Method of treating domestic water supply

Choosing technology is the most important step in water treatment techniques for domestic and industrial use. Choosing water treatment technology can be based on the following conditions

– Based on flow, composition, and nature of the water source.

– Requires treatment level to meet the Ministry of Health’s drinking water supply standards QCVN 01-1:2018/BYT

– Natural, meteorological and hydrological conditions in the area.

– Economic situation and financial capacity.
– Scale and development trend.
– Ability to meet equipment requirements for the treatment system.

– Construction investment, management, operation and maintenance costs. – Make the most of available works.
– Land fund and available premises area of factories

During water supply treatment, the following measures need to be taken:

Mechanical water treatment method

a. Reservoir and preliminary sedimentation

The function of the reservoir and preliminary sedimentation of raw water (surface water) is to: create favorable conditions for self-cleaning processes (settle out suspended sediment, reduce the amount of germs) due to the impact of environmental conditions. , performs oxidation reactions due to the effect of dissolved oxygen in water and is responsible for regulating the flow between the flow from the water source and the consumption flow from the raw water pumping station to the water treatment plant. .

b. Garbage screens and trash screens

– Screens and screens placed at the water inlet to the collection works are responsible for eliminating floating objects and floating objects in the water stream to protect equipment and improve the cleaning efficiency of the treatment works. Floating and suspended objects in water can be as small as floating toothpicks, or young tree branches, when passing through pumps into treatment works, can be crushed or rot, increasing sediment content and color. of water.

– Garbage screens are composed of steel bars with round section size 8 or 10, or rectangular section size 6 x 50 mm placed parallel to each other and welded to the steel frame. The distance between steel bars is from 40 ÷ 50 mm. The speed of water flowing through the barrier is about 0.4 ÷ 0.8 m/s. The trash screen is lifted and released by pulleys or hand winches arranged in the management compartment. The shape of the trash can can be rectangular, square or round. Flat trash screens are composed of a mesh stretched on a steel frame. The mesh is woven from steel wire diameter 1 ÷ 1.5 mm, mesh 2 x 2 ÷ 5 x 5 mm. In some cases, on the outside of the mesh, another mesh with a mesh size of 25 x 25 mm is woven with 2 – 3 mm diameter steel wire to enhance the load-bearing capacity of the mesh. The speed of water flowing through the mesh belt is taken from 0.15 ÷ 0.8 m/s.

c. Clarifier

– The sedimentation tank is responsible for preliminary cleaning before sending water into the filter tank to complete the water clarification process. According to the flow direction, sedimentation tanks are divided into: horizontal sedimentation tanks, vertical sedimentation tanks, thin layer sedimentation tanks and internal sedimentation tanks with suspended sediment layer.

– In a horizontal settling tank, wastewater flows horizontally through the tank at a speed not greater than 16.3 mm/s. Horizontal settling tanks are often used when water flow is greater than 3,000 m3/day. For vertical settling tanks, water moves vertically from the bottom up to the overflow wall at a speed of 0.3-0.5 mm/s. The settling efficiency of vertical settling tanks is usually 10 to 20% lower than that of horizontal settling tanks.

– Thin layer sedimentation tanks have the same structure as conventional horizontal sedimentation tanks, but different from horizontal sedimentation tanks in that in the settling area of thin layer sedimentation tanks, stainless steel or plastic partitions are placed. These partitions are inclined at an angle of 450 ÷ 600 compared to the horizontal plane and parallel to each other. Due to the additional structure of inclined baffles, thin-layer sedimentation tanks have higher efficiency than horizontal sedimentation tanks. The area of thin layer sedimentation tank is reduced by 5.26 times compared to a pure horizontal sedimentation tank.

– An internal sedimentation tank with a suspended sediment layer has the advantage of not needing to build a reaction tank, because the reaction and flocculation process occurs under contact flocculation conditions, right in the suspended sediment layer of the tank. settle. The treatment efficiency is higher than other settling tanks and requires less construction area. However, the internal settling tank has a complex structure and high operating techniques. The water velocity from bottom to top in the settling area is less than or equal to 0.85 mm/s and the water retention time is about 1.5 – 2 hours.

d. Filter tank

– The filter tank is used to filter part or all of the dirt in the water depending on the water quality requirements of the water users. The water filtration process is to let water pass through a layer of filter material with a certain thickness enough to retain on the surface or between the gaps of the filter material layer sediment particles and germs in the water. After a period of work, the filter material layer becomes clogged, increasing pressure loss and decreasing filtration speed. To restore the working ability of the filter tank, the filter tank must be blown with water or wind, combined with water to remove dirt from the filter material layer. Filtration speed is the amount of water filtered through a unit surface area of the filter tank in a unit of time (m/h). Filtration cycle is the time between two filter tank washes T (h).

– To carry out the water filtration process, several types of filter tanks can be used with different working principles, filter material layer structures and operating parameters. Filtration equipment can be classified in many different ways: according to characteristics such as: intermittent filtration and continuous filtration; according to the type of process such as thickening and clarification; according to the pressure during the filtration process such as vacuum filtration (pressure 0.085 MPa), pressure filtration (from 0.3 to 1.5 MPa) or filtration under the hydrostatic pressure of the liquid column; …

– In large capacity water treatment systems, it is not necessary to use high pressure filtration equipment but use filter tanks with granular filter material. The filter media that can be used is quartz sand, coke, or crushed gravel, even brown coal or wood charcoal. The choice of filter material depends on the type of wastewater and local conditions. The filtering process occurs according to the following mechanisms:

+ Screening to separate solid particles completely using mechanical principles; + Gravity settling;
+ Hold solid particles by inertia;
+ Chemical adsorption;

+ Physical adsorption;
+ Adhesion process;
+ Flocculation sedimentation process.

– Filters with particle beds can be classified into slow filters, fast filters, open filters and closed filters. The height of the filter material layer in open filtration equipment ranges from 1-2 m and in closed filtration equipment from 0.5 – 1 m.

Physiochemical water treatment method

a. Airy

– The essence of the aeration process is to dissolve oxygen from the air into water to oxidize divalent iron, divalent manganese into trivalent iron, and IV manganese to form trivalent iron hydroxyl compounds and Valence IV manganese hydroxyl Mn(OH)4 precipitates easily and can be removed from water by sedimentation and filtration.

– Aeration to remove CO2, H2S in water, increase water pH, create favorable conditions and accelerate the oxidation and hydrolysis of iron and manganese, increase the capacity of sedimentation and filtration works in the process. iron and manganese reduction. The aeration process increases the dissolved oxygen content in water, raising the redox potential of water to easily carry out the oxidation of organic substances in the process of deodorizing and deodorizing water.

* There are two ventilation methods:

– Putting water into the air: let water spray into jets or thin films flowing in the air in naturally ventilated systems, or let water spray into jets and thin films in closed containers and then blow air into the barrel as in forced ventilation units.

– Introducing air into water: guiding and distributing compressed air into small bubbles according to the distribution system located at the bottom of the water tank, the air bubbles rise, the water is aerated. – Mix the above two methods: aerate with a multi-step overflow trough and spray on the water surface.

b. Prechlorination

– Clo hóa sơ bộ là quá trình cho clo vào nước trước bể lắng và bể lọc. Clo hóa sơ bộ có tác dụng tăng thời gian khử trùng khi nguồn nước nhiễm clo bẩn nặng, oxy hóa sắt hòa tan ở dạng hợp chất hữu cơ, oxy hóa mangan hòa tan để tạo thành các kết tủa tương ứng, oxy hóa các chất hữu cơ để khử màu, ngăn chặn sự phát triển của rong, rêu, phá hủy tế bào của các vi sinh sản ra chất nhầy nhớt trên mặt bể lọc.

c. Coagulation – Creating Floc

– In water sources, a part of the particles usually exists in the form of fine dispersed colloidal particles, the size of the particles usually ranges from 0.1 to 10 mm. These particles neither float nor settle, and are therefore relatively difficult to separate. Because the particle size is small and their surface area to volume ratio is very large, surface chemistry becomes very important.

– As a rule, small particles in water tend to flocculate due to the VanderWaals attraction between the particles. This force can lead to adhesion between particles as soon as the distance between them is small enough through collision. Collisions occur due to Brownian motion and due to the effects of disturbances.

– However, in the case of colloidal dispersion, the particles maintain their dispersed state thanks to electrostatic repulsion because the surface of the particles carries an electrical charge, which can be negative or positive due to selective absorption. ions in solution or ionization of active groups. The suspended state of colloidal particles is stabilized thanks to electrostatic repulsion.

* The most commonly used coagulants are iron salts and aluminum salts such as:

Al2(SO4)3, Al2(SO4)2.18H2O, NaAlO2, Al2(OH)5Cl, KAl(SO4)2.12H2O, NH4Al(SO4)2.12H2O, FeCl3, Fe2(SO4)2.2H2O, Fe2(SO4)2.3H2O, Fe2(SO4)2.7H2O

* Aluminum Salt

– Among all types of alum, Al2(SO4)3 is the most widely used due to its good solubility in water, low cost and effective operation in the pH range = 5.0 – 7.5.

– Electrolysis and hydrolysis of Al2(SO4)3 occurs: Al3+ + H2O = Al(OH)2+ + H+
AlOH+ + H2O = Al(OH)2+ + H+
Al(OH)2+ + H2O = Al(OH)3(s) + H+

Al(OH)3 + H2O = Al(OH)4- + H+
– In addition, Al2(SO4)3 can react with Ca(HCO3)2 in water in a direction

following reaction:
Al2(SO4)3 + 3Ca(HCO3)2 → Al(OH)3 + 3CaSO4 + 6CO2

– In most cases, people use a mixture of NaAlO2 and Al2(SO4)3 in the ratio (10:1) – (20:1). The reaction occurs as follows:

6NaAlO2 + Al2(SO4)3 + 12H2O → 8Al(OH)3 + 2Na2SO4
– Using the above salt mixture allows to expand the optimal pH range of the lips

field as well as increase the efficiency of flocculation and flocculation process.

* Iron salts

– Iron salts used as coagulants have many advantages over aluminum salts because:

+ Better effect at low temperatures;
+ Has a wider range of optimal pH values of the environment; + Great durability;
+ Can deodorize H2S.

– However, iron salts also have the disadvantage of forming colored soluble complexes due to the reaction of iron ions with organic compounds. The flocculation process using iron salts occurs due to the following reactions:

FeCl3 + 3H2O → Fe(OH)3 + HCl

Fe2(SO4)3 + 6H2O →Fe(OH)3 + 3H2SO4 Under alkaline conditions:

2FeCl3 + 3Ca(OH)2 → Fe(OH)3 + 3CaCl2

FeSO4 + 3Ca(OH)2 →2Fe(OH)3 + 3CaSO4
– Flocculation Aids To increase the effectiveness of the flocculation process, people often

Use flocculation aids (flucculants). The use of flocculation aids allows to reduce the dosage of coagulant, reduce the flocculation process time and increase the settling speed of flocs. Commonly used natural coagulant aids are starch, dextrin (C6H10O5)n, ethers, cellulose, activated silicon dioxide (xSiO2.yH2O).

– Commonly used synthetic flocculation aids are polyacrylamide (CH2CHCONH2)n. Depending on the ionic groups when dissociating, coagulant aids have a negative or positive charge such as polyacrylic acid (CH2CHCOO)n or polydiallyldimethyl-ammon. The optimal dosage of coagulant used in practice is determined by the Jartest experiment.

River water treatment system by chemical sedimentation method was built and installed by Nam Viet

Method of treating domestic water supply by water disinfection

Water disinfection is a mandatory step in the process of treating domestic drinking water. Natural water contains many microorganisms and disinfectants. After mechanical treatment processes, especially water after passing through the filter tank, most germs are retained. But to completely destroy disease-causing germs, it is necessary to disinfect the water. Currently, there are many effective disinfection methods such as: disinfection with strong oxidants, physical rays, ultrasound, heat methods, heavy metal ions,…

a. Disinfection with chlorine and chlorine compounds

– Chlorine is a strong oxidizer in any form. When chlorine reacts with water, it forms hypochlorite acid (HOCl) which has a strong disinfectant effect. When chlorine is added to water, the disinfectant will diffuse through the microbial cell wall and react with the enzymes inside the cell, disrupting the metabolic process, leading to the destruction of the microorganisms.

– When chlorine is added to water, the reaction takes place: Al3+ + H2O → HOCl + HCl

– Or it can be in the form of a dissociation equation: Cl2+H2O→H++OCl- +Cl-

– When using lime chloride, the reaction takes place as follows: Ca(OCl)2 + H2O → CaO + 2HOCl
2HOCl → 2H+ + 2OCl

b. Use ozone for disinfection

– Ozone is a purple-colored gas that is rarely soluble in water and is very toxic to humans. In water, ozone decomposes very quickly into molecular and atomic oxygen. Ozone has stronger activation properties than chlorine, so its disinfecting ability is many times stronger than chlorine. The contact time is very short so the device surface area is reduced, does not cause unpleasant taste in the water even when the water contains phenol.

c. Sterilize by heat method

– This is a traditional disinfection method. Boiling water at 100 degrees Celsius can kill most bacteria in the water. Except for the group of bacteria, when exposed to high temperatures, they will transform into solid spores. However, this group of bacteria accounts for a very small proportion. The method of boiling water is simple, but consumes fuel and is cumbersome, so it is only used on a family scale.

d. Ultraviolet (UV) disinfection

– UV ultraviolet rays are electromagnetic radiation with a wavelength of about 4 – 400 nm, which has a very strong disinfectant effect.

– Use ultraviolet radiation lamps, placed in the water flow. The emitted ultraviolet rays will affect the protit molecules of microbial cells, disrupting their structure and losing their metabolic ability, so they will be destroyed.

– Disinfection effect is only achieved thoroughly when there are no organic substances and suspended residue in the water. Disinfection with ultraviolet rays does not change the smell or taste of water.

e. Ultrasonic sterilization

Ultrasonic current with an intensity of not less than 2W/cm2 for a period of over 5 minutes is capable of destroying all microorganisms in water.

f. Disinfect with silver ions

Silver ions can kill most germs in water. With a content of 2–10 ions g/l, it has a disinfectant effect. However, the limitation of this method is: if the water has high color, organic matter, many types of salt, etc., silver ions cannot promote disinfection ability.

2.4 Feed water treatment by ion exchange and reverse osmosis RO

In addition to the above treatment methods, when higher water quality is required, the following methods are also used in water treatment:

+ Deodorize and deodorize with air fresheners, strong oxidizers, and activated carbon;

+ Softening water by heat method, chemical method, ion exchange method;

+ Desalination and desalination of water by ion exchange, electrolysis, membrane filtration, heat or distillation.

Hệ thống xử lý nước RO — RO water treatment system installed by Nam Viet

a. Ion exchange method

Desalination of water by ion exchange method means filtering water through H-Cationite and OH-Anionite filter tanks. When filtering water through an H-Cationite filter tank, as a result of exchanging cations of salts dissolved in water with H+ ions of cationite particles, salts dissolved in water turn into corresponding acids:

RH + NaCl → RNa + HCl
2RH + Na2SO4 → 2RNa + H2SO4
2RH + Ca(HCO3)2 → R2Ca + 2CO2↑¬ + 2H2O

When further filtered, the water has been de-cationized in the H-Cationite Tank, through the OH-Anionite filter tank, the anionite particles will absorb from the water the anions of strong acids such as Cl-, SO42- (Carbon dioxide is removed from the water by airing before adding to the OH-Anionite tank) and releasing an equivalent amount of OH- anions into the water

[An]OH + HCl → [An]Cl + 2H2O

2[An]OH + H2SO4 → [An] 2SO4 + 2H2O

See details of water treatment method using ion exchange here

Ion exchange softening system installed by Nam Viet

b. Reverse osmosis method

-The essence of this method is: filtering water through a special reverse semi-permeable RO membrane. The membrane only allows water to pass through and the ions of salts dissolved in water are retained. To filter water through this membrane, it is necessary to create a residual pressure opposite to the direction of water movement by osmosis, which means creating a residual pressure in the source water that is higher than the osmotic pressure of the water passing through the membrane, so that the water is filtered. through the membrane does not return to the salt solution due to osmosis.

See details of water treatment method using ion exchange here

Mechanical water treatment method