Folded Plate/Grid Coagulation + Horizontal Flow Cellular Tube Composite Sedimentation Tank

This configuration is also commonly referred to by water plants as a composite sedimentation tank. It represents a classic combined process in water treatment plants: an upfront independent flocculation zone (folded plate/grid) + a middle horizontal flow pre-sedimentation zone + a rear honeycomb tube polishing sedimentation zone + an overhead collection water outlet system. It is an upgraded and efficiency-enhanced version of the traditional horizontal flow sedimentation tank, addressing issues such as the large footprint of conventional basins and insufficient removal of fine flocs.

1. Overall Process Flow

Raw water dosing and mixing → Folded plate / Grid coagulation-flocculation zone → Transition water distribution zone (perforated wall) → Horizontal flow pre-sedimentation zone → Honeycomb tube polishing sedimentation zone (counter-current flow) → Clear water zone → Perforated collection trough / V-notch weir outlet → Subsequent filter; the entire tank bottom is equipped with a sludge discharge system for unified collection and discharge.

2. Detailed Structure and Function of Each Zone

1. Coagulation-Flocculation Reaction Zone (At the very front of the tank, separate compartment)

Divided into a rapid mixing stage + slow flocculation stage. The media comes in two mainstream forms: folded plate flocculation and grid (lath) flocculation. One can be selected, or both can be combined.

1) Rapid Mixing Stage

After the coagulant (such as PAC) is dosed, vigorous agitation occurs via an inline mixer or a vertical shaft. The chemical disperses uniformly into the water almost instantly, destabilizing colloidal particles and forming micro-floc nuclei; the retention time is only a few tens of seconds.

2) Flocculation Reaction Stage

Folded Plate Flocculation Zone: Plates angled at 90-120° are installed inside the tank. Water flows back and forth and travels up and down between the plates, continuously generating micro-vortices that allow fine flocs to collide and grow. Flow velocity decreases progressively from fast at the front to slow at the back, starting at 0.25-0.35 m/s and reducing to 0.10-0.15 m/s in the latter section, preventing already formed large flocs from being sheared apart by high-velocity flow. The final section is changed to straight plates, where the flow becomes gentler and flocs are further compacted. Total flocculation retention time: 15-20 minutes, extended to 20-30 minutes for low-temperature, low-turbidity water.

Grid (Lath) Flocculation Zone: Plastic grid plates are installed in layers within vertical shafts. Water passing through the grids creates micro-turbulence, achieving high particle collision efficiency. Also divided into three stages, the velocity through the grids decreases gradually. Fits projects requiring high flow rates but limited footprint.

Zone-specific support: Perforated sludge discharge pipes are placed at the bottom of the flocculation tank to periodically remove small amounts of early settling sludge from the reaction zone, preventing sludge stagnation and anaerobic rising.

2. Transition Buffer Water Distribution Zone (Between flocculation and sedimentation zones)

The perforated wall (inlet baffle wall with openings) is a crucial component connecting the flocculation tank and the sedimentation zone. Water exiting flocculation passes through uniformly distributed openings in the wall, achieving even distribution across the entire cross-section of the sedimentation tank, dissipating energy, slowing velocity, breaking up flow vortices, preventing localized high-speed jets or short-circuiting, and ensuring smooth, uniform flow into the horizontal flow pre-sedimentation zone.

3. Horizontal Flow Pre-Sedimentation Zone (Middle section, rectangular horizontal flow segment)

This is the traditional horizontal flow sedimentation section, where water flows slowly and horizontally along the tank length at a typical horizontal velocity of 10-20 mm/s.

Function: Removes the vast majority of large and heavy flocs, achieving initial sludge-water separation. This significantly reduces the sludge load on the downstream honeycomb tubes, preventing sludge accumulation and blockage inside the tubes. This is the core advantage of the composite sedimentation tank; most sludge from high-turbidity influent settles here first.

Buffer zone: A 1.0-1.5m buffer zone is reserved between the water surface and the sludge layer at the bottom to prevent settled sludge from being resuspended by the horizontal flow.

Sludge discharge: A traveling bridge scraper at the tank bottom in this section scrapes settled sludge into a sludge hopper at the front for periodic gravity discharge.

4. Honeycomb Tube Polishing Sedimentation Zone (Rear of the tank, efficiency-boosting section)

Flow direction shifts from horizontal to vertically upward (counter-current flow), entering the tube water distribution area and then passing through the honeycomb tube media layer. This constitutes the secondary polishing sedimentation stage.

1) Water Distribution Zone: The 1.2-1.8m high space beneath the tube support. Water spreads uniformly again here, ensuring balanced flow into each individual tube.

2) Honeycomb Tube Media Layer:

Material: PP/PVC hexagonal honeycomb tubes; inclination angle of 60°, tube length of 1.0m, internal diameter of 35-50mm;

Principle: Based on the shallow sedimentation theory, remaining fine and difficult-to-settle residual flocs settle rapidly inside the tubes; sludge slides down along the inner walls of the tubes back to the bottom sludge zone; clear water moves upward through the tubes in the opposite direction, achieving sludge-water separation.

Control parameters: Typical upward flow velocity in the clear water zone above the tubes is 1.2-1.8 mm/s, with surface loading rates controlled at 5-6 m³/(m²·h).

Key operational point: Since most sludge is removed in the front horizontal section, there is measurably less sludge in the tube section, significantly reducing clogging risk and extending operational cycles.

5. Upper Clear Water Zone and Effluent Collection System (Above tubes)

The area between the top of the tubes and the effluent weir is the clear water zone, maintaining a height generally ≥1.0m to stabilize flow and dampen effluent fluctuations. Two common effluent collection methods exist:

Perforated collection troughs: arranged parallel across the tank surface, with outlet orifices on trough walls for uniform whole-tank collection;

V-notch overflow weirs with sawtooth edges; overflow water exits over them; Troughs channel water into the main effluent channel, flowing by gravity to filters. Weir loading is tightly controlled to prevent localized excessive flow from disturbing flocs on the tube surface and causing effluent turbidity.

6. Bottom Sludge Collection and Discharge System (Across entire tank bottom)

Covers the whole bottom beneath both horizontal and tube sections:

Horizontal section: traveling bridge scraper pushes sludge into hoppers;

Below slope tubes: sludge hoppers + pneumatic discharge valves for periodic gravity sludge removal; Sludge-laden water discharges into the sludge collection channel, sent to the sludge dewatering section.

3. Advantages and Disadvantages of This Combined Process

Advantages

Initial coarse sedimentation via horizontal flow, followed by polishing sedimentation via tube settlers; robust resistance against high-turbidity shock loads; superior effluent quality compared to basins with tubes alone;

Compared to full-length traditional horizontal flow sedimentation tanks, the rear tube section shortens overall tank length, saving footprint;

Thorough floc removal ensures stable settled water turbidity below 1-2 NTU, significantly lowering the filter burden and extending filter backwash cycles.

Disadvantages

Tube media has a finite service life and requires replacement periodically;

There is a potential for clogging in the tube section, requiring periodic draining for cleaning;

Basin structure is more complex than a simple horizontal flow sedimentation tank, slightly increasing civil construction costs.