1. What is dissolved air flotation (DAF)?

Dissolved air flotation (DAF) is a clarification technology widely used in physicochemical wastewater treatment and sludge concentration. This technology separates suspended solids, oils and grease from effluents by the addition of air in the form of microbubbles that float the particles to which they adhere.

2. How does a dissolved air flotation system work?

A DAF system operates continuously. The effluent is conveyed to the float tank, at the inlet of which it is mixed with part of the effluent flow saturated with pressurized air. The air microbubbles formed during depressurization adhere to the SS contained in the effluent and cause them to rise to the surface where they are evacuated by sweepers. The clarified effluent is collected in the upper zone of the float, below the floating layer.

What are the main applications of DAF systems?

DAF systems are used in water clarification, separation and concentration of pollutants in different industries. in different industriesfor both their products and waste. DAF systems offer excellent performance for the separation of suspended solids, oils, greases, emulsions and substances of low or medium density. In addition, the process removes organic matter and other contaminants (nutrients, metals, etc.) associated with the solids removed. Also, if the physicochemical process design is correct, salts and other components are precipitated and removed as solids.

• In wastewater, its main applications are:
• Sludge concentration
• Primary clarification
• Secondary clarifier for aerobic and anaerobic biological reactors
• Seawater desalination

4. Why is dissolved air flotation used in wastewater treatment?

Compared to other effluent clarification treatments, the DAF system allows working at high hydraulic loads (4 - 6 ^m3/m2/h) and high mass loads (3 - 5 ^Kg/m2/h), which favors a smaller footprint. In addition, pollutant separation yields of 95 - 99% are obtained and the resulting sludge after this process is of higher concentration (3 - 5%) than that obtained in other processes (sedimentation).

5. What industries commonly use DAF systems?

In general, DAF systems are an excellent technology for separating suspended solids in industrial wastewater. However, there are sectors in which their use is more common due to the nature of the solids present in the water to be treated, such as:

• Food industries (dairy, meat, beverages, oils and fats, etc.).
• Textile industry
• Metallurgical industry
• Pharmaceutical industry
• Paper industry
• Refineries and petrochemical plants
• Drinking water and urban wastewater treatment
• In general, as a system for sludge and product concentration

How does DAF technology compare with sedimentation?

These are two technologies for effluent clarification. To compare them we use their design parameters, as well as their operating costs and their capacity to produce sludge. The following table shows indicative values:

7. Are all suspended solids in the wastewater separated by flotation?

Suspended solids associated with air microbubbles introduced into the wastewater stream will tend to float due to their loss of bulk density; however, heavier solids will precipitate and remain at the bottom of the float. DAFs include a decanted sludge purge system. The separation efficiency of DAFs can exceed 95%, depending on the type of effluent.

Is DAF equipment expensive?

DAF floats are usually built in metallic materials (protected steels and stainless steels) or in plastic materials (PRFV, PP), and have an associated air compression and dissolution system by means of a pump and a pressurized boiler, with the corresponding decompression and injection system. The result is apparently more expensive than a conventional sedimentation system; however, if we take into account that we can eliminate a possible thickener of separated sludge, and that the space required is much smaller, the process is very profitable and efficient.

Is a DAF system efficient for the separation of emulsions, oils and hydrocarbons?

DAF systems are the most efficient technology for separating oils and hydrocarbons. This is due to the coalescent effect that air microbubbles have on these substances, which are immiscible with water, and which are usually presented as colloidal micelles. In addition, in many cases, no reagents are needed for separation, as they usually have low densities; therefore, the DAF process allows the recovery of many of these compounds.

What is the role of a DAF team in a desalination plant?

DAF flotation is very effective as a primary treatment in desalination, as it protects reverse osmosis membranes from fouling that shortens their lifetime and reduces their performance. It provides a high separation rate of suspended solids, algae cells, oils, fats and hydrocarbons that cannot be removed by sedimentation, thus protecting subsequent desalination processes. In addition, it minimizes algae by-products and their toxins that could reach the drinking water supply network.

How much space does a DAF system take up?

The size of DAF equipment varies depending on the volume of water to be treated, but a unit typically takes up less space than other wastewater clarification systems because it can operate at high hydraulic and mass loads. In addition, the need to install sludge thickening equipment is eliminated because DAF systems can also perform this function.

12. Is the assembly of a DAF system complex?

Dissolved air flotation equipment is usually supplied assembled from the workshop, which greatly simplifies installation at the final location. In addition, as the civil works required for its implementation is minimal, the implementation time is also minimal.

Only the discharge, pneumatic and electrical connections should be connected, and, at the most, the air compression and injection system (pumping/boiler/injector) should be connected to the float body, and the chemical conditioning set, which is usually supplied in a skid. (PFL Flocculator for coagulation-flocculation - SIGMADAF)

13. Is the start-up of DAF equipment complex?                             

DAF equipment is usually pre-assembled and tested in the workshop. This greatly simplifies the task of commissioning in the field, since most of the automatisms are already programmed in the PLC, and only the operating and recirculation pressures and flow rates have to be adjusted. Reagent dosages are adjusted on site by means of Jar test system, to optimize their performance.

1. What are the components of a DAF system?

The main components of a DAF system are:

• Flotation tank in which the separation of SS and flotants takes place.
• Air pressurization system in a part of the clarified effluent that is recirculated to the inlet. The system includes a pumping, and a pressurization boiler in which air is injected and dissolved under pressure.
• Diffusion system that allows the uniform formation of micro-bubbles of depressurized air.
• Extraction of floating SS composed of surface scrapers

2. What is the optimum pressure for operating a DAF?

The usual pressure at which air is introduced into the pressurization circuit to obtain the expected separation results is 3 - 6 bar, but it depends on several factors :

• Characteristics of the water to be treated: Factors such as the load and type of contaminants to be separated, or the temperature, are determining factors for the design of the equipment.
• Water-air ratio. There is a direct relation between the pressure to be supplied and the air dissolved in the water, (Approx. 0.05 Kg air /Kg SS/h, at 5 bar).
• Microbubble size. Pressure affects bubble size (desired is ≤50 µm).

3. What is the role of coagulants and flocculants in the dissolved air flotation process?

In many cases, the suspended solids present in effluents are in the form of colloids that remain in suspension due to their electrical charge. In order to separate them, coagulating reagents are first added to neutralize their polarity (_FeCl3, _{Al2 SO4}, PAC, etc.), and then flocculants, which are long-chain polymers that form separableflocs, are dosed.

4. How does air dissolve in water in a DAF system?

To dissolve pressurized air in water, part of the clarified water leaving the DAF tank is pumped into a small pressurized vessel (boiler), into which compressed air is also introduced at the same pressure. In this way, the effluent is supersaturated with air corresponding to that pressure and temperature, according to Henry's law.

5. How are air microbubbles formed in the effluent to produce flotation in a DAF?

The air-saturated effluent stream is recirculated to the inlet of the flotation tank and flows through a pressure reducing valve just as it enters a mixer with the raw wastewater. This causes the air to be released in the form of small bubbles, the size of which directly influences the upward velocity of the suspended solids with which they are associated.

What are the design flow rates for industrial DAF systems?

The design flow rates of DAFs depend on the type of effluents produced in the different industries. Thus, in industries that discharge important flows with low or medium SS load (drinking water treatment plants, food industries, petrochemical industry, etc.), it is possible to prefabricate equipment up to 250 ^m3/h (FPBC-EN.pdf file), while for the chemical industry it is usual to work with lower flows and higher SS loads (FPAC-EN.pdf file), and prefabricated equipment does not usually exceed 150 ^m3/h.

SIGMADAF manufactures DAF equipment that can treat from 3 ^m3/h to more than 1.000 ^m3/h.

What are the DAF system design parameters?

The parameters used for the design of dissolved air flotation equipment are:

• Design flow rate (^m3/h)
• SS /oils and fats (mg/l)                                                                     
• Hydraulic load (5 - 15 ^m3/m2/h)
• Holding time (15 -30 minutes)
• Recirculated flow (15 - 50 % of supply flow)
• Pressurized air supply (0.05 Kg air /Kg solids/hr)

The type of industry and the process where the effluent comes from is also basic for a correct design of the DAF equipment.

9. How will I receive a DAF system at my facility?

Installation is simple, but start-up requires specific knowledge of water treatment systems.

Our equipment leaves the workshop fully assembled and tested. Depending on the customer's needs, the equipment is usually delivered in modular assemblies that can be easily assembled on site, or completely assembled and interconnected (Plug&Play). For special applications, customized equipment can be built in the appropriate materials.

10. What materials are dissolved air flotation equipment made of?

This depends on the characteristics of the effluent to be treated. DAF systems are usually built in metallic materials: carbon steel, protected with epoxy resins, or stainless steel (AISI 304 /316). For highly corrosive or saline waters, DAFs can be made of plastic materials such as GRP, PP, PVC or HDPE.

How is wastewater treatment carried out in a DAF system?

The process consists of three basic stages:

• Physical - chemical conditioning (pH adjustment, coagulation / flocculation)
• Flotation tank and SS separation
• Air pressurization / depressurization system

The effluent is fed to a mixing system with the chemical conditioning reagents, where sufficient contact time will be available under the appropriate agitation conditions.

From there it is passed to the flotation tank where part of the effluent that has been supersaturated with pressurized air is recirculated to separate the SS that will already be flocculated and will float associated with the air microbubbles.

1. Why is my DAF system not achieving the expected separation efficiency?

It may be due to:

• Incorrect operating parameters. Check flow rates and operating pressures. 
• Incorrect air saturation. Verify that microbubbles are produced homogeneously. 
• Problems with the chemical parameters of the process. Due to inadequate pH or temperature, or incorrect dosage of coagulant / flocculant reagents.
• Concentration of contaminants. Check that SS and effluent oil and grease concentrations are within design limits.

2. What causes excessive sludge accumulation in a DAF system?

This situation can be caused by:

• Poor separation of floating sludge. Due to failure, poor regulation or obstructions in the surface scraping system.
• Problems with the chemical conditions of the system. Due to a bad adjustment in the dosage of coagulant and flocculant reagents, as well as pH control. incrustacion.PDF                          
• Pollutant load higher than the design load. 
• Inefficient bottom sludge purge. Due to clogging or short purge time.
• Incorrect operating/recirculation flow rates.

3. Why is there air leakage in my DAF unit?

The most common causes are the following:

• Low pressure in the recirculation circuit.
• Leaks in pipes or circuit equipment.
• Possible problems with the air compressor or recirculation pump.
•  Bubbles too large.
• Failures in the expansion valve or diffusion system.                       
• Clogging due to impurities in the supply air.                      
• Low effluent temperature.

4. How to prevent fouling in a DAF system?

To prevent fouling of DAF equipment, it is advisable to screen the effluent at the inlet of the float to separate solids that may affect the process.

Fouling is usually caused by chemical dosing errors or incorrect operating temperature. Control of pH and temperature, as well as adjustment of coagulants and flocculants prevent this problem.

Biological contaminations can also generate fouling.

Good maintenance, including periodic washing and even chemical washing if necessary, is the best way to prevent fouling.

5. How to clean a DAF system?

After electrically disconnecting the equipment and emptying it:

• Perform a clean water pressure wash for routine cleaning.
• If there are incrustations, proceed with acid washing.
• If there is dirt caused by oil, grease or organic matter, clean with NaOH or biodegradable detergents.
• If necessary, scrape or brush affected surfaces.
• In all cases, check and clean the diffusers and the air supply system, as well as the recirculation circuit and pumping.
• In order to achieve good contact and agitation in the washing process, the recirculation system of the equipment can be used.

The recommended frequency of washing is weekly, and a quarterly thorough washing, depending on the operating load.

6. Why do foul-smelling floats appear in a DAF system?

The most frequent causes are:

• Anaerobic decomposition of excess organic matter, which can occur due to poor sludge removal, which will require a review of the pipelines and the surface sweeping system for flotsam.
• pH control mismatch or coagulant/flocculant dosages
• Aeration adjustment problems (flow rate, pressure, bubble size).
• Possible presence of compounds such as sulfides, mercaptans or other potentially malodorous agents in the effluent.

7. Why do foams appear on the floats of a DAF?

Foams may appear for several reasons, among them:

• The presence of detergents or emulsions can generate foams on the surface of the DAF.
• Incorrect pH control or incorrect coagulation / flocculation.
• Excess of organic matter due to high concentration or deficiency in its elimination, which favors its decomposition and possible formation of floating foams.
• Incorrect regulation of air supply

If, in spite of the appropriate corrections, the foam persists, the dosage of a defoamer can be tried.

8. Why do large bubbles and bubbling form in a DAF?

In general, they are usually caused by a bad control of the air supply (deficit or excess), or by an incorrect pressure in the pressurized circuit. They can also appear due to failures in the pressurization system (valves/diffusers) due to clogging or incorrect regulation.

Poor coagulation / flocculation adjustment may be another cause, as air microbubbles will tend to agglutinate to form larger ones by not adhering to unformed flocs, or by the decomposition of deposited or floating organic sludge that may produce gases.

9. Why does the pressure and flow rate of the recirculation circuit vary in a DAF system?

These variations can condition the correct operation of the DAF. They can occur for several reasons, but basically there are changes in flow pressure due to leakage problems, clogging or incorrect operation of the water/air recirculation and pressurization circuit, both in pumping and in the pressurized tank, valves, diffusers and piping.

Another likely cause is the variation in effluent characteristics (temperature, SS, or viscosity).

10. Why can an effluent clarified in a DAF, and clear in appearance, contain high turbidity?

This can occur because of the presence of microparticles, colloids or oils, which are not visible to the naked eye. This can occur because the chemical adjustment of pH, coagulation / flocculation, has not been correct, or because the process of separation of these SS by means of air microbubbles has not been complete.

These parameters should be readjusted and the averaging instrument (turbidimeter) should also be checked. One way to detect this is the Tyndall effect.

11. Why does the DAF system not start or has stopped?

The causes can be many, but basically, it is necessary to check that the levels of the effluent input and output tanks are not at the minimum and/or maximum levels respectively, since these levels interlock the installation.

Other causes may be an electrical problem (power failure, thermal trip, inoperability of key equipment, alarm in automatic controls), or a mechanical problem that triggers an alarm (flow, pressure, etc.).

1. How does dissolved air flotation compare to induced air flotation?

DAF, IAF or CAF (induced air flotation) are solid-liquid separation processes, which have in common that they both use air bubbles for the separation of suspended solids present in the effluents.

In the DAF, micro air bubbles (50 - 100 µm) are generated by depressurization. These are homogeneously dispersed by suitable diffusion systems and are associated with the solids found in the wastewater, such as suspended matter, colloids, oils and fats.

On the other hand, CAF floats floats generate larger and more heterogeneous bubbles (100 - 1000 µm), which appear due to cavitation produced by a mechanical agitation system, or by the supply of air at low pressure. This technology is applied to water treatment lines with larger particles of solids and oils, such as wastewater from oil industries and refineries.

From the point of view of efficiency and energy consumption, the DAF is more efficient, but consumes more energy than the CAF.

2. What are the advantages of DAFs over traditional settlers?

The main advantages of DAF floats over settlers are:

• High separation performance of suspended solids, oils and grease (90 - 95%).
• High hydraulic and mass loads, with short residence times, which translates into less space occupied.
• High concentration of collected sludge (4 - 6 % DM).
• Possible elimination of sludge thickeners and downsizing of mechanical drying equipment.
• Separation of floating or low density material.

3. What are the limitations of DAF systems?

DAF flotation systems have some limitations in their use for wastewater treatment:

• Elevated effluent temperatures reduce the solubility of air while increasing the solubility of certain pollutants.
• If there are significant changes in the pollutant loads, the depressurization and dispersion of the air microbubbles must be fine-tuned. The doses of coagulant and flocculant reagents must also be adjusted.
• High density solids settle to the bottom of the DAF, which means they must be removed to prevent their accumulation and decomposition.
• DAF equipment is expensive compared to settling and thickening systems, while consuming more energy and requiring more maintenance.

4. Can DAF systems be used to treat drinking water?

Dissolved air flotation systems are used in drinking water treatment, as they are very effective in removing SS, algae and other contaminants present in surface water. In addition, DAF systems are used in seawater desalination plants, as they allow the separation of oils and greases and other contaminants that can clog filter membranes (UF/OI).

5. How is DAF equipment combined with other wastewater treatment technologies?

DAF flotation systems integrate effectively with other technologies in a wastewater treatment plant:

• Primary treatment of wastewater and drinking water, which conditions the effluent to be treated for subsequent purification processes, reducing its size, consumption and costs. Primary treatment is essential to ensure the efficiency of the entire treatment process.
• Secondary treatment, where the DAF is usually combined with biological processes. In this case, the DAF equipment replaces the secondary settling and sludge thickener.
• Other uses. It is also used in combination with advanced oxidation process (POA), or integrated in API and CPI type oil separation systems.

6. What is the efficiency of DAF systems with respect to filtration technologies?

The DAF process can separate on the order of 90% of suspended solids and ≥95% of oils and greases.

Filtration on sand beds, or other materials, achieves results of ≥ 95% for suspended solids, but oils and greases can produce serious fouling problems.

MF microfiltration, NF nanofiltration and UF ultrafiltration membranes have a filtration range that easily exceeds 99%, but the presence of oils and fats must be as low as possible to ensure proper operation.

The tangential filtration process with inorganic membranes (graphite/ceramic) is very efficient for all these contaminants, with separation efficiencies of >99.5 %. On the other hand, it is a more complex and expensive process.

7. Comparison of DAF flotation vs. API and CPI

Dissolved air flotation (DAF), inclined plate separators (API) and corrugated plate separators (CPI) are technologies used in wastewater treatment processes for the separation of oils, greases and solids.

• The DAF equipment works by supplying excess pressurized and dissolved air that floats solids, oils and greases, with a separation efficiency of up to 99%).
• API/CPI systems separate materials that are less dense than water, or oils, non-emulsified fats, but higher density suspended solids settle out. The efficiency of this process, which is between 40% and 80%, is lower than that of a DAF system.

DAF equipment takes up less space, but has higher installation and operating costs than API/CPI.

8. What are the advantages of a DAF versus a sludge thickener?

The sludge concentration achieved with a conventional settler is between 0.6 and 1% dry matter (DM), due to the carryover of water from the blowdowns. On the other hand, a DAF system can obtain values of 3 - 4% DM, which allows these sludges to be sent directly to the subsequent sludge dewatering process, after which they can be sent to landfill or used.

This process eliminates the need for sludge thickening equipment.

9. When should lamellae be included in a DAF system?

When the content of suspended solids, oils and greases is low and the flow rate is high, it is preferable to use DAF systems equipped with lamellae, since they increase the useful surface, considered as projected surface, which can be much larger than the geometric surface area. Thus we speak of equivalent upward velocity for floats equipped with lamellae.

In this way, the space occupied by the equipment in the plant is minimized at a lower cost.

10. Organic matter separation efficiency of a DAF equipment compared to the efficiency of a biological reactor.

Dissolved air flotation is applied to separate organic matter. A DAF unit and a biological reactor are perfectly complementary and it is very common to find them integrated in the same wastewater treatment plant.

• When the organic matter has a low density (oils, greases, detergents, suspended matter, colloids), the DAF process can be competitive (separates up to 60% of the OM).
• If the organic loads of a discharge are high, a float can be arranged prior to biological treatment, as it improves the purification performance of the biological process, while reducing the size of the reactor.
• The biodegradable organic matter removal performance of a biological reactor is higher (can exceed 99%), but the DAF system is more efficient for the reduction of poorly biodegradable matter.