New Technologies for Medical Wastewater Treatment And Future Demands

New Technologies for Medical Wastewater Treatment and Future Demands

Medical wastewater contains a large amount of bacteria, viruses, various drugs, radioactive substances, as well as oils and organic solvents. If not properly treated, it will pose a serious threat to the environment and human health. With the deepening of research, people have gradually realized the shortcomings of traditional treatment methods and begun to explore more effective treatment technologies.

Overview of Medical Wastewater

Medical wastewater is mainly classified into three types: pathogenic microorganism wastewater, radioactive wastewater, and ordinary domestic sewage. During the treatment process in hospitals, various chemical substances are used, which enter the water through patients' excretion in examination rooms, radiology departments, and wards, making medical wastewater a complex matrix containing a large amount of chemical substances and microorganisms. The content of pharmaceutical products is approximately 10 to 100 times that of ordinary urban wastewater. However, in the past, hospital wastewater was often considered to have the same pollutant load as urban wastewater and was treated together with urban domestic sewage after disinfection. However, studies have found that radioactive substances and micro-pollutants such as pharmaceuticals in medical wastewater are difficult to be completely removed by urban sewage treatment plants, posing a risk of entering the environment. Therefore, it is necessary to classify and treat medical wastewater.

Pathogenic microorganism wastewater comes from wards, clinics and other places, containing pathogenic microorganisms such as coliform bacteria, Staphylococcus aureus, viruses and eggs of parasites. If it flows into the environment, it will not only cause environmental pollution, but also may trigger large-scale infectious diseases. Therefore, appropriate methods should be adopted to disinfect and pretreat it. Radioactive wastewater mainly comes from sewage in clinics and medical equipment in contact with radioactive isotopes. If the radioactive nuclides in it flow into the environment without treatment, they will accumulate through the food chain and increase the risk of human diseases. Ordinary domestic sewage comes from wards, staff dormitories, canteens, etc. The pollutants include large suspended particles, oils, biochemical oxygen-consuming substances, antibiotic drugs and microorganisms such as bacteria and viruses.

Wastewater treatment for pathogenic microorganisms

For the disinfection of wastewater containing pathogenic microorganisms, traditional techniques include liquid chlorine method, chlorine dioxide method, ozone method, ultraviolet method, etc. Currently, there are also new technologies such as advanced oxidation process and nanomaterial disinfection method.

Liquid chlorine was one of the earliest disinfectants applied in wastewater treatment, with an effective chlorine content close to 100% and strong disinfection ability. When it is added to water, it hydrolyzes to form hypochlorous acid, a strong oxidant, which can penetrate the cell walls of bacteria and destroy their enzyme systems, causing bacterial death. However, liquid chlorine disinfection is easily affected by external factors and can react with substances in wastewater to form disinfection by-products such as trihalomethanes, which pose significant toxicological risks to human health. Therefore, researchers are considering the use of other disinfectants.

Chlorine dioxide has highly efficient and broad-spectrum bactericidal effects and a long-lasting action. Its diffusion speed and penetration ability in water are faster than those of chlorine, and it is less affected by external factors. It has a strong killing effect on pathogenic microorganisms in water. For example, low-concentration chlorine dioxide gas can significantly reduce the number of microbial colonies when used to disinfect wards and operating rooms. Chlorine dioxide also has a good effect on destroying the antigenicity of hepatitis B surface antigen. However, the disproportionation products of chlorine dioxide can cause poisoning reactions in animals, and the concentration of intracellular antibiotic resistance genes in wastewater treated with chlorine dioxide increases significantly. This is an urgent problem to be solved in the treatment of medical wastewater with chlorine dioxide.

The ozone method generates highly oxidative atomic oxygen to instantaneously decompose organic matter and microorganisms in water. It can also combine with water to form hydroxyl radicals, effectively eliminating pathogenic microorganisms and providing a continuous disinfection effect. However, the utilization efficiency of ozone is low, and it is greatly affected by water quality. If bromide ions are present in the wastewater, disinfection by-products will be generated. The catalytic ozonation process can overcome these problems to a certain extent.

Ultraviolet (UV) disinfection is a physical disinfection method. It does not require the addition of chemical reagents, can avoid secondary pollution, and is more convenient and safe. It does not produce DBPs and has a broad-spectrum killing effect on various microorganisms. UV disinfection works by irradiating microorganisms, inducing the entanglement of two adjacent thymine bases on one strand of the DNA double helix structure to form a new dimer, thereby disrupting the DNA double helix structure and affecting the normal replication and transcription of RNA. This prevents microorganisms from synthesizing the proteins necessary for their survival and reproduction, ultimately leading to the loss of their survival function and their decline or death. However, the effectiveness of UV disinfection depends on the depth of light penetration in water, which is affected by turbidity. When the concentration of suspended solids in water is above 20 mg/L and the particle size of suspended solids in wastewater is above 40 μm, UV disinfection is not applicable. Additionally, there is a phenomenon of disinfection revival in UV disinfection.

Radioactive and Domestic Sewage Treatment

Hospital radioactive wastewater mainly comes from sewage in diagnosis and treatment rooms, wastewater from washing items that have come into contact with radioactive isotopes, etc. Commonly used radioactive isotopes have short half-lives, low concentrations, and small volumes of water. Usually, the method of constructing a decay pool for storage and decay is adopted for treatment. This method utilizes the natural decay characteristics of radioactive nuclides, allowing their radioactivity levels to gradually decrease in the decay pool until they meet the safety standards for discharge. The design and operation of the decay pool need to take into account factors such as the type of radioactive nuclides, half-life, and volume of water to ensure the treatment effect.

Hospital sewage mainly comes from wards, staff dormitories, canteens, etc. The pollutants include large suspended particles, oils, biochemical oxygen-consuming substances, antibiotic drugs, and microorganisms such as bacteria and viruses. For the microorganisms like bacteria and viruses, chemical oxidation methods are usually adopted for disinfection treatment. As for the organic substances like oils in the sewage, they are generally removed by biological methods, such as aerated biological filters, periodic activated sludge process, moving bed biofilm reactors, etc. These biological treatment technologies utilize the growth of fungi, bacteria and other microorganisms to adsorb and oxidize the pollutants in medical wastewater, achieving good application effects. However, the antibiotic drugs in the sewage can inhibit the growth of microorganisms in the biological treatment process, leading to a decline in treatment efficiency. Therefore, advanced oxidation technologies such as photo-Fenton process and electrochemical oxidation are needed to treat the drugs contained in the sewage.

The demand for medical wastewater treatment technology

The wastewater quality from hospitals with different functions varies significantly and cannot be treated with a uniform process. When treating wastewater within a hospital, it is necessary to collect and treat it correctly based on its quality. The wastewater from the hospital's wards and non-ward areas should be separated, and the sewage and feces from infectious wards should be pre-disinfected and treated in dedicated septic tanks before being combined with other wastewater for treatment. This way, more appropriate treatment methods can be adopted according to the characteristics of different types of wastewater, improving the treatment effect and reducing the impact on the environment. For instance, the wastewater from infectious wards contains a large amount of pathogenic microorganisms, and separate treatment can better control the spread of pathogens.

In the disinfection of medical wastewater, it is necessary to balance the disinfection effect and the disinfection cost. Different projects can combine multiple disinfection methods, such as "ozone + sodium hypochlorite", which can reduce the dosage of disinfectants and the generation of disinfection by-products, and shorten the disinfection contact time.

Nanomaterial disinfection is a key research focus in the field of new disinfection technologies. Traditional wastewater disinfection methods have obvious drawbacks, such as the formation of harmful disinfection by-products and the regrowth of pathogens. Alternative methods like ozone oxidation and electrochemical disinfection are also constrained by low production rates and high energy consumption. Under these circumstances, nanomaterials such as silver nanoparticles, carbon nanotubes, and titanium dioxide nanoparticles, which possess unique biological, electronic, and catalytic properties, offer a feasible and effective option for wastewater disinfection. NAJAFPOORA et al. applied magnetic nanoparticles (MNPs) and silver-loaded magnetic nanoparticles (Ag-MNPs) to wastewater disinfection treatment. The results showed that when the nanoparticle dosage was 105 mg/L, the contact time was 70 minutes, and the pollution level was 55%, total coliforms (TC), fecal coliforms (FC), and heterotrophic bacteria (HB) could be removed by 65%, 48%, and 58% respectively, and the COD removal rate could reach 30.4%, demonstrating the broad application prospects of nanomaterials in wastewater treatment and disinfection. However, due to their small particle size, nanomaterials may be released into the wastewater during the treatment process, causing significant ecological toxicity and reducing disinfection performance and equipment lifespan. Therefore, developing universal and feasible strategies for the recovery and protection of nanomaterials is a key focus of subsequent research.

Nanometer disinfection technology and electrochemical disinfection technology have advantages such as high efficiency and environmental friendliness, but they are still in the research stage and need further exploration and improvement. In addition, it is also important to strengthen the research on electrode materials. Currently, the widely used coated electrodes have superior performance in wastewater treatment, but they have high production costs, strict process conditions, and it is difficult to manufacture large-area plates. The adhesion problem between the coating and the substrate has also become a bottleneck for application. Researchers need to develop new electrode film deposition processes to extend the service life of electrodes and reduce the cost of wastewater treatment.