A Comprehensive Analysis of High-Density Sedimentation Tank Technology: Types, Principles, and Application Characteristics

In water treatment engineering, sedimentation tanks are the core facilities for removing suspended solids from water. Traditional sedimentation tanks such as horizontal flow, radial flow, and inclined tube sedimentation tanks have long been widely used, but they generally suffer from low treatment efficiency, large footprint, and limited effectiveness in removing COD and SS, which increases the load on subsequent treatment stages and drives up overall treatment costs.

To address these pain points, a new sedimentation process with a compact structure and higher efficiency has emerged – the high-density sedimentation tank (Densadeg). This patented technology, developed by the French company Delimon, is not only the ultimate form of three generations of sedimentation technology, but also, due to its numerous advantages, has become the mainstream choice for primary enhanced treatment, advanced treatment, and drinking water production of urban wastewater. It is now being implemented in numerous water and wastewater treatment plants both domestically and internationally.

Main Content

1. Three Mainstream Models, Adapting to Different Water Treatment Scenarios

High-density sedimentation tanks are not a single structure, but are divided into three basic types: RL, RP, and RPL. The core differences lie in their configuration and applicable scenarios, precisely matching different treatment needs such as domestic water, municipal sewage, and industrial wastewater. Among them, the RL type is currently the most widely used model, accounting for 95%.

RL Type: Primarily designed for domestic water and sewage treatment, it comes standard with high-efficiency inclined tubes. The sedimentation process is divided into two stages—deep obstruction sedimentation below the inclined tube + shallow inclined tube sedimentation within the inclined tube. The upward flow velocity can reach 2~3m/h, resulting in more stable treatment effects.

RP Type: Without inclined tube configuration, suitable for scenarios with less stringent effluent discharge standards. It is only used for special needs such as filter backwash wastewater concentration, and its application range is relatively narrow.

RPL Type: Specifically designed for municipal sewage and industrial wastewater treatment, it can achieve centralized sludge storage and is only suitable for carbon removal processes (non-potable water) and special sedimentation processes for industrial wastewater.

2. RL Type Core Principle: Sludge Recirculation Doubles Sedimentation Efficiency

High-density sedimentation tanks are considered the ultimate form of three generations of sedimentation technology. The first two generations were 'static sedimentation' in the 1920s and 30s, and 'sludge contact layer sedimentation' in the 1950s. The core innovation of the 'sludge circulation type' high-density sedimentation tank, introduced in the 1980s, lies in the combination of external sludge circulation and high-efficiency inclined plates.

Its core working logic is simple: by recirculating concentrated sludge and precisely adding chemicals, suspended solids in the water combine with the sludge to form larger and denser flocs, increasing the density and radius of the flocs, thereby significantly improving the sedimentation speed. Simply put, it makes 'impurities clump together and settle faster.' For the same water volume treatment requirements, the volume of the sedimentation tank can be significantly reduced, while the treatment effect is superior.

Simultaneously, continuous external sludge recirculation ensures the solids concentration in the reaction tank, allowing for more uniform floc formation. Combined with the principle of inclined plate sedimentation, the area for sludge-water separation reaches several times that of ordinary sedimentation tanks, further enhancing the flocculation and sedimentation process.

3. Three Functional Zones, Forming an Integrated Treatment System

High-density sedimentation tanks (taking the mainstream RL type as an example) integrate coagulation, sedimentation, and concentration functions into one system. The entire system consists of a mixing zone, a reaction zone, and a sedimentation/concentration zone connected in series. Each zone performs its specific function, forming a highly efficient closed-loop water treatment system. The compact structure also reduces the land area cost of individual structures.

Mixing Zone: Equipped with a high-speed mixer, this zone rapidly and thoroughly mixes the added coagulants, such as ferric chloride, with the raw water, destabilizing suspended particles and initially forming fine flocs, laying the foundation for subsequent flocculation.

Reaction Zone: Composed of a slow-stirring reaction tank and a plug-flow reaction tank, this zone thoroughly mixes the pre-coagulated raw water, returned sludge, and coagulant aids. Slow stirring allows the fine flocs to gradually aggregate into large, dense flocs. The high concentration of sludge returned significantly enhances the flocculation effect.

Sedimentation/Concentration Zone: The flocs slowly enter this zone to avoid damage. The lower part collects sludge and completes concentration, while the upper inclined plate traps any remaining flocs, ensuring effluent quality. Some of the concentrated sludge is returned to the reaction zone via a circulation pump, while the remaining sludge is pumped out for subsequent dewatering. The bottom scraper further promotes sludge concentration, ensuring efficient sludge removal.

4 Seven Core Features

Compared to traditional sedimentation tanks, high-density sedimentation tanks have become the mainstream technology due to their comprehensive advantages in treatment efficiency, space utilization, and functionality. While there are a few areas for optimization, their overall characteristics can be summarized in seven points:

1. Excellent Flocculation Effect: Adding PAM organic flocculant as a coagulant aid, combined with sludge recirculation, forms large, high-density flocs, significantly improving settling performance;

2. High Sedimentation Efficiency: High-efficiency inclined plates allow the upward flow velocity in the sedimentation zone to reach 20-40 m/h, resulting in more thorough floc settling. The removal rate is approximately 85% for SS, a considerable COD removal rate, and a BOD removal rate as high as 92%;

3. Integrated Sludge Thickening: The sedimentation tank... 4. High sludge concentration eliminates the need for further concentration, allowing for direct dewatering and simplifying subsequent sludge treatment processes;

5. Compact structure and small footprint: Integrating three functions into one unit significantly reduces construction costs, saves land, and is well-suited to the current situation of limited urban land;

6. Excellent floc integrity: The rationally designed water flow velocity from the reaction zone to the sedimentation zone effectively prevents floc damage, ensuring optimal sedimentation;

7. Wide range of applications: Applicable to multiple fields such as drinking water production, municipal sewage, industrial wastewater, and sludge treatment, offering high flexibility;

8. Cost control required: The relatively large dosage of chemicals leads to higher operating costs. Optimizing the chemical dosing scheme is necessary to minimize overall operating costs.

5. ACTIFLO Derivative Process: Ultra-High-Speed Sedimentation Solution with Fine Sand Circulation

The ACTIFLO process is another operating mode of high-density sedimentation tanks, also known as ultra-high-speed sedimentation tanks. Developed by OTV of the French Velioa Group in the early 1990s, it was initially used in tap water production and has now expanded to the wastewater treatment field. Its core difference from conventional high-density sedimentation tanks lies in using fine sand circulation instead of direct external sludge circulation.

The process flow is as follows: After impurities are removed from the raw water, flocculants are added to form fine flocs. Then, polymers and fine sand are added to form large flocs. After maturation, these flocs enter the inclined plate sedimentation zone. The sludge and fine sand settle and are then returned. After separation by a hydrocyclone, the fine sand can be recycled again.

This process combines the advantages of accelerated coagulation with fine sand and inclined plate sedimentation, resulting in higher floc concentration and faster settling speed. However, it also has drawbacks: it does not utilize the flocculation function of sludge, leading to higher energy consumption and sludge production. Sludge treatment and disposal become subsequent challenges.

Conclusion

High-density sedimentation tanks, as the ultimate form of third-generation sedimentation technology, perfectly solve many pain points of traditional sedimentation tanks with their core advantages such as compact structure, high treatment efficiency, stable effluent quality, and small footprint. They are suitable for the increasingly scarce urban land and can meet the treatment needs of various scenarios, including drinking water, urban sewage, and industrial wastewater. They have been successfully applied in water plants and sewage treatment plants in many European countries and in cities such as Urumqi, Shijiazhuang, and Baoding in China.

From a technological development perspective, the core innovation of high-density sedimentation tanks lies in combining sludge recycling with inclined plate sedimentation, achieving integrated treatment of 'flocculation-sedimentation-concentration.' This optimization and upgrading approach to traditional processes provides important reference for the technological development of the water treatment industry.

With the continuous improvement of water treatment standards and the increasing demand for energy conservation, consumption reduction, and land reduction, high-density sedimentation tanks and their ACTIFLO derivative processes will be more widely used in the water treatment field in the future. At the same time, through optimized reagent dosing and energy consumption control, they will continuously make up for their shortcomings and become a more competitive core component in water treatment processes.