Sewage Treatment And How Treatment Plants Work
U.S. homes and businesses send about 34 billion gallons of wastewater to sewage treatment plants every day, according to the U.S. EPA. The wastewater contains nitrogen and phosphorus from human waste, food and detergents, as well as industrial chemicals and pesticides, pharmaceuticals, and other toxic waste. Eventually, in one fashion or another, this waste—solid and liquid—needs to be released or disposed of in the environment.
How effectively it’s treated, that is, how well pollutants are removed before discharge, impacts the water in which we swim and bathe, the water we drink, and the health of our environment and all forms of life. The methods used to treat wastewater depend on a several factors, including the source and volume of the wastewater, the cost involved, the energy available, and political and public will.
Methods also change as science advances. That includes the science used to treat wastewater—as well as the science used to create harmful substances that enter the wastewater stream (e.g., fire retardants and disinfectant byproducts.) SUEZ consistently rises to the challenge with advanced technology that has been used successfully at every stage of the sewage treatment process.
Ultrafiltration ensures a constant and perfect water quality regardless of variations in the quality and turbidity of the water to be treated. In addition, this method is more natural than other methods of purification, reducing the use of chemicals and waste treatment, while maintaining the mineral balance of the water.
Ultrafiltration assures the highest quality water available. The finished product is on average 200 times cheaper than bottled water and has far less of an environmental impact.
Ultrafiltration water treatment systems are used in a variety of settings, including:
- The production of high-quality water for product formulations or for cleaning equipment. For example, in the food and beverage industries, ultrafiltration is used to produce bottled water and other beverages and rinsing lines. This is typically referred to as process water.
- In the pharmaceutical, cosmetics and other industries, ultrafiltered water is used with ion-exchange resin or reverse osmosis during production. Ultrafiltration helps protect resins and reverse osmosis membranes.
- In the oil and gas industries, ultrafiltered water is used in pretreatment for desulfatation and seawater prefiltration.
- In energy systems, as boiler feed water pretreatment and cooling water.
- Ultrafiltered water is also used in instances of wastewater reuse. In these instances, it provides an excellent bacterial barrier and guarantees safety and cleanliness of tertiary water.
- Municipalities also use ultrafiltration for the production of drinking water. It removes turbidity, bacteria and viruses without the use of chemicals in the finished drinking water. It also produces a dependable supply of drinking water, regardless of the level of turbidity and bacterial contamination of the raw water. Ultrafiltration is also important to produce drinking water for remote areas, emergency situations and in small communities.
Often, one or more sedimentation tanks, also known as clarifiers, are used in the process. They are often large and circular or rectangular in shape. The waters are nearly still, but they do flow. An important aspect of the sedimentation stage is that the speed with which gravity pulls the solids vertically to the bottom of the tanks is greater than the speed with which the water travels vertically from ingress to egress. If several sedimentation tanks are used, the water becomes clearer as it moves from tank to tank.
Several types of sedimentation methods used:
Tube and plate settlers
Tube and plate settlers increase flow of water through rectangular tanks. The tube settlers consist of a series of tubes or plates that are installed at a 60-degree angle. The flow is directed up through the settlers. Because suspended solids typically flow at an angle different than water, they contact the tubes or plates at some point before reaching the top. After particles have been removed from the flow and collected on the tubes, they tend to slide down the tube or plate and back into a sludge zone.
Sludge Blanket and Solids-Contact Clarification
Solids-contact refers to units in which large volumes of sludge are circulated internally. Bringing the incoming raw water into contact with recirculated sludge improves the efficiency of the softening reactions and increases the size and density of the solids particles, known as floc. The solids-contact method is used in clarifiers with an upflow design, so called because its center is shaped like an inverted cone and pushes water upward through a suspended layer of previously formed floc. As floc becomes heavy enough, it sinks to the bottom.
In the solids-contact clarifier, raw water first undergoes coagulation and flocculation. Then this water is mixed so that small particles are trapped in the larger floc. Water passes out of the inverted cone into a settling zone. There, solids fall to the bottom and clarified water flows over a weir. Solids are drawn back into the primary mixing zone, causing recirculation of the large floc.
In some designs, water is pushed from the bottom of the clarifier through a blanket of suspended solids that acts as a filter. This is known as a sludge-blanket clarifier. As the water containing flocculated solids passes up through this blanket, the particles are absorbed onto the larger floc, which increases the floc size and drops it down to a lower level. It eventually falls to the bottom of the clarifier to be re-circulated or drawn off.
Dissolved Air Flotation (DAF)
Additionally, another technology, known as dissolved air flotation (DAF), is sometimes used in conjunction with sedimentation. DAF technology is an effective clarification method for low-density solids that sedimentation cannot remove in applications such as drinking water, process water, and wastewater treatment. As opposed to sedimentation, where solids sink to the bottom, DAF saturates solids so that they float to the top for removal.
Here’s how DAF works: Solids are floated in the clarifier by chemical coagulation and flocculation, and by adding microbubbles. The microbubbles are created in an unpacked saturator which combines 8 percent to 15 percent recycled from the clarified water with compressed air. The pressurized air/water mixture is sent through a row of nozzles or special injector depending on the type of DAF technology. For some applications, nitrogen is used as the flotation gas. A pressure drop brings the air out of solution and creates microbubbles, which adhere to the solids and float them to the top of the floatation zone. Clarified water is collected from the laterals at the bottom of the DAF.
In SUEZ’ Haworth, N.J., treatment plant, the DAF system cuts particulate matter during pretreatment by more than 90 percent.
While industrial wastewater can be treated extensively, it generally is not used to create potable water. More typically industrial wastewater is treated to a level where it can be safely discharged into the environment, which serves as a buffer. Before it is discharged, industrial wastewater is often subject to an industrial pretreatment program (IPP) tailored to a specific industry, such as oil and gas processing, and food and beverage production.
In addition to the IPP, industrial wastewater treatment typically follows these basic steps:
- Step 1: screening, first to remove large items and second grit.
- Step 2: primary clarification to separate solid organic matter.
- Step 3: aeration to encourage conversion of NH3 to NO3 and provide oxygen for bacteria to grow.
- Step 4: secondary clarification to allow remaining organic sediment to settle, often through chemical treatment.
- Step 5: disinfection using chlorination, UV, or other methods.
- Step 6: discharge of the water; and disposal of solids; reclamation of biosolids/biogas.
SUEZ’ ultrafiltration technology has been deployed in thousands of businesses and municipal operations around the globe. We offer a large catalog of membrane solutions, including hollow-fiber ultrafiltration membranes. Our smart ultrafiltration solutions for municipal and industrial clients to produce safe and clean water. These systems are adapted to all sizes of municipal or industrial installations from 50 to 100,000 m3/day.
Contact a representative to learn more about wastewater treatment solutions offered by SUEZ North America.