Application and Current Development of Carrier Flocculation Technology

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This post was last edited by liuquan1100 on 2018-2-11 at 14:30. Summary: It explains the working principle of carrier flocculation technology, focuses on the process flows, design parameters, and treatment effects of the two typical processes, Actiflo and DensaDeg, analyzes the current development status of this technology, and introduces several new types of devices and combined processes. It is pointed out that carrier flocculation is a compact and efficient rapid sedimentation technique, with advantages such as space savings, resistance to shock loads, and high treatment efficiency. It meets the requirements for building a resource-conserving society in China and holds good application prospects. Keywords: carrier flocculation ； Actiflo@rapid sedimentation tank ； DensaDeg@High-density Clarifier ； Fine sand. Chinese Library Classification Number: X703, 1. Document Code: B. Article Number: 1000—4602(2007)08—0001—04. Carrier flocculation is a rapid sedimentation technique characterized by the addition of high-density insoluble medium particles (such as fine sand) during the coagulation stage; the gravity-induced settlement of these particles, along with their adsorption properties, accelerates the \"growth\" of flocs and their sedimentation. Compared with traditional flocculation processes, this technology offers advantages such as a smaller footprint, lower construction costs, and resistance to shock loads. Since the 1990s, the West has **developed a variety of mature application technologies, which have been successfully used in over 100 large-scale water treatment plants around the world. At present, although there are examples of the application and modification of carrier flocculation processes in China, they have not been widely adopted, and there are also few reports on carrier flocculation technology. 1 Working Principle: The EPA in the United States defines carrier flocculation as a physicochemical treatment process that enhances the adsorption capacity of flocs by using continuously circulating medium particles and various chemical agents, thereby improving the sedimentation performance of suspended solids in water. Its working principle is to first add coagulants to the water (such as ferric sulfate) to destabilize the suspended solids and colloidal particles in it. Then, high-molecular-weight coagulant aids and carrier particles with higher density are added, so that the destabilized impurity particles use these carriers as nuclei for flocculation. Through the bridging adsorption effect of high-molecular-weight chains and the trapping effect of fine sand particles, floccules with higher density are formed rapidly, thereby **reducing the settling time, increasing the processing capacity of the clarifier, and effectively handling high shock loads. 2 Typical Processes 2.1 Acfiflo@ Process The Actiflo@ process was developed by OTV—Kruger Company (an engineering subsidiary of Veolia Water Group). It has been used in Europe for drinking water and wastewater treatment since 1991. Its feature lies in the use of fine sand with a particle size of 45–150 um as a carrier to enhance coagulation, as well as the use of inclined tube sedimentation tanks to accelerate the separation of solids from liquids. The surface load ranges from 80–120 m/h, with a maximum of up to 200 m/h; it is currently the most widely used carrier flocculation technology. Some water treatment plants in China have already adopted this technology; for example, the Linjiang project carried out by Veolia Water Shanghai Pudong in 2004 made use of the Actiflo@ rapid sedimentation process ； In response to the issues of low temperature, low turbidity, and high algae levels in the raw water, Beijing No. 9 Water Plant adopted the Actiflo@ high-efficiency sedimentation tank technology in the renovation project of its secondary sedimentation tanks. The Actiflo@ process flow is shown in Figure 1. The entire process is divided into three stages: mixing, aging, and high-speed precipitation. First, the raw water to which a coagulant (iron salt or aluminum salt) has been added enters the mixing tank, and after rapid stirring, it flows into the dosing tank ； An organic coagulant and fine sand carrier are added to the dosing tank to promote the “growth” of flocs; after a hydraulic retention time of 1–2 minutes, the mixture flows into the maturation tank ； Under slow stirring, the flocs with fine sand as their core further coalesce to form larger, denser flocs. Finally, the water enters an inclined tube sedimentation tank, where the gravitational settling of the fine sand flocs, combined with the rapid sedimentation effect of the inclined tubes, enables the floc particles to settle quickly. The sludge containing fine sand is returned to the hydrocyclone above the device, where centrifugal force is used to separate the slurry from the fine sand; the fine sand then returns to the flocculation tank for reuse. 2.2 DensaDeg@ Process The DensaDeg@ high-density clarifier was developed by the French company Degremont; it can be used for the clarification of drinking water, tertiary phosphorus removal, enhancement of primary sedimentation, as well as the treatment of combined sewer overflows (CSO) and domestic sewage overflows (SSO). This technology has now been adopted and applied in France, Germany, and Switzerland. In recent years, as major foreign water companies have entered the Chinese market, some domestic water treatment plants have also utilized this technology to expand and upgrade their existing processes; for example, this technology was employed in the expansion and renovation project of the Shidunzi Mountain Water Plant in Urumqi. The DensaDeg@ process combines technologies such as coagulation, inclined tube sedimentation, and sludge recirculation. Structurally, it is mainly divided into a reaction zone, a pre-sedimentation/concentration zone, and an inclined tube clarification zone (see Figure 2). It is characterized by the external circulation of the flocculated sludge, which serves as a carrier for flocculation; this accelerates the flocculation process and ensures the quality of the resulting flocs. The reaction zone is mainly divided into a rapidly stirred reaction tank and a slowly flowing reaction tank; the former ensures thorough mixing of the raw water with the coagulant, serving a purpose of pre-coagulation ； The latter allows the flocs to \"grow\" sufficiently through slow flow, thereby creating a mixed system within the entire reaction area that features good flocculation quality, high density, and excellent separation properties. The thoroughly coagulated mixture enters the pre-sedimentation/concentration zone for rapid separation. The supernatant water from the primary sedimentation stage goes to the inclined tube clarification zone to further remove any remaining flocs in the water. The sludge at the bottom, after being concentrated, is scraped into a sludge tank by sludge scrapers; part of this sludge is returned to the feed water, while the remainder is discharged into the sludge treatment system. Degremont has also developed the DensaDeg@4D clarifier, designed specifically for dealing with various types of wastewater overflows. Its working principle is similar to that of the DensaDeg@ process; it achieves water purification through functions such as the removal of sand and grit, the elimination of oils, an integrated coagulation and flocculation unit combined with inclined tube sedimentation, as well as the thickening and concentration of sludge. The working process involves the raw water to which coagulant has been added first entering a pre-coagulation tank, where air agitation ensures thorough contact and reaction between inorganic electrolytes and the particles in the water, allowing larger sand particles to settle directly at the bottom of the tank and be removed ； The water after pre-coagulation enters the flocculation tank, where it mixes thoroughly with the returned sludge and the added polymeric flocculants under mechanical stirring, forming dense flocs ； After thorough coagulation, the water enters an inclined tube clarifier, where most of the flocs separate from the water in the pre-sedimentation zone, and the remaining flocs are removed through the inclined tube sedimentation tank. The oil floating on the surface of the water is collected using skimmers to achieve oil removal ； Part of the sludge deposited at the bottom of the clarifier is recycled, while the remaining part is thickened and concentrated. 2.3 Treatment Effectiveness In Fort Worth, Texas, the United States, pilot studies were conducted on the aforementioned two carrier flocculation processes in order to develop a treatment scheme for peak wastewater flows; the results are shown in Table 1. In Columbus, Ohio, the United States, pilot studies were conducted using the Actiflo@ and DensaDeg@ carrier flocculation processes to address issues related to rainwater overflows and combined sewer overflows. The inflow rates for these two processes were 0.53–1.32 m³/min and 0.49–0.81 m³/min respectively, with average surface loads of 40 and 20 m/h. The overflow rates reached (2.0–3.1) and (1.4–1.6) m/min respectively, and the removal efficiency of suspended solids was >85%. The results show that both processes can effectively address issues such as high shock loads and large fluctuations in water quality caused by rainwater or sewage overflows. Furthermore, the amounts of different coagulants required to achieve an 85% TSS removal rate were determined through experiments, as shown in Table 2. As can be seen from these two examples, the Actiflo@ and DensaDeg@ carrier flocculation processes exhibit good performance in treating wastewater surges with high impact loads. By optimizing the amount of flocculant used, a TSS removal rate of over 85% can be achieved; moreover, under the same conditions, the amount of chemical required for the Actiflo@ process is lower than that required for DensaDeg@ ； The only drawback of these two processes is the significant variation in the BOD value of the effluent; this is because the flocculation process is a type of physicochemical treatment that makes it difficult to effectively remove dissolved organic matter from water. Overall, the carrier flocculation process effectively addresses the problems of sewage overflow and high shock loads that are difficult to handle with traditional processes. 2.4 Design parameters The design parameters for the Actiflo@ and DensaDeg@ carrier flocculation processes are shown in Table 3. 3 Development Status 3.1 Development of New Devices Binot Patrick and others in France have developed a carrier flocculation device that integrates multiple sedimentation modes; it can adjust its operation mode according to the quality and volume of the incoming water, making it suitable for treating wastewater whose flow rate and pollutant concentrations vary significantly throughout the year. Generally, the ratio of the average flow rate and peak flow rate of a municipal wastewater treatment plant on sunny days to the flow rate on rainy days is approximately 1:2:10. During the dry season, the plant operates in a normal sedimentation mode, while during the rainy season it operates in a coagulation mode that involves the addition of coagulants and fine sand. When the amount of wastewater falls between these two levels, conventional coagulation-sedimentation treatment is used. Its working process involves the raw water flowing sequentially through the chemical dosing area, the maturation area, and the inclined tube sedimentation area. In the chemical dosing area, the raw water mixes thoroughly with coagulants; the water after preliminary coagulation then enters the maturation area, where coagulant aids and fine sand are added. Once the flocs have fully \"grown,\" they proceed to the clarification area, where residual flocs are removed through inclined tube sedimentation. Fine sand usually settles at the bottom of the maturation tank, at which point the system is in normal sedimentation mode ； When the water volume increases suddenly, even beyond the operational capacity, the flow rate and mixing speed of the water rise, causing fine sand to rise from the bottom of the tank and act as floc nuclei, thereby achieving the purpose of carrier flocculation. Compared to conventional coagulation equipment, this device reduces unnecessary chemical consumption and thus has lower operating costs; however, it requires more control and monitoring units, resulting in higher fixed investment costs than traditional systems. Streat Philip in the United States designed a carrier flocculation device with a cubic structure; the mixing chamber, reaction chamber, and sludge chamber are all designed in a rectangular shape and are integrated organically within the flocculation device, resulting in a compact structure that saves a great deal of space. The process flow of this device is as follows: First, the water to which coagulants and fine sand have been added is thoroughly mixed in the mixing chamber, after which it enters the reaction chamber. There, under the action of vortex stirring, dense and large floccules are formed using the fine sand particles as nuclei. The walls between the reaction chamber and the sludge chamber are equipped with upper and lower connection ports, through which the formed floccules circulate between the two chambers. The larger and denser floccules settle directly to the bottom of the sludge chamber, while the lighter floccules return to the reaction chamber to continue growing. The sediment in the mud hopper is conveyed to the cleaning equipment via a transmission mechanism for separation; the fine sand resulting from this separation flows back from the cleaning equipment to the mixing chamber for reuse. The clarification chamber is located directly above the reaction chamber. The water after flocculation flows upward through the honeycomb-shaped turbulence control device into the clarification chamber; the particulate matter settles back into the reaction chamber, while the clear water is discharged through the overflow weir. The greatest advantage of this device is its compact design; there is no need for water transfer pipes or pumps between various components, which eliminates the problem of sand particles wearing out the pumps and pipes, thereby saving space and reducing equipment costs. 3.2 Combined Processes The coagulation process is an important component in water treatment processes, and it can be used in combination with other processes to achieve optimal treatment results. In 2005, Kruger applied for two patents related to carrier flocculation, both concerning the use of combined processes for wastewater treatment ； The Meriouse water treatment plant in Paris uses a carrier flocculation combined with nanofiltration process to supply 800,000 residents with high-quality drinking water at a rate of 14×104 m3 per day. Joyce Standley Perri and others proposed a combined process using electrocoagulation reactors along with carrier flocculation in order to address issues such as the toxicity and corrosiveness of the chemicals used in carrier flocculation processes. Its working process is as follows: the raw water first enters the electrocoagulation reactor. When an electric current is applied, the anode plates lose electrons, resulting in the formation of ions such as Al3+ and Fe2+. Under physical and chemical effects such as ionization, electrolysis, hydrolysis, and electromagnetic effects, the surface charges that keep pollutants dispersed are neutralized, thereby disrupting the stability of the dispersion of these pollutant particles ； The destabilized wastewater enters a carrier flocculation unit (such as an Actiflo@clarifier), passing successively through a mixing tank, a maturation tank, and an inclined tube sedimentation tank. Under the action of coagulants and fine sand carriers, the destabilized dispersed particles are adsorbed and \"grown\" into high-density, high-quality flocs; ultimately, solid-liquid separation is achieved through inclined tube sedimentation to produce clarified effluent. The advantages of this process are that it does not require the use of iron or aluminum salts as coagulants, which reduces chemical costs; it has a compact structure that saves space, and the quality of the treated water meets environmental requirements. To improve the removal efficiency of soluble BOD5 by the carrier flocculation process, Branden Hudson and others developed a combined treatment method using activated sludge and carrier flocculation, which can be operated in two modes: ① The water is first treated with activated sludge and then with carrier flocculation to obtain clarified effluent ； ②The water body is divided into two parts: one part enters the activated sludge tank, and after thorough mixing it goes into the carrier flocculation system; the other part enters the carrier flocculation system directly, and together with the mixture coming out of the activated sludge tank, it undergoes flocculation and sedimentation before being discharged in compliance with standards. In this process, the carrier flocculation equipment can be the Actiflo@ unit from OTV-Kruger Company. The flocculated activated sludge in the clarifier is partially recycled back to the reaction tank for reuse, while the remaining portion is sent to the sludge treatment facility. In this process, the activated sludge serves to degrade the soluble BOD5 in water; it also acts as a carrier and adsorbent in the flocculation tank, facilitating the \"growth\" of flocs and their precipitation, thereby reducing the amount of fine sand carriers required. At an MLSS of 800 mg/L and a contact reaction time of 30 minutes, the process achieves removal rates of 83.8%, 75.7%, and 89.4% for BOD5, soluble COD, and TSS, respectively, indicating that this process can effectively remove soluble organic matter and suspended solids from water. 4 Conclusions ① Carrier flocculation is an efficient sedimentation technique that features a compact structure and resource savings. Compared with traditional precipitation processes, it offers advantages such as high treatment efficiency, small footprint, resistance to shock loads, and low dosage of chemicals. ② The drawback of traditional carrier flocculation equipment is that the circulation of sand particles between pipes and pumps shortens the equipment’s lifespan. Flocculation treatment has limited effectiveness in reducing BOD and COD, as it is unable to remove dissolved organic matter from water. Compared to traditional devices, new carrier flocculation systems require more control equipment and components. ③ Currently, the development of this technology focuses on creating new devices designed to address the various shortcomings of traditional carrier flocculation equipment, or on using it in combination with other processes to achieve synergistic effects. For example, combining it with the activated sludge process can address the issue of low removal rates of dissolved organic matter in water bodies. ④ With the increasingly fierce competition in the water industry market and the growing scarcity of energy and resources, users are placing ever-higher demands on technological innovation. In the future, the development of equipment should primarily aim to enhance its automation level, conserve resources, and improve efficiency. Various devices developed based on carrier flocculation can meet these criteria; they align with China’s goal of building a resource-conserving society. These devices are expected to play a significant role in China’s water treatment sector, helping to narrow the gap between China and developed countries. participate