The Processes of a Water Purification Plant
A water purification plant provides clean, safe drinking water to the public. Through a series of processes including coagulation, filtration and disinfection, they remove chemicals, pathogens and other materials that could threaten human health.
Water is screened and pumped into storage tanks for pretreatment. During this process, chlorine is added to start the disinfection process.
Pretreatment
The water treatment process is a complex series of steps designed to remove contaminants and impurities from drinking water. Water purification plants provide clean water to residential and commercial customers and play a crucial role in maintaining public health. Water quality measurement instruments are used throughout the process to ensure the water meets public safety standards.
At our plant in Lake Erie, raw water pumps pull water into treatment tanks from the lake bed through a tunnel several miles long. From here, the water is screened to remove large debris like mussel shells and quagga mussels. It is also treated with Potassium Permanganate, an oxidant that more effectively removes particulates and helps control taste, odor, and color as well as preventing the growth of harmful algae and bacteria.
Next, the water goes through a rapid mix step where aluminum sulfate or alum is added and mixed quickly into the water to induce coagulation. This helps small particles clump together and become larger, heavier flakes called floc that can be more easily removed by filtration. The water then moves into a series of tanks called sedimentation basins where the heavy clumps of floc sink to the bottom and are removed.
After the coagulation and sedimentation processes, clear water is pumped through a secondary settling tank where it is mixed with low levels of chemicals including sodium hypochlorite for disinfection, zinc orthophosphate for corrosion inhibitors, sodium hydroxide for pH balance, and fluorosilicic acid to add fluoride to prevent tooth decay. These chemicals are all regulated by the EPA to ensure they meet all EPA drinking water standards.
Secondary Treatment
During this stage, the water is water purification plant filtered to reduce the soluble organic materials that escape from primary treatment. This step is important because dissolved organic material depletes the oxygen supply of receiving streams, rivers, and lakes. It also contributes to the degradation of water quality.
The sewage water that enters water purification plants contains simple organic materials such as sugars and fats. Microorganisms consume these molecules, converting them to carbon dioxide and water. This depletes the dissolved oxygen in the receiving river, lake, or stream and harms aquatic life. During secondary treatment, the bacteria in the wastewater are partially decomposed under anaerobic conditions in a tank without air. This removes the soluble organic matter and reduces Biochemical Oxygen Demand (BOD).
Wastewater pumped from the aeration basin is pushed into a primary clarifier where it is separated into water and sludge. Heavier particles float to the bottom and become thick sludge, while lighter substances sink to the top and are collected as clear water. The clarified wastewater is pumped to secondary treatment filters and disinfected.
To ensure that the water is safe for drinking, the treated sewage is sampled to monitor its quality. It is also tested for the presence of dangerous chemicals, such as heavy metals. The final water undergoes further treatment, water purification plant supplier including the addition of fluoride to prevent tooth decay and other treatments as necessary.
Tertiary Treatment
The final stages of water purification are needed to polish the final product to meet high quality standards and protect people, the natural environment and other organisms from harmful bacteria like Giardia and Cryptosporidium. Water at this stage is generally filtered again and sometimes disinfected using chlorine or UV radiation before being distributed to users.
Pumping and containment
The vast majority of water treated in a plant must be pumped and directed into pipes or holding tanks. This physical infrastructure must be made from suitable materials and constructed to avoid accidental contamination.
Screening
The first step in surface water treatment, this step removes large debris that may interfere with other processing steps. A screening system typically consists of screens or baskets which the water is forced through to physically remove larger particles. It can also involve a chemical precipitation process where aluminum sulfate or polymer is added to induce coagulation or the formation of larger particles. These larger particles can then be easily removed by a mechanical process in a sedimentation basin.
This stage of pretreatment is often referred to as “coagulation and flocculation.” In this process, chemicals are introduced into the raw water to induce coagulation or the formation of large particles which can then be easily separated by a mechanical process in a sedimentation tank. A rapid mix unit is often used in this step which creates turbulent mixing energies to help thoroughly disperse the primary coagulant, such as aluminum sulfate or polymer, into the raw water.
Filtration
Filtration is a physical process that separates insoluble solids, such as sand grains or organic matter from a liquid. It is a more efficient method of separation than decantation, which simply allows the insoluble material to sink to the bottom. Filtration can also be used to remove undesirable taste or odor from water. This is accomplished through additional treatment processes such as aeration and activated carbon adsorption.
After being screened, the raw water enters a clarifier basin where chemical processes take place to clarify and purify the water. These chemicals are usually inorganic metal salt coagulants, aluminum sulfate being the most commonly used, and synthetic organic polymers which act as aids to the coagulants or sometimes as replacements for the inorganic coagulants. Once the clear water reaches a turbidity level of less than 0.3 Nephelometric Turbidity Units (NTU), it is pumped to secondary settling basins where it will remain until the turbidity reaches an acceptable level to be used for drinking purposes.
Once the clear water reaches the secondary settling basins, it is treated with small amounts of disinfectant chemicals and fluoride in order to meet drinking water standards for human consumption. The disinfectants are mainly sodium hypochlorite and zinc orthophosphate, while the fluoride is added to reduce tooth decay. The water is also disinfected to kill any microorganisms left behind during the filtration and treatment processes and to provide residual disinfection throughout the distribution system.
