Sedimentation tanks are commonly used structures for separating suspended solids. The pretreatment used in biological treatment is known as the primary sedimentation tank. For general urban wastewater, the primary sedimentation tank can remove about 30% of the BOD5 and about 55% of the suspended solids. The secondary sedimentation tank, as part of the biological treatment process, is referred to as the secondary sedimentation tank.

Based on the direction of water flow, sedimentation tanks are typically categorized into three types: horizontal flow, vertical flow, and radial flow.

1. Horizontal flow sedimentation.

The pluvial sedimentation is rectangular. The wastewater flows out from one end of the tank, passes through the tank horizontally, and exits from the other end. The bottom of the pond is equipped with sediment buckets, while the rest of the bottom slopes towards the sediment buckets.

2. Vertical Flow Sedimentation Tank

The pools are mostly circular, with some being square or polygonal. Waste water enters from the central pipe in the middle of the pool and is evenly distributed across the cross-section of the pool by a reflector at the bottom of the pipe. The water troughs are positioned on the pool surface or around the pool walls, so the water flow direction is basically from bottom to top. Sludge is stored in the sediment bucket.

Radial Flow Sedimentation Tank

Also known as radiation precipitation. The ponds are mostly circular, with small ponds sometimes square or polygonal. The inlet and outlet arrangements of the storage tanks are similar to those of a rising flow tank, with the inlet centered and the outlet located around the perimeter. However, the radial flow pond has a diameter-to-radius ratio many times greater than that of a straight vertical pond. The water in the pond flows horizontally outward (jet flow). As the cross-sectional area increases continuously, the water velocity in the pond decreases gradually from the center to the edge. The bottom of the pond slopes towards the center, with a central sump for sediment. Sediments are generally removed using scraping/suction machinery.

The lagoon is divided into five sections: inflow, outflow, sedimentation area, sludge storage area, and buffer zone. The inflow and outflow sections are designed to ensure uniform and smooth water flow, enhancing precipitation efficiency. The sedimentation area is the primary component of the lagoon. The sludge storage area is where sludge is stored, concentrated, and discharged, serving as the sludge storage facility. The buffer zone is located between the sedimentation area and the sludge storage area. Its function is to prevent the sludge at the bottom of the lagoon from being washed away by water currents.

The operation of sedimentation tanks can be categorized into intermittent and continuous modes. In intermittent operation, the process is generally divided into three stages: inflow, sedimentation, and drainage. The settleable suspended solids in the wastewater are drained out through the drainage pipes at different heights on the pool wall after sedimentation. Continuous sedimentation tanks discharge wastewater without interruption. The settleable particles in the wastewater complete sedimentation as they flow through the tank. At this point, the settling particles are affected by gravity-induced sedimentation and flow velocity. Flow velocity is a significant factor influencing particle sedimentation.