Dissolved air flotation is a separation process in which suspended and emulsified impurities are carried to the surface by air bubbles released from a supersaturated solution. The floated layer is removed by a mechanical scraper and the clarified water leaves from the lower part of the chamber. The technology works with whatever is lighter than water or can be brought into that state by a reagent.
Principle
Part of the already clarified water is drawn off for recirculation and saturated with air in a saturator at 4–6 bar. The solubility of a gas is proportional to its partial pressure, so under pressure the water takes up many times more air than it would under atmospheric conditions. On throttling at the chamber inlet the solution becomes supersaturated and the excess air is released as bubbles tens of micrometres across.
A bubble attaches to the surface of a particle and forms an aggregate whose mean density is lower than that of water. The aggregate rises; the rise velocity follows Stokes' law and depends on the size of the aggregate and on the density difference. Float collects on the surface of the chamber and is removed by a chain scraper into the sludge launder, while clarified water is drawn from the lower part of the chamber.
A bubble can attach only to a hydrophobic surface, so reagent treatment is part of the process rather than an addition to it. In the coagulation and flocculation chambers ahead of the flotation chamber, a coagulant and a flocculant are dosed: the first neutralises the charge of colloidal particles and enlarges them, the second binds the microflocs into structures that hold bubbles. Residence time in those chambers and mixing intensity are set by calculation: excessive mixing breaks the flocs, insufficient mixing prevents them from forming.
Recycle flow is a design value that determines how much air enters the chamber. In the catalogue models it amounts to a noticeable share of the capacity: DAF-5 takes 2 m³/h at a capacity of 4–5 m³/h, DAF-100 takes 50 m³/h at 95–100 m³/h. These volumes pass continuously through the recycle pump and the saturator and account for most of the unit's energy consumption.
The capacity of a flotation chamber is limited not only by volume but also by the hydraulic loading on the surface area and by the solids loading. That is why the size is selected for the design flow rather than "with a margin": well below the design flow the distribution within the chamber and the sludge removal regime break down, while above it the residence time shortens and part of the aggregates fail to rise.
Principle
Flotation is not settling in reverse. A particle rises not because its density is lower than that of water but because a bubble has attached to it and the resulting aggregate has become buoyant relative to the medium. Attachment is possible only where the surface is sufficiently hydrophobic, and it is reagent treatment that makes it so. It follows that a flotation unit without a matched coagulant does not merely "work less well": for most parameters it does not work at all, and the effluent passes through the chamber virtually unchanged.
Bubble size matters more than bubble count. Bubbles tens of micrometres in diameter are the effective ones: they rise slowly, stay in contact with particles longer and have time to attach. Large bubbles rise quickly, their probability of attachment is low, and the upward flow they create stirs the contents of the chamber and breaks up aggregates that have already formed. For that reason increasing the air supply above the design value degrades treatment rather than improving it, and the saturator regime belongs among the parameters that should not be altered arbitrarily.
A flotation unit concentrates pollutants rather than destroying them. The removed matter does not disappear: it passes into float amounting to some one to three per cent of the effluent flow at 94–97 % moisture. In dry solids that is little; in transport volume it is substantial, because what is hauled is mostly water. Sludge dewatering is therefore planned together with the flotation unit: without it the task has not been solved but moved to the next stage of waste handling.
Practice
A flotation unit is installed ahead of biological treatment. For activated sludge, fats and light solids are not a nutrient substrate but a load: they impair mass transfer, promote sludge bulking and reduce the performance of structures designed for dissolved organics.
On a single site the unit serves as the step that brings effluent within the discharge limits of the municipal sewer for suspended solids, fats and part of the COD. Parameters relating to dissolved matter remain unchanged, and where the exceedance concerns them, flotation alone does not resolve the task.
The reagent regime is established for the specific effluent by jar testing. The type and dose of coagulant, the need for a flocculant and the required pH correction are determined. Effluents from one industry are similar in character but not identical: differences in recipe, washing schedule and detergents change the dose several-fold.
The unit is supplied skid-mounted and comprises coagulation and flocculation chambers, the flotation chamber, a saturator, a recycle pump, a scraper mechanism and a dosing station. The site has to provide the effluent inlet, outlets for clarified water and sludge, power supply and access for servicing the scraper.
Performance is assessed from comparable samples at the inlet and the outlet, not from the appearance of the clarified water. For fats and petroleum products the achievable reduction is usually 80–95 %, for suspended solids 70–90 %; the actual figures depend on the reagent regime, the temperature and the stability of the effluent composition.
Limits
Practice
The cases are typical examples, not site reports.
Innovations
Scheme
A flotation unit is not applied as a stage in its own right. It requires a screen or sieve ahead of it to retain coarse matter and a balancing tank to even out flow and composition: the reagent side is tuned to a certain range, and leaving that range means losing performance at precisely the moment of peak load.
The dosing station is part of the unit. Coagulant and flocculant doses are established by jar testing and revised when production changes; where the pH deviates substantially, a correction unit is introduced, since the effectiveness of coagulants is confined to a narrow range.
Downstream of the unit the scheme divides into two lines. Clarified water goes to discharge or to biological treatment where limits on dissolved matter are not met. The float goes to dewatering: hauling sludge at over 94 % moisture is not economically justified.
For cold sites the temperature regime is taken into account: air solubility and water viscosity both depend on temperature, and operation away from design values changes the saturator regime and the rise velocity of the aggregates.
Scheme
Operation
Energy
Specific energy consumption is of the order of 0.1–0.3 kWh per cubic metre of effluent. Most of it goes to the recycle pump and the compressor that saturate the water with air; the scraper and stirrer drives consume substantially less. In the catalogue models the total installed drive power ranges from 3 to 15 kW depending on the size, which in continuous operation makes a predictable and comparatively modest item of operating cost next to the price of chemicals.
Consumables
What goes to drain
Float is produced at some one to three per cent of the effluent flow at 94–97 % moisture. It contains fats, suspended solids and reagent flocs, that is, everything removed from the water. Further handling follows from its composition: dewatering on a screw or belt press, storage and haulage by a licensed contractor or, for certain food industries, transfer for processing. The hazard class and the permitted route are determined by the effluent composition, not by the type of equipment.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
Reagents are part of the process. A bubble attaches only to a hydrophobic surface, and it is the coagulant and the flocculant that create one while enlarging the particles to a size at which the aggregate rises within the residence time of the chamber. Without reagent treatment only free-floating fat is removed, and the figures for suspended solids and COD remain practically unchanged.
No. Flotation removes suspended and colloidal matter from the water and turns it into sludge, whereas dissolved organics, ammonium nitrogen and mineral salts pass through the unit unchanged. In a typical scheme the flotation unit sits ahead of the biological stage and reduces the load on it, which allows the structures to be smaller and their operation more stable.
The volume is some one to three per cent of the effluent flow at 94–97 % moisture. The sludge is dewatered on a screw or belt press and then transferred to a licensed contractor in accordance with its hazard class. Handling has to be planned together with the choice of the unit; otherwise what the plant hauls away is mostly water.
A margin on flow is justified within the limits set by the hydraulic loading of the chamber. Flotation also has a lower bound: well below the design flow the distribution of the flow and the sludge removal regime break down, and performance falls. It is sounder to build the margin into the balancing tank and the duty cycle and to select the size for the design flow.
The choice follows from the character of the solids. A lamella clarifier works with particles denser than water: mineral solids, scale, sand. It does not remove fats, oils or light flocs. Flotation is designed precisely for those and requires a reagent side. Where the effluent carries both, the stages are applied in sequence.
Temperature affects the process in two ways. As the temperature falls the solubility of air increases, which is favourable in itself, but the viscosity of water rises, the rise velocity of the aggregates falls, and coagulation becomes less effective and calls for a higher reagent dose. For unheated sites insulation and heating of the dosing equipment are provided: freezing of the reagent solutions stops the unit entirely.
Tell us about your site — our engineer will prepare a process design, a specification and a quotation within one business day.