The characteristics of industrial wastewater depend directly on the production process. Textile wastewater may contain fibres, suspended solids, colloids and organic substances; in the food industry, fats, proteins and product residues may predominate; in automotive and metalworking processes, the main challenge may instead be machining oils, emulsions and suspended particles.
These differences are crucial to treatment design. Not all contaminants exhibit the same physical behaviour: some settle rapidly, while others have a density close to that of water or dimensions small enough to remain stably dispersed.
It is in this context that DAF in industrial wastewater treatment finds one of its main applications.
DAF stands for Dissolved Air Flotation. The process uses microbubbles to increase the upward velocity of suspended particles, oils, greases and flocs, allowing them to be separated at the top of the unit.
The dissolved air flotation process remains unchanged across different sectors; what changes is the wastewater to be treated and, consequently, how the system is sized and integrated into the treatment plant.
Flow rate, TSS, particle size distribution, oil and grease concentration, COD fractionation, pH, temperature and load variability directly influence system performance and configuration.
HOW AN INDUSTRIAL DAF SYSTEM WORKS
In an industrial DAF system, a portion of the clarified effluent is normally recirculated and pressurised. Air is dissolved into the stream under pressure and, when the mixture is subsequently returned to atmospheric pressure in the DAF inlet zone, microbubbles are generated.
The bubbles come into contact with suspended solids and previously formed flocs, promoting their rise to the surface. The floated material is then concentrated at the top of the unit and removed by skimming, while the clarified effluent is sent to the downstream treatment stages.
Separation efficiency depends on the interaction among several parameters: hydraulic loading, recycle rate, saturation pressure, air-to-solids ratio, microbubble size and floc characteristics.
DAF is particularly suitable when conventional sedimentation is hindered by the low density of contaminants or their low settling velocity.
COAGULATION AND FLOCCULATION: CONTROLLING THE FLOC
Not all particles in the wastewater can interact effectively with the microbubbles. Colloids, in particular, may be extremely small and carry surface charges that keep them stably dispersed.
For this reason, DAF is often preceded by a coagulation and flocculation stage. Coagulation destabilizes colloidal particles, while flocculation promotes their aggregation into larger structures.
In a DAF process, simply producing a floc that is “large” is not enough: it must have characteristics compatible with flotation. It must provide sufficient surface area for interaction with microbubbles while maintaining enough mechanical strength during mixing and transfer.
For this reason, pH, reagent type and dosage, mixing gradients and contact times must be optimised for the actual wastewater matrix being treated.
WHERE IS DAF APPLIED?
The main applications of DAF in industrial wastewater treatment vary according to the nature of the pollutant load:
- Textile industry: fibres, TSS, oils and colloidal particles can be separated before biological treatment. DAF acts mainly on the suspended and aggregable fraction of the load and can help reduce the COD fraction associated with these components. It does not, however, directly remove salts, TDS or fully dissolved organic compounds.
- Food & Beverage: wastewater can contain high loads of fats, oils, proteins and organic solids. When a significant fraction of COD is associated with suspended or floatable matter, DAF can remove it before biological treatment, reducing the organic and solids load transferred to the downstream process.
- Dairy industry: fats, proteins and milk residues may require preliminary pH adjustment, coagulation and flocculation to promote the formation of aggregates that can be more readily separated by DAF. Managing emulsions and the protein fraction is one of the most important aspects of pretreatment.
- Meat processing: fats, proteins and organic solids can represent a significant share of the overall load. Flotation allows the physically removable fraction to be separated upstream, avoiding transfer of the entire load to the biological treatment stage.
- Automotive and metalworking: machining oils, lubricants, TSS and oil-water emulsions are common contaminants. Free oils may be relatively easy to separate, while emulsions generally require preliminary destabilization before coagulation, flocculation and subsequent flotation.
- Pulp and paper industry: fibres, fillers and suspended solids may have characteristics favourable to flotation. DAF can be used both to clarify the stream and, in certain configurations, to recover part of the fibrous material.
- Chemical and manufacturing industries: DAF can be applied to separate suspended solids, oily phases, colloids or precipitates generated by a preceding physico-chemical treatment. In this case, flotation is the final separation step in a sequence that may include pH adjustment, precipitation, coagulation and flocculation.
PLANT INTEGRATION AND REDUCED CIVIL WORKS
One of the advantages of a DAF system is that the unit can be supplied as a compact, prefabricated system, significantly reducing civil works.
This characteristic provides greater flexibility when integrating the unit into existing plants. The DAF can be installed at plant floor level or, subject to structural load verification, on platforms, mezzanines or upper levels.
This is particularly valuable in retrofits of existing plants or in facilities where available ground-level space is limited. The compact configuration makes it possible to add a new separation stage without necessarily building new tanks or significantly expanding civil works.
POSITIONING THE DAF WITHIN THE PROCESS
Correct DAF sizing cannot be separated from an assessment of the technologies installed downstream.
Upstream of biological treatment, the objective may be to reduce TSS, oils, greases and the COD fraction associated with the separable fraction. Before filtration, ultrafiltration or membrane systems, the objective may instead be to reduce particulate and oily loads in order to limit fouling and cleaning frequency.
This means that the most important parameter is not necessarily the maximum removal efficiency of the DAF itself, but the wastewater quality required by the downstream stage.
ONE TECHNOLOGY, BUT NOT A STANDARD SOLUTION
The physical process of dissolved air flotation remains the same, but the design changes significantly from one application to another.
Flow rate, TSS, particle size distribution, oils and greases, COD, temperature, pH, emulsions, floc characteristics and load variations must be assessed together.
The value of DAF in industrial wastewater treatment therefore lies in its ability to selectively separate the fraction of contaminants best suited to flotation, reducing the load transferred to downstream units.
From textile to Food & Beverage, from automotive to the pulp and paper industry, the starting point remains the same: correctly characterise the wastewater and distinguish what can be floated, what can settle, and what must be treated by biological, physico-chemical or membrane processes.
