There are many methods for disinfecting hospital wastewater, broadly categorized into physical and chemical methods.
Physical methods include radiation, ultraviolet light, heating, and freezing. Physical methods are generally suitable for smaller volumes of wastewater, and their treatment effect is often less pronounced than chemical methods. However, a significant advantage is the absence of secondary pollution. Ultraviolet light disinfection is a commonly used physical method, offering advantages such as speed, simple equipment, easy maintenance, and no secondary pollution. However, its disadvantages include strict pretreatment requirements, small treatment volume, susceptibility to organic interference, and lack of continuous disinfection.
Chemical methods include treatment with halogens, ozone, heavy metal ions, and cationic surfactants. Among these, chlorination and ozone disinfection are commonly used. Ozone disinfection has excellent sterilization effects, but ozone preparation and maintenance costs are high, and the equipment is difficult to manage.
1. Chlorination Method Chlorination treatment is further divided into liquid chlorine and chlorine dioxide methods, depending on the type of chlorine added.
1.1 Liquid chlorine disinfection is widely used for disinfecting tap water and hospital wastewater due to its strong disinfection ability and low price. Liquid chlorine has a high chlorine concentration, with an effective chlorine content of over 99%, which is 5-10 times higher than sodium hypochlorite solution. However, chlorine gas is a yellow, toxic gas with an irritating odor, requiring specialized storage and chlorination equipment. Liquid chlorine (Cl2) reacts with ammonia to produce monochloramine, dichloramine, and trichloramine, consuming the liquid chlorine. It can also form trihalomethanes (THMs), which have carcinogenic effects. Combined with the incompleteness of liquid chlorine, its disinfection capabilities are limited.
1.2 Chlorine dioxide (ClO2) has a solubility in water that is 5 times that of chlorine, and its oxidizing power is approximately 215 times that of chlorine gas, making it a strong oxidizing agent. It is internationally recognized as the only highly effective disinfectant among chlorine-containing disinfectants. It can kill all microorganisms, including vegetative bacteria, spores, fungi, mycobacteria, and viruses. It can effectively destroy trace organic pollutants in water, such as benzo[a]pyrene, anthraquinone, chloroform, carbon tetrachloride, phenols, chlorophenols, cyanide, hydrogen sulfide, and organosulfur compounds. It can also effectively oxidize some reduced-state metal ions in water, such as Fe2+, Mn2+, and Ni2+. The biggest advantage of chlorine dioxide is that it reacts with humic substances and organic matter without producing any dispersive organic halides, does not generate and inhibits the formation of carcinogenic trihalomethanes, and does not react with ammonia or amino compounds.
2. Ozone Method The manufacturing cost varies depending on the amount of ozone generated. Generally, ozone generators are relatively expensive. Because hospital wastewater discharge standards explicitly state that hospital wastewater must not contain enteropathogenic bacteria, tuberculosis bacteria, or total coliforms exceeding 500 per liter, or when using chlorination for disinfection, the standard is based on contact time and residual chlorine content in the effluent from the contact tank. Furthermore, ozone disinfection is more expensive than chlorination. Therefore, most hospitals use chlorination to treat their wastewater.
