（2）Theoretical oxygen demand (thOD): a theoretical oxygen demand for an organic matter in water. It usually refers to the theoretical value of the oxygen requirement of carbon and hydrogen in organic matter to carbon dioxide and water i. e., the oxygen demand calculated by the complete oxidation reaction formula.
（3）Total oxygen demand (TOD): refers to the substance that can be oxidized in the water, mainly the oxygen amount required by the organic matter when it becomes a stable oxide in combustion. The result is expressed as the mg/L of O2.
（4）Chemical oxygen demand (COD): it is a chemical measure of the amount of reducing substances that need to be oxidized in water samples. Oxygen equivalent of wastewater, wastewater treatment plant effluent and contaminated water that can be oxidized by strong oxidants (generally organic matter).
（5）Total organic carbon (TOC): refers to the total amount of carbon content of dissolved and suspended organic matter in the water body.
5. When should use the biochemical method?
Once the sewage’s BOD / COD is higher than 0.3 is suitable for biochemical treatment.
6. What are the sanitary standards of drinking water?
（1）Microbiological Quality: No detectable levels of pathogens like E. coli, coliforms, or Giardia.
（2）Chemical Quality: Limits on contaminants such as lead, nitrates, pesticides, and heavy metals, ensuring they are below harmful levels.
（3）Physical Quality: Water should be clear, free from unpleasant odors or tastes, and have appropriate pH and turbidity levels.
（4）Radiological Quality: Limits on radioactive substances to ensure safety.
Standards vary by country but are often guided by organizations like the World Health Organization (WHO) or the Environmental Protection Agency (EPA).
7. What is water eutrophication?
（1） Water eutrophication is a natural phenomenon of sudden overproliferation in algae caused by excessive nitrogen, phosphorus and potassium content in fresh water.
（2）The main reason for the formation of water eutrophication is that nitrogen, phosphorus, potassium and other elements are discharged into the surface water body with slow flow rate and long renewal cycle, which makes algae and other aquatic organisms grow and reproduce in large numbers, so that the production rate of organic matter far exceeds the consumption rate, and the accumulation of organic matter in the water body destroys the aquatic ecological balance.
8. What is dissolved oxygen?
The oxygen dissolved in the water body is called dissolved oxygen. The organisms and aerobic microorganisms in the water, the oxygen they live on is dissolved oxygen. Different microbes have different requirements for dissolved oxygen.
9. What are the basic methods of modern sewage treatment?
Modern sewage treatment technology, according to the principle of action, can be divided into physical treatment method, chemical treatment method, biological treatment method.
10. What is the stability of the colloid?
Colloid stability: refers to the characteristics of colloidal particles in water for a long time.
11. What is the electric power position?
Dynamic potential: the potential on the colloidal sliding surface is the ζ potential.
12. How is how to form large particles?
For hydrophobic colloids, in order to make the colloids collide through Brownian motion and form large particles, it is necessary to reduce or eliminate the exclusion energy peak. The way to reduce the exclusion energy peak is to reduce or eliminate the ζ potential of the colloidal particles.
13. What is the role of the adsorption bridge?
Adsorption bridge refers to the adsorption and bridge connection of polymer substances and adhesive particles.
14. What is the role of the grille?
The role of the grille: used to intercept the larger suspended matter or floating impurities.
15. What are the main factors affecting the coagulation effect?
The main factors affecting the coagulation effect are: water temperature, the PH value and alkalinity of the water, the concentration of suspended matter in the water and the hydraulic conditions.
16. What are the first few types of precipitation? What are they?
There are four types of precipitation, which are:
Free precipitation: the particles are in a discrete state during the precipitation process, their shape, size and quality do not change, the sinking speed is not disturbed, and each completes the precipitation process independently.
Dock sedimentation: in the process of precipitation, the size, mass and sedimentation speed of particles increase with the increase of depth.
Crowded precipitation: the concentration of particles in the water is large, interfering with each other in the process of sinking, forming an obvious interface between clear water and muddy water, and gradually moving downward.
Compressed precipitation: the concentration of particles in water is very high. During the precipitation process, the particles contact with each other and are mostly supported by the compressors, and the gap of the lower particles is squeezed out.
17. According to the direction of the water flow in the pool, which types of sedimentation tank can be divided into?
According to the different direction of the water flow in the pool, the sedimentation tank can be divided into advection flow type, oblique flow flow type, radial flow type and vertical flow type.
18. What are the rules of impurity distribution in the filter material layer?
Filter material layer impurity distribution rule: at the beginning of the filter, filter material is clean, pore is larger, water shear force is smaller, the adhesion is stronger, at this point, the water particles are the surface filter retention, as the filtration time, the filter layer impurities, porosity gradually decreased, especially the surface filter material, water shear force increases, shedding off, finally by adhesion particles first off to the lower layer and moved by the lower filter material.
The result is: under a certain filter head, the filter speed will be sharply reduced, or at a certain filter speed, the loss of the water head reaches the limit value, or due to the uneven force on the filter surface and the mud film cracks, a large number of water flow out from the crack, resulting in water impurity particles through the filter layer to make the water quality deterioration.
19. What are the ways to improve the filtration efficiency?
The way to improve the filtration efficiency: in order to change this situation and improve the pollution ability of the filter layer, there is a "reverse size" filtration, that is, the flow direction, the filter material size from large to small, due to the complex structure of upward flow and bidirectional flow filter pool, inconvenient to wash and other reasons.
20. What is the homogeneous filter material composed of?
Homogeneous filter material composition: homogeneous filter material refers to any cross section of the depth direction of the whole filter layer, the filter material composition and the average particle size are uniform, rather than the filter material particle size is exactly the same.
21. What is the negative water head phenomenon? What are the ways to avoid it?
Negative head phenomenon occurs during the filtration process when a filter layer accumulates a large amount of impurities. This causes the water head loss at a certain depth below the sand surface to exceed the water depth at that point.
To avoid negative head phenomenon, you can:
（1）Increase the water depth above the sand surface: This can help maintain a positive water head and prevent the negative head situation.
（2）Ensure the filter outlet is at or above the surface of the filter media: By positioning the outflow at or above the filter media surface, you can avoid negative head conditions.
（3）Use siphon filters or valve-free filters: These types of filters are designed to prevent negative head because their specific construction and drainage mechanisms help maintain appropriate water head levels.
22. How ways to supply backwashing water in ordinary fast filter?
There are two ways to supply backwashing water in ordinary fast filter tank: flushing water pump and water tower.
23. What is chlorination breakpoint?
Chlorination breakpoint is an effective method for disinfecting drinking water through chlorination. When the organic matter in the water mainly consists of ammonia and nitrogen compounds, the actual chlorine demand is met. Initially, as chlorine is added, both the amount of added chlorine and the residual chlorine increase, but the latter grows more slowly. After some time, if the chlorine dose is increased further, the residual chlorine may actually decrease. However, as more chlorine is added beyond this point, the residual chlorine will rise again. This point, where the residual chlorine starts to increase despite the continued addition of chlorine, is known as the breakpoint. Continuing chlorination beyond this point improves the disinfection effect, which is referred to as breakpoint chlorination.
24. What are the several systems of the activated sludge method?
Activated sludge method is composed of aeration tank, sedimentation tank, sludge reflux and residual sludge removal system.
25. What is the sludge sedimentation ratio?
Sludge sedimentation ratio (SV%): refers to the volume ratio (%) of the sedimentation sludge in a 1000ml measuring cylinder.
26、 What is the sludge index?
Sludge index (SVI): refers to the volume per gram of dry sludge of the mixed liquid at the outlet of the aeration tank for 30min, calculated in mL.
The low SVI value indicates that the mud particles are small and tight, with many inorganic substances, lack of activity and adsorption capacity. The SVI value is too high indicates that the sludge is difficult to settle and separate, and is about to swell or has expanded, so the cause must be identified and measures should be taken.
27. What is the sludge swelling, disintegration, decay, floating and foam?
Sludge expansion: when the sludge is deteriorated, the sludge is not easy to precipitate, the SVI value increases, the structure of the sludge is loose, the volume expands, the moisture content rises, the clarifier is less, and the color also changes.
Sludge disintegration: the treatment water quality is turbid, sludge orderly condensation and micro refinement, the treatment effect is the phenomenon of sludge disintegration.
Sludge rot: in the secondary sedimentation tank may be due to sludge long-term retention and anaerobic fermentation, gas generation, resulting in the phenomenon of large sludge floating.
Sludge floating: the phenomenon of sludge floating in the secondary sedimentation tank.
Foam problem: foam in the aeration tank is mainly due to the existence of a large amount of synthetic washing or other foaming substances in the sewage.
28. What is the growth curve of activated sludge?
Activated sludge microorganism is a mixed group of multiple bacteria, and its growth law is relatively complex, but it can also be expressed by its growth curve. The curve expresses the law of the proliferation and decay of microorganisms over time when the environmental conditions such as temperature and dissolved oxygen meet the growth requirements of microorganisms and have a certain amount of initial microbial inoculation after the sufficient amount of microorganisms over time.
The change of the growth rate of activated sludge is mainly caused by the ratio of nutrients or organic matter to microorganisms (usually expressed by F/M). F/M value is also an important factor affecting the degradation rate of organic substrate, oxygen utilization rate, the condensation and adsorption performance of activated sludge.
The four stages of the growth curve of activated sludge are: adaptation period, logarithmic growth period, deceleration growth period (the most biomass), and endogenous respiration period (the best water quality treatment effect).
29.What is the activated sludge purification processes?
Activated sludge purification of wastewater is completed in three stages:
（1） In the first stage, the wastewater is purified through the adsorption of activated sludge. The adsorption is very rapid, usually within 30min, and the BOD 5 removal rate can be as high as 70%. At the same time, it also has the effect of partial oxidation, but the adsorption is the main effect.
（2）The second stage, also called the oxidation phase. It is mainly to continue to decompose the organic matter adsorbed and absorbed in the pre-oxidation stage, while continuing to adsorb some residual dissolved substances.
（3）The third stage is the muddy-water separation stage. During this stage, the activated sludge is separated in a secondary sedimentation tank. Both the anabolism and catabolism of microorganisms can remove organic pollutants from sewage, but the products are different.
30. What are the characteristics of the secondary sedimentation tank?
The characteristics of the secondary sedimentation tank: in the function of the characteristics, it is in addition to the separation of muddy water, but also for sludge concentration, and due to the change of water quality, water quantity, but also temporarily stored sludge.
31. What is the sewage slow water filtration system?
Sewage slow leachate is to let the sewage slowly through the land, due to the natural infiltration filtration and sewage purification. Suitable for good permeable soil and small evaporation, humid climate areas.
32. What is the sewage fast water filtration system?
For very permeable soil. Such as sand, gravel sand, etc. The sewage tank to the surface of the rapid seepage field quickly seeps into the ground, and finally enters the groundwater layer.
33. What are the stages of the anaerobic reaction? What are they?
The anaerobic reaction is divided into three stages:
The first stage is the decomposition of organic matter into fatty acid machine products under the action of hydrolysis and fermentation of bacteria.
The second stage is that acetic acid is further converted into hydrogen, carbon dioxide and acetic acid under the action of bacteria.
The third stage is the methane fermentation stage (alkaline fermentation stage) through two different groups of methanogens, one is hydrogen and carbon dioxide into methane, the other is acetic acid to produce methane.
34. What is the two-phase digestion?
Two-phase digestion is the separation of the acid production stage and the methane production stage of anaerobic digestion of organic substrates.
35. What is the material composition of the sludge?
The composition of substances in sludge can be divided into organic sludge and inorganic sludge.
According to the source of sludge, it can be divided into primary sedimentation sludge, residual activated sludge, humic sludge, ripe sludge and chemical sludge.
36. What water is contained in the sludge?
The water contained in the sludge is divided into four categories: void water between the particles, capillary water, water adsorbed by the sludge particles and water inside the particles.
Removal method: gravity, air flotation, and centrifugation.
37. What does mechanical dehydration include?
Mechanical dehydration: vacuum filtration dehydration, pressure filtration dehydration, rolling dehydration, centrifugal dehydration of sludge.
38. What is the purpose of sludge stability?
The stable purpose of sludge is: to eliminate the odor from sludge and kill the pathogenic microorganisms in sludge.
39. What is the adsorption effect?
The use of porous solid (such as activated carbon) or flocculent substances (such as polyiron) will be toxic and harmful substances in wastewater adsorption on the surface of solid or flocculent or micropores, to achieve the purpose of purifying water quality, this treatment method is called as adsorption treatment. The object of adsorption can be insoluble solid or soluble.
40. What are the characteristics of physical adsorption and chemical adsorption?
Physical adsorption characteristics: adsorption heat is small, low temperature can be carried out, adsorption is reversible, adsorption is basically no selectivity.
Characteristics of chemical adsorption: adsorption heat is large, adsorption is irreversible, adsorption is selective.
41. What is the resin density?
Resin density: generally refers to the wet true density and wet-optic density two expression methods. The wet density is related to the backwash strength and expansion rate of the resin layer and the resin stratification of the mixed bed and double bed. The wet density is used to calculate the amount of wet resin required for the ion exchanger.
42. What is the role of the water filler?
The role of the water filler: it is to splash the water droplets of the water distribution system into small water droplets or water film for many times, increase the contact area of water and air, prolong the contact time, so as to ensure the good heat and mass exchange of air and water.
43. What is a mixed liquid volatile suspended solid?
Mixed volatile suspended solid (MLVSS) refers to the weight of volatile substances in the dry sludge contained in the biochemical tank mixture per unit volume, and the unit is also mg/L. Because it does not include inorganic matter in activated sludge, it can accurately represent the number of microorganisms in activated sludge.
44. Why is there any residual sludge produced?
During the biochemical treatment process, the microorganisms in the activated sludge constantly consume the organic substances in the wastewater.
The consumption of organic material, part of the organic material is oxidized to provide microbial life energy, another part of the organic material is used by microorganisms to synthesize new cytoplasm, so that the microbial reproduction, microorganisms in metabolism at the same time, and part of the old microbial death, therefore produce the residual sludge.
45. What is the needle charcoal technology?
Iron carbon treatment method, also known as iron carbon micro-electrolysis method or iron carbon internal electrolysis method, is an application form of metal iron treatment wastewater technology, using iron carbon method as a pretreatment technology to treat toxic, harmful, high concentration COD wastewater has a unique effect.
46. Why should the effluent pH of the neutralization sedimentation tank be adjusted to above 9?
Iron carbon effluent contains a large amount of ferrous sulfate, if not removed, will affect the growth and reproduction of microorganisms in the subsequent biochemical pool.
Therefore, we must use lime to adjust the pH value of the wastewater from 5-6 to above 9, so that the water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate, and then precipitate them through the method of coagulation settlement, so as to ensure that the wastewater into the biochemical tank does not contain ferrous sulfate.
47. What are the types of air flotation?
Air flotation is divided into: dissolved gas flotation (vacuum dissolved gas flotation and pressurized dissolved gas flotation), dispersed gas flotation, electrolytic gas flotation.
48. What is flocculation?
Flocculation is the addition of polymer coagulation agent in wastewater. After the dissolution of polymer coagulation agent, polymer polymer will form. The structure of this high polymer is a linear structure, with one end of the line pulling a small particle, and the other end pulling another small particle, which plays a bonding bridge between two particles far apart, making the particles gradually larger, and finally form a large particle flocculation (commonly known as alum flower), to accelerate the settlement of the particles.
49. Why does the wastewater use polypolyiron for flocculation and adsorption treatment?
The formation of iron hydroxide flocs in the process of coagulation has a good adsorption capacity of organic substances in wastewater. The experimental data show that the wastewater can remove about 10% -20% of COD in wastewater, which can greatly reduce the operation burden of the biochemical tank and is conducive to the standard discharge of wastewater treatment.
In addition, coagulation pretreatment with polyiron can remove the trace substances that are toxic and inhibitory to microorganisms in the wastewater, so as to ensure the normal operation of microorganisms in the biochemical pool. In many coagulation agents, the price of polypolyiron is relatively cheap (25-300 yuan / ton), so the treatment cost is relatively low, which is more suitable for the pretreatment of process wastewater.
50. Why are the colloidal particles in the wastewater not easy to precipitate naturally?
（1）Small Particle Size: Colloidal particles typically range in size from 1 nm to 1 mm. Their small size makes them subject to Brownian motion, which keeps them suspended in water and less affected by gravity.
（2）Surface Charge: Colloidal particles often carry an electrical charge on their surfaces, which creates electrostatic repulsion between them. This repulsion prevents the particles from coming together to form larger aggregates that could settle more easily.
（3）Stable Hydration Layer: The surface of colloidal particles may be surrounded by a layer of water molecules, forming a hydration layer. This layer stabilizes the particles and hinders their settling.
（4）Low Density Difference: The density of colloidal particles is often similar to that of water, so gravity has less of an effect on them, resulting in slower settling rates.
（5）High Surface Area to Volume Ratio: Colloidal particles have a large surface area relative to their volume, which increases the interaction with water molecules and further inhibits their ability to settle.
Due to these factors, colloidal particles in wastewater require external aids like coagulants or flocculants to aggregate into larger particles that can settle more effectively.
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