Water Disinfection: What Are the Physical Methods for Water Sterilization?

Microscopic inhabitants of aquatic environments can be divided into two groups based on their impact: 1. Bacteria, fungi and algae that cause clogging of pipes, heat exchangers, water tanks, filter components, etc., are usually introduced into water from surface sources and reproduce under favorable conditions.
2. Pathogenic and conditionally pathogenic microorganisms, such as viruses, bacteria, fungi and helminth eggs, can cause infectious diseases in humans and animals.

Water disinfection methods are classified according to their principle of action:
- Physical or non-chemical methods, in which disinfection occurs through the influence of physical factors (boiling, ultraviolet, electrolysis, reverse osmosis);
- Chemical or reagent methods, in which certain reagents (chlorine, ozone, non-oxidizing agents) are added to the water;
- Combined methods involve combining both technologies, such as ultrafiltration and chlorination.

Physical Methods of Water Disinfection
Disinfection without the use of chemicals has its advantages. The main advantage of these methods is usually the absence of secondary contamination from disinfectant solutions, but all these methods have a significant drawback — water can be reinfected with microorganisms, as the methods have no prolonged effect.

Boiling Water
This is probably the simplest way to disinfect water in domestic and field conditions. Under the influence of high temperature, the DNA structure of most pathogenic microorganisms is damaged, and they are unable to continue reproducing. Boiling is effective against all spore-forming microorganisms.

It is important to note that for disinfection, water should not merely be brought to boiling point, but should be boiled for at least five minutes.

ADVANTAGES:
- Simplicity of execution;
- No need for additional equipment;
- Effectiveness against most pathogenic microorganisms;
- In addition to disinfection, it reduces water hardness and turbidity.

DISADVANTAGES:
- Significant increase in energy consumption as water volume increases;
- Long processing time;
- Possibility of secondary contamination.

Ultraviolet
Since ancient times, mankind has known about the beneficial effects of sunlight. Thanks to ultraviolet radiation, which is one of the components of the UV spectrum, it is capable of breaking down the thymine structure in the DNA of microorganisms. As a result, bacteria and viruses lose their ability to reproduce both in water and in the human body.

The simplest method of UV water disinfection is the SODIS method. Water, filtered to remove large mechanical particles greater than 50 microns, is poured into PET (polyethylene terephthalate) bottles, which are placed on a surface exposed to direct sunlight. The main disadvantage of this method is the need for active sunlight exposure. The method is most effective in the band between 35 degrees north and south latitude, where disinfection takes approximately six hours. As the intensity of sunlight decreases, the duration of disinfection increases.

Water disinfection devices are produced in the form of cylindrical mechanical tubes with a quartz emitter. Water enters the housing and flows around the sleeve, thereby being exposed to UV radiation and disinfected. The wavelength in most of these devices is approximately 250 nm.

Today, such devices are used for preventive disinfection of drinking water and household water after complex purification systems. Installing such an emitter prevents clogging of pipes, filter elements, washing machines, etc.

ADVANTAGES:
- Easy-to-use method;
- No bulky equipment required;
- No constant reagent dosing required;
- Does not introduce secondary pollution into the water, unlike reagent-based disinfection;
- Low energy consumption.

DISADVANTAGES:
- Not effective against a wide range of microorganisms; combined methods are recommended for water contaminated with pathogens;
- Regular replacement of the irradiation source is required;
- Water must be free of mechanical particles before passing through the device, as these can reduce the method's effectiveness by 50%;
- No prolonged action.

Reverse Osmosis and Ultrafiltration
The pore size of reverse osmosis membranes is 4,000 times smaller than that of the smallest bacterial cells and 200 times smaller than that of viral particles. Therefore, such filters are capable of retaining 100% of microorganisms. The technology is used primarily for drinking water purification. It is the basis for household filters, vending machines, water kiosks and even food and beverage production.

ADVANTAGES:
- 100% removal of viruses and bacteria;
- Compact size and high productivity;
- Environmentally friendly;
- Removal of other toxins in addition to microbiological pollutants: heavy metals, organic substances, chlorine, etc.

DISADVANTAGES:
- Relatively high cost of the technology;
- Large amount of wastewater — between 20 and 70% of output, depending on membrane size and pressure;
- No prolonged action, which limits water use directly at the point of consumption. Alternatively, water requires combined disinfection.

Ultrafiltration membranes have relatively large pore sizes that may partially allow viral particles to pass through, so this method can only be used in combination with reagents or ultraviolet light.