Primary Treatment

Wastewater contains suspended matter that has a higher specific gravity than the liquid and is in a relatively quiescent state, the particles will settle out because of gravity. Grit chambers remove grit, including gravel, sand, and heavy particulate matter such as corn kernels, bone chips, coffee grounds, and broken glass. All of this accounts for only a portion of the inert suspended solids in raw sewage. After grit removal, the wastewater still contains suspended solids, which are then removed by sedimentation.

Raw wastewater

Raw wastewater

Prevention Pollution Policy

Figure 2 Stages of wastewater treatment.

Figure 2 Stages of wastewater treatment.

1. Grit Removal

For design purposes, grit is defined as 0.2-mm particles with a specific gravity of 2.65 and a settling velocity of 0.075 fps (0.0225 m/sec). Grit removal in municipal waste treatment protects mechanical equipment and pumps from abnormal abrasive wear, prevents pipe clogging, and reduces accumulations in the settling tanks. Two types of grit removal units that are used in wastewater treatment plants are the horizontal flow type and the aerated type.

Settling Tanks

Horizontal flow type. In sewers, the flow velocity is generally not less than 2.00 fps (0.6 m/sec), which is "self-cleaning velocity." Most of the gritty material settles out of the flow by gravity if the velocity is reduced to 1 fps (0.3 m/sec). Therefore, to prevent scouring of the settled grit and other material, the horizontal velocity of the grit removal unit is controlled at approximately 1 fps (0.3 m/sec). Figure 3 diagrams a chain-and-bucket grit collector.

Aerated type. Aerated-type grit-removal units may be square or rectangular hopper-bottom tanks with the inlet and the effluent weir on opposite sides of the tank. The shearing action of the air bubbles strips the inert grit of the organic material that adheres to its surface. These units are often designed to keep the organic solids in suspension while the grit settles out. Design of an aerated hopper-bottom grit tank is based on a detention time of approximately 1 min at peak hourly flow. Settled matter is removed from the hopper bottom by gravity flow, pumping, screw conveyor, or bucket lift.

The grit removed from the grit chambers is high in organic content. In most plants, the grit is washed before disposal in the landfill. Washing can be done by a counterflow grit washer, which functions like a screw conveyor. The waste organics are returned to the plant influent. Another method is to use a centrifugal pump to lift the slurry from the grit pit to a centrifugal cyclone, which separates the grit from the organic material and discharges it to a classifier for washing. The wash water with organics is returned to the wastewater.

Primary settling tanks are used to remove the readily settleable solids preceding further treatment. With chemical treatment and flocculation, the primary settling tank (clarifier) can remove a significant amount of colloidal particles. Intermediate and final settling tanks are

Inlet Settling Zone


Effluent Weir

Outlet Zone

Inlet Outlet Zone Flocculation

Figure 4 (a) Horizontal settling tank, (b) Upflow settling tank.

Figure 4 (a) Horizontal settling tank, (b) Upflow settling tank.

used to remove the settleable solids following biological treatment processes. These settling tanks can be either circular or rectangular and are designed to operate on a continuous flow-through basis.

Figure 4 is a sketch showing the four zones—inlet, settling, outlet, and sludge storage. The inlet zone distributes the flow evenly across the cross section of the settling zone. It consists of inlet pipes and baffles. Wastewater enters at the center behind a stilling baffle and travels down and outward toward the effluent weir. The inlet line usually terminates near the surface, but the wastewater must travel down behind the baffle before it enters the actual settling zone. The baffle reduces the velocity and forces downward motion of the solids.

Following the inlet zone is the settling zone. A good inlet design is important for efficient removal of suspended solids in the settling zone. The flow velocity decreases in the settling zone, and suspended solids settle out and accumulate in the sludge zone. The configuration and depth of the sludge storage zone depends upon the frequency of cleaning, method of cleaning, and estimates of sludge production. Mechanically cleaned tanks have bottom scrapers. The sludge is continuously scraped to the hopper, from which it is pumped out. The tank bottom has a 1% slope in the direction of the sludge removal point. The sludge hopper is designed with sides at a vertical-to-horizontal slope of between 1.2:1 and 2:1.

The function of the outlet zone is to remove the settled water from the basin without carrying particles with the effluent. The outlet zone should prevent short-circuiting and loss and destruction of floe. A proper outlet design can prevent scouring and thus loss of floe.

In the design of a sedimentation tank, the settling velocity (Vs) of the particles to be removed should be greater than the overflow rate (V0). The determination of the particle-settling velocity is different for different types of particles. Settling properties of particles are characterized into the following classes.

Type I sedimentation is characterized by particles that settle in a discrete manner at a constant velocity, such as sand and grit material. Particles do not come into contact with each other. Type I settling occurs in grit chambers. Type U sedimentation is characterized by particles that flocculate while settling. Since floc-culation constantly changes the size of particles, the velocity of the particles is also changing. These types of particles are present in the settling tanks following trickling filtration. In Type III sedimentation, the particles tend to settle as a mass, leading to a separation of a clear zone and a sludge zone. Zone settling occurs in the tank following an activated sludge unit and sludge thickeners.

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