Ultrapure Water Perforated Coagulation Tank and Mesh Bar Coagulation Tank

A perforated vortex coagulation tank consists of several cubic cells, typically no fewer than six. Perforations are located along the tank wall in the partition walls between each cell, with the orifices staggered vertically. Water enters tangentially to the tank wall, creating a vortex. The first cell has the smallest orifice size, the highest flow velocity, and the greatest rotational speed within the tank. Subsequent cells have gradually larger orifice sizes, decreasing flow velocities, and a correspondingly decreasing velocity gradient (G) to accommodate floc growth. Generally, the initial orifice velocity should be 0.6–1.0 m/s, and the final orifice velocity should be 0.2–0.3 m/s, with a coagulation time of 15–25 minutes.

The perforated vortex coagulation tank can be considered similar to a CSTR-type reactor, but is significantly affected by variations, resulting in poor coagulation performance and a tendency for sludge accumulation at the bottom. Its advantages include simple construction, convenient installation, low cost, and suitability for small to medium-sized water plants or combined with other types of tanks.

A stylized, bar-type coagulation tank is designed as a multi-cell vertical shaft recirculation system. Each vertical shaft is equipped with several layers of grids or bars, with staggered openings in the partition walls between shafts. The number of grids or bars in each shaft gradually decreases from the inlet to the outlet, generally controlled in three sections: a dense grid at the beginning, a sparse grid in the middle, and no grid at the end. A group of flocculation tanks consists of nine shafts (nine in total), with a total of 27 grid layers. When water flows through the grid, it successively contracts and expands, forming vortices and causing particle collisions. The flow velocity through the openings between shafts and beyond the grid layers gradually decreases according to the flocculation law.

The turbulence caused by grid and bar flocculation tanks closely resembles local isotropic turbulence; therefore, isotropic turbulence theory is more suitable for application to grid and bar flocculation tanks. Grid flocculation tanks are effective, with low head loss and short flocculation time. However, based on the operational experience of existing grid and bar screen flocculation tanks, sludge accumulation at the bottom of the initial tank remains a problem, and a few water plants have observed algae growth and mesh blockage on the grids.

Grid and bar screen flocculation tanks are still under continuous development and improvement. Flocculation tanks should ideally be built in conjunction with sedimentation tanks, generally arranged in a Class C interconnected configuration. The design flow rate for each group is generally between (1.0–2.5) x 10⁴ m³/d.