Polluted water contains harmful substances like chemicals, microorganisms, or waste materials, making it unsafe for consumption or environmental health. Cleaning this water is a fundamental step in protecting human health, preventing waterborne diseases, and maintaining ecological balance in aquatic ecosystems. This process also helps preserve biodiversity and supports sustainable access to clean resources worldwide.
Fundamental Principles of Water Treatment
Water treatment relies on a combination of physical, chemical, and biological processes to remove various contaminants. Physical processes often involve separating impurities based on their size or density. Sedimentation, for instance, allows heavier particles to settle out of the water naturally over time, while filtration physically strains water through a barrier to trap suspended solids.
Chemical processes introduce specific substances to alter contaminants, making them easier to remove or neutralizing their harmful effects. Coagulation involves adding chemicals like aluminum sulfate or ferric chloride, which cause tiny suspended particles to clump into larger, heavier flocs. Disinfection, another chemical process, uses agents such as chlorine to kill or inactivate pathogenic microorganisms, ensuring water is safe for consumption.
Biological processes use microorganisms to break down organic pollutants. Beneficial bacteria consume dissolved organic matter, converting it into harmless byproducts like carbon dioxide and water. This approach is effective for removing biodegradable contaminants and plays a role in many large-scale treatment facilities.
Household and Community-Level Purification Methods
Boiling is a common method to purify water, killing most bacteria, viruses, and parasites by heating water to a rolling boil for at least one minute, or three minutes at elevations above 6,500 feet. This process does not remove chemical pollutants or heavy metals. Simple filtration techniques, such as sand or ceramic filters, physically remove larger particles and some microorganisms. Sand filters use layers of sand and gravel, while ceramic filters feature tiny pores that prevent the passage of bacteria and protozoa.
Solar disinfection (SODIS) uses sunlight to purify water stored in clear plastic bottles. Exposure to direct sunlight for at least six hours, or two days in cloudy conditions, allows UV-A radiation and heat to inactivate pathogens like bacteria, viruses, and protozoa. This method is useful in areas with limited access to fuel or electricity. Chemical disinfectants, such as household bleach (sodium hypochlorite) or iodine tablets, also kill harmful microorganisms. For bleach, add two drops of unscented household bleach (5.25% to 6% sodium hypochlorite) per liter of clear water, allowing it to stand for 30 minutes before consumption.
Iodine tablets are another option, requiring one tablet per liter of water with a 30-minute waiting period. While effective against many pathogens, these chemical methods may leave a slight taste and are not effective against all types of cysts, like Cryptosporidium. Long-term use of iodine is not recommended for pregnant women or individuals with thyroid conditions.
Industrial and Large-Scale Water Treatment Processes
Large-scale water treatment plants begin with screening, removing large debris like leaves, sticks, and rags from incoming raw water using metal screens. This initial step protects equipment from damage and prevents blockages in subsequent stages. Following screening, water undergoes coagulation and flocculation, where chemicals such as aluminum sulfate or ferric chloride are added to neutralize suspended particles, causing them to clump into larger aggregates called flocs.
These heavier flocs settle out during sedimentation, a process where water flows slowly through large basins, allowing gravity to pull flocs to the bottom. The clarified water then moves to filtration, passing through layers of sand, gravel, and sometimes activated carbon. Rapid sand filters remove remaining suspended particles, protozoa, and some bacteria. More advanced methods, such as membrane filtration (including reverse osmosis or ultrafiltration), use semi-permeable membranes with microscopic pores to physically block contaminants like dissolved solids, viruses, and bacteria.
The final stage is disinfection, which ensures any remaining harmful microorganisms are inactivated before the water is distributed. Chlorination, the most widely used method, involves adding chlorine gas or chlorine compounds to the water, effectively killing bacteria and viruses. Ultraviolet (UV) light disinfection exposes water to UV radiation, which damages the DNA of microorganisms, preventing them from reproducing. Ozonation, another disinfectant, uses ozone gas (O3) to oxidize and destroy pathogens, as well as break down organic compounds that cause taste and odor issues. Each of these disinfection methods serves to provide a final barrier against waterborne diseases, ensuring the safety of the treated water for public consumption.