Basic Q&A on Wastewater Treatment Technologies

Thread Content

This post was last edited by zhaolijun on 2016-5-10 08:37. 1. What measures should be taken when the biochemical treatment tank is subjected to a load shock and the microorganisms are damaged during the biochemical treatment of wastewater? 　　During the operation of the biochemical tank, once the microorganisms are subjected to stress due to changes in water volume or concentration, the COD removal efficiency drops sharply; in severe cases, sludge detaches from the biological fillers, causing the effluent to become turbid. At this point, water inflow should be immediately halted, and powdered activated carbon should be added to the biochemical tank to reduce the sludge load. The recommended dosage is 10 kilograms of powdered activated carbon per 100 m³ of the biochemical tank’s volume. Once the sedimentation properties of the sludge have improved, the rapid proliferation method for sludge acclimatization can be employed: domestic wastewater can be added to the biochemical tank, or waste alcohol or wet paste made from cooked dry flour can be used. The addition rate is 5–10 kilograms of dry flour per 100 m3 of the biochemical tank’s volume. Water is introduced after 2–3 days, with the amount of water added increasing day by day until the microorganisms return to normal activity. 　　2. How should the biological treatment tank operate when there is no production wastewater due to holidays or temporary shutdowns? 　　At *** Company, situations where no industrial wastewater is generated due to holidays or temporary shutdowns may occur frequently. In such cases, we can add domestic sewage to the biochemical tank, or pump in river water along with a paste made from cooked dry flour, in order to sustain the growth and reproduction of microorganisms. In the biochemical tank, 5-10 kilograms of dry flour can be added per 100 cubic meters of volume, or waste alcohol can be added in proportion; aeration should take place for 4-8 hours per day. 　　 　　4. How does the biochemical tank function in winter? As we know, the optimal temperature range for microorganisms to grow and reproduce is 16–30°C. When the temperature drops below 10°C, the efficiency of wastewater purification significantly decreases. Generally, for every 10°C drop in temperature, the removal rate of COD decreases by 10%. So, how do the biochemical tanks function in winter? One method is to introduce steam into the regulating tank to raise the temperature of the biochemical feedwater ； Another approach is to add biological sludge to the biochemical tank in order to increase the sludge concentration and reduce the sludge load; as long as the water temperature can be maintained at 6–7°C, the activated sludge can still perform its purification functions effectively. 　　5. How is the sludge in the sludge tank dewatered? 　　The main methods for sludge dewatering include vacuum filtration, pressure filtration, centrifugation, and natural drying. Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. uses the filtration by pressure method; specialized equipment, namely plate and frame filter presses, is employed to carry out pressure filtration of the chemical sludge and excess sludge generated by the system. After dehydration, the moisture content of the sludge typically ranges from 80% to 85%. 　　6. How to discharge the excess sludge from the SBR biochemical tank into the sludge tank? 　　The excess sludge in the SBR biological tank should be regularly discharged into the sludge tank; otherwise, it will affect the proper operation of the SBR biological tank and the quality of the wastewater treated by this system. During sludge discharge, first open the pipe valve between the SBR biochemical tank and the sludge tank; the pressure from the water level in the SBR tank is then used to push the remaining sludge into the sludge tank. After sludge discharge is complete, the valve in the sludge pipeline between the SBR tank and the sludge tank should be closed. 　　7. How much potassium dihydrogen phosphate should be added to the biochemical tank? 　　Converted at a ratio of 100:1 for carbon to phosphorus (by weight); strictly speaking, the carbon referred to here is BOD5. Therefore, if the inflow to the biological treatment tank is 240 tons per day and the BOD5 concentration is 250 mg/L, the daily amount of BOD5 in the influent water will be 240×0.25 kg/ton = 60 kg. The daily phosphorus requirement is 60÷100 = 0.6 kg. The amount of potassium dihydrogen phosphate that needs to be added is calculated as follows: 0.6×136÷31 = 2.6 kg/day. 　　For ease of calculation, we can use the following simplified formula. 　　W = BOD5 × Q × 0.044 ÷ 1000 　　W = COD × B/C × Q × 0.044 ÷ 1000 　　Where: 　　COD refers to the COD value in the biochemical influent, with the unit being mg/L ； 　　BOD5—refers to the BOD5 in the biochemical influent, with the unit being mg/L ； 　　B/C—is dimensionless ； 　　Q—Is the amount of biochemical wastewater input, in tons per day ； 　　W—is the daily dosage of potassium dihydrogen phosphate, expressed in kilograms per day ； 　　8. What should be done when a large number of microorganisms die? 　　When microorganisms are severely damaged, die in large numbers, and rescue efforts prove ineffective, it is necessary to report the situation to the local environmental protection authorities immediately and replace the activated sludge right away. Then identify the cause to prevent similar accidents from occurring again. 　　9. How much urea should be added to the biochemical tank each day? 　　The appropriate nutritional ratio is: carbon:nitrogen:phosphorus = 100:5:1. Calculated based on a carbon-to-nitrogen ratio of 100:5 (on a weight basis); strictly speaking, the carbon referred to here is BOD5. Therefore, if the inflow to the biological treatment tank is 240 tons per day and the BOD5 concentration is 250 mg/L, the daily amount of BOD5 in the influent water will be 240 tons × 0.25 kg/ton = 60 kg. The daily nitrogen requirement is 60 ÷ 100 × 5 = 3 kg. The amount of urea that needs to be added is: 3 × 44 ÷ 14 = 9.4 kg/day. 　　For ease of calculation, we can use the following simplified formula. 　　W = BOD5 × Q × 0.157 ÷ 1000  W = COD × B/C × Q × 0.157 ÷ 1000 Where:  COD is the COD value in the biochemical influent, expressed in mg/L ； 　　BOD5—refers to the BOD5 in the biochemical influent, with the unit being mg/L ； 　　B/C—is dimensionless ； 　　Q—Is the amount of biochemical wastewater input, in tons per day ； 　　W—is the daily dosage of urea, in kilograms per day ； 　　Since there is already a certain amount of nitrogen present in the wastewater from *** Company, there is no need to add urea during operations. 　　10. Why should the COD concentration of wastewater in the regulating tank be kept below 700 mg/L? 　　The wastewater from the regulating tank serves as the biochemical feedwater, and its COD concentration is generally determined based on experimental values and design parameters. For wastewater that is easy to treat biologically, the COD level of the wastewater in the regulation tank can generally be kept at around 1000 mg/L. In the case of industrial wastewater, especially that which is difficult to biodegrade, the COD level of the wastewater fed into the biological treatment process is usually maintained within the range of 500–800 mg/L. Otherwise, it is difficult to ensure the stable operation of the biochemical system, as well as to guarantee that the wastewater treated by this system meets the specified discharge standards. The COD concentration of the biochemical wastewater from Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. (700 mg/L) was determined based on experimental values. 　　11. How to cultivate and acclimate sludge? 　　The cycle time for biochemical bacterial cultivation depends on the water temperature and quality of the wastewater. When the water temperature is above 15°C, the process of culturing bacteria proceeds more rapidly; whereas when the temperature is below 15°C, the time required for sludge acclimatization is longer. Therefore, sludge cultivation and acclimatization should be carried out preferably between May and November (in the Yangtze River basin). In terms of wastewater quality, for wastewater that is non-toxic, harmless, and easily biodegradable, the biochemical treatment time is generally 10–20 days. On the other hand, toxic, harmful, and difficult-to-biodegrade wastewater requires a longer processing time, of around 30–60 days or even more. 　　After the water flushing and commissioning are completed, for wastewater with good biodegradability, microorganisms can be acclimatized directly using the wastewater ； For chemical industrial wastewater or wastewater with poor biodegradability, a stepwise bacterial inoculation method should be adopted. The specific steps are as follows: (1) Rapid proliferation. The purpose of rapid proliferation is to enable the sludge to quickly grow on the filler. Generally, during the dewatering or transportation of the purchased sludge, the microorganisms suffer varying degrees of damage. They need a process to recover and grow in the new environment, and therefore require a favorable living condition. If acclimatization is carried out directly using chemical wastewater at this time, it will inevitably lead to the massive death of microorganisms. Therefore, in the first phase, domestic wastewater, glucose, or cooked paste made from dry flour can be used to cultivate microorganisms (during the initial 3–5 days, 5–10 kilograms of dry flour can be added per 100 m3 of bioreactor volume). Aeration is carried out twice a day, for 8 hours each time in the aerobic tank, to enable rapid recovery, growth, and reproduction of the microorganisms; this method is known as the rapid proliferation method. During the rapid proliferation period, the wastewater in the biochemical tank can be discharged through the sludge acclimation pipe. Before draining, aeration should be stopped; after allowing the sludge to settle for 4–8 hours, the water can then be drained. The rapid proliferation period is generally 7-10 days. 　　During the operation of the biochemical tank, when microorganisms are subjected to a load shock and the COD removal rate or SV drops suddenly, the rapid proliferation method can also be employed to help the microorganisms recover and grow. 　　(3) Wastewater acclimatization. After the sludge has grown on the filler, the amount of dry flour added daily to the 100 m³ biochemical tank can be increased to 20–30 kg. At the same time, biochemical influent or wastewater is pumped into the tank. The initial flow rate of wastewater can be pumped in at a rate of 1-2% of the volume of each 100 m3 of the biological treatment tank. Thereafter, the amount of wastewater pumped in should be increased gradually by 2% every two days, until the designed wastewater flow rate is reached. As the amount of wastewater pumped in gradually increases, the amount of glucose or dry flour added, as well as the amount of domestic sewage pumped in, should be reduced accordingly until addition is stopped; alternatively, waste alcohol can be added in proportion (1 kilogram of waste alcohol corresponds to 1.5 kilograms of COD). 　　During the culture and acclimation period, COD must be measured daily; if a sudden drop in the COD removal rate or SV is observed, the amount of wastewater fed should be stopped immediately until the COD removal rate rises above 50% and the SV stops declining. 　　When wastewater is properly fed into the aerobic tank, the COD removal rate can be maintained above 80%, and if the COD concentration of the treated effluent is below 200 mg/L, it can be considered that the biological treatment tank is functioning properly. 　　During the sludge acclimation period, it is crucial to avoid any sudden increases in load (such as high water volume or high concentration). Once the microbial cultivation is complete, normal operation can be carried out. 　　12. How to add sludge to the biochemical tank? How to apply a film? 　　If the dry sludge culture method is used, first fill the aeration tank with clean water or river water and aerate it, while slowly adding the prepared dry sludge into the aeration tank. After all the components have been added, aeration continues for 2–4 hours. Once aeration is complete, the mixture is left to stand for 2 hours before the supernatant is removed. This process can be repeated 2–3 times until the supernatant becomes clear and transparent, without any turbidity. This process is known as sludge washing, sludge activation, or sludge biofilm formation. After the sludge is activated, acclimation is initiated using nutrient-rich water or low-concentration wastewater. 　　13. How much sludge should be added to the biochemical tank for the first time? 　　If the dry sludge cultivation method is used, we must maintain a sludge concentration in the biological tank of around 3 g/L, or 3 Kg/m3. Since the moisture content of dry sludge is 80%, at least 15 Kg/m3 of dry sludge must be added to the aeration tank; in other words, for a tank with a volume of 100 m3, about 1.5 tons of dry sludge should be added. 　　14. What type of activated sludge should be added to the biochemical tank? 　　The so-called cultivation of activated sludge involves providing certain growth conditions for the microorganisms that constitute activated sludge. Under these conditions, over time, activated sludge is formed, its quantity gradually increases, and it eventually reaches the concentration required for wastewater treatment. 　　The microbial cultivation process for domestic wastewater treatment plants is relatively simple, whereas it is more challenging to cultivate microorganisms in toxic and hazardous industrial wastewater; the time required for sludge acclimatization is also longer. Generally, for industrial wastewater, the dry sludge cultivation method is used – that is, dried sludge obtained from a properly operating wastewater treatment plant (with a moisture content of around 80%), without the addition of any chemicals during dehydration, is used as the source of microorganisms for cultivation. To enable the microorganisms to adapt as quickly as possible to toxic and harmful industrial wastewater, it is best to use dry sludge of the same type, or from the same type of wastewater treatment plant after dewatering, as a source of microorganisms. 　　15. How is sludge discharged from a neutralization sedimentation tank? 　　After the wastewater in the neutralization sedimentation tank has been completely coagulated and settled by adding lime, the sludge and water have clearly separated, with the chemical sludge settling at the bottom of the reaction tank. When discharging sludge, first open the sludge pipeline valves at the bottom of the sedimentation tank as well as those in the sludge tank itself; use the pressure from the water level to push the sludge out of the reaction tank and into the sludge tank. After the sludge has been discharged, close the sludge pipeline valves in both tanks. Then open the sewage valve to pour the clear liquid into the equalization tank. 　　The neutralization sedimentation tank is equipped with a decanter. Its structure consists of a rubber ring beneath which a hose is attached; the other end of the hose is connected to the sewage outlet pipe at the bottom of the tank. It works by having a rubber ring float on the water surface, rising and falling with it. Since the separation of sludge and water always begins at the water surface, as soon as clear liquid forms on the surface, it flows out of the tank through a hose. Therefore, drainage occurs simultaneously with the separation process; there is no need to wait until the sludge and water are completely separated before discharging them. This saves operational time. However, during operation, when mixing the concrete, the decanter must be lifted out of the water to prevent mud from entering the hose. 　　16. Why must the pH of the effluent from the neutralization sedimentation tank be adjusted to above 9? 　　The effluent from the iron-carbon process contains large amounts of ferrous sulfate; if this is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biological treatment tanks. Therefore, we must use lime to raise the pH value of the wastewater from 5-6 to above 9, thereby converting soluble ferrous sulfate into insoluble ferric hydroxide and calcium sulfate. These substances are then precipitated through coagulation and sedimentation, ensuring that the wastewater entering the biological treatment tanks does not contain ferrous sulfate. 　　Whether ferrous hydroxide precipitate can form depends mainly on the pH value of the wastewater. When the pH of the wastewater reaches 6.5, some ferrous hydroxide begins to precipitate; however, for complete precipitation of ferrous hydroxide in the wastewater, its pH must reach 9.7. Therefore, it is essential to adjust the pH of the wastewater to above 9 during neutralization, so as to keep the ferrous ions in the wastewater entering the biological treatment tank at a very low level. 　　17. How to prepare dilute sulfuric acid? 　　First, add clean water to the waste acid preparation tank, then slowly pour in 98% concentrated sulfuric acid until a dilute sulfuric acid solution with a concentration of 50%-60% is obtained. 　　When preparing dilute sulfuric acid, the following three points should be kept in mind: Both 98% concentrated sulfuric acid and the dilute acid that is prepared afterward are highly corrosive. 98% sulfuric acid is also highly absorbent and can cause burns to the skin. Therefore, protective equipment must be worn at all times during operation. 　　Concentrated sulfuric acid generates a large amount of heat during dilution; therefore, it is absolutely forbidden to pour water into concentrated sulfuric acid – instead, the concentrated sulfuric acid must be poured into water. When carrying out this operation, it is necessary to add the concentrated sulfuric acid to the water slowly and gradually. 　　Since the dilution of concentrated sulfuric acid is a highly exothermic process, plastic components in the mixing tank (such as water pumps and pipes) should be removed in advance to prevent them from deforming due to heat and getting damaged. 　　18. What chemical agents are required for wastewater treatment? 　　***List of chemical materials used at the treatment station (for reference)
Name of material, Specification, Required amount, Market price
98% sulfuric acid, industrial grade, 20 Kg/day, 0.5 yuan/kg
Calcium oxide, CaO >93%, packaged in 25 Kg units, 12.5 Kg/day, 0.75 yuan/kg
Potassium dihydrogen phosphate, 2-3 Kg/day, 5 yuan/kg
Granular activated carbon, 17# type, particle size Φ3-4, length = 4-8 mm, 5 tons per 1-2 years, 2500 yuan/kg
Powdered activated carbon, type 670, packaged in 20 Kg units, 100 Kg per package, 6000 yuan/kg
Cast iron scrap, 25 tons per 1-2 years, 1100 yuan/kg
Biochemical sludge, with 81% moisture content, 10 tons (one-time use), 150 yuan/kg
19. What are the main technical parameters of the sludge dewatering system? 　　Sludge treatment capacity: 8 tons/day. Sludge concentration before dewatering: 3%. Sludge concentration after dewatering: 20%. 20. What are the main technical parameters for the biochemical treatment process? 　　(1) Regulating tank
Maximum water storage capacity: 120 tons
pH: 6–8
COD range: 700 mg/L
BOD5 range: 250 mg/L

(2) Biological contact oxidation tank
Operation mode: Continuous flow operation
Maximum treatment capacity: 120 tons/day
Hydraulic retention time: 20 hours
Aeration time: ≥16 hours
COD in effluent: ≤300 mg/L

(3) SBR biological tank
Operation mode: Batch operation
Maximum treatment capacity: 120 tons/day
Hydraulic retention time: 20 hours
Inflow time: 6 hours
Aeration time: 4 hours
Outflow time: 2 hours
COD in effluent: ≤100 mg/L
BOD5 in effluent: ≤30 mg/L

21. What are the main technical parameters for the pretreatment process? 　(1) Iron-carbon electrolytic cell Process wastewater volume: 20 tons/day Inlet pH: 2–3 Outlet pH: 5–6 Reaction time: >8 hours (2) Neutralization and sedimentation tank Process wastewater volume: 20 tons/day Inlet pH: 5–6 Outlet pH: >9 Lime powder dosage: 0.8 kilograms per ton of wastewater Neutralization time: 2 hours 22. How many stages are there in the entire wastewater treatment process? 　　The entire wastewater treatment process is divided into two stages: the pretreatment stage (iron-carbon microelectrolysis–neutralization coagulation) and the biological treatment stage (biological contact oxidation tank–SBR biological treatment). 　　23. What is the biochar method (PACT method)? 　　For certain pharmaceutical wastewater that is difficult to biodegrade, it is quite challenging to keep the COD level in the effluent from biological treatment below the **first-class discharge standard (100 mg/L)**. Therefore, it is essential to employ granular activated carbon adsorption technology on the effluent from biological treatment to ensure compliance with the discharge standards. However, the granular activated carbon adsorption treatment method has a fatal weakness: its high treatment cost. The fundamental reason for this is that the dynamic adsorption capacity of granular activated carbon for COD is around 10% (on a weight basis); in other words, one ton of activated carbon can only absorb approximately 100 kilograms of COD from wastewater. Due to the difficulty in regenerating granular activated carbon and the high treatment costs, the application of granular activated carbon treatment technology is not yet widespread in China. So, is it possible to develop a new technology that can significantly increase the dynamic adsorption capacity of activated carbon, thereby effectively reducing the costs associated with wastewater treatment? 　The Powdered Activated Carbon Treatment Process, first developed by DuPont, is one of the representatives of this new technology. The biochar method, also known as the “PACT method” or “PACSBR biochemical method”, is considered by foreign experts to be the most promising new biochemical treatment process for wastewater. Powdered activated carbon is added to the biochemical wastewater (or within the aeration tank), where it mixes with the recycled carbon-containing sludge; the excess sludge discharged from the sludge concentration tank is then sent to a sludge dewatering device. In the aeration tank, activated sludge adheres to the surface of powdered activated carbon. Due to the large specific surface area and strong adsorption capacity of powdered activated carbon, the adsorption capacity of the sludge is enhanced; in particular, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and powdered activated carbon increase significantly, which in turn improves the rate of COD degradation and removal. Generally, in PACT systems, the dynamic adsorption capacity of activated carbon for removing COD is between 100–350% (on a weight percentage basis); that is, one kilogram of powdered activated carbon can absorb and remove 1.0–3.5 kilograms of COD. Moreover, the PACT method can handle toxic and harmful organic pollutants that are difficult for organisms to degrade. 　　Based on our experience in engineering commissioning, regularly adding powdered activated carbon in fixed amounts directly into the SBR aerobic biological tank every 15–30 days can achieve excellent treatment results. In fact, the adsorption mechanism for both powdered activated carbon and granular activated carbon is the same; however, using powdered activated carbon in SBR biological tanks offers the following advantages: it reduces investment costs ； 　　Flexible and easy to operate ； 　　High utilization rate of activated carbon ； 　　It overcomes the drawback of granular activated carbon, where biofilms tend to form, leading to blockages and affecting the effluent flow rate. In the powder activated carbon–activated sludge system, the activated sludge adheres to the surface of the powder activated carbon. Thanks to the large specific surface area of the powder activated carbon and its strong adsorption capacity, the concentrations of dissolved oxygen and degradation substrates at the interface between the activated sludge and the powder activated carbon increase significantly, thereby improving the efficiency of COD degradation and removal. Generally, COD removal (depending on the type of wastewater) can be increased by 10-40% ； 　　Since the toxic and harmful organic substances in the wastewater are adsorbed by powdered activated carbon, the concentration of such substances in the wastewater can be maintained at a low level, thereby ensuring the proper operation of the biochemical treatment system ； 　　It is highly effective in preventing the rebound of ammonia nitrogen levels and ensuring that the ammonia nitrogen levels in the effluent meet the specified standards. 　　We once used the PAC-SBR method to treat the wastewater generated by *** factory, and the results showed that this method achieved a fairly significant treatment effect, with the effluent from biological treatment meeting the **first-class discharge standards. 　　For *** company’s wastewater treatment system, if the effluent from the SBR biological treatment process does not meet the discharge standards, it is possible to add a small amount of powdered activated carbon to the SBR tank in order to improve the efficiency of biological treatment and ensure that the resulting effluent meets the specified discharge standards. 24. How can the amount of excess sludge generated be estimated? 　　During the metabolism of microorganisms, some organic substances (BOD) are utilized by them to synthesize new cytoplasm in order to replace the dead microorganisms. Therefore, the amount of excess sludge generated is related to the amount of BOD that has been decomposed; there is a connection between the two. 　　In engineering design, it is generally assumed that for every kilogram of BOD5 treated, 0.6–0.8 kilograms of excess sludge are generated (100%); when converted to dry sludge with an moisture content of 80%, this amounts to 3–4 kilograms. 　　25. 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. 　　26. What is the ratio of various nutrient elements required by microorganisms in wastewater? 　Like animals and plants, microorganisms also require necessary 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 nutrients in wastewater; for aerobic biological treatment, the ratio is generally C:N = 100:5:1 (on a weight basis). 　　27. 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 wastewater, due to the simplicity of the products manufactured, 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 microbial metabolism and promote the synthesis of microbial cells. It’s like how people need to consume enough vitamins while eating rice and flour. 　　28. Who provides the oxygen that microorganisms need in biochemical processes? 　　The oxygen required by microorganisms in biochemical processes is mainly supplied by rotary blowers. 　　29. 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 used 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. 　　30. What does dissolved oxygen (DO) represent? Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water, and it is expressed in mg/L. Different biochemical treatment methods have varying requirements for dissolved oxygen. During anaerobic-biological treatment, the dissolved oxygen level in water is generally between 0.2–2.0 mg/L, whereas during SBR aerobic biological treatment, it is usually between 2.0–8.0 mg/L. Therefore, during the operation of the aerated tank, 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. 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 it 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 is already expanded. 32. What is the Sludge Settling Ratio (SV)? 　　The sludge settling ratio (SV) refers to the volume percentage (%) of settled sludge to the mixed liquid in a 100-milliliter measuring cylinder, after the mixture has been allowed to settle at rest for 30 minutes; therefore, it is sometimes denoted as SV30 as well. 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. 　　33. 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, with the unit also being 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. 　　34. 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 maintain the MLSS level in the SBR biological tank at around 2000–4000 mg/L. 　　35. 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. 　36. How to evaluate the activated sludge in the activated sludge process and the biofilm process? 　The identification and evaluation of activated sludge growth in the activated sludge process and the biofilm process are different. 　　In the biofilm process, the evaluation of activated sludge growth is primarily based on 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). 　　37. 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 needed for their activities and to build themselves. 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 sediment particles with the ability to degrade organic matter are called 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. 　　38. 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 configurations for biochemical treatment; their main visual difference lies in the fact that in the former, the microorganisms do not require a filler medium, as the biological sludge remains suspended, whereas in the latter, the microorganisms are attached to the fillers. 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 shows certain similarities. Furthermore, in the biofilm process, the microorganisms being fixed on the packing materials can form a relatively stable ecosystem. Their energy requirements for survival and metabolism 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 of Shanghai Xinyi Bailuda Pharmaceutical Co., Ltd. uses the biofilm process, while the SBR biochemical tank employs the activated sludge process. 　　39. What are the applications of biological treatment in wastewater treatment projects? 　　There are two main categories of biological treatment techniques that are most widely used and practical in wastewater treatment projects: one is called the activated sludge process, and the other is called 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 present 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. 　　40. 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. 　　Aerobic microorganisms can adapt to wastewater with high COD concentrations; the inlet COD concentration can reach over 2000 mg/L, with a COD removal rate generally ranging from 50% to 80% ； Aerobic microorganisms can only adapt to wastewater with low COD concentrations; the COD concentration in the incoming water is generally kept below 1000–1500 mg/L, and 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 drawback of anaerobic biochemical treatment is that the treatment time is very long; typically, the retention time of wastewater in the anaerobic bioreactor needs to be over 40 hours. 　　41. Why do high-concentration saline wastewater have a particularly significant impact on microorganisms? 　　Let’s first describe an experiment on osmotic pressure: Two salt solutions of different concentrations are separated by a semipermeable membrane. Water molecules from the solution with the lower concentration will pass through the semipermeable membrane into the solution with the higher concentration, while water molecules from the solution with the higher concentration will also pass through the membrane into the solution with the lower concentration, but in smaller quantities. As a result, the liquid level on the side of the solution with the higher concentration rises. Osmosis stops when the pressure generated by the difference in liquid levels between the two sides becomes sufficient to prevent further water flow; this pressure is what is known as osmotic pressure. Generally, the higher the salt concentration, the greater the osmotic pressure. 　　The situation of microorganisms in saline solutions is similar to the osmotic pressure experiment. 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 considering 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. In daily life, people use table salt (sodium chloride) to pickle vegetables and fish in order to sterilize and preserve food, applying this very principle. 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 8600 mg/L, it causes the sludge volume to expand, large amounts of foam to form on the surface, and the microorganisms die one after another. 　　However, through long-term domestication, microorganisms gradually adapt to growing and reproducing in high-concentration saltwater. People have already domesticated microorganisms capable of adapting to chloride or sulfate concentrations of over 10,000 mg/L. However, the principles of osmotic pressure tell us that microorganisms adapted to growing and reproducing in high-concentration saltwater have cell fluids with a very high salt concentration. Once the salt concentration in the wastewater is low or very low, water molecules from the wastewater will flood into the microorganisms, causing their cells to swell; in severe cases, this leads to cell rupture and death. Therefore, microorganisms that have undergone long-term domestication and can gradually adapt to growing and reproducing in high-concentration saline solutions require a salt concentration in the biochemical feedwater to remain at a fairly high level; it cannot fluctuate greatly, otherwise a large number of these microorganisms will die. 　　42. What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms? 　　Oxygen dissolved in water is called dissolved oxygen. The organisms in water bodies and aerobic microorganisms rely on dissolved oxygen to survive. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require an adequate supply of dissolved oxygen; generally, it is advisable to maintain the dissolved oxygen level at 3 mg/L, with a minimum level of not less than 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. 　　43. What should be the optimal pH range for microorganisms? 　　The life activities and metabolic processes of microorganisms are closely related to pH value. Most microorganisms can adapt to 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 strict requirements regarding pH, which should be between 6.7 and 7.4. 　　44. Within what temperature range do microorganisms grow and reproduce most efficiently? 　　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 increases accordingly, and the COD removal rate also rises by about 10% ； Conversely, for every 10°C decrease in temperature, the COD removal rate drops by 10%. Therefore, during winter, the biochemical removal rate of COD is significantly lower than in other seasons. 　　45. 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 the environmental conditions are abnormal, it will affect the metabolic activities of microorganisms, and they may even mutate or die. 　　46. How do microorganisms break down and remove organic pollutants from wastewater? 　　Due to the presence of organic substances such as carbohydrates, fats, and proteins in wastewater, 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, and other substances (catabolic metabolites). In this process, the organic pollutants in the wastewater are removed through degradation by microorganisms. 　　47. What is the biochemical treatment of wastewater? 　　The biochemical treatment of wastewater is one of the most important processes in wastewater treatment systems, commonly referred to as biochemical treatment. Biochemical treatment utilizes the life activities of microorganisms to effectively remove soluble organic substances and some insoluble organic substances from wastewater, thereby purifying the water. In fact, we are not unfamiliar with biochemical treatment. In natural water bodies, there exists a food chain: larger fish eat smaller fish, smaller fish eat shrimp, shrimp eat small insects, small insects eat microorganisms, and microorganisms consume sewage. Without this food chain, nature would fall into chaos. In natural rivers, there are large numbers of microorganisms that rely on organic matter for survival. These microorganisms oxidize or reduce the organic substances that people discharge into the rivers day and night – such as industrial wastewater, pesticides, fertilizers, feces, and other organic materials – converting them into inorganic substances. Without these microorganisms, the rivers around us would become polluted and smelly within a few months at the earliest, or even one to two years at the latest. It’s just that these microorganisms are so tiny and dispersed that they cannot be seen with the naked eye. The biochemical treatment of wastewater is an enhancement of this process under artificial conditions. People concentrate countless microorganisms in one tank, creating an environment highly suitable for their reproduction and growth (such as temperature, pH level, oxygen, and nutrients like nitrogen and phosphorus), thereby allowing the microorganisms to multiply in large numbers and increasing the speed and efficiency with which they can break down organic matter. Then, wastewater is pumped into the tank, allowing the organic substances in the wastewater to be oxidized and degraded during the life activities of microorganisms, thereby purifying and treating the wastewater. Compared with other treatment methods, the biochemical method features low energy consumption, no need for chemicals, good treatment efficiency, and low treatment costs. 　　48. How to estimate the generation amount of chemical sludge? 　　Sludge generated through chemical reactions (such as neutralization) and physicochemical treatments (such as coagulation with chemicals) is commonly referred to as chemical sludge. The sludge formed after neutralization and coagulation treatment of the iron-carbon effluent is mainly composed of ferrous hydroxide and calcium sulfate. The amount of sludge generated can be calculated based on the amounts of sulfuric acid and lime powder added. In engineering, estimates can also be made using experience. Generally speaking, if the pH of the iron-carbon treated influent is around 2, the amount of chemical sludge produced per ton of wastewater after neutralization and coagulation (with a moisture content of 80%) is approximately 50 kg. 　　49. Why is lime powder used for neutralization treatment of the water coming out of the iron-carbon process? 　　After the wastewater, whose pH has been adjusted to 2 using sulfuric acid, is treated with iron and carbon, sulfuric acid is converted into ferrous sulfate, and the pH of the wastewater rises from 2 to 5–6. Then why is lime powder still used for neutralizing the effluent resulting from this iron-carbon treatment? Or could we use less lime powder during the neutralization process? 　　The effluent from the iron-carbon process contains large amounts of ferrous sulfate; if this is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical tanks. Therefore, we must use lime to raise the pH value of the wastewater from 5-6 to above 9, thereby converting the water-soluble ferrous sulfate into insoluble ferric hydroxide and calcium sulfate. These substances are then precipitated through coagulation and sedimentation, ensuring that the wastewater entering the biochemical tanks does not contain ferrous sulfate. 　　Can less lime powder be used during the neutralization process? We can conduct a comparative experiment in the laboratory. An equal amount of iron-carbon influent water (with a pH of around 2) and iron-carbon effluent water (with a pH of 5–6) was placed in two beakers respectively, after which lime powder was added in measured amounts for neutralization and coagulation. When the pH value of the wastewater in both beakers was adjusted to 9, it was found that the same amount of lime powder had been added to each beaker. This is because iron is not a neutralizing agent; ferrous sulfate, which is formed from sulfuric acid, remains an acidic substance. Lime powder is essential for the neutralization process, as it is used to convert ferrous sulfate into ferrous oxide and calcium sulfate. Therefore, when neutralizing iron-carbon effluent, lime powder must not be underdosed. 　　50. What is adsorption? 　Adsorption treatment involves using porous solids (such as activated carbon) or floc materials (such as polyferric compounds) to adsorb toxic and harmful substances in wastewater onto the surface or within the micropores of these solids or flocs, thereby purifying the water quality. The substances to be adsorbed can be either insoluble solid substances or soluble substances. Adsorption treatment is highly efficient and produces high-quality effluent, which is why 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). 　　51. 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. 　　52. Why is polyferric chloride used for flocculation and adsorption pre-treatment of wastewater? 　　Ferric polymers form ferric hydroxide flocs during the coagulation process, and these flocs possess excellent capacity to adsorb organic substances in wastewater. Experimental data show that after wastewater is treated using ferric polymer flocs for adsorption, approximately 10%-20% of the COD in the wastewater can be removed. This helps to reduce the operational load on biological treatment tanks, thereby facilitating the proper treatment of wastewater before it is discharged. Furthermore, coagulation pretreatment using polyferric salts can remove trace substances in wastewater that are toxic and inhibitory to microorganisms, thereby ensuring the proper functioning of the microorganisms in the biochemical tank. Among various coagulants, polyferric sulfate is relatively inexpensive (25–300 yuan per ton), resulting in low treatment costs; it is therefore suitable for the pretreatment of process wastewater. 　　Polyferric acid is an acidic substance with strong corrosivity; therefore, the treatment equipment must be properly protected against corrosion. 　　53. What is flocculation? 　　Flocculation involves adding polymer coagulants to wastewater; once these polymers are dissolved, they form polymer complexes. 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, causing the particles to grow larger over time and eventually forming large particle flocs (commonly known as alum flowers), which accelerates the settling of the particles. Common flocculants include polyacrylamide (PAM), polyferric compounds (PE), etc. 　54. What is cohesion? 　　When coagulants carrying positive ions are added to wastewater, a large number of these positive ions exist between the colloidal particles, thereby eliminating the electrostatic repulsion between them and allowing 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. 　　55. How can colloidal particles be precipitated? To precipitate colloidal particles, it is necessary to cause them to come into contact with each other, thus forming larger particles; in other words, they need to aggregate so that their specific gravity exceeds 1 and they can settle. 　　There are many methods available, and the commonly used techniques in engineering include coagulation, flocculation, and coacervation. 　　56. 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 those 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 sedimentation: (1) Generally, colloidal particles carry a negative charge, and due to the repulsion between like charges, this prevents the colloidal particles from coming into contact with each other and sticking together, allowing them to remain suspended in water. (2) The surface of the colloidal particles is also tightly surrounded by a layer of molecules; this hydration layer prevents and isolates contact between the colloidal particles, stopping them from sticking together and allowing them to remain suspended in water.