Water treatment removes contaminants and disease-causing elements from drinking water. Pollutants can include harmful bacteria, heavy metals such as lead, cadmium and arsenic, sediment and chemicals.
For a simple home water treatment, fill clean clear bottles with clear water and leave them in the sun. The UV rays kill infectious germs such as viruses, bacteria and parasites (Giardia and Cryptosporidia). This is called solar disinfection.
Biological Treatment
Biological treatment involves using bacteria, nematodes or other microorganisms to help break down organic materials in wastewater. Wastewater contains a buffet of organic substances such as garbage, wastes and partially digested foods plus heavy metals, toxic chemicals and pathogenic organisms. If this untreated wastewater is released into the environment, it can damage wildlife and create water-borne diseases and ecological imbalances.
During the biological drinking water treatment process, organic matter in the wastewater is broken down by free-floating bacteria and other microorganisms that use it for food. Complex organic molecules are systematically broken down and disassembled to simpler organic molecules, such as carbon dioxide and water. Once the organic material is decomposed, the resulting sludge is removed from the wastewater by sedimentation. The water then goes through secondary clarification and disinfection to meet environmental discharge standards before it is pumped to storage or reused.
Biological wastewater treatment processes are based on the growth of bacterial communities that mediate oxidation-reduction reactions (see Figure 1). These reactions can be heterotrophic or autotrophic. Heterotrophic reactions require an organic electron donor; autotrophic reactions use an inorganic one, such as hydrogen. Biological wastewater treatment is effective and less expensive than many mechanical or chemical treatments. Biological processes also have the added advantage of producing sludge that is easy to handle and dispose of.
Chemical Treatment
Chemical water treatment uses a variety of chemicals to treat water. These chemicals are used to disinfect water and make it safe to drink, to kill algae and to remove harmful substances from the water supply. They also help to break down foams and prevent corrosion in boilers.
The first step in a chemical water treatment process is to screen out coarse contaminants with screens. This reduces the amount of sediment and other large particles in the wastewater. The next step is to add chlorine, which disinfects the water and kills bacteria. This is followed by aeration or circulation tanks to slow down the velocity of the water, so that it can settle.
Other chemical treatments include coagulation and sedimentation to remove coarse suspended materials from the water, chemical softening and demineralization (cation and anion exchange). Chemicals can also be added to improve the characteristics of the waste water, such as microbial impregnation or carbonation.
Another common type of water treatment is to oxidize the pollution in the wastewater, using chemical oxidants such as chlorine and hydrogen peroxide. These reactions cause electrons to travel from the oxidant into the pollutants in the wastewater, modifying their structure and making them less harmful. Oxidation is also used to destroy organic compounds like antibiotics and cytostatic drugs that might be in the wastewater.
Physical Treatment
The physical treatment of water is a process used to separate out contaminants from the water. This is done by using a variety of methods such as coagulation and flocculation, sedimentation, centrifugal separation, and filtration. These processes involve the use of equipment like racks, screens, comminutors, clarifiers (sedimentation and flotation) and filters.
The simplest physical method involves passing water through a filter media that has a specific pore size. This allows impurities that are smaller than the pore size to pass through and concentrates those with higher molecular weight, thereby producing clean water. The coagulation method uses various chemicals that cause the impurities to stick together and form larger particles, or flocs. These are then separated from the water by gravity or adsorption (selective absorption).
Chemical treatments can also be applied during this phase of water treatment. Examples of these include disinfection to inspect or inactivate pathogens and phosphate removal, which is important because our wastewater is often contaminated with phosphates from detergents, fertilisers, food additives, and faeces. When they remain in the wastewater, they lead to overfertilisation and useless plant growth – a process known as eutrophication – that is harmful to our environment.
Other methods of physical treatment include removing bulky matter with sieves, grit and debris from the surface of water, or reducing the amount of fat, oil and grease in sewage through a de-greasing process. Another method is membrane separation, such as reverse osmosis and nanofiltration. This forces water at high pressure through semi-permeable membranes that prevent the passage of some substances based on their molecular weight.
Mechanical Treatment
Water treatment plants often use mechanical processes to physically separate contaminants from water. This step is important, as it protects the rest of the treatment plant from debris that could clog pipes and disrupt other processes. Screens are used to remove leaves, twigs, paper and other materials that may pose a risk of obstructing or damaging equipment. This is also known as Pre-Treatment.
Next, coagulation occurs to remove fine particles from the water. Chemicals with a positive charge are added to the water to neutralize its negative charge, which causes the particles to bind together and form larger, more solid pieces (flocs). Common chemicals used for this process include specific types of salts, aluminum and iron. The flocs are then separated from the water by sedimentation and filtration. The filtered water is then disinfected using chlorine or other chemicals.
Some water treatment plants may choose to skip this step if they are using a source of water that already has low levels of contaminants. This method is used by small water utilities that may have limited technical and financial capacity.
In biological treatment, bacteria consume contaminants by reacting with them in a vessel or basin called a bioreactor. This process is aided by adding an electron donor and nutrients to the water. The bacteria then create biomass and other nontoxic byproducts.