Water treatment plants are located throughout the world and are used to treat raw water. These treatment plants are used to clean water for specific objectives such as:
- Drinking Water
- Irrigation Water
- Industrial Water
- Environmental Water
Most municipal water treatment plants are for potable drinking use as they are the area’s supply of clean drinking water per federal or state water regulations. Irrigation water does not have to meet as strict federal and State requirements as drinking water does. Typically irrigation water will not need to go through the process of post-treatment (secondary treatment) which involves disinfection. The regulations for irrigation water are very similar to those of industrial and environmental water.
Water Treatment Plant Processes
Filtration is the first step in the water treatment process and typically includes screening techonologies to remove larger particles. Two of the most common types of screens are coarse screens and fine screens. These screens are placed at the inlet of the treatment train. Coarse screens are used to remove any debris in the water such as leaves and plastics. Fine screens are typically placed after the coarse screens to remove smaller particles such as dirt and other small particles in the water.
Water conditioning is the process of bringing the water to certain conditions to optimize treatment processes such as flocculation and membrane processes. Conditioning water typically targets specific criteria such as:
- pH level
Typical methods for conditioning water include injecting chemicals to modify the above listed characteristics.
Simply put, coagulation is the process of injecting chemicals into water in order to remove: colloidal, suspended, and dissolved matter. Some common coagulants used in the coagulation process are:
- ferric chloride
- ferric sulfate hydrolyze
- aluminum sulfate (alum)
- synthetic polymers
Coagulation works by destabilizing the particles by reversing the charges. These charges are made negative and repel from the negatively charged water surface. This makes removal of these particles much easier.
Flocculation is the process of removing particles from water by the process of aggregation. This typically follows the coagulation stage and involves a series of mixers with decreasing rotational speeds. The purpose of having mixers with decreasing speeds is to not disturb the floc that form from the contact of particles within the water. As the water moves through the mixers the floc get bigger and are more susceptible to breaking apart. By the end of the flocculation process the floc are scraped off the top of the water.
Sedimentation is the part of the water treatment plant process that involves particles settling out of the water. These particles that fall to the bottom of the sedimentation chamber are removed through a hole at the bottom of the chamber. This is typically used to remove smaller particles that fine screens and flocculation missed.
Membrane filtration is used in many different ways to obtain specific water quality objectives. This can be by physically straining particles out of the water, reverse osmosis, or to treat brackish water. The main purpose of membrane filtration is to remove microorganisms from water.
This membrane process helps in removing solutes from water. This process works by having a pressured water line that pushes water through a semipermeable material. This helps remove smaller particles in the water such as salt.
Rapid Granular Filtration
Rapid Granular Filtration is used for removing particles from water. This is done by having layers of granular mediums or sand. The water is then placed above the medium layers and uses gravity to travel through them. This process removes many small particles and micro organisms that the other processes may of missed.
Disinfection is the process of inactivating or removing microorganisms that are determined to be harmful to human health. This process is typically used to treat surface water as many chemicals and other disease causing agents are present. One of the most common disinfecting agents used is chlorine gas and chloramine.
Over the past decade residual management has become of upmost importance in constructing water treatment plants. Residual mangement is the process of managing, reusing, and disposing of byproducts from the treatment process. The main objective of residual management is to make the process more environmentally friendly and to reduce the costs of treating water. Typical residuals are sludge, treatment chemicals, and treatment media.
Treatment Train Overview
Treatment trains don’t necessarily use all of the mentioned processes. Instead they use specific treatment systems based on the water input and output.
Water Input (Raw Water) – Typically surface water or ground water
Water Output (Water Quality Objective) – potable water, irrigation water, industrial water, environmental water