Question

Describe how water can be disinfected by (a) membrane filtration, (b) reverse osmosis, and (c) ultraviolet irradiation.

Short answer

Water can be disinfected using membrane filtration, reverse osmosis, and ultraviolet irradiation by removing or inactivating contaminants through different mechanisms.

Step-by-step solution

Membrane Filtration Description

In membrane filtration, water is passed through a filter that has tiny pores. These pores are small enough to block the passage of microorganisms such as bacteria and protozoa but allow water molecules to pass through. The size of the pores dictates which contaminants are removed. This method is effective for removing particles or bacteria from water.

Reverse Osmosis Overview

Reverse osmosis involves a semi-permeable membrane that separates contaminants from water. Water is forced through this membrane under high pressure. The pores in the membrane are small enough to remove a wide range of contaminants, including viruses, salts, and dissolved organic substances, effectively purifying the water.

Ultraviolet Irradiation Explanation

Ultraviolet (UV) irradiation uses UV light to kill or inactivate microorganisms. When water flows past the UV light source, the DNA of bacteria and viruses absorbs the light, which disrupts their reproductive capacity, effectively disinfecting the water. This method is safe and leaves no chemical residues in the water.

Key concepts

Membrane Filtration

Membrane filtration is an efficient water disinfection method that utilizes a physical barrier to cleanse water from impurities.
This process involves passing water through a selectively permeable membrane that contains minute pores. These pores are tiny enough to block contaminants like bacteria, protozoa, and suspended particles from passing through.
The pore size of the membrane plays a crucial role in determining which impurities are filtered out.

• The smaller the pores, the more effective the filtration process in removing microscopic contaminants.
• This method does not remove dissolved substances, which may require additional treatment.

Membrane filtration is particularly beneficial as it does not require chemical additives, thus ensuring the natural taste and quality of water are preserved.
Moreover, it is relatively easy to operate and maintain, making it suitable for both large-scale and household water purification.

Reverse Osmosis

Reverse osmosis is a powerful water purification technique that removes a broad spectrum of impurities.
This method operates by using a semi-permeable membrane through which water is forced under high pressure. The membrane allows water molecules to pass while trapping larger molecules of contaminants such as salts, viruses, and various dissolved organic materials.

• Reverse osmosis effectively reduces the concentration of total dissolved solids (TDS), providing cleaner and safer water.
• This process can also desalinate seawater, making it drinkable.

While reverse osmosis is highly effective, it can result in substantial water wastage, as a portion of the input water is needed to flush away contaminants.
It's also important to frequently replace the membrane to maintain the system's efficacy, which can be a potential drawback for some users.

Ultraviolet Irradiation

Ultraviolet (UV) irradiation is a chemical-free method of disinfecting water by utilizing UV light to neutralize pathogens.
When water is exposed to UV light, it penetrates the cells of microorganisms such as bacteria and viruses, disrupting their DNA and rendering them unable to reproduce or cause harm.

• UV irradiation is quick, with disinfection occurring almost instantly as water flows past the UV light source.
• It leaves no chemical residues, making it an environmentally friendly option.

However, it is crucial for the water to be clear and free of large particles before UV treatment, as these can shield microorganisms from the light, reducing effectiveness.
This method does not remove any physical or chemical contaminants, so it is often used in conjunction with other filtration methods to ensure comprehensive water purification.