1、 Introduction to Ozone

Ozone, with the chemical formula O3, also known as triatomic oxygen or superoxide, is named after its fishy odor and can be self reduced to oxygen at room temperature. Specific gravity is higher than oxygen, easily soluble in water, and prone to decomposition. Due to the fact that ozone is composed of oxygen molecules carrying an oxygen atom, it is only in a temporary state. The carried oxygen atoms are used up by oxidation, and the remaining ones combine to form oxygen and enter a stable state. Therefore, ozone does not cause secondary pollution.

The decomposition rate of ozone in aqueous solution is faster than its decomposition rate in gas phase. The half-life of ozone decomposition in water is related to temperature and pH. As the temperature increases, the decomposition rate accelerates. When the temperature exceeds 100 ℃, the decomposition is severe; When the temperature reaches 270 ℃, it can be immediately converted into oxygen. The higher the pH value, the faster the decomposition. Decompose in air at normal temperature and pressure, with a half-life of about 15-30 minutes.

2、 Application of Ozone in Water Treatment

The treatment of wastewater by ozone oxidation uses air or oxygen containing low concentrations of ozone. The main process facilities consist of ozone generators and air-water contact equipment. Ozone oxidation method is mainly used for water disinfection, removal of pollutants such as cyanide in water, decolorization of water, removal of metal ions such as iron and manganese in water, and removal of odors and unpleasant odors.

1. Disinfection of water:

Ozone is a broad-spectrum and fast acting fungicide that has better killing effects on various pathogenic bacteria, as well as resistant spores, viruses, etc. than chlorine. After ozone disinfection, the physical and chemical properties of water, such as turbidity and color, have significantly improved. Chemical oxygen demand (COD) can generally be reduced by 50-70%. Ozone oxidation treatment can also remove carcinogenic substances such as benzo (a) pyrene.

2. Remove pollutants such as phenol and cyanide from water:

The actual amount of ozone and reaction rate required for treating phenol and cyanide containing wastewater using ozone method are related to the amount of pollutants such as sulfides in the water and the pH value of the water, so necessary pretreatment should be carried out. To oxidize phenols in water into carbon dioxide and water, the theoretical requirement for ozone is 7.14 times the phenol content. Using ozone to oxidize cyanide, the first step is to oxidize cyanide into slightly toxic cyanates, and the required amount of ozone is theoretically 1.84 times the cyanide content; The second step is to oxidize cyanate into carbon dioxide and nitrogen, and the theoretical requirement for ozone is 4.61 times the cyanide content. Ozone oxidation method is usually used in combination with activated sludge method. The activated sludge method is first used to remove most of the pollutants such as phenol and cyanide, and then the ozone oxidation method is used for treatment. In addition, ozone can also decompose pollutants such as sodium alkylbenzenesulfonate (ABS), proteins, amino acids, organic amines, lignin, humus, heterocyclic compounds, and chain unsaturated compounds in wastewater.

3. Water decolorization:

Printing and dyeing wastewater can be decolorized using ozone oxidation method. This type of wastewater often contains chromophores such as diazo, azo, or cyclic compounds with benzene rings. Ozone oxidation can break the divalent bonds of dye chromophores and destroy the cyclic compounds such as benzene, naphthalene, and anthracene that make up the chromophores, thereby decolorizing the wastewater. Ozone has a fast decolorization rate and good effect on hydrophilic dyes, but a slow decolorization rate and poor effect on hydrophobic dyes. Wastewater containing hydrophilic dyes can achieve a decolorization effect of over 95% by treating it with 20-50 mg/L ozone for 10-30 minutes.

4. Remove metal ions such as iron and manganese from water:

Metal ions such as iron and manganese can be separated from water by ozone oxidation to form metal oxides. In theory, the ozone consumption is 0.43 times that of iron ions and 0.87 times that of manganese ions.

5. Removing odors and unpleasant odors:

The odor and foul smell in surface water and industrial recycled water are produced by the decomposition products of actinomycetes, molds, and algae, as well as pollutants such as alcohols, phenols, and benzene. Ozone can oxidize and decompose these pollutants, eliminating unpleasant odors and odors. Meanwhile, ozone can be used for deodorization in sewage treatment plants, sludge and garbage treatment plants.

6. Improve wastewater B/C

For some wastewater containing complex organic compounds that are difficult to biodegrade, the oxidizing property of ozone can be used to decompose the complex organic compounds into simple organic compounds with certain biodegradability, and then undergo biochemical treatment. It is generally used for front-end pretreatment of certain chemical wastewater.

3、 Introduction to Ozone Generator

Ozone generator is a device used to produce ozone gas (O3). Ozone is prone to decomposition and cannot be stored, so it needs to be produced and used on-site (in special cases, short-term storage can be carried out). Therefore, any place where ozone can be used must use an ozone generator.

According to the way ozone is generated, there are currently three main types of ozone generators: high-voltage discharge, ultraviolet irradiation, and electrolysis.

High voltage discharge type is divided into tube type and plate type according to the structure of the ozone generator discharge chamber, which are also common types of ozone generators.

4、 Ozone equipment selection

1. Determination of ozone dosage for difficult to degrade C0D

Calculated based on experience using a dosage of COD: ozone=1:4

2. Example:

A certain printing and dyeing wastewater produces 20m ³/h, with a COD index of 300mg/L treated to 100mg/L

(1) Take a dosage of 1:3

(2) Reduce COD absolute value by 300-100=200mg/L

(3) Unit ozone required=200 * 3=600mg/L=600g/m ³

(4) The total amount of ozone required per hour is 600g/m ³ * 20m ³=12kg

(5) Ozone utilization rate of 90%, required ozone production per hour=12kg/0.9=13.3kg

(6) The safety factor is 1.2, and the required ozone selection per hour is 13.3 * 1.2=16kg/h

The above are some project experience dosages. Due to different production processes and varying wastewater quality, in order to avoid economic losses or treatment processes that do not achieve ideal results, it is best to confirm the dosage of ozone through small-scale testing when selecting ozone equipment.

Calculation formula for selecting ozone level:

Determine the ozone dosage based on the sewage quality and treatment process, determine the ozone usage based on the ozone dosage and hourly treatment water volume, and select the number and model of ozone generators based on the hourly ozone usage. The calculation formula is as follows:

G=q*g

In the formula: G - the amount of ozone used per hour, g/h

Q - maximum hourly sewage treatment capacity, m³/h

G - Ozone dosage, g/m ³ of sewage

5、 Design of Ozone Contact Reactor

The contact reactor for ozone treatment is a device that provides ozone dissolution in water and ensures ozone reaction time. Therefore, the ozone contact reactor should have the following two functions: allowing ozone to have a higher dissolution rate (higher ozone absorption rate) and a higher reaction rate (higher pollutant removal rate).

Design requirements for ozone contact tank:

(1) The contact pool is composed of two to three contact chambers connected in series, separated by vertical partitions;

(2) Each contact chamber consists of a gas distribution zone and a subsequent reaction zone, separated by a vertical guide baffle;

(3) Ozone gas should diffuse directly into the water through a microporous aeration disc located at the bottom of the gas distribution area, and the number of gas injection points should be consistent with the number of sections in the contact chamber;

(4) The arrangement of the aeration disc should ensure uniform air distribution during the process of air distribution changes, with the air distribution in the first section accounting for about 50% of the total air distribution;

(5) The design water depth of the contact pool should be 5.5-6m, and the ratio of depth to length of the gas distribution area should be greater than 4;

(6) The clear distance between the diversion partitions should not be less than 0.8m;

(7) A residual ozone monitoring device must be installed at the outlet of the contact tank.