1. What is water self purification?

Self purification of water body: the polluted river reduces or transforms the concentration of pollutants through physical, chemical and biological effects, and the water body returns to its original state, or reduces from exceeding the water quality standard to equal to the water quality standard.

2. What are the basic methods of sewage treatment?

The basic method of sewage treatment is to use various means and technologies to separate, remove and recycle the pollutants in sewage, or convert them into harmless substances to purify sewage. It is generally divided into water supply treatment and sewage treatment.

3. What are the current sewage treatment technologies?

Modern sewage treatment technology can be divided into physical treatment method, chemical treatment method and biological treatment method according to the action principle.

4. Five water measurement indicators

Biochemical oxygen demand (BOD): refers to the oxygen required to degrade organic matter under aerobic conditions due to the action of microorganisms. It is a comprehensive index indicating that sewage is polluted by organic matter.  

Theoretical oxygen demand (thod): the theoretical oxygen demand of an organic matter in water. It usually refers to the theoretical value of oxygen required for complete oxidation of carbon and hydrogen in organic matter to carbon dioxide and water (i.e. oxygen demand calculated according to complete oxidation reaction formula).

Total oxygen demand (TOD): refers to the substances that can be oxidized in water, mainly the oxygen required when organic substances become stable oxides during combustion. The result is expressed in mg / L of O2.

Chemical oxygen demand (COD): refers to the amount of reducing substances to be oxidized in water samples measured by chemical methods. Oxygen equivalent of waste water, effluent from waste water treatment plant and polluted water, substances (generally organic substances) that can be oxidized by strong oxidants.

Total organic carbon (TOC):   It refers to the total carbon content of dissolved and suspended organic matter in water.

5. What is the biochemical treatment?

It is generally considered that sewage with BOD / COD value greater than 0.3 is suitable for biochemical treatment.  

6. What is the hygienic standard for drinking water?

Physical indexes of sanitary standard for drinking water: color, turbidity, odor and taste.

7. What is water eutrophication?

Water eutrophication is a natural phenomenon that occurs in fresh water, and the sudden excessive proliferation of algae is caused by the high content of nitrogen, phosphorus and potassium in water.

The main reason for water eutrophication is that nitrogen, phosphorus, potassium and other elements are discharged into the surface water with slow flow rate and long renewal cycle, which makes algae and other aquatic organisms grow and reproduce in large quantities, so that the production rate of organic matter far exceeds the consumption rate, and the accumulation of organic matter in the water body destroys the aquatic ecological balance.

8. What is dissolved oxygen?

Dissolved oxygen in water is called dissolved oxygen. For organisms and aerobic microorganisms in water, the oxygen they rely on for survival is dissolved oxygen. Different microorganisms   The requirements for dissolved oxygen are different.

9. What are the basic methods of modern sewage treatment?

Modern sewage treatment technology can be divided into physical treatment method, chemical treatment method and biological treatment method according to the action principle.

10. What is colloidal stability?

Colloidal stability: refers to the characteristics of colloidal particles in water in a dispersed and suspended state for a long time.

11. What is potentiodynamic potential?

Potentiodynamic potential: the potential on the colloidal sliding surface, i.e ζ Potential.   

12. How do hydrophobic colloids form large particles?

For hydrophobic colloids, in order to make colloids collide with each other and form large particles through Brownian motion, the repulsive energy peak must be reduced or eliminated. The way to reduce the repulsive energy peak is to reduce or eliminate the repulsive energy peak ζ Potential.  

13. What is the role of adsorption bridging?

Adsorption bridging refers to the adsorption and bridging of polymer and colloidal particles.

14. What does the grille do?

Function of Grid: it is used to intercept coarse suspended solids or floating impurities.

15. What are the main factors affecting the coagulation effect?

Main factors affecting coagulation effect: water temperature, pH value and alkalinity of water, suspended solids concentration in water and hydraulic conditions.  

16. How many types of precipitation are there? What are they?

There are four types of precipitation:

Free precipitation: the particles are discrete in the precipitation process, their shape, size and quality do not change, and the sinking speed is not disturbed. They complete the precipitation process independently.

Turbulent sedimentation: during the sedimentation process, the size, mass and sedimentation rate of particles increase with the increase of depth.

Crowded sedimentation: the concentration of particles in water is large, which interferes with each other in the process of sinking, forms an obvious interface between clean water and muddy water, and moves downward gradually.

Compression precipitation: the concentration of particles in water is very high. In the precipitation process, the particles contact each other and are mostly supported by compressed materials, and the gap of lower particles is extruded.  

17. According to the flow direction in the tank, what kinds of sedimentation tanks can be divided into?

According to the different flow directions in the tank, the sedimentation tank can be divided into advection type, oblique flow type, radial flow type and vertical flow type.

18. What is the law of impurity distribution in the filter material layer?

Distribution law of impurities in the filter material layer: at the beginning of filtration, the filter material is relatively clean, the pores are large, the flow shear force is small and the adhesion is strong. At this time, the particles in the water are first intercepted by the surface filter material. With the extension of filtration time, the impurities in the filter layer increase and the porosity gradually decreases. Especially for the surface fine filter material, the flow shear force increases and the shedding effect increases, Finally, the adhered particles first fall off and move down the layer and are intercepted by the lower filter material.

The result is that under a certain filtering head, the filtering speed will decrease sharply, or when the loss of the water head reaches the limit value at a certain filtering speed, or when the mud film cracks due to uneven stress on the surface of the filter layer, a large amount of water flows out of the cracks, causing impurity particles in the water to penetrate the filter layer and deteriorating the effluent quality.

19. What are the ways to improve filtration efficiency?

Ways to improve filtration efficiency: in order to change this situation and improve the pollution capacity of the filter layer, there is "reverse particle size" filtration, that is, along the water flow direction, the particle size of the filter material from large to small. Due to the complex structure of the upward flow and two-way flow filter, it is inconvenient to wash.

20. What is the composition of homogeneous filter material?

Composition of homogeneous filter material: homogeneous filter material refers to any cross section in the depth direction of the whole filter layer, and the composition of filter material is uniform and consistent with the average particle size, rather than the particle size of filter material is exactly the same.

21. What is the negative head phenomenon? What are the ways to avoid?

Negative head phenomenon: in the process of filtration, when a large amount of impurities are intercepted by the filter layer, so that the head loss at a certain depth below the sand surface exceeds the water depth there.

The method to avoid negative head is to increase the water depth on the sand surface, or the outlet position of the filter is equal to or higher than the surface of the filter layer. The siphon filter and valveless filter will not have negative head, which is the reason.

22. How many backwashing water supply modes are there for ordinary fast filter?

There are two supply modes of backwash water for ordinary fast filter: flushing water pump and water tower.

23. What is break point chlorination?

When the organic matter in the water is mainly ammonia and nitride, when the actual chlorine demand is met, the chlorination amount increases and the residual chlorine amount increases, but the latter increases slowly. After a period of time, the chlorination amount increases and the residual chlorine amount decreases. Thereafter, the chlorination amount increases and the residual chlorine amount increases. After this break point, the free residual chlorine appears, and the disinfection effect of continuous chlorination is the best, that is, break point chlorination.

24. What are the systems of activated sludge process?

Activated sludge process is composed of aeration tank, sedimentation tank, sludge return and excess sludge removal system.

25. What is the sludge sedimentation ratio?

Sludge sedimentation ratio (SV%): refers to the volume ratio of precipitated sludge to mixed liquid (%) after standing in 1000ml measuring cylinder for 30min.  

26. What is the sludge index?

Sludge index (SVI): refers to the volume of each gram of dry sludge after the mixed liquid at the outlet of the aeration tank is settled for 30min, in ml.     

The SVI value is too low, indicating that the mud particles are small and compact, there are many inorganic substances, and lack of activity and adsorption capacity; If the SVI value is too high, it indicates that it is difficult to settle and separate the sludge, which is about to swell or has expanded. The causes must be found out and measures must be taken.        

27, what are sludge bulking, disintegration, decay, floating and foam?

Sludge bulking: when the sludge deteriorates, the sludge is not easy to precipitate, the SVI value increases, the structure of the sludge is loose, the volume expands, the moisture content increases, the clarifier is less, and the color also changes.     

Sludge disintegration: the phenomenon of sludge disintegration is that the treated water quality is turbid, the sludge is turbulent and slightly refined, and the treatment effect is deteriorated.

Sludge decay: in the secondary sedimentation tank, anaerobic fermentation may occur due to the long-term retention of sludge to generate gas, resulting in the floating of large sludge.

Sludge floatation: sludge floats in blocks in the secondary sedimentation tank.

Bubble problem: foam in the aeration tank is mainly caused by a large amount of synthetic washing or other foaming substances in the sewage.

28. What is the activated sludge growth curve?

Activated sludge microorganism is a mixed population of multiple strains, 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 attenuation of the number of microorganisms with time after a full addition of nutrients when the environmental conditions such as temperature and dissolved oxygen meet the growth requirements of microorganisms and a certain amount of initial microbial inoculation.

The change of activated sludge growth rate is mainly caused by the ratio of nutrients or organic matter to microorganisms (usually expressed in F / M). F / m value is also an important factor affecting the degradation rate of organic substrate, oxygen utilization rate, coagulation and adsorption performance of activated sludge.

Four stages of activated sludge growth curve: adaptation period  、 Logarithmic growth period  、 Deceleration growth period (maximum biomass)  、 Endogenous respiration period (the best treatment effect of water quality)   。

29. How many processes can activated sludge purification be divided into?

Activated sludge purification of wastewater is completed in three stages:

In the first stage, the wastewater is purified mainly through the adsorption of activated sludge. The adsorption is carried out very quickly, generally completed within 30min, and the BOD5 removal rate can be as high as 70%. It also has the function of partial oxidation, but adsorption is the main function.

The second stage, also known as oxidation stage. It is mainly to continue to decompose the organics adsorbed and absorbed in the pre oxidation stage, and continue to adsorb some residual dissolved substances.

The third stage is the mud water separation stage. In this stage, the activated sludge is precipitated and separated in the secondary sedimentation tank. Microbial anabolism and catabolism can remove organic pollutants from sewage, but the products are different.

30. What are the characteristics of secondary sedimentation tank?

Characteristics of secondary sedimentation tank: in addition to sludge water separation, it also concentrates sludge, and temporarily stores sludge due to changes in water quality and quantity.

31. What is sewage slow infiltration system?

Slow infiltration of sewage is to let the sewage slowly pass through the land and purify the sewage due to natural infiltration and filtration. It is suitable for soil with good water permeability and areas with low evaporation and humid climate.

32. What is sewage rapid infiltration system?

It is suitable for soil with very good permeability. Such as sandy soil, gravelly sandy soil, etc. After the sewage tank reaches the surface of the rapid infiltration field, it soon infiltrates into the underground and finally into the underground water layer.

33. How many stages of 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 mechanical products under the action of hydrolysis and fermentation bacteria.

The second stage is hydrogen production and acetic acid production, which is further transformed into hydrogen, carbon dioxide and acetic acid under the action of bacteria.

The third stage is methane fermentation stage (alkaline fermentation stage). Through the action of two groups of methanogens, one group converts hydrogen and carbon dioxide into methane, and the other group deacetylates acetic acid to produce methane.

34. What is two-phase digestion?

Two phase digestion is to separate the acidogenic stage and methanogenic stage of anaerobic digestion of organic substrates.

35. What are the material components of sludge?

The 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, cooked sludge and chemical sludge.

36. What moisture does the sludge contain?

The water contained in sludge is divided into four categories: void water between particles, capillary water, sludge particle adsorption water and water inside particles.

Removal methods: gravity, air flotation and centrifugation.  

37. What does mechanical dehydration include?

Mechanical dewatering: vacuum filtration dewatering, pressure filtration dewatering, rolling dewatering and centrifugal dewatering of sludge.

38. What is the purpose of sludge stabilization?

The purpose of sludge stabilization is to eliminate the odor and kill pathogenic microorganisms in sludge.

39. What is adsorption?

Porous solids (such as activated carbon) or flocs (such as Polyferric) are used to adsorb toxic and harmful substances in wastewater on the surface or micro surface of solids or flocs   This treatment method is called adsorption treatment. The adsorbed object can be insoluble solid matter or soluble solid matter   Hydrolytic substances.

40. What are the characteristics of physical adsorption and chemical adsorption?

Physical adsorption characteristics: the adsorption heat is small and can be carried out at low temperature. The adsorption is reversible and there is basically no selectivity.

Chemical adsorption characteristics: large adsorption heat, irreversible adsorption and selective adsorption.       

41. What is resin density?

Resin density: generally refers to wet true density and wet apparent density. The wet true density is related to the backwashing strength and expansion rate of the resin layer and the resin stratification of the mixed bed and double bed. The wet apparent density is used to calculate the amount of wet resin to be filled in the ion exchanger.

42. What is the function of drenching filler?

Function of water spraying filler: it is to scatter the water droplets splashed by the water distribution system into fine water droplets or water film for many times, increase the contact area between water and air and prolong the contact time, so as to ensure good heat and mass exchange between air and water.

43. What is a mixture of volatile suspended solids?

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

44. Why is there excess sludge?

In the process of biochemical treatment, microorganisms in activated sludge constantly consume organic substances in wastewater.

Among the consumed organic substances, some organic substances are oxidized to provide the energy required for microbial life activities, and the other organic substances are used by microorganisms to synthesize new cytoplasm, so as to make microorganisms reproduce. While microorganisms metabolize, some old microorganisms die, so excess sludge is produced.

45. What is iron carbon treatment technology?

Iron carbon treatment method, also known as iron carbon micro electrolysis or iron carbon internal electrolysis method, is an application form of metal iron wastewater treatment technology. Using iron carbon method as pretreatment technology to treat toxic, harmful and high concentration cod wastewater has a unique effect.

46. Why should the effluent pH of neutralization sedimentation tank be adjusted to above 9?

Iron carbon effluent contains a large amount of ferrous sulfate, which will affect the growth and reproduction of microorganisms in the subsequent biochemical tank if it is not removed.

Therefore, we must use lime to increase the pH value of wastewater from 5-6 to more than 9, so as to convert water-soluble ferrous sulfate into insoluble ferrous hydroxide and calcium sulfate, and then precipitate them through coagulation sedimentation, so as to ensure that the wastewater entering the biochemical tank is free of ferrous sulfate.

47. What are the types of air flotation?

Air floatation is divided into: dissolved air floatation (vacuum dissolved air floatation and pressurized dissolved air floatation), dispersed air floatation and electrolytic air floatation.

48. What is flocculation?

Flocculation is to add polymer coagulant into wastewater. After the polymer coagulant is dissolved, polymer will be formed. The structure of this polymer is linear   Type structure, one end of the line pulls a small particle and the other end pulls another small particle, which plays the role of bonding and bridging between the two particles far away, so that the particles gradually become larger, and finally form a floc of large particles (commonly known as alum flower) to accelerate particle settlement.

49. Why should the wastewater be treated by flocculation and adsorption with polyferric chloride?

The ferric hydroxide floc formed by Polyferric in the coagulation process has a good ability to adsorb the organic substances in the wastewater. The experimental data show that after the wastewater is flocculated and adsorbed by Polyferric, about 10% - 20% of COD in the wastewater can be removed, which can greatly reduce the operation burden of the biochemical tank and is conducive to the standard discharge of the treated wastewater.

In addition, coagulation pretreatment with Polyferric can remove trace substances that are toxic and inhibitory to microorganisms in wastewater, so as to ensure the normal operation of microorganisms in biochemical tank. Among many coagulants, the price of Polyferric is relatively cheap (25-300 yuan / ton), so the treatment cost is low   It is relatively cheap and suitable for the pretreatment of process wastewater.

50. Why is it difficult for colloidal particles in wastewater to precipitate naturally?

Many impurities, suspended solids, large particles and easily settled suspended solids with a specific gravity greater than 1 in wastewater can be removed by natural sedimentation, centrifugation and other methods.  

However, the suspended solids with specific gravity less than 1, small or even invisible to the naked eye are difficult to settle naturally. For example, the colloidal particles are micro particles with the size of 10-4-10-6mm   Grain is very stable in water. Its settling speed is very slow. It takes 200 years to settle 1m.