Background
The implementation of DAF systems across Europe appears to be steadily developing, with applications becoming more varied in nature. Applications employing DAF systems range from potable water treatment to industrial effluent treatment and sludge (e.g. activated sludge) thickening. The use and development of DAF has particularly been expanding in the industrial field with a variety of applications currently being employed. DAF systems are commonly applied to remove suspended solids, fats, oils and greases and associated BOD and COD from a variety of waters and wastewaters.
A key application of DAF units includes the removal of free and emulsified hydrocarbons from petrochemical and similar wastewaters upstream of biological processes. This has been done in an effort to prevent toxic or inhibitory materials from hindering the biological processes downstream of the DAF unit. Other industrial applications of DAF systems include the treatment of concentrated fish farming wastewaters, the pre-treatment of food and meat processing effluents and the treatment of effluents generated by the pulp and paper industries. In certain circumstances, DAF can also be a substitute for gravity settlement of solids generated by a biological treatment process.
Advantages linked with DAF systems include the fact that they are high-rate processes when compared with more traditional gravity-based settlement systems. This means that a reduction in space requirements can be achieved, and in terms of sludge thickening, a thicker sludge can be produced. Additionally, DAF systems offer the operator some degree of flexibility, subject to design, with regard to the system's operating parameters.
Basic Operation
DAF is a purely physical process which operates based on a reasonably simple design philosophy. Incoming effluent may require pre-treatment as necessary, e.g. the addition of chemical coagulant(s) and/or flocculant(s) may be required with associated mixing and coagulation/flocculation stages. Adjustment of pH may also be a consideration to ensure optimum conditions for coagulation and flocculation.
DAF systems may be designed for pressurisation and air dissolution of the total flow or, more commonly, the incoming effluent enters the flotation vessel where it comes into contact with a portion of recycled, treated effluent (sometimes termed whitewater). The percentage of the total effluent flow into which air is dissolved under pressure and subsequently recycled will be determined by several factors. Increasing the pressure within the vessel where the air is being dissolved ensures that a higher concentration of air dissolves into the liquid phase than is possible at atmospheric pressure. Once this portion of saturated effluent enters the flotation tank, the pressure is released back to atmospheric pressure. This immediately results in the recycled flow becoming supersaturated, resulting in the generation of microbubbles as the dissolved air comes back out of solution. These bubbles attach to, and form within, the solids or chemical flocs entering the vessel, causing them to float to the surface where they are retained and subsequently removed by a mechanical skimmer.
In the case of rectangular flotation tanks, the skimmer mechanism consists of a series of paddles or 'flights' which run on a belt or chain and skim just below the surface of the tank removing the 'float' into a trough for further treatment or, in some instances, recovery of materials. The alternative of circular DAF tanks may incorporate rotating skimmer blades feeding a 'float' trough or involve use of a circulating, revolving scoop. In cases where some gross solids may be present and there is risk of gradual accumulation of sludge build-up on the flotation tank floor, the design may also incorporate a floor scraper.
There are limits to what can efficiently be removed by applying flotation technology. It would seem a logical step therefore to apply DAF systems to effluents where the solids present are of approximately neutral or perhaps even positive buoyancy so that the bubbles produced are working with gravity rather than against it. Under these circumstances DAF systems would, on first approximation, appear to be a process worthwhile of consideration should standard sedimentation systems not provide the required removal of contaminants.
Key Design Parameters
Inevitably the design details for any given effluent treatment system will be dependent on a number of specific factors. There are, however, several key design parameters which are commonly applied when considering and assessing the design of a DAF system. The parameters listed below are, where applicable, accompanied by design figures. These figures are provided as an indication of the range of figures one may encounter.
Air:Solids Ratio
The Air:Solids (A:S) ratio may be reported as a volume:mass ratio or a mass:mass ratio and will be application specific. To give an idea of the range of A:S ratios commonly applied, typical values range between 0.005 – 0.06 ml/mg which, at 20oC and atmospheric pressure (say 1.0133 bar) is equivalent to 0.006 mg – 0.072 mg of air per mg of solids to be removed.
Hydraulic Loading Rate
The DAF hydraulic loading rate is a measurement of the volume of effluent applied per unit effective surface area per unit time. This results in process design figures expressed as equivalent upflow velocities with units of m/h. This figure should be application specific but as a general guide the figures which should be expected would be between 2 m/h and 10 m/h. A key consideration with regard to this design parameter is whether the loading rate includes the recycled volume as well as the influent wastewater volume being applied per unit area of the system.
Typical Solids Loadings
Solids loadings are normally given in units of mass per unit area per unit time (kg/m2.h). Typical figures encountered range from around 2 kg/m2.h up to15 kg/m2.h, although again the design will be application specific, depending on the nature of the solids to be removed and the extent to which chemical aids are used.
Recycle Ratio
The recycle ratio is determined as the fraction of the final effluent produced which is returned and saturated under pressure prior to entering the flotation vessel where the pressure is subsequently released and the bubbles are generated. The recycle ratio can vary immensely with recycle ratios being typically 15-50% for water and wastewater treatment application. However, for activated sludge flotation thickening, up to 150-200% recycle rates have been applied. Air dissolution rates are proportional to absolute pressure (i.e. system gauge pressure + atmospheric pressure) in accordance with Henry' Law of partial pressures of gases adjacent to liquids. Thus, for a given application, the higher the operating pressure of the air/water saturation vessel, the lower the required percentage recycle – and vice-versa. Operating pressures can therefore vary widely but are typically in the range 3-7 barg.
Saturation of Effluent
The production of saturated water from which the micro-bubbles are generated is normally achieved in two ways. The first which is common to potable water treatment involves passing the required flow of treated effluent through a packed bed system which is pressurised using a pump which is often a centrifugal pump. In systems where solids are likely to be encountered, e.g. sludge treatment, the saturation vessel is likely to be empty to prevent the fouling of any packing materials. The percentage of saturation which can be achieved will depend on the design of the system but, with good design, saturation efficiencies of up to 80-95% can be expected.
Flow Regime
To ensure that DAF systems operate as designed it is important to ensure that the system does not encounter sudden changes in the flow regime. For this reason some form of flow balancing or regulation is recommended to ensure a consistent flow rate. Another consideration is to develop a flow path through the flotation tank which ensures the maximum removal of solids via their entrainment in the air microbubbles generated.
Summary
DAF units can be applied successfully to a range of different waters, wastewaters and sludges and demonstrate certain specific advantages over more conventional solids removal processes. DAF plants can be designed to be small, compact and robust systems with a high rate of operation. DAF systems are capable of coping with reasonable variations in influent water quality, and to some extent variations in flow. Disadvantages of DAF systems may include the increased service and maintenance costs when compared with traditional sedimentation systems and the increased operating costs due to the energy requirements of the system.
Key References
Kiuru, H and Vahala, R (Editors). Dissolved Air Flotation in Water and Waste Water Treatment. Wat. Sci. Tech. 43 (8).