The high-density sedimentation tank is a summary and development of coagulation and sedimentation calculations. This process primarily combines clarification technology with sludge thickening technology, allowing for further removal of pollutants such as SS, TP, and some COD from secondary effluent. The sedimentation tank is mainly divided into three areas: reaction zone, sedimentation zone, and effluent zone. In the reaction zone, mixing is achieved through rapid mixers to achieve an internal recirculation rate of 6-9.5 times the influent, with the impeller of the mixer typically consisting of three layers.

Propeller blades or two layers (determined by pond depth) react, agitate, and mix the suspended solids in the water, forming large, easy-to-settle flocs. In the sedimentation zone, high-density suspended particles settle rapidly, while the fine flocs are captured by the inclined pipes, resulting in high-quality effluent collected and discharged through the water collection channel at the top of the pond.

Wastewater first enters the rapid mixing tank, where it is quickly mixed with added coagulants, which can be aluminum or iron salts. After mixing, the wastewater flows into the flocculation reaction tank, where high molecular weight flocculants, typically polyacrylamide, are added. It is then slowly mixed with污泥 returned from the sedimentation tank, completing the flocculation reaction. This accelerates the solid recycling process and promotes the formation of dense, uniform floc particles. Subsequently, the water flows through the push flow zone from the flocculation tank into the sludge concentration area, with clear water entering the top collecting trough via inclined tubes/plates; most suspended solids are directly separated in the sludge-water separation zone, while the finer flocculated particles are intercepted and settled in the inclined plates/tubes. A bottom scraper with grates is installed to concentrate the sludge. After concentration, part of the sludge is recycled back to the outlet of the rapid mixing tank, and the remainder is discharged. Compared to traditional sedimentation tanks, high-density sedimentation tanks have the following characteristics.

① The system features sludge recirculation, accounting for 1.5% to 12% of the treated water volume, with flocculation and mixing functions. ② In the flocculation zone and the recirculated sludge, the use of flocculants and organic high-molecular-weight flocculants as coagulants enhances overall coagulation efficiency and accelerates the separation of sludge from water. ③ The sedimentation area is equipped with inclined plates to increase the surface hydraulic load, further separating fine particles from the water. ④ Automation of the entire system can be achieved by monitoring key operational conditions. This can be done by adjusting the speed of the flocculation mixer, dosage of chemicals, recirculated sludge volume, and sludge disposal volume to achieve different operational effects. ⑤ The mixing tank and the flocculation tank are mechanically stirred to facilitate control under various operating conditions. ⑥ The tank is fitted with a concentrator scraper, which effectively increases the sludge settling concentration and speed, achieving sedimentation and concentration simultaneously. The active sludge effluent has good performance and high concentration, and the sludge-water mixture is further concentrated in a static pressure sludge pool or sludge concentration tank. In some cases, the concentrated sludge can be directly transported to the dewatering room for direct dewatering.

2. High-density sedimentation tank design parameters

The main technology of the high-density sedimentation tank is carrier flocculation technology, a rapid sedimentation technique characterized by adding high-density, insoluble medium particles (such as fine sand) during the coagulation stage. This utilizes the gravity settlement of the medium and the adsorption effect of the carrier to accelerate the "growth" and sedimentation of the flocs.

The definition of carrier flocculation is a physical-chemical treatment process that improves the settling performance of suspended solids in water by reinforcing the adsorption of flocs using continuously recycled medium particles and various chemicals (usually PAV and PAM ferric salts, etc.). Its working principle is to first add coagulants (such as polyaluminum chloride) to the water to destabilize the suspended solids and colloidal particles, and then add high molecular weight flocculating aids and denser carrier particles. These cause the destabilized impurity particles to form flocs around the carrier as the nucleus. Through bridging adsorption by the high molecular chains and the trapping action of fine sand particles, a dense floc is rapidly formed, significantly shortening the settling time, enhancing the treatment capacity of the clarification pond, and effectively dealing with high shock loads. Compared with traditional flocculation processes, this technology offers advantages such as a smaller footprint, lower construction costs, and resistance to shock loads.

High-density sedimentation tank process flow:

Concrete Tank

Coagulant is added to the raw water, where it rapidly mixes with suspended solids in the wastewater under the action of a high-speed stirrer. By neutralizing the negative charge on the particle surface, the particles become "unstable," forming small flocs which then enter the clarification pool. Simultaneously, the phosphorus in the raw water reacts with the coagulant to form phosphates, achieving the objective of chemical phosphorus removal.

② Feed Tank

Fine sand and floc formation through coagulation are rapidly mixed in the agitator, with the fine sand at the core to form denser and heavier flocs, facilitating rapid sedimentation in the sedimentation tank.

③ Maturation Tank (Floculation Tank)

Flocculants cause the small flocs entering to form larger ones through adsorption, electrical neutralization, and bridging between them. The slow stirrer ensures that the chemicals and flocs are thoroughly mixed without disrupting the formed large flocs.

④ Inclined Plate Sedimentation Tank

After flocculation, the water enters the bottom of the inclined plate in the sedimentation basin and flows upwards to the upper collecting area. Particles and flocs settle on the surface of the inclined plate and slide downwards under gravity. The higher upward flow rate and the 60° inclination of the plate can form a continuous self-scrubbing process, preventing the accumulation of flocs on the plate. Fine sand moves downward along with the sludge on the plate surface and settles at the bottom of the sedimentation basin. Then, the circulation pump transports the fine sand and sludge to the hydrocyclone, where they are separated under centrifugal force: the fine sand flows out from the bottom directly back to the addition pool, while the sludge overflows from the top and is then gravity-fed to the sludge treatment system. The water after sedimentation is collected and discharged by the stainless steel collecting channels distributed at the top of the inclined plate sedimentation basin.

1 million tons sedimentation pond

40,000 tons settling pond

3,000-ton settling pond