The Sedimentation Process in Water Treatment
There are several methods of sedimentation, and they are often used with other techniques that expedite the process of solid removal. But the principle of sedimentation is that gravity is the prime mover.
When coagulants are used, and they typically are, they are most often one of three common chemicals: aluminum sulphate, polyaluminum chloride (also known as liquid alum or PAC), and ferric sulphate. These coagulants help the suspended solids bond to each other, and thus, become one heavier, single mass.
The addition of the coagulants and the gravitational pull of the solids to the bottom of the holding basin happens slowly, often over four to six hours. When the solids settle to the bottom of the basins—more than one basin is typically used—sludge is then either hydraulically removed over a stationary weir by raising the water level on a set interval, or mechanically removed with a scraper.
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.
How is the sedimentation process done? Sedimentation, the steps in which solids fall to the bottom of a tank and are removed, occurs after coagulation and flocculation. Different coagulants can be used, but in general, coagulation is accomplished through the addition of inorganic salts of aluminum or iron. These inorganic salts neutralize the charge on the particles causing raw water turbidity. Coagulation can also be caused by the addition of water-soluble organic polymers.
Flocculation, the fusing of suspended particles in the water, agglomeration of destabilized particles into large particles, occurs when particles whose charge has been neutralized during coagulation, collide and fuse to form larger particles.
Potable Water Treatment
After source water—the influent from rivers, reservoirs, and streams—passes through weirs for removal of debris, primary treatment begins. In the production of potable water, the sedimentation process is often accelerated through the use of coagulation and flocculation, as discussed above. A secondary clarification tank is often used before the water is filtered (through membranes, sand, or other materials) and disinfected before discharge.
The treatment of wastewater typically uses two or more sedimentation tanks or basins. Because of the volume of biomass found in wastewater, coagulation and flocculation is usually not used, although it can be used in later stages. If a secondary sedimentation tank is used, some of the settled biomass is removed for production and treatment as sludge. The remainder is recycled to aeration tanks where they provide micro-organisms that consume nutrients in the primary effluent.
When used in tandem with coagulation and flocculation, sedimentation can be an effective first step in the removal of material containing protozoa, bacteria, and viruses, particularly when a polyelectrolyte polymer is used to initiate flocculation.
Sedimentation is an essential step in all water and wastewater treatment. But the specifics of sedimentation vary according to the quality of the water or wastewater entering the plant and its agreed-upon quality after it leaves the plant. Other factors affecting which sedimentation process must be used include physical constraints (how much room is available for the treatment plant), budgetary concerns, and environmental proscriptions.
SUEZ brings to bear its global scientific resources and experience to design water and wastewater sedimentation systems for businesses and municipalities. Our repair, protection, cleaning, and rehabilitation solutions are custom-tailored to the chemical and mechanical aggressiveness of each system’s process. Our services include tank sediment cleaning services.
Contact a representative to learn more about wastewater treatment solutions offered by SUEZ North America.