Received: August 2, 2021
Accepted: November 10, 2021
Available: December 16, 2021
The current issues of climate change and high freshwater demand worldwide have promoted the implementation of wastewater reclamation technologies. This study aims to review the efficiency of the dissolved air flotation (DAF) technique in a wide variety of applications in the agricultural, industrial, domestic, and municipal sectors, which have high freshwater consumption worldwide. We made a systematic review of the DAF technique in wastewater treatment in 2015-2021. We reviewed six indexed databases and governmental statistical reports; we used the keywords: dissolved air flotation, microbubbles, wastewater treatment, and the main operating and design parameters involved in the effectiveness of the flotation process. Additionally, we conducted a review of the most common synthetic coagulant studies used with DAF, as well as natural coagulants that promise to mitigate current climate change. Finally, we discussed advantages, disadvantages, and potential future studies. DAF to have considerable potential for wastewater treatment, as well as for waste utilization. The generation of large quantities of DAF sludge is a breakthrough for clean energy production, as it allows the use of this waste for biogas production.
Keywords: Dissolved air flotation, Microbubbles, Wastewater treatment, Design and operating parameters, coagulation-flocculation.
La actual problemática de cambio climático y alta demanda de agua dulce a nivel mundial ha promovido la implementación de tecnologías para la regeneración de las aguas residuales. El objetivo de este estudio es revisar la eficiencia del sistema de flotación por aire disuelto (DAF) en una amplia variedad de aplicaciones en los sectores agrícola, industrial, doméstico y municipal, los cuales presentan un elevado consumo de agua dulce en el mundo. Por tal motivo se realizó una revisión sistemática de la técnica de DAF utilizada para el tratamiento de aguas residuales en el periodo 2015-2021. Se revisaron seis bases de datos indexadas y reportes estadísticos gubernamentales, las palabras claves fueron flotación por aire disuelto, microburbujas, tratamiento de aguas residuales y los principales parámetros de operación y diseño que intervienen en la eficacia del proceso de flotación, junto con un análisis de los estudios de coagulantes sintéticos más comunes utilizados con DAF, así como de los coagulantes naturales que prometen mitigar el cambio climático actual. Por último, se discuten las ventajas, los inconvenientes y los posibles estudios futuros. Se observó que DAF tiene un potencial considerable para el tratamiento de aguas residuales, así como para la utilización de residuos. La generación de grandes cantidades de lodos de DAF es una brecha para la producción de energía limpia, pues permite utilizar estos residuos para la producción de biogás.
Palabras clave: Flotación por aire disuelto, microburbujas, tratamiento de aguas residuales, parámetros de diseño y operación, coagulación-floculación.
The effects of climate change and the growing demand for water worldwide have generated an alarming water deficit. The global population is expected to reach nearly 8.6 billion by 2030, and water demand is expected to grow annually by 1 %, resulting in a cumulative increase of 20 % to 30 % by 2050 [
Numerous water recovery methods have been developed. They include chemical, biological, and physical techniques such as gravity separation, membrane filtration, flotation, and electrocoagulation [
This has been proven in wastewater treatment [
Dissolved Air Flotation (DAF) is a reliable method for wastewater treatment [
DAF is effective in removing low-density components present in elements and chemical compounds, plant compounds, microorganisms, and parasites. In eutrophic waters, DAF has maintained stable removal efficiencies in the removal of turbidity, total phosphorus, chlorophyll, and COD of 80 %, 72 %, 71 %, and 61 %, respectively. In addition, dissolved oxygen in polluted urban rivers could be raised from 0.2 to 2 mg/L to 3 to 3.5 mg/L due to air dissolved in water, mitigating the negative impact on the environment [
The efficiency of DAF in the removal of antibiotics has been evaluated in wastewater from the pharmaceutical sector, showing COD and penicillin G potassium removal percentages of 70.41 % and 67.45 %, respectively, under optimal conditions in the operation of the system [
Hybrid treatment methods combined with biological and physicochemical treatment processes have been adopted with high efficiency in wastewater treatment plants (WWTPs) [
DAF is an excellent complementary stage. A study tested the efficiency of DAF in conjunction with a pilot-scale high-speed activated sludge system of the municipal water treatment plant in Aartselaar (Belgium). Sand was first removed by sedimentation, and then large particles (size 1 mm) were removed by a drum filter. Finally, the liquid was transferred to the DAF unit, where special conditions were used to further clean the water. The results showed a removal of 78 % of TSS and 68 % of COD [
Another study recently evaluated the ability of DAF as a possible new mechanism to diagnose intestinal parasites in feces. The results showed parasite recovery efficiencies of 73.27 %, 58.12 %, 37.85 %, and 91.89 % for Ascaris lumbricoides, Hymenolepis diminuta, Giardia duodenalis, and Strongyloides stercoralis, respectively. This reveals a promising line of research for DAF, as intestinal parasitosis is a severe public health problem globally [
In general, the DAF technology is efficient in water treatment; it also has favorable operating costs and can be very attractive if it integrates renewable energies in all its processes. Studying the trends of DAF allows us to foresee its efficiency in future studies because some environments present more complex compositions than others.
Therefore, reviewing different DAF applications enables us to indirectly identify the strengths and limitations of these systems to optimize the technique.
This paper reviews the literature published between 2015 and 2021 about the applications of DAF in the agricultural, industrial, and domestic and municipal sectors. It examines the functionality of DAF systems (including coagulation-flocculation) and their applications in wastewater treatment. It also identifies the main design and operating parameters that determine the efficiency of the cleaning process in conventional DAF. Finally, the advantages and disadvantages of the technology, its advances, and areas of possible future work are highlighted.
This literature review adopted the systematic search method [
However, this literature review also includes eight papers published before 2015 and two website references that corroborate the vast experience of the DAF technique since its beginnings.
3.1 Dissolved Air Flotation
In the 1970s, the DAF method was already being used in drinking and wastewater treatment in Finland, Sweden, and South Africa [
DAF separates fine particles from the flow by means of MBs. Conventional DAF involves four processes: MB generation, chemical pretreatment of the wastewater, flotation, and sludge removal. Structurally, the technique consists of a flotation tank divided into two zones (contact and separation) by a baffle. The contact zone guarantees the collision and adhesion of the particles to the MBs, which are then conveyed to the separation zone. This MB-particle agglomeration rises to the surface by the force of buoyancy, forming sludge or a foam layer.
In many cases, depending on the wastewater to be treated, a coagulation-flocculation process is used before the flotation tank to ensure better adhesion of the microparticles to the bubbles [
Microbubbles are generated by the precipitation caused by the sudden depressurization at high pressures (usually 300-700 kPa) of a mixture of water and air in a saturator (a tank or Venturi tube). The generated bubbles are driven to the contact zone [
The optimal size of the MBs in DAF is usually between 20 μm and 100 μm [
Computer vision and machine learning algorithms [
3.2 Applications of DAF in water treatment
This subsection describes DAF applications classified by sector: agricultural, industrial, and domestic and municipal.
3.2.1 Agricultural sector
Multiple publications have studied the efficiency of DAF in the treatment of wastewater from the dairy, slaughterhouse, and poultry industries. Dairy wastewater is characterized by high FOG, COD, and BOD contents, as well as the formation of sludge with strong butyric acid odor attributed to the decomposition of casein [
No. | Wastewater type | Principal physicochemical parameters Removal efficiency (%) | Ref. | |||||
Oil and grease | TSS | COD | BOD | Color | Turbidity | |||
1 | Dairy | - | - | 87.5 | - | 83.1 | 97.8 | [ |
2 | Dairy | - | 62.5 | 87.8 | 91.4 | - | - | [ |
3 | Slaughterhouses | - | - | 90.0 | 90.0 | - | - | [ |
4 | Poultry | - | - | 97.9 | 98.6 | - | - | [ |
5 | Poultry | 91.1 ± 5.2 | - | 97.9 ± 1.0 | 98.6 ±1.0 | - | - | [ |
6 | Harvesting of Chlorella sorokiniana | - | 88.6-92.5 | - | - | 91.7-92.3 | 93.7-96.2 | [ |
7 | Dairy | 66.1 | 95.0 | 54.2 | 76.2 | 83.2 | 88 | [ |
8 | Dairy | - | - | 48.3-50.3 | - | - | 91.1-91.5 | [ |
Wastewater from the slaughterhouse industry is characterized by high levels of Organic Matter (OM), nutrients, pathogens, detergents, FOG, and sometimes antibiotics and heavy metals [
3.2.2 Industrial sector
DAF has been applied in wastewater treatment in the paper manufacturing [
No. | Wastewater type | Physicochemical parameters Removal efficiency (%) | Ref. | |||||
Oil and grease | TSS | COD | BOD | Color | TPH | |||
1 | Biodiesel | 98.0-99.6 | 98.0-100.0 | 80.0-90.0 | 80-90 | No color | - | [ |
2 | Metalworking fluids | - | - | 99.8 | - | - | 98.9 | [ |
3 | Emulsified oil | >90.0 | - | - | - | - | - | [ |
4 | Wastewater from the petrochemical and food processing industries | 99.7 | - | 95.0 | - | - | >90.0 | [ |
5 | Wastepaper industry | - | 98.1 | 39.0 ± 10.0 | - | 33 ± 20 | - | [ |
6 | Metalworking wastewater | 99.7 | - | 98.5 | 61 | - | 70.0 | [ |
7 | Water emulsions | - | - | 100.0 | - | - | - | [ |
8 | Oilfield wastewater | 96.6 | 88.8 | 95.7 | - | - | - | [ |
In some cases, DAF as primary treatment is insufficient to meet the removal parameters established by environmental regulations; therefore, it is necessary to incorporate other methods, which typically include biological treatments [
Other important removal parameters for industrial wastewater include heavy metals, radioactive waste, and OM. The removal of heavy metals and other chemical compounds in wastewater is a problem encountered in the metallurgical [
DAF has shown effective removal results for such elements and other chemical components (see Table 3).
No. | Element | Removal efficiency (%) | Reference |
1 | Sulfate ions | 82.0 | [ |
2 | Fe (OH)3 | 99.0 | [ |
3 | Amina | 80.0 | [ |
4 | Lead and zinc sulfide (particles < 44 µm) | 89.0-96.0 | [ |
5 | Fe ions | 95.0 | [ |
6 | Cd, Ni, Mn, and Pb | 29.0, 27.0, 31.0, and 29.0 | [ |
7 | Sodium oleate (NaOL) | 97.6 | [ |
8 | Cu, Cd, and Zn | >95.0 | [ |
9 | Acrylonitrile-butadiene-styrene | >90.0 | [ |
10 | Nanoparticles of TiO2 | 95.0-100.0 | [ |
In some cases, ultrafine metallic particles are difficult to remove in the cleaning process, making it necessary to use flocculants and/or coagulants that facilitate the adhesion of the particles to the bubbles [
The nuclear industry generates gaseous, liquid, and solid radioactive wastes that require specialized treatment due to their high degree of toxicity and contamination [
Finally, another important removal parameter is OM [79]. It has been shown that DAF can remove more than 98 % of the OM [
3.2.3 Domestic and municipal sector
DAF has been used in the treatment of a variety of wastewater types generated by the domestic and municipal sector (see Table 4), showing efficient results in combination with other methods [
No. | Wastewater type | Removal efficiency (%) | Reference |
1 | Phosphorus and organic matter removal | 82 (Total phosphorus | [ |
2 | Pharmaceutical products removal | 71 (Diclofenac) | [ |
3 | Organic compounds (domestic wastewater) | 78 (TSS) | [ |
4 | Residential/municipal wastewater | - | [ |
5 | Restaurants | 92.4 (Oil) | [ |
6 | Giardia andCryptosporidium protozoa | 95.38-99.81 (Total Coliforms, TC, and E. coli) | [ |
In general, physicochemical removal parameters such as OM, nitrogen, organic carbon, phosphorus, and FOG (which are derived from the use of cleaning or pharmaceutical products and other domestic activities) are treated with bacterial cultures in activated sludge systems [
Municipal and domestic wastewater also contains microorganisms that can be highly infectious. The effect of DAF in the presence of coagulants to removeGiardia and Cryptosporidium has been studied in [
DAF’s recovery of biodegradable particles has been studied by evaluating its TSS removal efficiency in screened municipal wastewater effluents [
As for potable water, there are no recent studies that encourage the use of DAF. However, the literature reports the application of DAF as a clarifier for drinking water in the 1960s [
3.2.4 Operating and design parameters of DAF
Identifying the parameters of DAF systems requires an understanding of the stages of the cleaning process. In the case of conventional DAF systems, there are three important processes: chemical pretreatment (coagulation-flocculation), MB generation, and flotation.
They present a wide variety of operating and design parameters. This paper proposes a selection of main DAF parameters to highlight their relevance in the assessment of wastewater treatment efficiency.
The working principle of DAF systems is the flotation process, which, in essence, uses air MBs to capture microparticles and then separates the emulsions produced based on their physicochemical properties [
No. | Reference | Diameter MB (μm) | Air pressure (psi) | % R | Removal efficiency (%) |
1 | [ |
40-50 | 65.27-87.02 | 5-20 | - |
2 | [ |
- | 72.52 | 20 | 100 (algal cells), 99.99 (Adenosine Tri-Phosphate (ATP)) |
3 | [ |
- | 72.52 | - | 70 (DQO) |
4 | [ |
40 | 72.52 | 20 | 98 (Chlorella sorokiniana) |
5 | [ |
- | 87.02 | 20-25 | 89-96 (Particles < 44 µm) |
6 | [ |
40-80 | Patm | - | 52 (Particle removal) |
7 | [ |
45 | 14.50-58.02 | 20-29 | 76.6 (Oil droplets) |
8 | [ |
- | 116.03 | 20 | 91.5 (Turbidity in dairy wastewater) |
9 | [ |
- | 145.04 | 20 | > 90 (Turbidity in dairy wastewater) |
10 | [ |
- | 58.02-145.04 | 20-100 | 97.8 (Turbidity in dairy wastewater) |
11 | [ |
40-60 | 89.92 | 40 | 99 (Ni, Cu, Cr, Pb liquid radioactive waste) |
Indeed, the study of MB characteristics in the flotation process helps to ensure MB stability and removal efficiency [
DAF parameters are directly related to the setup of DAF processes and, therefore, vary when coagulation or flocculation is used [
On the other hand, coagulation and flocculation are chemical pretreatment processes that affect the performance of DAF since they alter the properties of the wastewater to be treated.
The coagulation process consists of the electrical destabilization of the particles, and flocculation is the process in which the particles collide with each other to form larger clots, which facilitates the removal of turbidity, color, and pathogenic microorganisms, among other things [
Typically, in DAF systems, two units are integrated for coagulation-flocculation as a pretreatment of the wastewater before the flotation process is applied [
No. | Coagulant | Flocculant | pH | Mixing velocity [rpm] | Mixing time [min] | Floc. time [min] | Removal efficiency (%) | Ref. |
1 | Ferrous sulfate (FeSO4) | Cationic polymer TanFloc | 4.0 | 120 | 5 | - | 97.8 (Turbidity) | [ |
2 | Chitosan/ polyaluminum ferric chloride (PAFC) | - | 6.0 | 50-200 | 5-15 | - | 64.3 (Organic pollutants) | [ |
3 | Aluminum sulphate, ferric chloride, Tanfloc SG and Zetag 8185 | - | 7.0 | - | - | 15-30 | 89.4-90.9 (Total phosphorus) 90.2-92.6 (Total Kjeldahl nitrogen, TKN) | [ |
4 | Combined polyacrylamide (PAM) | Tanfloc | 7.6 | 120 | 5 | 5 | >90 (Turbidity) | [ |
5 | Liquid ferrate/ Ferric chloride | - | (8.0±0.20) | 100 | 2 | 20 | 92 (Organic matter) | [ |
6 | Ripe Okra (Abelmoschus esculentus) / passion fruit (Passiflora edulis) | - | 9/5 | 200 | 1 | 15 | 91.1 (Turbidity) | [ |
7 | Ferric chloride (FeCl3) | PACl | 2.5-3.0 | - | - | - | 98.9 (Total petroleum hydrocarbon) | [ |
8 | - | Dismulgan V3377 | 7.0 | 9000-144000 | 1-5 | - | >99 (Oil removal) | [ |
9 | Ferric chloride (FeCl3) | PACl | 7.48 | - | 1 | - | 95 (COD and oil) | [ |
10 | Polyaluminum chloride (PACl) | Cationic polyacrylamide (C-PAM) | 6.9 | 20-80 | 2-20 | 15 | 98.1 (TSS, wastepaper) | [ |
11 | - | Polyacrylamide | 4.5 | 120 | 1-3 | 5 | 80-82 (Sulfate-sodium salts) | [ |
12 | FeCl3 and PACl | - | 7/6 | - | 0.25 | 20 | 96.38/95.38 (E. coli) | [ |
In particular, ferric chloride (FeCl3) and polyaluminum chlorides (PAC) are coagulants of inorganic character used very frequently in the treatment of water with high color and low turbidity [
Currently, the study of coagulants, flocculants, and other substances of organic-type chemical action is an area of interest to researchers since these compounds tend to be environmentally friendly and could replace conventional chemicals [
DAF design parameters vary according to the hydrodynamics of the flotation tank, i.e., its geometry [
Another design parameter of DAF is the air-to-solid ratio (A/S), which is directly related to the required removal efficiency and indicates the mass of air required in the pressurization mechanism per unit mass of particles [
where Cs is the theoretical solubility of air (mg/L); P, the absolute saturation pressure (atm); f, the fraction of air dissolved at pressure P, usually 0.5; R, the recirculation rate; Xf, the concentration of solids to be removed; and Q, the flow that enters the flotation cell (L/s).
In DAF design, the required amount of air is determined by the concentration and size of the suspended particles [
Other parameters are also involved, such as Hydraulic Loading Capacity (HLC) (5-15 m/h in conventional DAF) [
No. | Ref. | Inlet flow rate (m3/h) | HRT (min) | HLC (m/h) | Air pressure (psi) | A/S (ml/m) | % R | Removal efficiency (%) |
1 | [ |
0.5 | - | - | 43.51-72.52 | - | 10 | _ |
2 | [ |
- | 120 | - | 14.50-75.52 | - | - | 99 (NH4+-N) |
3 | [ |
1.8-2.4 | 11-14 | 11.4-15 | 87.02 | 20-25 | 95 (Pb2+ and Cu2+) | |
4 | [ |
- | 4320 | - | 72.52 | - | 20 | 89.4-90.95 (Total phosphorus) 90.2-92.6 (Total Kjeldahl nitrogen TKN) |
5 | [ |
1.6-4 | 25-60 | 10-22 | 43.51-87.02 | 0.5 | 12-40 | 94-96 (TSS) |
6 | [ |
- | - | 11.8-23.4 | - | 0.01 | - | _ |
7 | [ |
- | 1440 | - | 29.00-58.02 | 0.026 | 30 | 91.1 ± 5.2 (Oil and grease) 98.6 ± 1.0 (DBO) |
8 | [ |
- | 10-20 | - | 89.92 | 0.35 | 40 | 99 (Ni, Cu, Cr, Pb liquid radioactive waste) |
Salinity is another important parameter that could influence different aspects of DAF, such as air dissolution, bubble characteristics, and system hydrodynamics [
An important tool to design DAF systems is Computational Fluid Dynamics (CFD). In numerical simulations of DAF using CFD, the flow regime is an essential parameter for flotation tank design, analysis, and control because it helps to improve the performance by directing heat and mass transfer [
Based on this literature review, Table 8 details the most relevant design and operating parameters in the three main DAF processes (i.e., chemical pretreatment, bubble generation, and flotation). The pretreatment of the wastewater at the coagulation-flocculation stage facilitates bubble-particle interaction and floc formation. The size of the bubbles generated is highly influential during the flotation process. Therefore, it should be set rigorously according to the characteristics of the generation device. Finally, the flotation process is directly determined by tank hydrodynamics, which influences the effectiveness of the operation.
No. | Chemical pretreatment | Bubble generation | Flotation process |
1 | pH | Temperature | SLR |
2 | Coagulation-flocculation | % R | A/S |
3 | Zeta potential | A/S | % R |
4 | Injected air flow | MB size | |
5 | Air pressure | MB rise velocity | |
6 | Salinity | Number of MBs | |
7 | Flow regime | HRT | |
8 | MB generator device characteristics | HLC | |
9 | pH | Air pressure | |
10 | Gas holdup | Flotation tank hydrodynamics | |
11 | Zeta potential | Particle size and concentration | |
12 | Wastewater density | ||
13 | Surface tension |
The literature has demonstrated the ability of DAF to remove high OM loads, particularly in the separation of oil and grease and FOG emulsions [
The number of publications between 2015 and 2021 (see Figure 4-a) examining the use of DAF for wastewater suggests that its application is less frequent in the agricultural sector, which represents 24 % of the total documents. This could be because agricultural types of waste do not represent a complex problem in water treatment since they can become organic sources of coagulation, being an alternative to conventional metallic coagulants [
In contrast, the industrial sector was the most representative, with 44 % of all the publications. This is likely due to the high amounts of oil and grease and microparticles in industrial wastewater, whose removal is facilitated by MBs due to the hydrophobicity of the medium and the density difference between the waste fluid (oil and grease in other chemical substances) and water [
DAF has been in continuous development in recent years and has shown to be highly efficient in combination with other cleaning techniques [
No. | Advantages | Disadvantages |
1 | Efficient removal of OM, algae, oil and grease, FOG, microparticles, and microorganisms from wastewater. | The flotation tank must be protected from low and high temperatures. |
2 | Fast system start-up, high separation efficiency, and operational versatility. | High energy consumption and increased operating costs. |
3 | Generation of thicker sludge. | DAF foam generation. |
4 | Compact system, low HRT, lower construction costs, low operation cost, and high loading rates. | Use of coagulants and flocculants. |
One particularity of DAF is that, in very cold climates, the system must be protected from low temperatures to avoid freezing the float and causing sedimentation of solids that have already been driven to the surface by the effect of the bubbles. This is because rising or falling temperatures in the flotation tank can affect gas retention and MB stability. In the case of a temperature increase, larger MBs break into smaller bubbles and the viscosity of the liquid is reduced, resulting in decreased stability due to oxygen reduction [
Technological advances in DAF systems have stimulated new research into ways to optimize control, automation, operating times, system size, and energy costs, thus improving the efficiency of the overall process [
In addition, CFD allows researchers to examine various aspects of the flotation process by increasing the removal efficiency and decreasing investment and operating costs [
These mathematical models provide accurate information to design systems using optimal operating parameters. The results of CFD simulation can provide more practical information about the design and establish appropriate operating conditions [
Mathematical models and flow simulation of MBs using CFD have reduced the limitations of DAF systems and enabled advances in their operation and design to make them more efficient. For this reason, modern DAF system designs tend to be faster, more compact, and less expensive in operation [
Studying the hydrodynamic characteristics of the flotation system is fundamental in the design, control, analysis, optimization, operation, and modeling of DAF equipment because these factors significantly affect the efficiency of the process [
The utilization of DAF sludge is an interesting avenue for future research as it can boost sustainable development and cleaner energy. A study [
Finally, another interesting application of DAF is microalgae harvesting [
Wastewater treatment systems are increasingly facing new restrictions on emissions as well as tighter regulations on energy efficiency and resource recycling due to growing environmental pollution. An adequate configuration of the main design and operating parameters in DAF systems and their optimization at start-up can overcome many of these difficulties.
In general, the trend in DAF has been oriented towards improving removal efficiency and optimizing plant operating and energy costs. In this regard, flotation hydrodynamics is fundamental for the design, control, analysis, optimization, operation, and modeling of DAF [
The effects of DAF’s operating and design parameters on the characteristics of the bubbles are remarkable, as they ensure the efficiency of bubble-particle floc formation and flocculation of the bubbles in the waste removal stage. In this respect, the parameters of both the tank hydrodynamics and the bubble-generating device must be considered in the design of DAF systems.
In order to establish optimal DAF operating parameters, it is essential to understand MB flow and interaction with surrounding particles [
DAF has proven its high performance in hybrid technologies, particularly in combination with biological methods, that meet the standards required by environmental legislation. DAF’s effectiveness in reducing removal parameters (particularly turbidity, COD, BOD, TSS, and oil and grease, among other pollution indicators) makes it a recommendable technique for the recovery of water resources and the reduction of environmental toxicological risks.
This paper presented a systematic review of literature about the DAF method for wastewater treatment. This method has proven to be effective in treating wastewater of different characteristics generated by activities in the agricultural, industrial, and domestic and municipal sectors. Its MB-based technology has enabled the elimination of difficult-to-remove pollutants such as oil and grease, FOG, OM, and fine particles that are a problem in water reclamation processes. Over the past decade, significant progress has been made to increase the effectiveness of cleaning processes in DAF systems.
The selection of a set of design and operating parameters for DAF is directly related to wastewater characteristics. MB size, saturation pressure, dissolved oxygen concentration, recycle flow rate, and coagulation or flocculation are considered the main parameters in DAF due to their strong influence on system start-up. Currently, the most frequently used coagulants or flocculants in wastewater treatment are ferrous sulfate (FeSO4) and polyacrylamide (PAM); however, natural alternatives such as ripe okra and passion fruit seed have shown promising results as replacements for artificial coagulants.
The advantages and disadvantages of DAF in wastewater treatment were also identified here to encourage new DAF studies in different areas of agribusiness, industry, and wastewater treatment, from the cleaning of facilities and equipment used in production processes to the treatment of drinking water. For instance, the generation of large amounts of DAF sludge can fill a gap in clean energy production since this waste can be used for biogas production and alleviate the waste discharge. Future research could enable the use of DAF sludge in other high-value activities. Finally, future work should focus on developing DAF technologies that use renewable energy, examine ways to reduce the energy consumption of elements (such as pressure pumps, motors, air compressors, and mechanical systems), and continue searching for natural coagulants and flocculants.
The authors would like to thank the Universidad del Cauca for supporting this study.
All the authors declare that they have no conflicts of interest.
Jeimmy Adriana Muñoz-Alegria: Conceptualization, methodology, data curation, and writing/original draft preparation.
Elena Muñoz España: Data visualization, research, supervision, writing, revising, and editing.
Juan Fernando Flórez-Marulanda: Data visualization, research, supervision, writing, revising, and editing.