The water treatment process is comprised of a series of steps to remove or minimize impurities from water and make the water suitable for a particular purpose.
The purpose can anything from recreational use of water to drinking purposes. The quality criteria and standards are determined in relation to the suitability of the water for that particular purpose
The chain of processes involved in water treatment include chemical, biological and physical processes. The combination of steps, devices and processes is usually referred to as the ‘treatment train’.
The required treatment train is governed by the specific characteristics of the source water that is to be treated, and by the quality criteria for the intended use. Additional considerations include capital for the initial investment, operating costs and residuals.
For example, treating surface water for potable purpose may include coagulation, flocculation, sedimentation, filtration and disinfection. Ground water, on the other hand, usually require a lower level of treatment, because it contains less sediments and is less exposed to pollutants. However, ground water usually contains higher level of dissolved solids, iron and manganese. Therefore, the treatment process may include softening, oxidation, desalination etc.
Treating wastewater requires a more complex process and treatment may also include processes such as grit removal, activated sludge, carbon adsorption and denitrification.
Let’s briefly describe some of the main water treatment processes and unit operations:
Coagulation-flocculation – In the coagulation process, a chemical that neutralizes the negative charges of the particles and allows them to aggregate is added to the water.
Flocculation refers to a gentle mixing of the water, that brings the particles to bind together and form larger particles, which can be removed by sedimentation or conventional filtration.
Filtration – Filtration is a water treatment process of separation and removal of solid particles and other impurities, such as color and taste, from water. This is achieved by passing the water through a porous media.
There are several filtration methods, including slow sand filtration, rapid sand filtration, membrane filtration and more.
Sedimentation – This process is used to remove suspended solids from water. Decreasing the velocity of the water allows suspended solid particles to settle out by gravity, as the water flows slowly through the sedimentation tank.
Biological water treatment processes – There are several biological treatments that are used to remove organic contaminants from water. For example, activated sludge, trickling filters and membrane bioreactors. Activated sludge is the most common process used in wastewater treatment. This is an aerobic biological process in which bacteria are used to biodegrade organic material.
As the bacteria grow, they settle to the bottom of the tank. This biological floc is referred to as “activated sludge”. Some of the settled sludge is returned to the tank in order to keep the process continuous.
Disinfection – The disinfection process is aimed to inactivate microorganisms, such as fungi, bacteria and viruses. Disinfection methods include chemical disinfection (e.g. with chlorine, chlorine dioxide or ozone), disinfection with ultraviolet light and thermal disinfection.
Desalination – Refers to the removal of dissolved salts from water. Desalination methods include reverse osmosis, ion exchange, nanofiltration, distillation and other methods. Ion exchange remove specific, undesired ions from water by exchanging them with other ions. For example, it is used for water softening. Reverse osmosis, on the other hand, removes most ions from water and no ‘new’ ions are added to the water.
There are many water treatment processes and unit operations available. Designing the ‘right’ process requires knowing the intended use of the water, the water quality criteria for the specific intended use and the composition and properties of the water to be treated.