Introduction
As residents continue to raise their standards of living, the impact of secondary treated wastewater on urban water bodies has garnered increased attention, prompting concerns about health and safety issues. Disinfection is one of the fundamental methods to inactivate these pathogenic microorganisms. Therefore, wastewater disinfection has become a crucial step in sewage treatment, and water treatment professionals are continually exploring the best methods for wastewater disinfection.
I. Various Disinfection Methods
1. Physical Disinfection Method – Ultraviolet (UV) Disinfection
1.1 Principles of UV Disinfection
Ultraviolet (UV) disinfection is a physical disinfection method that does not kill microorganisms but rather renders them inactive by disrupting their ability to reproduce. The primary principle of UV disinfection involves using UV light to destroy the genetic material, DNA, or RNA of microorganisms, preventing them from dividing and replicating. Additionally, UV light can also cause damage to other structures within microorganisms. UV light falls within the wavelength range of 136nm to 400nm, with the most effective germicidal wavelength at approximately 260nm. Currently, UV lamps primarily emit at a wavelength of 253.7nm. This wavelength accounts for 90% of the UV energy in top-tier UV lamps worldwide, contributing to 30% of the total energy. Due to the existence of the high-intensity and efficient UV-C spectrum, UV technology has become a competitive disinfection technique in the field of water disinfection.
1.2 Structural Forms of UV Disinfection Units
1) Open Structure: In open UV disinfection units, the water to be disinfected flows through the UV disinfection unit under the influence of gravity, effectively eliminating microorganisms in the water.
2) Closed Structure: Closed UV disinfection units are pressure vessels that enclose the disinfected water, utilizing UV lamps equipped with quartz sleeves.
2.Chemical Disinfection Methods
2.1 Liquid Chlorine Disinfection
1) Principles of Liquid Chlorine Disinfection: When liquid chlorine or hypochlorite (e.g., NaClO) solution is added to water for disinfection, the following reactions occur:
Cl2 + H2O = HCl + HClO (Hypochlorous acid)
ClO- + H2O = HClO + OH-
The sum of HClO and ClO-, known as active free chlorine, has hypochlorous acid (HOCl) as the most effective component. When discharged into water bodies, chlorine reacts with ammonia-nitrogen and organic nitrogen to produce less effective inorganic and organic chloramines, known as combined chlorine. Total residual chlorine refers to the sum of active free chlorine and combined chlorine. The disinfection efficacy of chlorine is influenced by contact time, dosage, water quality (nitrogen compound concentration, suspended solids concentration), temperature, pH, and control systems.
2) Chlorination Systems: Common chlorination systems include chlorine dosing units, contact tanks, mixing equipment, and chlorine cylinders.
2.2 Ozone Disinfection
1) Principles of Ozone Disinfection: Ozone (O3) is an allotropic form of oxygen (O2), typically a blue gas at room temperature and pressure. Ozone possesses strong oxidative capabilities (second only to fluorine) and can oxidize most organic substances. The ozone disinfection process involves physical, chemical, and biological reactions, offering the following effects:
a. Ozone can oxidatively break down enzymes essential for glucose oxidation inside bacteria, rendering them inactive.
b. It directly interacts with bacteria and viruses, damaging their cell walls, DNA, and RNA, disrupting bacterial metabolism, leading to their demise.
c. Ozone infiltrates cell membranes, affecting the outer membrane’s lipoproteins and the inner lipopolysaccharides, causing bacterial cell permeability changes and dissolution. Therefore, ozone is effective for algal disinfection and exhibits substantial inactivation effects on highly resilient microorganisms such as viruses and spores.
2) Wastewater Ozone Treatment Process: Ozone is a powerful oxidant, stable and unaffected by NH3 or pH in wastewater. Its final byproducts are carbon dioxide and water, eliminating the formation of carcinogenic substances.
2.3 Chlorine Dioxide Disinfection
Chlorine dioxide has five times the solubility in water compared to chlorine, with an oxidation capacity around 2.5 times that of chlorine gas. It is a potent oxidizing agent and is recognized internationally as an efficient disinfectant. Chlorine dioxide primarily functions as an oxidizer and does not exhibit chlorinating behavior. Consequently, it typically does not produce carcinogenic substances. The disinfection efficiency of chlorine dioxide is similar to that of chlorine gas. However, in wastewater with high NH3-N concentrations, chlorine consumption significantly increases, but chlorine dioxide is not affected by NH3, so its dosage remains unaffected. Additionally, chlorine dioxide disinfection is not pH-dependent. Due to its instability and explosive nature, chlorine dioxide must be generated on-site and used immediately. Chlorine dioxide generation involves the use of sodium chlorite (NaClO2) as the raw material, making it more complex and expensive than other disinfection methods. As a result, the use of chlorine dioxide disinfection is currently limited to small and medium-sized wastewater treatment projects.
II. Comparative Analysis of Disinfection Methods
The table below summarizes the current application of various disinfection methods in China. Liquid chlorine disinfection is widely used in large sewage treatment plants due to its low operating cost, simplicity, mature technology, easy access to chemicals, accurate dosage, residual disinfection effect, and minimal equipment requirements. Small and medium-sized wastewater treatment plants primarily employ chlorine dioxide and UV disinfection. However, UV disinfection may not be suitable for water with low UV transmittance, such as untreated or primary-treated wastewater and wastewater with high suspended solids (>30 mg/L). In these cases, UV disinfection may increase energy consumption and yield subpar disinfection results. For secondary-treated wastewater and reclaimed water with a UV transmittance of 40% to 80%, UV disinfection remains a favorable choice. Ozone disinfection is predominantly used in water treatment, offering strong disinfection and color removal capabilities. It is often complemented by chlorine dosing to ensure compliance with residual chlorine requirements.
Comparative Analysis of Disinfection Methods
| Disinfection Method | How It’s Used | Advantages | Disadvantages |
|---|---|---|---|
| Chlorination | Chemical dosing of chlorine gas, sodium hypochlorite, or chlorine dioxide into water. | – Strong disinfection capability | – Formation of potentially harmful disinfection by-products. |
| – Proven technology | – Safety concerns associated with chlorine gas. | ||
| – Residual disinfection effect | – Increasingly restricted use due to disinfection by-product concerns. | ||
| Ultraviolet (UV) | Exposure of water to UV light in specially designed reactors. | – No chemical dosing required | – Limited effectiveness in water with low UV transmittance. |
| – No addition of odors or tastes | – Equipment maintenance costs. | ||
| – No toxic by-products | – Potential for microorganism reactivation. | ||
| – Fast and efficient disinfection | |||
| – Simple and safe operation | |||
| Ozone | Injection of ozone (O3) gas into water. | – High oxidation potential | – Ozone generation on-site. |
| – Strong disinfection capability | – Potential explosion risk. | ||
| – Efficient color and odor removal | – Complexity and cost of on-site ozone generation. | ||
| – Inactivation of a wide range of microorganisms | |||
| Chlorine Dioxide | Injection of chlorine dioxide (ClO2) gas into water. | – Strong oxidizing capacity | – Complex and costly on-site generation. |
| – Does not produce chlorine by-products | – Limited to small and medium-sized wastewater treatment projects. | ||
| – Minimal pH dependence |
Conclusion
The choice of wastewater disinfection method is influenced by various factors, including water quality, treatment plant size, safety concerns, and operational costs. UV disinfection offers the advantage of not requiring chemical dosing and has proven to be efficient and cost-effective for wastewater with moderate UV transmittance. Liquid chlorine disinfection is widely used for its affordability and strong disinfection capabilities, although concerns about disinfection by-products have led to restrictions on its use. Ozone disinfection is suitable for water treatment, providing excellent disinfection, color, and odor removal, though it necessitates on-site ozone generation. Chlorine dioxide disinfection has potent oxidizing capabilities but is limited to small and medium-sized wastewater treatment projects due to the complexity and cost of on-site generation.
Selecting the appropriate disinfection method should be based on a thorough evaluation of specific wastewater characteristics and treatment plant requirements to ensure effective and safe water disinfection.
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