Complete Guide to High-Density Sedimentation Tank Technology
As water treatment requirements continue to rise, flocculation and sedimentation—a traditional water treatment technology—has also undergone continuous innovation and development. Traditional sedimentation tanks have limited ability to remove pollutants such as COD and SS, and only provide a single sedimentation and clarification function, requiring a flocculation reaction tank upstream, which increases the footprint and construction cost. To improve COD and SS removal, a high-density sedimentation tank process integrating flocculation, sedimentation and clarification has emerged. It effectively improves COD and SS removal while featuring a compact structure that reduces the footprint, and can be flexibly applied to primary and advanced treatment of raw drinking water, industrial and domestic sewage, and rainwater.
The high-density sedimentation tank (HDST) consists mainly of three parts: the rapid mixing tank, the flocculation reaction tank, and the sedimentation separation tank, thereby integrating the flocculation and sedimentation processes.
◎ Rapid Mixing Tank
Influent enters the rapid mixing tank through the pipeline while flocculant is added, and rapid stirring achieves fast flocculation while preventing alum floc sedimentation.
◎ Flocculation Reaction Tank
The effluent from the rapid mixing tank is then discharged into the flocculation reaction tank, where a coagulant aid is dosed into the lower part of the reaction tank through a dosing device to form large alum flocs;
at the same time, the stirring rate in the reaction tank is controlled (lower than that of the rapid mixing tank) to prevent the alum flocs from being broken up and from settling within the reaction zone.
◎ Sedimentation Separation Tank
Wastewater carrying alum flocs enters the sedimentation separation tank, where most of the alum flocs settle and are thickened.
A sludge scraper at the bottom of the sedimentation separation tank continuously scrapes to promote thickening of the settled sludge. Part of the sludge is returned to the flocculation reaction tank through the sludge return pipe to maintain the required sludge concentration in the flocculation reaction tank, promote the growth of alum flocs during flocculation and increase their density, while the excess sludge is discharged for further treatment.
It should be specially noted that an inclined-plate (lamella) settling device is installed in the upper part of the sedimentation separation tank to remove the remaining fine alum flocs and ultimately produce water meeting standards.
2. Characteristics of the High-Density Sedimentation Tank
◎ Small Footprint
The high-density sedimentation tank is highly integrated with a compact process structure. Inclined-plate/tube settling allows the sedimentation separation zone to maintain a high upward flow velocity, reducing the system footprint.
◎ High and Stable Effluent Quality
Multi-stage flocculation is set up inside the high-density sedimentation tank. By adjusting the stirring intensity in different operating units, regulating the hydraulic conditions of each flocculation stage, and controlling the velocity gradient in each unit, a good flocculation effect is achieved and the effluent turbidity is stabilized.
◎ Reduced Flocculant Dosage
Sludge return promotes the flocculation reaction, reducing the flocculant dosage.
◎ Effectively Improved Operating Load and Sludge Thickening
The inclined-plate settling in the sedimentation separation tank provides good settling of alum flocs, increasing the operating load;
the lower part of the sedimentation separation tank is designed as a sludge thickening tank structure, greatly improving the sludge thickening effect, and the discharged sludge can be directly dewatered.
◎ High Degree of Operational Automation
By developing an advanced integrated control system, automatic control of the high-density sedimentation tank operation can be achieved, improving commissioning and operating efficiency;
it can also provide independent control of mixing, flocculation and sedimentation separation in the tank, facilitating targeted process optimization of each operating unit.
◎ Wide Range of Applications
It is suitable for the treatment of wastewater with high suspended solids, and for water purification in water supply and drainage systems.
02 Applications of High-Density Sedimentation Tanks in Water Treatment: Densadeg, Multiflo, Actiflo, and Central-Feed Type
In China, the widely applied options in water treatment projects include Degremont's Densadeg sedimentation tank, Veolia's Multiflo and Actiflo sedimentation tanks, and the central-feed high-density sedimentation tank designed by the Shanghai Municipal Engineering Design Institute (Group) Co., Ltd.
1. Densadeg High-Density Sedimentation Tank
Degremont was the first to develop the Densadeg high-density sedimentation tank, which includes a mixing tank, a reaction tank, and a sedimentation-thickening tank.
A draft tube is installed in the reaction tank to control the directional flow of water. The mixing tank effluent is slowly stirred within the draft tube to mix evenly and form larger flocs, then flows slowly through the plug-flow reaction tank into the sedimentation zone to achieve sludge-water separation.
It is worth mentioning that Degremont has developed various models of high-density sedimentation tanks for different application scenarios.
Among them, the most versatile Densadeg 4D high-density sedimentation tank has four functions, integrating grit and scum removal and providing grit removal, oil removal, sedimentation and sludge thickening, with a removal capacity of over 95% for grit and oil/grease.
It is understood that Densadeg high-density sedimentation tanks have been widely applied in domestic municipal and industrial wastewater treatment and disposal. For example:
◎ At the Tangshan Steel wastewater treatment plant with a treatment capacity of 720,000 m3/d, the mixed wastewater from the circulating cooling water system blowdown, process drainage and filter backwash water contains high levels of SS, oil and hardness. The "high-density sedimentation tank + V-type filter" process is adopted, which ensures effluent turbidity below 3 NTU and oil content below 0.5 mg/L.
◎ The Shanghai Taihe wastewater treatment plant, with a treatment capacity of 400,000 m3/d, adopts a fully underground "high-density sedimentation tank + denitrifying deep-bed filter" process for advanced treatment of domestic sewage, enhancing the removal of SS, ammonia nitrogen, total nitrogen and total phosphorus, with effluent quality exceeding Grade 1A.
◎ A wastewater treatment plant in Chongqing with a treatment capacity of 60,000 m3/d adopts the "high-efficiency sedimentation tank + denitrifying biological filter + biological aerated filter" process to treat domestic sewage, and the effluent quality of the high-efficiency sedimentation tank and biological filter processes can stably meet the Grade 1A discharge standard.
2. Multiflo and Actiflo High-Density Sedimentation Tanks
The Multiflo high-density sedimentation tank was developed by Veolia in the late 1960s and, through continuous improvement, has formed a variety of sedimentation tank types. For influent SS ranging from 10 to 4,000 mg/L, the treated effluent SS can be reduced to below 6 mg/L.
◎ The Urumqi Hedong wastewater treatment plant, with a treatment capacity of 400,000 m3/d, adopts the "two-stage biological treatment + Multiflo high-density sedimentation tank + UV" process for advanced treatment of secondary effluent from municipal sewage. The Multiflo tank removes part of the SS, organic pollutants and most of the phosphorus from the water, and the overall process effluent has COD below 50 mg/L, BOD5 below 10 mg/L, total phosphorus below 0.4 mg/L, and SS below 8 mg/L, which can be reused for industrial production and road landscaping in industrial parks.
The Actiflo ultra-high-efficiency sedimentation tank is a high-density sedimentation tank that uses microsand circulation instead of sludge return to promote flocculation, comprising a dosing tank, a maturation tank and a sedimentation separation tank.
The Actiflo tank is suitable for the clarification and purification of drinking water, the treatment of municipal and industrial wastewater, the production of industrial process water, and stormwater sedimentation. In wastewater treatment, its removal rate for TSS and total phosphorus exceeds 85%, and the COD removal rate can reach 60%.
◎ The Kunming No. 3 wastewater treatment plant, with a treatment capacity of 400,000 m3/d, is the first plant in China to apply the Actiflo process for the treatment of combined sewer overflow pollution. In the dry season, the Actiflo process serves as an advanced treatment unit, keeping the effluent SS below 10 mg/L; in the rainy season, when the influent SS surges, the SS removal rate can exceed 70%.
◎ The influent of the Tongxiang Chongfu wastewater treatment plant is a mixture of leather, printing and dyeing, chemical wastewater and municipal discharge. The "A2O + Actiflo high-density sedimentation tank" process is adopted for treatment, and a certain amount of activated carbon is dosed in the Actiflo process, improving the adsorption and removal of organic matter in the water and significantly reducing the SS and COD content.
3. Central-Feed High-Density Sedimentation Tank
To address problems such as uneven water distribution, low treatment capacity and flow direction defects, the Shanghai Municipal Engineering Design Institute (Group) Co., Ltd. designed and developed the central-feed high-density sedimentation tank based on the high-density sedimentation tank.
◎ The Jiangsu Changjiang Water First Water Plant, with a treatment capacity of 400,000 m3/d, adopts the "biological contact oxidation + central-feed high-density sedimentation tank + ozone catalytic oxidation + biological activated carbon filter + V-type filter" process to treat raw water.
In response to changes in influent water quality in different seasons, by adjusting process parameters such as inflow rate, chemical dosage, sludge discharge and stirring speed, the sludge return ratio of the high-density sedimentation tank is controlled at 3–5% and the sludge concentration in the flocculation tank at 500–800 mg/L, forming large and dense alum flocs, ensuring the settling effect, and keeping the effluent turbidity below 1 NTU to meet the influent requirements of subsequent treatment units.
◎ The Shaoxing Cao'e River Water Plant, with a treatment capacity of 200,000 m3/d, adopts the third-generation central-feed high-density sedimentation tank process, with multi-point dosing of PAM, an enlarged sludge collection zone at the tank bottom, and air-lift technology to raise the sludge to the sludge transfer tank. The returned sludge concentration is increased by half and the return ratio is reduced by half, keeping the effluent turbidity below 1 NTU.
03 Problems of the High-Density Sedimentation Tank: Four Aspects Needing Improvement and Optimization
In summary, as an efficient and compact water treatment facility, the high-density sedimentation tank can be flexibly applied to the upgrading and renovation of water treatment plants, temporary water treatment systems and new water treatment projects, achieving efficient removal of SS and COD with a small footprint, high effluent quality and a high degree of automation.
However, no process or facility is perfect. Likewise, in practical engineering applications, the high-density sedimentation tank also has some problems, specifically manifested as follows:
