms in the biochemical process is mainly provided by the Roots blower.

39. Why is it necessary to frequently supplement nutrients in wastewater during the biochemical process? The method of using biochemical processes to remove pollutants mainly utilizes the metabolic processes of microorganisms, and the cell synthesis and other life processes of microorganisms all require sufficient amounts and types of nutrients (including trace elements). For chemical wastewater, due to the single nature of the products produced, the composition of the wastewater quality is also relatively single, lacking the necessary nutrients for microorganisms. For example, the production wastewater of *** company only contains carbon and nitrogen but not phosphorus. This wastewater cannot meet the needs of microbial metabolism, so phosphorus must be added to the wastewater to improve the microbial metabolic process and promote the synthesis of microbial cells. This is like a person eating rice and flour while also ingesting sufficient amounts of vitamins.

40. What is the ratio of various nutrient elements required by microorganisms in wastewater? Like animals and plants, microorganisms also need necessary nutrients to grow and reproduce. The nutrients required 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 biochemical processes, it is generally C:N:P = 100:5:1 (weight ratio).

41. Why is there excess sludge? In the biochemical treatment process, the microorganisms in the activated sludge continuously consume the organic matter in the wastewater. Of the organic matter consumed, part of it is oxidized to provide energy for the life activities of microorganisms, and the other part is used by microorganisms to synthesize new cytoplasm, thus enabling microorganisms to proliferate and reproduce. While microorganisms are metabolizing, some old microorganisms die, thus producing excess sludge.

42. How to estimate the amount of excess sludge produced? In the metabolism of microorganisms, some organic matter (BOD) is used by microorganisms to synthesize new cytoplasm to replace dead microorganisms. Therefore, the amount of excess sludge produced is related to the amount of BOD decomposed, and there is a correlation between the two. In engineering design, it is generally considered that for every kilogram of BOD5 treated, 0.6-0.8 kilograms of excess sludge (100%) are produced. Converted to dry sludge with a water content of 80%, this is 3-4 kilograms.