Thanks to favorable policies, China's environmental protection industry has developed rapidly, and the market space in subdivided fields has been expanding. Among them, the water treatment industry is favored by policies and markets, showing a high-speed development trend and broad market prospects, and the trillion-level water treatment market has exploded. Below, let's review these knowledge points about water treatment with the editor:

1. What is water self-purification? 

Self-purification of water bodies: The polluted rivers reduce or transform the pollutant concentration through physical, chemical, biological and other effects, and restore the water body to its original state, or reduce it from the initial 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: it is to use various means and technologies to separate and remove the pollutants in the sewage, recycle them, or convert them into harmless substances, so that the sewage can be purified. 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 matter due to the action of microorganisms under aerobic conditions. It is a comprehensive indicator that indicates that sewage is polluted by organic matter. 

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

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

Chemical oxygen demand (COD): It is a chemical measurement of the amount of reducing substances in a water sample that needs to be oxidized. The oxygen equivalent of substances (usually organics) that can be oxidized by strong oxidants in wastewater, wastewater treatment plant effluent and contaminated water.

Total Organic Carbon (TOC): Refers to the total amount of carbon contained in dissolved and suspended organic matter in a body of water.

5. What is the use of biochemical treatment? 

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

6. What is the hygienic standard for drinking water?

Physical indicators of sanitary standards for drinking water: color, turbidity, odor and taste.

7. What is eutrophication of water bodies? 

Water eutrophication is a natural phenomenon that occurs in freshwater, caused by excessive levels of nitrogen, phosphorus, and potassium in the water body, resulting in sudden overgrowth of algae.

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

8. What is dissolved oxygen?

Oxygen dissolved in a body of water is called dissolved oxygen. The organisms and aerobic microorganisms in the water body depend on dissolved oxygen for their survival. 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 colloidal stability?

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

11. What is an electromotive potential? 

Kinetic potential: The potential on the sliding surface of the colloid, that is, the ζ potential.

12. How do hydrophobic colloids form large particles?

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

13. What is the function of adsorption bridging? 

Adsorption bridging refers to the adsorption and bridging of polymer substances and colloids.

14. What is the function of the grille? 

The function of the grille: it is used to intercept coarse suspended solids or floating impurities.

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

The main factors influencing the coagulation effect: water temperature, pH and alkalinity of water, concentration of suspended solids in water, and hydraulic conditions.

16. How many types of precipitation are there? 

There are four types of precipitation, and they are: 

Free precipitation: The particles are in a discrete state during the precipitation process, their shape, size and quality do not change, and the sinking speed is not disturbed, and the precipitation process is completed independently. 

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

Crowded precipitation: the concentration of particles in water is large, and they interfere with each other in the process of sinking, forming an obvious interface between clear water and muddy water, and gradually moving downward. 

Compressive precipitation: The concentration of particles in water is very high, and during precipitation, the particles are in contact with each other and are mostly supported by the compression, and the gaps of the lower particles are extruded.

17. According to the direction of water flow in the pool, what are the types of sedimentation tanks?

According to the different directions of water flow in the pool, 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 the distribution of impurities in the filter layer?

The distribution law of impurities in the filter material layer: at the beginning stage of filtration, the filter material is relatively clean, the pores are also larger, the water flow shear force is small, and the adhesion effect is stronger, at this time, the particles in the water are first intercepted by the surface filter material, with the extension of the filtration time, the impurities in the filter layer increase, and the porosity gradually decreases, especially the surface fine filter material, the water flow shear force increases, the shedding effect is enhanced, and finally the particles on the adhesion first fall off and move to the lower layer and are intercepted by the lower filter material.

The result is: under a certain filter head, the filtration rate will be sharply reduced, or the loss of the water head at a certain filtration rate reaches the limit value, or when the mud film cracks due to the uneven force on the surface of the filter layer, a large amount of water flows out of the cracks, causing the impurity particles in the water to penetrate the filter layer and deteriorate 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 dirt-containing capacity of the filter layer, there is "reverse particle size" filtration, that is, along the direction of the water flow, the particle size of the filter material is from large to small, due to the complex structure of the upward flow and bidirectional flow filter, it is inconvenient to flush and other reasons.

20. What is the composition of homogeneous filter material? 

Homogeneous filter material composition: Homogeneous filter material refers to any cross section in the direction of the depth of the entire filter layer, and the filter material composition and average particle size are uniform and consistent, not that the particle size of the filter material is exactly the same.

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

Negative head phenomenon: In the filtration process, when a large number 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 at that place. 

The way to avoid negative heads is to increase the water depth on the sand surface, or to increase the water outlet of the filter at or above the surface of the filter layer. This is the reason why siphon and valveless filters do not have negative heads.

22. How many ways are there to supply backwash water to ordinary fast filters? 

There are two ways to supply backwash water to ordinary fast filters: flushing pump and water tower.

23. What is this point of chlorination? 

When the organic matter in the water is mainly ammonia and nitride, the actual chlorine demand is satisfied, the amount of chlorine increases, the amount of residual chlorine increases, but the latter grows slowly, after a period of time, the amount of chlorine increases, the amount of residual chlorine decreases, and then the amount of chlorine increases, and the amount of residual chlorine rises, after this breakpoint, the free residual chlorine appears, and the effect of continuous chlorination disinfection is the best, that is, the breakpoint chlorine.

24. What are the systems of activated sludge method? 

The activated sludge method is composed of an aeration tank, a sedimentation tank, sludge reflux, and a residual sludge removal system.

25. What is sludge sedimentation ratio?

Sludge sedimentation ratio (SV%): refers to the volume ratio (%) of the sedimented sludge to the mixed solution in the aeration tank that is precipitated in a 1000ml graduated cylinder for 30min.

26. What is the sludge index?

Sludge index (SVI): refers to the volume of each gram of dry sludge occupied by the mixture at the outlet of the aeration tank after 30 minutes of static precipitation, calculated in mL. 

If the SVI value is too low, it means that the mud particles are fine and compact, there are many inorganic substances, and there is a lack of activity and adsorption capacity. If the SVI value is too high, it means that the sludge is difficult to settle and separate, and is about to expand or has expanded, and the cause must be identified and measures must be taken.

27. What is sludge expansion, disintegration, decay, floating and foaming? 

Sludge expansion: 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 rises, the clarifier is less, and the color is also abnormal. 

Sludge disintegration: the treatment water quality is turbid, the sludge is fine, and the treatment effect deteriorates. 

Sludge decay: In the secondary sedimentation tank, there may be anaerobic fermentation due to long-term retention of sludge, which will generate gas, resulting in the phenomenon of large sludge floating. 

Sludge floating: the phenomenon that sludge floats in a block in the secondary sedimentation tank. 

Foam Problem: Foam occurs in aeration tanks, mainly due to the presence of large amounts of synthetic scrubbing or other foaming substances in the effluent.

28. What is the activated sludge growth curve?

Activated sludge microorganisms are mixed populations with multiple strains, and their growth rules are complex, but their growth curves can also be used to represent certain laws. The curve expresses the proliferation and decay of the number of microorganisms over time when the environmental conditions such as temperature and dissolved oxygen meet the growth requirements of microorganisms, and there is a certain amount of initial microbial inoculation. 

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

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

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

Activated sludge wastewater is purified in three stages:

In the first stage, the wastewater is purified mainly by adsorption of activated sludge. The adsorption process is carried out very quickly, generally completed within 30 minutes, and the BOD5 removal rate can be as high as 70%. At the same time, it also has the effect of partial oxidation, but adsorption is the main effect.

The second stage is also known as the oxidation stage. It mainly continues to decompose the organic matter that was adsorbed and absorbed in the pre-oxidation stage, and at the same time continues to adsorb some residual dissolved substances. 

The third stage is the slurry separation stage. In this stage, the activated sludge is sedimentated and separated in a secondary sedimentation tank. Both the anabolism and catabolism of microorganisms can remove organic pollutants from wastewater, but the products are different.

30. What are the characteristics of the secondary sedimentation tank?

The characteristics of the secondary sedimentation tank: in addition to the separation of sludge and water, it also carries out sludge thickening, and due to the change of water quality and quantity, it also needs to temporarily store sludge.

31. What is a slow percolation system for sewage?

Slow seepage of sewage

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

The anaerobic reaction is divided into three phases: 

The first stage is the decomposition of organic matter into fatty acid machine products under the action of hydrolysis and fermentation bacteria.

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

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

34. What is two-phase digestion?

Two-phase digestion is the separation of the acid-producing and methanogenic stages of anaerobic digestion of organic substrates.

35. What are the material components of 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, humus sludge, cooked sludge and chemical sludge.

36. What kind of water is contained in sludge? 

The water contained in sludge is divided into 4 categories: inter-particle void water, capillary water, sludge particle adsorption water, and internal particle water. 

Removal method: gravity, air flotation, centrifugation.

37. What does mechanical dehydration include?

Mechanical dehydration: vacuum filtration dehydration, press filtration dewatering, rolling dewatering, centrifugal dewatering of sludge.

38. What is the purpose of sludge stabilization? 

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

39. What is adsorption?

Utilize porous solids (such as activated carbon) or floc substances (such as polyiron) to adsorb toxic and harmful substances in wastewater on the surface of solid or floc or in micropores, achieve the purpose of purifying water quality, and this treatment method is called adsorption treatment. The object of adsorption can be either an insoluble solid substance or a dissolved substance.

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

Physical adsorption characteristics: the heat of adsorption is small, it can be carried out at low temperature, the adsorption is reversible, and the adsorption is basically non-selective. 

Chemical adsorption characteristics: the heat of adsorption is large, the adsorption is irreversible, and the adsorption is selective.

41. What is resin density?

Resin density: Generally refers to two representations: wet true density and wet apparent density. The wet apparent density is related to the backwash strength of the resin layer, the expansion rate, and the resin delamination of the mixed and double beds, while the wet apparent density is used to calculate the amount of wet resin that needs to be filled with the ion exchanger.

42. What is the function of water-spraying filler?

The function of the water filler is to splash the water droplets of the water distribution system into tiny water droplets or water films after many splashes, so as to increase the contact area between water and air and 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 solids (MLVSS) refers to the weight of volatile substances in the dry sludge contained in the mixed liquid of the biochemical tank per unit volume, and the unit is also mg/L, because it does not include the inorganic substances in the activated sludge, so it can more accurately represent the number of microorganisms in the activated sludge.

44. Why is there residual sludge? 

During the biochemical treatment process, the microorganisms in the activated sludge continuously consume the organic matter in the wastewater.

Among the organic matter consumed, part of the organic matter is oxidized to provide the energy required for the life activities of microorganisms, and the other part of the organic matter is used by microorganisms to synthesize new cytoplasm, so that microorganisms can reproduce, and while microorganisms are metabolizing, some old microorganisms die, so that residual sludge is produced.

45. What is needle charcoal technology?

Iron and carbon treatment method, also known as iron and carbon micro electrolysis method or iron carbon internal electrolysis method, it is an application form of metal iron treatment wastewater technology, and the iron carbon method is used as a pretreatment technology to treat toxic and harmful and highly concentrated COD wastewater with a unique effect.

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

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

Therefore, we must use lime to increase the pH value of the wastewater from 5-6 to more than 9, so that the water-soluble ferrous sulfate is converted into insoluble ferrous hydroxide and calcium sulfate, and then they are precipitated by coagulation and sedimentation to ensure that the wastewater entering the biochemical tank does not contain ferrous sulfate.

47. What are the types of air flotation? 

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

48. What is flocculation?

Flocculation is the addition of polymer coagulation agent to wastewater, and after the polymer coagulation agent is dissolved, a polymer polymer will be formed. The structure of this polymer is a linear structure, one end of the line pulls a tiny particle, the other end pulls another tiny particle, plays the role of bonding and bridging between two particles far apart, makes the particles gradually become larger, and finally forms a floc of large particles (commonly known as alum flowers), accelerates the sedimentation of particles.

49. Why should wastewater be treated with polyiron for flocculation adsorption?

The experimental data show that after the effluent is adsorbed by polyiron flocculation, it can remove about 10%-20% of the COD in the wastewater, which can greatly reduce the operating burden of the biochemical tank and is conducive to the standard discharge of treated wastewater. 

In addition, coagulation pretreatment with polyiron can remove trace substances that are toxic to microorganisms and have inhibitory effects 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, and it is more suitable for the pretreatment of process wastewater.

50. Why are colloidal particles in wastewater not easy to precipitate naturally?

Many impurities with a specific gravity greater than 1, large particles, and suspended solids that are easy to settle in wastewater can be removed by natural sedimentation, centrifugation and other methods.

However, the specific gravity is less than 1, tiny and even invisible suspended solids particles are difficult to settle naturally, such as colloidal particles are 10-4-10-6mm size particles, which are very stable in water, and its sedimentation speed is extremely slow, and it takes 200 years to settle 1m.