However, it is widely accepted that treatment facilities that use aluminum based coagulants often have higher levels of aluminum in their treated water, but not by much. Generally, water treatment facilities have the coagulation process set up so that the coagulant chemicals are removed with the floc. The majority of municipal water treatment plants use aluminum sulphate as the coagulation chemical. And if aluminum sulphate is used, aluminum and sulphate are added. If ferric chloride is used, iron and chloride are added. If ferric sulphate is used, iron and sulphate are added to the water. The particular residuals depend on the coagulant that is used. Residuals are the by-products that remain in the water after substances are added and reactions occur within the water. With accurate dosages and proper application, the residuals of the added chemicals generally do not pose a problem. A municipal water treatment plant can save money by using less chlorine, and the water will be safer, because trihalomethanes (THMs) are a dangerous by-product that results from the reaction of chlorine with NOM. Because coagulation removes some of the dissolved substances, less chlorine must be added to disinfect the water. It is, however, an important primary step in the water treatment process,īecause coagulation removes many of the particles, such as dissolved organic carbon, that make water difficult to disinfect. In the picture below, the coagulants have been added to the water, and the particles are starting to bind together and settle to the bottom.Īs coagulation does not remove all of the viruses and bacteria in the water, it cannot produce Usually, the pathogens that are removed from the water are removed because they are attached to the dissolved substances that are removed by coagulation. In an international report published in 1998, it was found that coagulation and sedimentation can only remove between 27 and 84 percent of viruses and between 32 and 87 percent of bacteria.
While coagulation can remove particles and some dissolved matter, the water may still contain pathogens. A large amount of DOC can give water an unpleasant taste and odour, as well as a brown discolouration. Coagulation can also remove suspended particles, including inorganic What is Removed During Coagulation? What is Added?Ĭoagulation can successfully remove a large amount of organic compounds, including someĭissolved organic material, which is referred to as Natural Organic Matter (NOM) or Dissolved In a settling tank, or clarifier, the heavy particles settle to the bottom and are removed, and the water moves on to the filtration step of the treatment process. The coagulated water can either be filtered directly through a medium filter (such as sand and gravel), a microfiltration or ultrafiltration membrane, or it can be moved to a settling tank. In a water treatment facility, the coagulant is added to the water and it is rapidly mixed, so that the coagulant is circulated throughout the water.
The following diagram illustrates the basic reactions and processes that occur during coagulation. This settling process is called sedimentation. The larger particles, or floc, are heavy and quickly settle to the bottom of the water supply. When this reaction occurs, the particles bind together, or coagulate (this process is sometimes also called flocculation). The positive charge of the coagulant neutralizes the negative charge of dissolved and suspended particles in the water. These chemicals are called coagulants, and have a positive charge. The coagulation process involves adding iron or aluminum salts, such as aluminum sulphate,įerric sulphate, ferric chloride or polymers, to the water. For more information about the disinfection portion of the treatment process, see the Chlorination fact sheet. Worldwide, a combination of coagulation, sedimentation and filtration is the most widely applied water treatment technology, and has been used since the early 20th century. Many water treatment plants use a combination of coagulation, sedimentation, filtration and disinfection to provide clean, safe drinking water to the public. CONVENTIONAL WATER TREATMENT: COAGULATION AND FILTRATION FACT SHEET What is the Conventional Method to Treat Water?