Wastewater treatment

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1. What is COD (Chemical Oxygen Demand)? Chemical oxygen demand (COD) refers to the amount of oxygen required to oxidize the substances in wastewater that can be oxidized by chemical oxidants, measured in milligrams of oxygen per liter. It is currently one of the most commonly used methods for determining the organic content in wastewater. The oxidants commonly used in COD analysis are potassium permanganate (Mn method for CODMn) and potassium dichromate (Cr method for CODCr); currently, the potassium dichromate method is more frequently employed. Wastewater is used to oxidize organic substances under conditions of strong acid, heating, boiling, and reflux; the use of silver sulfate as a catalyst can increase the oxidation rate of most organic substances to 85-95%. If the wastewater contains a high concentration of chloride ions, mercury sulfate should be used to shield these ions in order to reduce interference with the COD measurement. 2. What is BOD5 (Biochemical Oxygen Demand)? Biochemical oxygen demand can also be used to indicate the degree of organic pollution in wastewater. The most commonly used value is the 5-day biochemical oxygen demand, denoted as BOD5; it represents the amount of oxygen required for the biochemical degradation of wastewater over a period of 5 days in the presence of microorganisms. Moving forward, we will frequently use the 5-day biochemical oxygen demand. 3. What is the relationship between COD and BOD5? Some organic substances can be biologically oxidized and degraded (such as glucose and ethanol), some can only be partially biologically oxidized and degraded (such as methanol), while others cannot be biologically oxidized or degraded and are also toxic (such as ginkgolides, ginkgic acid, and certain surfactants). Therefore, we can divide organic matter in water into two categories: biodegradable organic matter and non-biodegradable organic matter. It is generally believed that COD can essentially represent all organic matter in water. BOD refers to the biodegradable organic matter in water; therefore, the difference between COD and BOD represents the organic matter in wastewater that is not biodegradable. 4. What is B/C? What does B/C mean? B/C is an abbreviation for the ratio of BOD5 to COD, and this ratio can indicate the biodegradability of wastewater. Therefore, the BOD5/COD ratio is often used as an indicator to evaluate the biodegradability of organic matter. https://bbs.hcbbs.com/data/attachment/forum/202012/04/100913x4l7op46hfihzuzh.png 5. What is the pre-treatment of wastewater? What purposes should preprocessing achieve? The treatment prior to biochemical treatment is generally customarily called pretreatment. Due to the relatively low treatment costs and stable operation of biochemical methods, these are commonly used to treat industrial wastewater, and they serve as the primary approach for wastewater treatment. However, wastewater contains certain organic substances that inhibit or are toxic to microorganisms. Therefore, necessary pretreatment must be carried out on the wastewater before it enters the biochemical tank. The purpose is to minimize or remove those substances that have inhibitory or toxic effects on microorganisms, thereby ensuring the normal functioning of the microorganisms in the biochemical tank. There are two purposes for pretreatment: one is to minimize and remove, or convert into substances that are harmless or beneficial to microorganisms, those substances in the wastewater that have an inhibitory or toxic effect on them, so as to ensure the proper functioning of the microorganisms in the biochemical tank ； The second is to reduce the COD load during the pretreatment process in order to lessen the operational burden on the biochemical tank. 6. What is the purpose of the wastewater collection tank? The function of a wastewater collection tank is to gather, store, and balance the quality and quantity of wastewater. The amount and quality of wastewater generated by various production workshops are generally uneven; wastewater is produced during manufacturing processes, but not when there is no production. There can even be significant variations within a single day or between shifts. This is especially true for wastewater from the fine chemical industry. If wastewater with high and low pollution levels is not separated, then the quality and quantity of the concentrated wastewater and that of less polluted wastewater vary greatly. Such variations are detrimental to the proper operation of wastewater treatment facilities and equipment, as well as to the effectiveness of the treatment process. Therefore, before wastewater enters the main sewage treatment system, a wastewater collection tank with a certain capacity must be installed to store the wastewater and homogenize it, thereby ensuring the proper operation of the wastewater treatment equipment and facilities. 7. Why do colloidal particles in wastewater not settle naturally? Many impurities in wastewater with a specific gravity greater than 1, as well as large particles and suspended matter that settle easily, can be removed through methods such as natural sedimentation or centrifugation. However, tiny suspended particles with a density of less than 1, which are even invisible to the naked eye, find it difficult to settle naturally. For example, colloidal particles are particles sized between 10-4 and 10-6 mm; they are very stable in water, and their settling speed is extremely slow – it takes 200 years for them to settle 1 meter. There are two reasons for the slow settlement: First, colloidal particles all carry a negative charge, and due to the repulsion between like charges, this prevents the colloidal particles from coming into contact with each other, preventing them from sticking together and remaining suspended in water. II. The surface of colloidal particles is also surrounded by a layer of molecules; this hydration layer prevents and isolates contact between the colloidal particles, preventing them from sticking together and remaining suspended in water. 8. How to make colloidal particles precipitate? To make colloidal particles precipitate, it is necessary to encourage them to come into contact with each other, thereby forming larger particles—that is, causing them to coagulate—so that their specific gravity becomes greater than 1 and they can then precipitate. There are many methods available, and the commonly used techniques in engineering include coagulation, flocculation, and coagflocculation. 9. What is cohesion? Coagulant agents carrying positive ions are added to wastewater; the presence of a large number of positive ions between colloidal particles eliminates the electrostatic repulsion between them, thereby causing the particles to aggregate. This process of causing colloidal particles to aggregate through the addition of positive ion electrolytes is known as coagulation. Common coagulants include aluminum sulfate, ferrous sulfate, alum, ferric chloride, etc. 10. What is flocculation? Flocculation involves adding high-molecular-weight coagulants to wastewater; once dissolved, these coagulants form high-molecular-weight polymers. The structure of this polymer is linear; one end of the chain is attached to a tiny particle, and the other end is attached to another tiny particle. It acts as a bonding bridge between these two particles that are far apart from each other, allowing the particles to grow larger over time and eventually forming large particle flocs (commonly known as alum flowers), which accelerates the sedimentation of the particles. Common flocculants include polyacrylamide (PAM), polyferric sulfate (PF), etc. 11. What is coagulation? The process of combining coagulation and flocculation is known as coagulation-flocculation. Coagulation is frequently used in experiments and engineering applications: first, chemicals such as ferrous sulfate are added to water to eliminate the electrostatic repulsion between colloidal particles; then polyacrylamide (PAM) is added, causing the particles to grow larger and form flocs visible to the naked eye, which eventually settle. 12. What is adsorption? Porous solids (such as activated carbon) or flocculent substances (such as polyferric compounds) are used to adsorb toxic and harmful substances in wastewater onto the surface or into the pores of these solids or flocs, thereby purifying the water quality. This treatment method is known as adsorption treatment. The substances to be adsorbed can be either insoluble solid substances or soluble substances. Adsorption treatment is highly efficient and produces high-quality effluent; therefore, it is often used for advanced wastewater treatment. Adsorption treatment can also be introduced into the biochemical treatment unit to improve its efficiency (the PACT method is one such example). 13. What is the biochemical treatment of wastewater? The biochemical treatment of wastewater is one of the most important processes in wastewater treatment systems; it is commonly referred to as biochemical treatment. Biochemical treatment utilizes the metabolic processes of microorganisms to effectively remove soluble organic substances as well as some insoluble organic substances from wastewater, thereby purifying the water. 14. In what way do microorganisms break down and remove organic pollutants from wastewater? Since wastewater contains organic substances such as carbohydrates, fats, and proteins, these inorganic organic compounds serve as food for microorganisms. Some of them are degraded and synthesized into cellular materials (anabolic metabolites), while others are degraded and oxidized into water, carbon dioxide, etc. (catabolic metabolites). In this process, the organic pollutants in the wastewater are removed through degradation by microorganisms. 15. What factors are related to microorganisms? In addition to nutrients, microorganisms also require appropriate environmental factors such as temperature, pH value, dissolved oxygen, and osmotic pressure in order to survive. If environmental conditions are abnormal, it will affect the metabolic activities of microorganisms, and they may even mutate or die. 16. What temperature range is most suitable for microorganisms to grow and reproduce? In biological wastewater treatment, the optimal temperature range for microorganisms is generally 16–30°C, with the maximum temperature being 37–43°C; when the temperature drops below 10°C, microorganisms cease to grow. Within the appropriate temperature range, for every 10°C increase in temperature, the metabolic rate of microorganisms rises accordingly, and the COD removal rate also increases by about 10% ； Conversely, for every 10°C decrease in temperature, the COD removal rate decreases by 10%; therefore, during winter, the biochemical removal rate of COD is significantly lower than in other seasons. 17. What is the optimal pH range for microorganisms? The life activities and metabolic processes of microorganisms are closely related to pH value. Most microorganisms can tolerate a pH range of 4.5–9, with the optimal pH range being 6.5–7.5. When the pH is below 6.5, fungi begin to compete with bacteria; at a pH of 4.5, fungi gain complete dominance in the biochemical tank, which severely affects the sedimentation of sludge ； When the pH exceeds 9, the metabolic rate of microorganisms is hindered. Different microorganisms have different requirements for the pH range to which they can adapt. In aerobic biological treatment, the pH can vary between 6.5 and 8.5 ； In anaerobic biological treatment, microorganisms have relatively strict requirements regarding pH, which should be between 6.7 and 7.4. 18. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms? The oxygen dissolved in water is called dissolved oxygen. The oxygen upon which aquatic organisms and aerobic microorganisms depend for survival is dissolved oxygen. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require an adequate supply of dissolved oxygen. Generally, the level of dissolved oxygen should be maintained at 3 mg/L; the minimum acceptable level is 2 mg/L ； Facultative anaerobic microorganisms require a dissolved oxygen level between 0.2–2.0 mg/L ； Anaerobic microorganisms require a dissolved oxygen level of below 0.2 mg/L. 19. Why do high-concentration saline wastewater have a particularly large impact on microorganisms? The basic structural unit of microorganisms is the cell; the cell wall functions as a semipermeable membrane. When the chloride ion concentration is 2000 mg/L or less, the osmotic pressure that the cell wall can withstand is 0.5–1.0 atmospheres. Even taking into account the certain toughness and elasticity of both the cell wall and the cytoplasmic membrane, the osmotic pressure that the cell wall can tolerate does not exceed 5–6 atmospheres. However, when the chloride ion concentration in the aqueous solution exceeds 5000 mg/L, the osmotic pressure rises to approximately 10–30 atmospheres. Under such high osmotic pressure, water molecules within the microorganisms penetrate into the external solution in large quantities, causing the cells to lose water and undergo plasmolysis; in severe cases, the microorganisms die. Engineering experience data shows that when the chloride ion concentration in wastewater exceeds 2000 mg/L, microbial activity is suppressed, and the COD removal rate drops significantly ； When the chloride ion concentration in wastewater exceeds 8,000 mg/L, it causes sludge volume expansion, produces abundant foam on the water surface, and leads to the successive death of microorganisms. 20. What is aerobic biological treatment? What is aerobic biochemical treatment? What is the difference between the two? Based on the different oxygen requirements for microbial growth, biochemical treatment can be divided into two main categories: aerobic biochemical treatment and anaerobic biochemical treatment. Anaerobic biochemical treatment can further be divided into facultative anaerobic biochemical treatment and anaerobic biochemical treatment. During aerobic biological treatment, aerobic microorganisms must grow and reproduce in the presence of large amounts of oxygen, thereby reducing the organic matter in wastewater ； During the anaerobic-oxidizing biological treatment process, anaerobically oxidizing microorganisms require only a small amount of oxygen to grow and reproduce as well as to degrade the organic substances in wastewater. If there is too much oxygen in the water, these microorganisms cannot grow properly, which in turn affects their efficiency in dealing with organic substances. Oligotrophic microorganisms can adapt to wastewater with a relatively high COD concentration; the influent COD concentration can be raised to over 2,000 mg/L. The COD removal rate generally ranges from 50% to 80% ； Aerobic microorganisms can only thrive in wastewater with low COD concentrations; the COD level in the influent water is generally kept below 1000–1500 mg/L. The COD removal rate is usually between 50% and 80%. Neither anaerobic nor aerobic biological treatment processes take very long, typically ranging from 12 to 24 hours. Taking advantage of the differences as well as the similarities between anaerobic and aerobic biological treatment processes, these two methods are combined. Wastewater with a high COD concentration is first treated using anaerobic biological treatment, and the effluent from this anaerobic tank is then used as the feedwater for the aerobic tank. This combined treatment approach allows for a reduction in the volume of the biological treatment tanks, thereby saving on environmental protection investments as well as reducing daily operating costs. The principles and functions of anaerobic biological treatment are the same as those of facultative aerobic biological treatment. The difference between anaerobic biological treatment and aerobic biological treatment is that anaerobic microorganisms do not require any oxygen for their reproduction and growth, nor for the degradation of organic matter; moreover, they can cope with wastewater containing higher COD concentrations (4000–10000 mg/L). The disadvantage of anaerobic biological treatment is that the biological treatment process takes a long time; the retention time of wastewater in the anaerobic biological tank generally requires more than 40 hours. 21. What are the applications of biological treatment in wastewater treatment projects? There are two main categories of biological treatment technologies that are most widely and practically used in wastewater treatment engineering: one is called the activated sludge process, and the other is known as the biofilm process. The activated sludge process is a form of aerobic wastewater treatment that relies on the biochemical metabolism of suspended biological populations. During their growth and reproduction, microorganisms can form microbial flocs with a large surface area. These flocs are capable of aggregating and adsorbing large amounts of suspended colloidal or dissolved pollutants in wastewater, taking them into the cells. With the presence of oxygen, these substances are completely oxidized, releasing 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, forming a gelatinous, interconnected biofilm. Biofilms generally have a fluffy flocculent structure, with numerous micropores and a large surface area, giving them strong adsorption capabilities that facilitate microorganisms in further decomposing and utilizing these adsorbed organic substances. During the treatment process, the flow of water and the agitation of air keep the surface of the biofilm in constant contact with water. Organic pollutants and dissolved oxygen in the wastewater are absorbed by the biofilm, and the microorganisms on the biofilm continuously break down these organic substances. As these organic substances are oxidized and decomposed, the biofilm itself also undergoes continuous metabolism; the aged biofilm then falls off and is carried away by the treated water out of the biological treatment facility, where it separates from the water in a sedimentation tank. The sludge concentration in the biofilm process is generally 6–8 g/L. To increase the sludge concentration and thereby improve treatment efficiency, the activated sludge process can be combined with the biofilm process by adding fillers to the activated sludge tank. Such a bioreactor, which contains both microorganisms attached to the fillers and suspended microorganisms, is known as a composite bioreactor; it features a very high sludge concentration, typically around 14 g/L. 22. 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 types for biochemical treatment; their main visual difference lies in the fact that the microorganisms in the latter do not require a packing medium, as the biological sludge remains suspended, whereas in the former, the microorganisms are fixed on the packing. However, their mechanisms for treating wastewater and purifying water quality are the same. Furthermore, the biological sludge of both is aerobic activated sludge, and the composition of the sludge also exhibits certain similarities. Furthermore, in the biofilm process, since the microorganisms are fixed on the fillers, a relatively stable ecosystem can be formed. Their metabolic energy requirements and energy consumption are not as high as those of the microorganisms in the activated sludge process; as a result, the amount of excess sludge generated in the biofilm process is less than that in the activated sludge process. The contact oxidation tank uses the biofilm process, while the SBR biological tank employs the activated sludge process. 23. What is activated sludge? From a microbial perspective, the sludge in a biochemical tank is a biological community composed of various microorganisms with biological activity. If the sludge particles are observed under a microscope, it is possible to see various microorganisms within them—bacteria, molds, protozoa, and metazoans (such as rotifers, insect larvae, and worms). These organisms form a food chain; bacteria and molds can break down complex organic compounds to obtain the energy necessary for their activities and to build their own bodies. Protozoa feed on bacteria and molds, and they in turn are consumed by metazoans, which can also live off bacteria directly. These microbial-rich, floc-like particles with the ability to degrade organic matter are known as activated sludge. In addition to being composed of microorganisms, activated sludge also contains some inorganic substances and organic matter that is adsorbed on the activated sludge and cannot be biodegraded any further (i.e., metabolic residues of microorganisms). The moisture content of activated sludge is generally 98-99%. Activated sludge, resembling flocs, has a large surface area, which gives it strong adsorption capacity and the ability to oxidize and decompose organic matter. 24. How to evaluate the activated sludge in the activated sludge process and the biofilm process? The judgment and evaluation of activated sludge growth in the activated sludge process and the biofilm process are different. In the biofilm process, the evaluation of the growth condition of activated sludge mainly involves direct microscopic observation of the biological community. In the activated sludge process, in addition to direct microscopic observation of the biological community to assess the growth of the activated sludge, common evaluation indicators include: Mixed Liquor Suspended Solids (MLSS), Mixed Liquor Volatile Suspended Solids (MLVSS), Sludge Settling Ratio (SV), and Sludge Settling Index (SVI). 25. When observing the biological phase under a microscope, which type of microorganisms indicates that the biochemical treatment has been effective? The presence of microzoa (such as rotifers and nematodes) indicates that the microbial community is thriving and that the ecosystem of the activated sludge is relatively stable; at this point, the biochemical treatment efficiency is at its best. It’s similar to a river where large fish can often be caught, allowing small fish and shrimp to thrive as well. 26. What is Mixed Liquor Suspended Solids (MLSS)? Mixed liquor suspended solids (MLSS), also known as sludge concentration, refers to the weight of dry sludge per unit volume of the mixed liquor in the biochemical tank; it is expressed in milligrams per liter and is used to indicate the concentration of activated sludge. It consists of organic and inorganic substances. Generally, it is appropriate to keep the MLSS level in the SBR biological tank at around 2000–4000 mg/L. 27. What are Mixed Liquor Volatile Suspended Solids (MLVSS)? Mixed liquor volatile suspended solids (MLVSS) refer to the weight of volatile substances in the dry sludge contained per unit volume of the biochemical tank mixture, also expressed in milligrams per liter. Since it does not include the inorganic substances in the activated sludge, it can more accurately represent the number of microorganisms in the activated sludge. 28. Sludge settling ratio (SV)? The sludge settling ratio (SV) refers to the volume percentage (% of sedimented sludge to the mixed liquid) after the mixture in the aeration tank has been allowed to settle statically in a 100-milliliter measuring cylinder for 30 minutes; therefore, it is sometimes also denoted as SV30. Generally, the SV in the biochemical tank ranges between 20-40%. Determining the sludge settling ratio is relatively simple; it is one of the important indicators for evaluating activated sludge. It is often used to control the discharge of excess sludge and to address abnormal conditions such as sludge bulking in a timely manner. Obviously, SV is also related to the sludge concentration. 29. Sludge Volume Index (SVI)? SVI stands for Sludge Volume Index; it represents the volume in milliliters that 1 gram of dry sludge occupies in its wet state. The formula for calculating SVI is as follows: SVI = SV * 10 / MLSS. By eliminating the influence of sludge concentration, SVI provides a better indication of the coagulation and sedimentation properties of activated sludge. Generally, it is considered that when 60 < SVI < 100, the sludge has good sedimentation properties; when 100 < SVI < 200, the sedimentation properties are average; when 200 < SVI < 300, the sludge shows a tendency to expand; and when SVI > 300, the sludge has already expanded. What does DO stand for? DO stands for Dissolved Oxygen, which refers to the amount of oxygen dissolved in water, with the unit being mg/L. Different biochemical treatment methods have varying requirements for dissolved oxygen. During anaerobic-biological processes, the dissolved oxygen level in water is generally between 0.2–2.0 mg/L, whereas in SBR aerobic biological processes, it is usually between 2.0–8.0 mg/L. Therefore, during the operation of the aerotank, the aeration volume should be low and the aeration time should be short ； During the operation of the SBR aerobic tank, the aeration volume and aeration time are much greater and longer; whereas we use contact oxidation, with the dissolved oxygen level controlled at 2.0–4.0 mg/L. 31. What factors are related to the dissolved oxygen content in wastewater? The concentration of dissolved oxygen in water can be expressed by Henry’s law: when dissolution equilibrium is reached, C = KH*P, where C represents the solubility of oxygen in water at that equilibrium state ； P is the partial pressure of oxygen in the gas phase ； KH is the Henry coefficient, which is dependent on temperature ； Increased aeration efforts are made to achieve oxygen dissolution equilibrium, while activated sludge also consumes oxygen in the water. Therefore, the actual dissolved oxygen level in wastewater is related to factors such as water temperature, effective water depth (which affects pressure), aeration volume, sludge concentration, and salinity. 32. Why is it necessary to regularly replenish nutrients in wastewater during biochemical processes? The method of removing pollutants using biochemical processes relies primarily on the metabolic activities of microorganisms; moreover, life processes such as cellular synthesis in microorganisms require an adequate amount and variety of nutrients, including trace elements. For chemical industry wastewater, due to the homogeneity of the products produced, the composition of the wastewater is also relatively simple, lacking the necessary nutrients for microorganisms. For example, the production wastewater from *** Company contains only carbon and nitrogen, with no phosphorus; such wastewater cannot meet the metabolic needs of microorganisms. Therefore, phosphorus must be added to the wastewater in order to facilitate the metabolic processes of these microorganisms and promote the synthesis of their cells. It’s like how people need to consume enough vitamins while eating rice and flour. 33. What is the ratio of various nutrient elements required by microorganisms in wastewater? Like animals and plants, microorganisms also require essential nutrients in order to grow and reproduce. The main nutrients needed by microorganisms are carbon (C), nitrogen (N), and phosphorus (P). There are specific requirements regarding the proportion of these nutrient elements in wastewater; for aerobic biological treatment, the ratio is generally C:N:P = 100:5:1 (by weight). 34. Why is excess sludge generated? During the biochemical treatment process, the microorganisms in the activated sludge continuously consume the organic substances in the wastewater. Among the organic matter that is consumed, part of it is oxidized to provide the energy required for the metabolic activities of microorganisms, while another portion is used by these microorganisms to synthesize new cytoplasm, thereby enabling them to reproduce. As microorganisms carry out their metabolism, some of them die, which results in the formation of excess sludge. 35. What is water eutrophication? Water eutrophication is a natural phenomenon that occurs in freshwater, where an excessive amount of nitrogen, phosphorus, and potassium in the water leads to a sudden overgrowth of algae. The main cause of water eutrophication is the release of elements such as nitrogen, phosphorus, and potassium into surface waters with slow flow rates and long renewal cycles. This leads to excessive growth and reproduction of aquatic organisms like algae, resulting in a rate of organic matter production that far exceeds its rate of consumption. As a result, organic matter accumulates in the water, disrupting the aquatic ecological balance. ——Original: Eco-Friendly Water Circle & Ruijietech