The dissolved air floaters produced have been used for many years in the treatment of waste water, for the separation of suspended solids, oils and fats, fibers and other low-density materials, as well as for the thickening of activated sludge and sludge. products from chemical flocculation.
The flotation-separators can be used in civil wastewater treatment plants to remove floatable and sedimentable solids, improving the efficiency of existing treatment systems and promoting greater flexibility of the entire system. In chemical treatment plants, a flotation-separator can replace the final clarifier downstream of flocculation, improving performance.
In industrial wastewater treatment plants, the flotation device can be used to recover products lost in the process water and reduce pollution. Materials such as light chemical flakes, organic wastes, light and delicate solids, fats and oils can be recovered without breakdown or emulsification.
The flotation-thickeners are instead used to obtain high concentrations of solids, in the thickening of activated sludge or other biological sludge. With activated sludge it is possible to obtain concentrations of 4%, significantly reducing treatment costs. In many applications the sludge volume can be reduced by 90%.
DISSOLVED AIR FLOTATION THEORY
Flotation derives from the word to float which means to rise to the surface, a physical phenomenon that has always existed. As a separation method, flotation has long been used in the mining industry. In 1930 the wood and paper industry developed a technique in which air was dissolved in water under pressure; a method that was perfected later and revolutionized the application of flotation. In general, water treatment involves separating suspended material.
This can be achieved directly or following one or more transformation processes such as chemical precipitation. Dissolved organic substances can also be transformed by microorganisms into biological flakes. With the aid of chemical precipitation other dissolved impurities, such as phosphorus, iron manganese, can be transformed into a state which then allows them to be separated. Both biological and chemical flakes normally tend to settle and can thus be separated by settling, but since the their density is generally similar to that of water, they settle slowly.
This means that both separation time and removal efficiency can be improved considerably with the flotation technique. The low density of air compared to water will cause air bubbles to rapidly rise to the surface. Along their ascending path, the bubbles will stick to the flakes and other particles and will form an air-flake aggregate which will have a rising speed higher than the sedimentation speed of the particles alone, this is the system for inverting gravity. Dissolved air flotation It has been used with different systems applying the solubility of air at various pressures and temperatures in order to achieve maximum pressurization.
With total pressurization, the entire water flow is saturated with air at a pressure of 2-3 bar. The pressure is then reduced at the inlet of the flotation unit and the air bubbles produced attach to the particles present and make them rise to the surface.
This system is only suitable for the separation of solid materials due to its violence first in the passage into the pumps and then in the sudden drop in pressure. In the partial pressurization system only part of the water to be treated is pressurized and saturated with air, and then rejoined to the main flow. With the recycling pressurization system, a part of the treated water is recirculated and saturated with pressurized air. It is then added to the influent water to be treated before it enters the flotation unit. This system works very well with chemical flakes and significantly expands the possibilities for using flotation.
THE FLOTCAR SYSTEM
The FLOTCAR system is a real revolution in dissolved air flotation (DAF) as it is combined with a chemical reaction process, breaking down the colloidal particles. The FLOTCAR system is a real revolution in dissolved air flotation (DAF) as it is combined with a chemical reaction process, breaking down the colloidal particles and then separating them with the flotation effect, in short, excellent purifying effects are obtained at an operating cost reduced by 70% compared to a conventional system.
The raw slurry, after the usual preliminary treatments (screening, possible shredding, sand removal, possible de-oiling), is mixed very intensely with one or more chemical reagents (rapid mixing): the colloidal particles (organic and inorganic), stable suspension due to the The action of mutual repulsion determined by the electric charges of the same sign (generally negative) that they possess, are destabilized by the added chemical reagents, with a complex mechanics, with consequent coagulation (more specifically «perikinetic or electrokinetic coagulation»), which occurs with very high speed at the very moment of contact with the chemical reagents (coagulation phase with rapid mixing): it manifests itself in a mutual approach of the colloidal particles, being able, once the electric charge due to mutual repulsion has been canceled or reduced, the forces of molecular mutual attraction to predominate between individual particles (Van der Waals forces).
The destabilized particles, in the form of “microflakes” (to which are also added the flakes formed by chemical reagents, normally consisting of complexes of gelatinous hydroxides, equipped with effective adsorbent characteristics, are subjected to a subsequent flocculation phase (or “orthokinetic coagulation”): the the mixture is gently agitated, in order to favor the opportunity for (statistical) collision of the particles, and consequently the “coalescence”, i.e. the aggregation and growth of the microflakes, which bind each other due to adsorption phenomena, and at the same time provide for mechanically incorporating those colloidal particles still remained in suspension.
The volume and specific weight of the flakes are consequently increased, making them clearly visible to the naked eye. The result of coagulation-flocculation is therefore the transformation of non-sedimentable colloidal substances into sedimentable substances, i.e. into flakes which, in a subsequent flocculation phase, can be easily collected on the surface with the effect of dissolved air flotation, in the form of mud.
To accelerate the flocculation effect, the flotation technique using dissolved air is used, where the low density of air compared to water will cause the air bubbles to rapidly rise to the surface. Along their ascending path, the bubbles will stick to the flakes and other particles and will form an air-flake aggregate which will have an ascent speed higher than the sedimentation speed of the particles alone, this is the system for inverting gravity. With total pressurization, the entire water flow is saturated with air at a pressure of 4/6 bar. Depending on the degree of saturation that is intended to be achieved, the pressure is then reduced at the inlet of the flotation unit and the air bubbles produced attach to the particles present and make them rise to the surface. This system is suitable for the separation of solid materials and obviously also chemical flakes produced during flocculation.
In the pressurization system only part of the water to be treated is pressurized and saturated with air, and then rejoined to the main flow; the recycling pressurization system is designed to recycle a part of the treated water by introducing it back into the circuit after being saturated with pressurized air. The recycled and saturated water is mixed together with the wastewater and chemical flakes previously produced with the chemical reaction adopted to then pass the cylinder or diflotation compartment.
COLLOIDAL DESTABILIZATION (Perikinetic and Orthokinetic coagulation)
In the initial phase of the purification process, a coagulant is dosed into the wastewater, in our case it is ferric chloride or aluminum chloride in a manner entirely proportional to the volume of water. The necessary condition for the destabilization of the colloidal particles is that the wastewater has a PH range higher than 8.0 and therefore a PH detection instrument is installed connected to a proportional dispenser, which comes into operation if the PH is lower than the minimum threshold set. The chemical product chosen to increase the PH is liquid caustic soda.
In addition to the dosing, a fast mixing called Perikinetic mixing takes place with a 540 RPM agitator equipped with a motor, mixing rod reducer with marine blade propeller. The optimal pH “range” for an effective product reaction is between 8.5 and 11, and this is why NaOH is added as a pH corrector. In the second phase of the purification process, a flocculant is dosed in the wastewater, in our case it is Polyelectrolyte, in a manner entirely proportional to the volume of water. In addition to the dosing, a slow mixing called orthokinetic mixing takes place with a 70 RPM agitator equipped with a motor, mixing rod reducer with marine blade propeller.
FLOTATION OF POLLUTING PARTICLES
The particular construction of the inlet chamber allows the complete mixing of the influent with the pressurized flow. The back pressure valve is positioned so as to reduce the pressure of the pressurized flow inside the mixing chamber. This avoids the reduction in efficiency due to the coalescence of air bubbles in the external pipes and fittings. The flotation units are equipped with a flotation collection compartment, with an inclined bottom.
This provides sufficient storage capacity which eliminates the need for a separate collection pan. The tank is made of steel concrete, just as existing tanks can be used, with a wide choice of sizes, for each application. The pressurization system is chosen according to the required application and, thanks to its particular conception, requires the minimum flow rate to be pressurized, helping to reduce costs, both initial and operational. The possibility of varying the speed of the foamer, together with the regulation of the liquid level, ensure maximum control in the removal of the floatate.
The suggested dissolved air reactor consists of a circular-shaped tank in which the water level is approximately 2 m. . The inlet, outlet and sludge removal mechanisms are located in both the surface and bottom section. This section’s cleaning blades move across both the surface and bottom of the tank at a speed synchronized with the flow of water entering.
In the influent pipe, the water to be treated is mixed with the microbubbles produced by the pressurization system. The microbubbles are trapped inside the flocs and bring them to the surface very quickly as they enter the flotation room. The distribution tube is provided with openings which can be adjusted to give uniform diffusion of water into the flotation chamber. The floating particles are accumulated on the surface and concentrated by the buoyancy given by the micro air bubbles.
These solid particles are dragged by the cleaning blades and discharged by gravity into the sludge discharge channel characterized by a screw which sends them to the subsequent treatment. The heaviest particles will quickly accumulate on the bottom of the tank, the mechanically operated attached cleaning blades scrape the bottom of the tank and discharge the accumulated particles into a channel where the sludge is sent to the next treatment through one and a lifting pump.
FLOATED SLUDGE EXTRACTOR
The main function of the surface scraper is to convey the floated substances present in the tank into a collection and transport network. This equipment is made of stainless steel and consists of a control gearmotor unit which rotates the shaft. The spoon works in a rotational way, taking the floated particles from the surface of the flotation tank, while the advancement on the surface is given by a variable speed gearmotor which advances on the edge of the tank.
EXTRACTION OF SEDIMENTED SLUDGE (R version)
The main function of the bottom scraper is to collect and convey the mud that has settled on the bottom into a transverse channel located at one end of the rectangular tank. This equipment, made mainly of plastic material (thermoplastic resin with glass fiber added), is substantially made up of a control gearmotor unit which, via a first driving chain, rotates the drive shaft of a pair of operating chains. The two operating chains are guided by a series of pairs of toothed pinions positioned on lateral supports fixed to the walls.
On the bottom and on the walls of the tank there are wear sliding supports for the scrapers. An adequate number of scrapers is mounted between one chain and the other. The system is equipped with an automatic tensioning mechanism for the two operating chains. All Inside the transversal channel, there is a horizontal transport screw which receives the mud taken from the scrapers and conveys it towards a collection area where a pump transports the material towards the treatment plant. The auger is equipped with a gearmotor unit, positioned outside the tank, which rotates the transport system with a driving chain.
The pressurization system is composed of a medium pressure pump, a pressurization reactor called G.D.A. saturator. (gas dissolving reactor) and an air compressor. Inside the G.D.A the saturation of the air in the water takes place. The air required to further saturate the water is injected through a special pore membrane by an air compressor. The air saturation phase is very important, because it is here that the production of the micro bubbles that will float the particles occurs. In the G.D.A we have the saturation of the air in the water at a pressure of 6 bar.
The saturated water, before entering the process, passes through a friction valve which will bring the water to atmospheric pressure and facilitate the formation and development of micro bubbles. These will have a diameter of 40-100 µm. The significance of the quantity of micro air bubbles released by the gas dissolving reactor is evident. It is necessary to design the flotation unit so that the relationship between the air and the solids (A/7) has a correct flotation reaction. The theoretical calculations are made by taking into consideration the relationship between the number of air bubbles, the number of successful collisions with the union between the particles and the bubbles and the number of particles present in the water. Field experience demonstrated that the theoretical A/S relationship is not easily calculable.
This is due to various factors: it is highly dependent on the degree of muscle building, on decision factors, on the coalescence of the air bubbles and on the lack of union between the particles and the air bubbles. For this reason it is always necessary to provide a quantity of air higher than that predicted by theory. Usually the objective is to ultra saturate the water gently, using in this reactor pressurization systems that purge out the excess water supplied. In any case it is necessary to have control systems and regulate the amount of air given to the water because an excess could damage the flotation process.
CHARACTERISTICS OF MACHINES WITH CIRCULAR SURFACE (FLOTCAR “C”)
The tanks can be made of reinforced concrete or steel.
Standard machines up to 20 m in diameter are foreseen, which allows maximum results
The tanks can be made of reinforced concrete or steel. Standard machines up to 20 m in diameter are available, which allows maximum performance to be achieved at minimum costs. The units can be centrally or peripherally driven, the motorization can be variable or constant speed .
With timed operation you can adjust the amount of flotation removed and ensure maximum concentration. All units are equipped with a bottom scraping system for the removal of non-floating materials. For each application the correct system and type of pressurisation is selected. The pressure of the pressurized flow is reduced just before entering the unit, in order to ensure complete mixing of the influent with the flow itself, with minimal turbulence and coalescence of the free air. The air-water-solids mixture is distributed by the diffusion chamber, uniformly throughout the entire area of the circular unit. The correct hydraulics of the system have been tested in various applications, both clarification and thickening.
FEATURES OF RECTANGULAR SURFACE MACHINES (FLOTCAR “R”)
The particular construction of the inlet chamber allows the complete mixing of the influent with the pressurized flow. When chemical flotation aids are required, these must be added into the mixing chamber. The back pressure valve is positioned to reduce the pressure of the pressurized flow inside the mixing chamber. This avoids the decrease in efficiency due to the coalescence of air bubbles in the external pipes and fittings. The flotation units are equipped with a flotation collection compartment, with an inclined bottom.
This provides sufficient storage capacity which eliminates the need for a separate collection tray. The tank can be made of reinforced concrete or steel, just as existing tanks can be used, with a wide choice of sizes, for each application. The pressurization system is chosen depending on the required application and, thanks to its particular conception, requires the minimum flow rate to be pressurized, helping to reduce costs, both initial and operational. The possibility of varying the speed of the foamer, together with the regulation of the liquid level, ensure maximum control in the removal of the floatate.
An optional bottom scraper is provided for the removal of non-floating materials. Also on request, an auger can be supplied for transporting the mud to one side of the tank. To remove the floated material, a foamer with blades equipped with rubber flaps is provided.
FLOTCAR R50 with reed valve pack
Low maintenance is ensured by the direct drive of the scrapers, the bottom scraper drive shafts do not have stuffing boxes. Compact flotation units are shipped fully assembled and require minimal intervention to start up
GENERAL PROJECT CONSIDERATIONS
Design variables for flotation units include:
- pressurization system
- recycling rate
- hydraulic load
- solids load
- detention time
The solids load becomes the fundamental variable when sizing a flotation device for sludge thickening (from 10 to 25 kg/h/m2).
The degree of pressurization is generally maintained at 4-5 bar.
For the pre-treatment of oil-rich wastewater, a recycling ratio of 30-40%, a hydraulic load of 2.4 to 9.6 m3/h/m2 (including recycling) and a detention time of between 20 and 40 minutes
Technical table of “circular surface flotcar” models
|Circular units Diameter (mt.)
|Flow rate m3/h 2.4 m3/h/m2
|Flow rate m3/h 3.6 m3/h/m2
|Flow rate m3/h 4.8 m3/h/m2
|Solids load (Kg/h)
10 lcg/h m2
Technical table of “rectangular surface flotcar” models
|Rectangular units Dimensions (m)
|Flow rate m3/h 2.4 3/h/m2
|Flow rate m3/h 3.6 m3/h/m2
|Flow rate m3/h 4.8 m3/h/m2
|Solids load (Kg/h)
10 lcg/h m2
|1,0 x 3,0
|1,50 x 4,0
|2,0 x 5,0
|2,0 x 6,0
|2,5 x 7,5
|2,5 x 9,0
|2,5 x 10,5
|2,5 x 12,0