There Are So Many Types of Disinfectants, How to Choose? I've Organized this Mind Map!

After primary or secondary treatment of wastewater, the water quality is improved, and the bacterial content is significantly reduced. However, the absolute number is still considerable, and there may be pathogenic bacteria. Therefore, wastewater should be disinfected before being discharged into water bodies.

1、Selection of Disinfectants

At present, it is widely known that chlorination disinfection can produce harmful substances that affect human health. This is because chlorine reacts with organic substances in water through both oxidation and substitution reactions. The oxidation reaction can help remove organic matter, while the substitution reaction involves chlorine combining with organic substances to form halides with mutagenic or carcinogenic activity. The United States stipulates that the maximum concentration of trihalomethanes (THMS) is 100μg/L, while Germany, Canada, and Japan have set it at 25μg/L, 350μg/L, and 100μg/L respectively. China's 1985 edition of the "Sanitary Standards for Drinking Water" also stipulates that the upper limit of chloroform is 60μg/L. In view of this, for wastewater disinfection, first, the appropriate dosage should be controlled; second, other disinfectants such as chlorine dioxide, ozone, and ultraviolet radiation should be used instead to reduce the production of harmful substances.

The advantages, disadvantages, and applicable conditions of various disinfectants are shown in the mind map. With reference to this map, the appropriate disinfectant can be initially determined.

Mind Map: Advantages, Disadvantages and Applicable Conditions of Various Disinfectants

2、Types of DisinfectantsCommonly used disinfectants include hypochlorous acid compounds, chlorine dioxide, ozone, and ultraviolet radiation. Hypochlorous acid disinfectants exist in forms such as liquid chlorine, bleaching powder, high-test hypochlorite, chlorine tablets, and sodium hypochlorite, and mainly exert their disinfection effect through HOCl. The weakness of hypochlorous acid disinfectants is that they easily react with organic substances in water to form chlorinated hydrocarbons, which have been confirmed to be extremely harmful to human health. At the same time, the treated water may have some unpleasant odors. The dust of hypochlorous acid disinfectants and the released chlorine gas have a strong stimulating effect on human respiratory tract, eyes, and skin. If they accidentally splash into the eyes or touch the skin, they should be rinsed immediately with a large amount of water. The storage environment should be cool, ventilated, and dry, away from heat sources and fire sources. They should not be stored or transported together with organic substances, acids, and reducing agents. During transportation, they should be protected from rain and sunlight exposure. When loading and unloading, handle with care to avoid collision and rolling.Disinfection with hypochlorous acid disinfectants often involves substitution reactions, which is the fundamental reason for the production of chlorinated hydrocarbons. In contrast, disinfection with ozone and chlorine dioxide involves pure oxidation reactions, which can destroy the structure of organic substances, improve the biodegradability of wastewater (BOD5/CODCr ratio) while sterilizing, and remove part of the CODCr in water. Compared with ozone or ultraviolet disinfection, chlorine dioxide disinfection has a lower one-time investment but higher operating costs (about 0.1 yuan/m³); the latter has a higher one-time investment but lower operating costs (about 0.02 yuan/m³).Ozone disinfection and ultraviolet disinfection can achieve disinfection effects in a very short time. The effluent from the secondary sedimentation tank or reclaimed water after ozone disinfection and ultraviolet disinfection can meet the requirements for microbial indicators such as total bacterial count and total coliforms. However, their disadvantage is that they are instantaneous reactions and cannot maintain the effect to resist the growth and reproduction of microorganisms in the pipeline. Therefore, when these two disinfection methods are used in reclaimed water systems, it is often necessary to add 0.05-0.1mg/L of chlorine dioxide or 0.3-0.5mg/L of chlorine to the effluent to ensure sufficient residual chlorine at the end of the pipe network.3、Introduction to Common Disinfectants1、ChlorineWhat are the physical and chemical properties of chlorine? Chlorine is a yellow-green gas under normal pressure. Its density is 3.2mg/mL at 0°C and one atmosphere, which is about 2.5 times that of air, and it has a strong pungent odor. Chlorine is generally produced by electrolyzing brine, and then compressed and cooled to obtain liquid chlorine. Liquid chlorine is highly volatile, with a boiling point of -34.5°C. Pressurized liquid chlorine becomes a yellow-green transparent liquid. 1kg of liquid chlorine can vaporize into a volume of 300L. Chlorine is very reactive and soluble in water, and its solubility decreases with the increase of water temperature. Chlorine is a strongly irritating asphyxiating gas that can cause damage to the human respiratory system, eyes, and skin. The maximum allowable concentration in the air is 1mL/m³. Although it is not flammable, it can support combustion. When mixed with other flammable gases in sunlight, it can burn and explode, and can react with most substances. Chlorine is a strong oxidant with the advantages of strong bactericidal ability, low price, and easy availability, and is the longest-used disinfectant in the water treatment industry. The mechanism of chlorine disinfection relies on the hydrolysis product hypochlorous acid (HOCl) diffusing into the cell walls of microorganisms and reacting with cellular proteins to form chemically stable N-Cl bonds. In addition, chlorine can oxidize certain active substances in microorganisms, inhibiting and killing them.How to prevent chlorine poisoning? Although the maximum allowable concentration in the air is 1mL/m³, long-term work in an environment below this value can also lead to chronic poisoning, manifested as chronic bronchitis, chronic gastroenteritis, gingivitis, stomatitis, skin pruritus, etc. Short-term exposure to a high-chlorine environment can cause acute poisoning. Mild acute poisoning is characterized by dry throat, chest tightness, and rapid pulse, while severe acute poisoning is characterized by bronchospasm and edema, and even coma or shock. Measures to prevent chlorine poisoning can be summarized as follows:(1) Workers who are often exposed to chlorine may have reduced sensitivity to the smell of chlorine, and they may be harmed by chlorine before they can smell it. Therefore, the operator's duty room should be set separately from the chlorination room, and monitoring and alarm devices should be installed in the chlorination room to detect the chlorine concentration at any time.(2) The chlorination room should be close to the chlorination point, with a distance of no more than 30m. The chlorination room should be strong, fireproof, frost-resistant, heat-insulating, well-ventilated, with doors opening outward, and strictly separated from other workrooms without any direct connection. Since chlorine is heavier than air, when chlorine leaks in a room, it will displace the air and accumulate in the lower part of the closed room and gradually diffuse upward. Therefore, the bottom of the chlorination room must be equipped with forced ventilation facilities, and the air intake should be set at a high place.(3) Maintenance tools, gas masks, and rescue equipment should be prepared outside the door of the chlorination room. The switches of lighting and ventilation equipment should also be set outside. Before entering the chlorination room, ventilation should be carried out first. The pressure water pipe leading to the chlorination room must ensure uninterrupted water supply and stable water pressure, and there must be measures to deal with sudden water cuts. An alkali solution pool should be set in the chlorination room, and regular inspections should be carried out to ensure that the alkali solution is effective at all times. If a serious leak is found in the chlorine cylinder, put on a gas mask and quickly move the chlorine cylinder into the alkali solution pool.(4) When someone is found to have acute chlorine poisoning on site, try to quickly transfer the poisoned person to a place with fresh air. For those with difficulty breathing, they should be given oxygen, and artificial respiration is strictly prohibited. A 2% sodium bicarbonate solution can be used to wash their eyes, nose, mouth, etc., and they can also be allowed to inhale atomized 5% sodium bicarbonate solution.What are the precautions for using liquid chlorine cylinders? Liquid chlorine is the most widely used disinfectant at home and abroad, and it also has an oxidation effect in addition to disinfection. Liquid chlorine is usually stored and transported in steel cylinders. When in use, liquid chlorine is converted into chlorine gas and added to water.(1) The pressure in the chlorine cylinder is generally 0.6-0.8MPa, so it should not be exposed to the sun or near stoves or other high-temperature heat sources to avoid excessive pressure during vaporization and explosion. Liquid chlorine and dry chlorine gas have no corrosive effect on metals such as copper, iron, and steel, but when in contact with water or moisture, their chemical activity increases and they can corrode most metals. Therefore, the chlorine storage cylinder must maintain a residual pressure of 0.05-0.1MPa and cannot be completely used up to prevent water from entering.(2) Liquid chlorine needs to absorb heat to become chlorine gas. About 289kJ of heat is required for 1kg of liquid chlorine to become 1kg of chlorine gas. In low temperatures, the heat absorbed by the chlorine cylinder from the air is limited, and the amount of liquid chlorine vaporized is limited, so it is necessary to heat the chlorine cylinder. However, it is strictly forbidden to directly heat the chlorine cylinder with an open flame or steam, and it is not advisable to increase the temperature of the chlorine cylinder too much or too quickly. Generally, the method of continuously sprinkling the chlorine cylinder with warm water at 15-25°C can be used to heat the chlorine cylinder.(3) The connection between the chlorinator and the chlorine cylinder should be checked for leaks with 10% ammonia water frequently. If a blockage is found in the chlorine pipe of the chlorinator, do not flush it with water. It can be unblocked with a steel wire, and then the debris can be blown off with an air pump or compressed air.(4) Before opening, check whether the placement position of the chlorine cylinder is correct. It must ensure that the outlet is upward, that is, the gas released is chlorine gas rather than liquid chlorine. When opening the main valve of the chlorine cylinder, first open it slowly half a turn, then check for leaks with 10% ammonia water, and then open it gradually after everything is normal. If the valve is difficult to open, do not hit it with a hammer or use a long wrench to force it, to avoid breaking the valve stem.2、Sodium HypochloriteSolid sodium hypochlorite (NaClO) is a white powder with a pungent odor, extremely unstable in the air, and decomposes rapidly when heated. The available chlorine content of commercial solid sodium hypochlorite is generally 10%-12%. The common preparation method is the liquid caustic chlorination method, which involves passing chlorine gas into a sodium hydroxide solution with a concentration below 30% for reaction. Commercial solid sodium hypochlorite is easy to use, but its disinfection effect is worse than that of chlorine, and the cost is higher than that of chlorine disinfection. Since sodium hypochlorite is easily decomposed by sunlight and temperature, it is generally prepared on-site by a sodium hypochlorite generator and added immediately. Using a titanium anode to electrolyze brine (seawater can be used as the salt solution in coastal areas), the obtained sodium hypochlorite solution is a pale yellow transparent liquid with an available chlorine content of 6g/L-11g/L. Generally, to produce 1kg of available chlorine, the salt consumption is about 3-4.5kg, and the power consumption is 5-10kWh, which is usually lower than the consumption of disinfection with bleaching powder. Solid or liquid sodium hypochlorite should not be stored for a long time, and must be stored in a dark and low-temperature environment. It is best to produce and use the electrolytic sodium hypochlorite solution as needed. When the temperature is below 30°C, the available chlorine loss is 0.1-0.15mg/L per day; if the temperature exceeds 30°C, the available chlorine loss can reach 0.3-0.7mg/L per day. Therefore, if a certain reserve is needed for backup, the storage time is generally no more than one day in summer and no more than 7 days in winter.3、Bleaching PowderBleaching powder (CaCl2·Ca(OCl)2·2H2O) is a white powder with a chlorine odor, containing 20%-25% available chlorine. Bleaching powder is easily hygroscopic and extremely unstable. Exposure to sunlight and heat can cause it to deteriorate and reduce the available chlorine content. Mixing with organic substances and flammable liquids can cause heat and spontaneous combustion, and it will explode when exposed to high heat. Chlorine tablets are made from high-test hypochlorite (3Ca(OCl)2·2Ca(OH)2·2H2O) and contain 60%-70% available chlorine. Chlorine tablets and high-test hypochlorite are more stable than bleaching powder and can be stored at room temperature for more than 200 days without decomposition. Their disinfection effect is the same as that of chlorine, and parameters such as chlorine dosage (based on available chlorine) and contact time can refer to liquid chlorine. When using bleaching powder as a disinfectant, it needs to be prepared into a solution for dosing, and a mixing tank is generally required. Each 50kg bag of bleaching powder is first mixed with 400-500kg of water to form a 10%-15% solution, and then water is added to adjust it to a 1%-2% concentration solution. The mixing tank usually has two types: baffle type and blast type. For disinfection with chlorine tablets, wastewater flows into a special chlorine tablet disinfector, soaks and dissolves the chlorine tablets, mixes with them, and then enters the contact tank.4、Chlorine DioxideWhat are the physical and chemical properties of chlorine dioxide? Chlorine dioxide (ClO2) is a yellow-green gas with extremely unstable properties, a pungent odor similar to chlorine, and higher toxicity than chlorine, with a relative density of 2.4. Chlorine dioxide can be compressed into a liquid at room temperature and is very volatile. Chlorine dioxide is highly explosive. It may explode when the temperature rises, exposed to light, or in contact with and friction against certain organic substances, and liquid chlorine dioxide is more explosive than gaseous chlorine dioxide. Explosion will occur when the volume concentration in the air exceeds 10% or the concentration of chlorine dioxide in water exceeds 30%. Chlorine dioxide is easily soluble in water, and its solubility in water is 5 times that of chlorine, but ClO2 does not react with water, is highly volatile in water, and is prone to photochemical decomposition under light. The content of ClO2 in an aqueous solution stored in an open container will drop rapidly. Therefore, chlorine dioxide is not suitable for storage and is best prepared and used on-site. Commercial stable chlorine dioxide solutions sold on the market are mostly colorless or slightly yellow-green transparent liquids with a chlorine dioxide content generally around 2%, and a certain amount of special stabilizers (such as aqueous solutions of sodium carbonate, sodium borate, and perchlorides, or diethylenetriamine pentamethylenephosphonic acid, etc.) must be added. However, attention should still be paid to avoiding high temperatures and strong light during transportation and storage. Therefore, when using chlorine dioxide for disinfection, it is best to produce and use it on-site.After sterilization, chlorine dioxide decomposes into non-toxic substances and causes no pollution to environmental water bodies.

5、Ozone

What are the physical and chemical properties of ozone?Ozone (O₃), an allotrope of oxygen, was discovered over a century ago. The ozone layer, formed by solar ultraviolet radiation 15–25 km above Earth, acts as a protective shield for life. Named for its fishy odor (noticeable after thunderstorms), ozone is a colorless gas at low concentrations under normal temperature and pressure, turning pale blue at 15% concentration.

With a boiling point of -112°C and density of 2.144 g/m³ (1.65 times that of oxygen), ozone supports more vigorous combustion than oxygen, producing higher temperatures. It is 10 times more soluble in water than pure oxygen. Ozone is highly unstable, decomposing easily into oxygen—faster in water than in air. Its half-life in air is 20–50 minutes (accelerated by higher temperature and humidity) and ~35 minutes in water (varying with water quality and temperature). Impurities, especially metal ions, catalyze its decomposition in water.

What are the applications of ozone in water treatment?Ozone has a higher redox potential (2.07 V) than chlorine (1.36 V), making it a stronger oxidant and biocide (second only to hydroxyl radicals ∙OH and fluorine). As a disinfectant, it achieves better bactericidal and virucidal effects with shorter contact times: e.g., a ozone dosage-contact time product of 5 achieves the same effect as a chlorine product of 1440.

A key advantage is that ozone does not form toxic organochlorines (unlike chlorine) when organic matter is present. Its strong oxidizing power enables simultaneous disinfection, deodorization, decolorization, and phenol removal. Due to its rapid decomposition and lack of residues, it is ideal for disinfecting micro-polluted surface water (drinking water sources) and advanced treated wastewater.

Ozone oxidizes Fe²⁺, Mn²⁺, and other metal ions to higher oxidation states, which then precipitate as hydroxides. It also oxidizes toxic reducing inorganic substances (cyanides, sulfides, nitrites) into harmless or less toxic substances (CO₂, N₂O, SO₄²⁻, NO₃⁻).

In reactions with organic compounds, ozone breaks unsaturated bonds, splitting molecules into carboxylic acids. Ozonides decompose into carboxyl compounds and amphoteric ions (unstable, forming acids and aldehydes). This property is used to pretreat non-biodegradable organic wastewater before secondary biological treatment.

UV/ozone photochemical systems enhance ozone decomposition to produce ∙OH radicals, improving oxidation efficiency for complete mineralization of organics (e.g., xylene into water and CO₂).

What are the precautions for using chlorine dioxide?(1) In water treatment, the dosage of chlorine dioxide is generally 0.1–1.5 mg/L, with specific amounts varying based on raw water properties and application purposes:

0.1–1.3 mg/L when used solely as a disinfectant;

0.6–1.3 mg/L when used as both a disinfectant and deodorant;

1–1.5 mg/L when used as an oxidant to remove iron, manganese, and organic matter.

(2) As a strong oxidant, chlorine dioxide must be transported and stored using corrosion-resistant, oxidation-resistant inert materials. Contact with reducing agents should be avoided to prevent explosions.

(3) When producing chlorine dioxide on-site, measures must be taken to prevent excessive accumulation of chlorine dioxide in the air (which may cause explosions). Equipment for collecting and neutralizing gas released or leaked during production is generally required.

(4) Work areas and product storage rooms must be equipped with ventilation systems, monitoring and alarm devices, and protective gear should be available outside the doors.

(5) Stabilized chlorine dioxide solution is non-toxic but releases chlorine dioxide only after activation. Activation must control reaction intensity to avoid explosive gas accumulation.

(6) Chlorine dioxide solution should be sealed in dark plastic barrels, stored in a cool, ventilated area away from direct sunlight and air contact. During transportation, avoid high temperatures, strong light, and ensure stable handling.

What are the preparation methods of chlorine dioxide?Chlorine dioxide can be prepared through various methods. In water treatment, it is typically produced by reacting chlorine, hydrochloric acid, or dilute sulfuric acid with sodium chlorite or sodium chlorate, or by acidifying sodium hypochlorite and reacting it with sodium chlorite. The reaction formulas are as follows:

2NaClO₃ + 2NaCl + 2H₂SO₄ → 2ClO₂ + Cl₂ + 2Na₂SO₄ + 2H₂O

Cl₂ + 2NaClO₂ → 2ClO₂ + 2NaCl

5NaClO₂ + 4HCl → 4ClO₂ + 5NaCl + 2H₂O

10NaClO₂ + 5H₂SO₄ → 8ClO₂ + 5Na₂SO₄ + 4H₂O + 2HCl

NaClO + 2HCl + 2NaClO₂ → 2ClO₂ + 3NaCl + H₂O

For chlorine-sodium chlorite synthesis, a chlorine solution with pH < 2.5 is first prepared, then mixed with a 10% sodium chlorite solution in a reaction chamber to generate chlorine dioxide. Theoretically, 10g of sodium chlorite reacts with 3.9g of chlorine to produce 7.5g of chlorine dioxide. Excess chlorine (beyond the theoretical amount) is usually added to prevent unreacted sodium chlorite from entering the water.

Other methods follow similar procedures. To ensure safety, acids, sodium chlorate, or sodium hypochlorite are prepared as aqueous solutions, and excess acid is added to improve conversion rates. The resulting ClO₂ solution is directly dosed into water for disinfection at appropriate concentrations.

Domestic markets offer electrolytic devices claiming to produce chlorine dioxide, but these actually generate a mixture of chlorine dioxide and chlorine. They cannot eliminate chlorinated hydrocarbons (a issue with chlorine-based disinfectants) and have even caused explosions in some cases.

What are the precautions for using ozone?(1) Ozone is toxic, strongly irritating to eyes and respiratory organs. Normal atmospheric ozone concentration is (1–4)×10⁻⁸ m³/m³; concentrations of (1–10)×10⁻⁶ m³/m³ cause headaches and nausea. China’s Hygienic Standard for Industrial Enterprises (GBZ1–2002) limits workplace ozone to 0.3 mg/m³.

(2) Ozone is highly unstable, decomposing into oxygen with heat release under normal conditions. Decomposition accelerates with higher temperature and concentration in air; in water, it decomposes faster, catalyzed by hydroxide ions (faster at higher pH). Thus, it cannot be stored or transported and must be produced on-site.

(3) Ozone is highly corrosive, reacting with almost all elements (except Pt, Au, Ir, F). Containers, pipes, and diffusers in contact with ozone must use corrosion-resistant materials (stainless steel, ceramics, PVC) or be coated with anti-corrosion layers.

(4) Ozone solubility in water is only 10 mg/L, so not all ozone injected into wastewater is utilized. To improve efficiency, contact reactors are often deep (5–6 m) or closed, multi-stage in series, with tubular or plate micro-porous diffusers for ozone dispersion.

What are the preparation methods of ozone? Ozone is produced via chemical, electrolytic, ultraviolet, irradiation, and silent discharge methods. Water treatment primarily uses silent discharge: high-voltage AC (15,000–17,500 V) is applied across a dielectric (e.g., special glass) with a discharge gap, creating a uniform blue-purple corona. Purified, dried air or oxygen passing through this gap undergoes electron excitation, forming ozone (O₃) from O₂ molecules. Production consumes 20–30 kWh per kg of ozone.

Ozone generators use air or oxygen as feedstock. Air must be purified and dried to prevent electrode damage and yield reduction. Oxygen with 92–99% purity gives the highest ozone output (impurities may act as catalysts). Foreign generators achieve up to 250 kg/h, enabling large-scale disinfection.

To enhance ozone dissolution, contact tanks are often 5–6 m deep, with ozone introduced as micro-bubbles for rapid mixing. Injection methods include static mixers, Venturi tubes, and micro-porous aeration, with contact times of several minutes and dosages of 1–5 mg/L (depending on water quality). Ozone oxidation systems typically include air purification/drying units, generators, and water-ozone contact tanks.

Principle of UV sterilization

UV sterilization destroys or alters microbial DNA, killing bacteria or inhibiting reproduction. UVC (especially ~253.7 nm) is most effective, as it is readily absorbed by DNA.

UV germicidal lamps emit primarily 254 nm (directly damaging DNA) and 185 nm (converting O₂ to ozone) wavelengths. Ozone, a strong oxidant, complements UV’s line-of-sight limitation by diffusing to sanitize shadowed areas.

UV sterilization is a physical method with advantages: simplicity, broad-spectrum efficiency, no secondary pollution, and ease of automation. New lamp designs have expanded its applications.

Structure of UV germicidal lamps

UV lamps are low-pressure mercury lamps, similar to fluorescent tubes, using low-pressure mercury vapor (<10⁻² Pa) excited to emit UV. Unlike fluorescent tubes (which use glass blocking 254 nm UV, converted to visible light via phosphors), germicidal lamps use quartz glass (transmitting 80–90% of all UV wavelengths).

Lamps include hot-cathode and cold-cathode types, with various shapes and power ratings. Due to differing thermal expansion coefficients, quartz cannot be sealed with aluminum bases; lamp bases are instead made of bakelite, plastic, or ceramics.

Tubes of UV germicidal lamps

Cost and application needs may lead to using high-borosilicate glass (UV transmittance <50%) instead of quartz. Cheaper to produce (similar to energy-saving lamps), high-borosilicate tubes suffer rapid UV decay (50–70% of initial intensity after 数百 hours), compared to quartz (80–70% after 2,000–3,000 hours).

Some glass types offer higher UV transmittance than high-borosilicate (but lower than quartz) but with greater decay and no ozone production. Philips uses such glass in certain germicidal lamps.

Types of UV germicidal lamps

Emitting 254 nm (DNA damage) and 185 nm (ozone generation) wavelengths, quartz glass can be doped with titanium (Ti) to block <200 nm UV while transmitting 254 nm. Controlling Ti levels adjusts 185 nm emission, producing low-ozone (ozone-free), standard ozone, and high-ozone lamps.

Applications of UV germicidal lamps

(1) Each microorganism has a specific lethal UV dose (dose = intensity × time, K = I·t). High intensity for short durations equals low intensity for long durations.

(2) Quartz tubes age over time, reducing UV intensity. Regular testing is required; replace lamps when intensity is insufficient.

(3) UV travels in straight lines with weak penetration—paper, lead glass, or plastic significantly reduce intensity. Ensure full exposure of surfaces; clean tubes regularly to maintain transmittance.

(4) UV harms human skin and eyes. Avoid using UV lamps in occupied areas or staring at lit tubes. Shortwave UV cannot penetrate ordinary glass, so goggles offer protection.

(5) Ozone lamps are unsuitable for occupied spaces: ozone promotes hemoglobin coagulation, causing oxygen deprivation, dizziness, or nausea. Concentrations >0.3 ppm (mg/m³) are harmful.

(6) The bluish-purple glow of low-pressure lamps reflects mercury vapor pressure, not directly indicating UV intensity (which cannot be judged visually).

(7) Reflectors concentrate UV energy and protect workers. Use materials with high reflectivity for 253.7 nm UV (e.g., polished aluminum, optimal for shortwave UV).