The battle for environmental protection is still raging. I hope that this year's haze control will be effective.

Water is not only the source of life, but also extremely important to human beings, and the pollution is so severe. Long-term drinking of poor water quality can lead to poor physical fitness and natural weakening of resistance, or even cancer. Therefore, we should prevent and protect water resources, and use water reasonably.

Q&A Summary of Water Treatment Knowledge

1. What is water self-purification

Self-purification of water body: the polluted river reduces or transforms the concentration of pollutants through physical, chemical, biological and other aspects, and the water body returns to its original state, or from the initial exceeding the water quality standard 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 and remove the pollutants in the sewage, recycle them, or convert them into harmless substances to purify the 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, chemical treatment and biological treatment according to the principle of action.

4. Five water measurement indicators

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

Theoretical oxygen demand (THOD): the theoretical oxygen demand of an organic substance in water. Generally, it refers to the theoretical value of oxygen required for the complete oxidation of carbon and hydrogen elements in organic matter to carbon dioxide and water (that is, the oxygen demand calculated according to the complete oxidation reaction formula).

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

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

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

5. When is biochemical treatment adopted?

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

6. What is the sanitary standard for drinking water?

Physical indicators of the 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 excessive proliferation of algae is caused by the high content of nitrogen, phosphorus and potassium in the water.

The main reason for the formation of water eutrophication is that nitrogen, phosphorus, potassium and other elements are discharged into the surface water with slow flow rate and long renewal period, which makes aquatic organisms such as algae grow and multiply in large numbers, making the rate of organic matter production far exceed the consumption rate, and the accumulation of organic matter in the water body, which destroys the aquatic ecological balance.

8. What is dissolved oxygen?

Oxygen dissolved in water is called dissolved oxygen. The oxygen that organisms and aerobic microorganisms in the water depend on to survive is dissolved oxygen. Different microorganisms have different requirements for dissolved oxygen.

9. What is the basic method of modern sewage treatment?

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

10. What is the stability of colloid?

Colloid stability: It refers to the characteristics of colloidal particles in water to keep dispersion and suspension for a long time.

11. What is the electric position?

Dynamic potential: the potential on the colloid sliding surface is ζ Potential.   

12. How do hydrophobic colloids form large particles?

For hydrophobic colloid, if the colloid collides with each other through Brownian motion to form large particles, the repulsion energy peak must be reduced or eliminated. The way to reduce the repulsion energy peak is to reduce or eliminate the colloidal particles ζ Potential.  

13. What is the role of adsorption bridging?

Adsorption bridging refers to the adsorption and bridging of macromolecular substances and colloidal particles.

14. What is the function of grille?

The function of the grating is to intercept the larger suspended solids or floating impurities.

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

The main factors affecting the coagulation effect include water temperature, PH value and alkalinity of water, concentration of suspended solids in water and hydraulic conditions.   

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

There are four types of precipitation:

Free sedimentation: The particles are in a discrete state during the sedimentation process. Their shape, size and quality are not changed, and the sedimentation speed is not disturbed. They complete the sedimentation process independently.

Turbulence sedimentation: In the process of sedimentation, the size, mass and sedimentation rate of particles increase with the increase of depth.

Congestion sedimentation: The concentration of particles in the water is large, and they interfere with each other during the sinking process, forming an obvious interface between clean water and muddy water, and gradually moving downward.

Compression sedimentation: The concentration of particles in water is very high. During sedimentation, particles contact with each other and are mostly supported by compressed materials, and the gap of lower particles is squeezed out.  

17. According to the flow direction in the tank, which types of sedimentation tank can be divided into?

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

18. What is the rule 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 pore is also 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 to the lower layer and are intercepted by the lower filter material.

The result is that under a certain filtration head, the filtration rate will decrease sharply, or the loss of the water head at a certain filtration rate will reach the limit value, or when the mud film cracks due to the uneven stress on the surface of the filter layer, a large amount of water will flow out from the cracks, causing the impurity particles in the water to penetrate the filter layer and worsen the water quality.

19. What are the ways to improve filtration efficiency?

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

20. What is the composition of homogeneous filter material?

Homogeneous filter material composition: Homogeneous filter material refers to any cross section in the depth direction of the whole filter layer. The filter material composition and average particle size are uniform, but not the same.

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

Negative water head phenomenon: the phenomenon occurs when the filter layer intercepts a large amount of impurities, so that the water head loss at a certain depth below the sand surface exceeds the water depth at that point.

The method to avoid negative water 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 filter surface. This is the reason why negative head will not occur in siphon filter and valveless filter.

22. What are the supply modes of backwash water for ordinary rapid filter?

There are two kinds of backwash water supply modes for common rapid filter: flushing water pump and water tower.

23. What is this chlorination?

When the organic substances in the water are mainly ammonia and nitrogen compounds, and the actual chlorine demand is met, the amount of chlorine added will increase, and the amount of residual chlorine will increase, but the latter will increase slowly. After a period of time, the amount of chlorine added will increase, and the amount of residual chlorine will decrease. After that, the amount of chlorine added will increase, and the amount of residual chlorine will rise again. After this break point, the free residual chlorine will appear, and the effect of continuous chlorination is the best, that is, the break point chlorination.

24. What are the systems of activated sludge process?

The 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 the mixed liquid in the aeration tank, which is settled in a 1000ml measuring cylinder for 30min.  

26. What is sludge index?

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

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

27. What are sludge bulking, disintegration, corruption, floating and foam?

Sludge bulking: when sludge deteriorates, sludge is not easy to precipitate, SVI value increases, sludge structure is loose, volume expansion, water content increases, less clarifier, and color changes.     

Sludge disintegration: the treatment water quality is turbid, the sludge turbulence is slightly refined, and the treatment effect is deteriorated.

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

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

Foam problem: foam is produced in the aeration tank, which is mainly caused by a large number 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 bacteria, and its growth law is relatively complex, but its growth curve can also be used to express certain laws. This curve shows the growth and decay rule of the number of microorganisms with time after a sufficient 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 microorganisms are inoculated.

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), and 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.

There are four stages of the activated sludge growth curve: adaptation period, logarithmic growth period, deceleration growth period (the largest biomass), and endogenous respiration period (the best water treatment effect).

29. How many processes are involved in the purification of activated sludge?

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 process is very fast, which is generally completed within 30 minutes, 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 is also called 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, activated sludge is precipitated and separated in the secondary sedimentation tank. Both anabolism and catabolism of microorganisms can remove organic pollutants in sewage, but the products are different.

30. What are the characteristics of secondary sedimentation tank?

Characteristics of secondary sedimentation tank: in terms of function, it not only carries out mud water separation, but also carries out sludge thickening, and also temporarily stores sludge due to changes in water quality and quantity.

31. What is the sewage slow infiltration system?

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

32. What is the sewage rapid infiltration system?

It is suitable for soil with very good permeability. Such as sand, gravelly sand, etc. After the sewage tank reaches the surface of the rapid infiltration field, it quickly infiltrates into the ground and finally enters the groundwater layer.

33. How many stages are there in anaerobic reaction? What are they?

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 bacteria.

In the second stage, hydrogen and acetic acid are further converted 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 different groups of methanogens, one group converts hydrogen and carbon dioxide into methane, and the other group is 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 substrate.

35. What is the material composition 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, mature sludge and chemical sludge.

36. What moisture is contained in sludge?

The water content in sludge can be divided into four categories: interstitial water between particles, capillary water, water absorbed by sludge particles 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, sludge centrifugal dewatering.

38. What is the purpose of sludge stabilization?

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

39. What is adsorption?

Use porous solid (such as activated carbon) or floc material (such as polyiron) to adsorb toxic and harmful substances in wastewater on the surface of solid or floc or in micropores to purify water quality. This treatment method is called adsorption treatment. The object of adsorption can be insoluble solid substances or soluble 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 the adsorption is basically non-selective.

Characteristics of chemical adsorption: the adsorption heat is large, the adsorption is irreversible, and the adsorption is selective.       

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 backwash strength and expansion rate of the resin layer and the resin layering of the mixed bed and the double bed. The wet apparent density is used to calculate the amount of wet resin required to be filled in the ion exchanger.

42. What is the function of water filling?

The function of the water-pouring filler is to make the water droplets splashed down by the water distribution system become tiny water droplets or water film through multiple splashes, increase the contact area of water and air, and extend the contact time, so as to ensure the good heat and mass exchange between air and water.

43. What is volatile suspended solid of mixed liquid?

Mixed liquid volatile suspended solids (MLVSS) refers to the weight of volatile substances in dry sludge contained in the mixed liquid of unit volume biochemical tank, and the unit is also mg/L. Because 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 continuously consume organic substances in wastewater.

Among the consumed organic substances, some organic substances are oxidized to provide the energy needed for the life activities of microorganisms, while the other organic substances are used by microorganisms to synthesize new cytoplasm, so that microorganisms multiply and reproduce. At the same time, some old microorganisms die, resulting in excess sludge.

45. What is needle carbon technology?

Iron-carbon treatment method, also known as iron-carbon micro-electrolysis or iron-carbon internal electrolysis, 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 above 9?

The iron-carbon effluent contains a large amount of ferrous sulfate. If it is not removed, it will affect the growth and reproduction of microorganisms in the subsequent biochemical tank.

Therefore, we must use lime to adjust the pH value of the wastewater from 5-6 to above 9, so that water-soluble ferrous sulfate can be converted into insoluble ferrous hydroxide and calcium sulfate, and then precipitate them by coagulation and sedimentation 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 (including vacuum dissolved air floatation and pressurized dissolved air floatation), dispersed air floatation and electrolytic air floatation.

48. What is flocculation?

Flocculation is the addition of macromolecular coagulant in wastewater, which will form macromolecular polymer after dissolution. The structure of this polymer is linear. One end of the line pulls a small particle and the other end pulls another small particle. It plays a role of bonding and bridging between the two particles far away, making the particles gradually larger, and finally forming a large particle floccule (commonly known as alum flower) to accelerate particle sedimentation.

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

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

In addition, coagulation pretreatment with polyferric chloride can remove trace substances that are toxic and inhibitory to microorganisms in the wastewater, so as to ensure the normal operation of microorganisms in the biochemical tank. Among many coagulants, the price of polyiron 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 waste water difficult to settle naturally?

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

However, suspended particles with a specific gravity of less than 1, small and even invisible to the naked eye are difficult to settle naturally. For example, colloidal particles are particles with the size of 10-4-10-6 mm, which are very stable in water. Their settling speed is extremely slow, and it takes 200 years to settle 1m.