Science in a beer bottle reveals the natural attractions of DAF liquid-solid separation

Most commonly known for the treatment of industrial wastewater, DAF has found its place in many other areas such as waste activated sludge (WAS) thickening, polishing of tertiary treated wastewater and process water treatment. DAF used in potable water treatment is usually combined with an integrated filtration stage. This combination is referred to as DAFF.

Although DAF systems come in many shapes and sizes, the principles of operation are very similar. Separation occurs by allowing air bubbles to attach to particles in a polluted stream forcing them to the surface of a vessel, which is fitted with a scum removal mechanism.

The primary advantage of DAF over other forms of solid liquid separation is that air bubble attachment reduces the specific gravity of particles that might normally sink, forcing them to float. This results in two distinct, easily separable phases, being the treated water and surface sludge.

DAF was developed commercially in the 1950s. Early DAF designs predominantly used circular separation tanks as they were basically a development from settling clarifiers. It is suspected that air was introduced into such units when difficulties were encountered with suspended or floating material. Though relatively inexpensive to manufacture, inefficiencies in floated sludge removal and feed flow dynamics have seen the circular design virtually phased out.

Over the last few decades, there have been considerable improvements made in DAF technology. Modern DAF systems are generally rectangular due to the more compact sizing, modular expansion capabilities, better sludge removal dynamics and the ease of combining chemical reaction tanks into packaged designs. All DAF systems are fitted with a surface sludge scraper, usually in the form of a chain driven blade assembly. A current trend leans strongly towards scraping the sludge in a counter-current direction that has proved highly beneficial in overall performance. DAF systems are also fitted with bottom (or settled sludge) removal systems when required.

The performance and reliability of DAF systems is dependent on the continuous supply of a stream of micro fine air bubbles. This is heart of the DAF process with the key being the simplicity and reliability of the bubble generation circuit.

The DAF principle

The easiest way to imagine how dissolved air is produced is to think of a bottle of beer. When you open the top, you release the pressure inside the bottle. The CO₂ that was originally held in a saturated state due to pressure in the sealed bottle begins to nucleate out of solution and can be observed as bubbles rising to the surface. In conducting this experiment you may have also noticed that some bubbles adhere to the sides of the bottle demonstrating the natural surface tension attraction of fine bubbles to surfaces. This may not be particularly important to the taste of your beer but the underlying physics of air bubble dissolution and surface tension adherence is critical to DAF designers.

In a DAF system, bubble precipitation is accomplished by firstly pressuring a recycle stream of treated effluent from the discharge end of the DAF in a dissolved air contactor (DAC).

Compressed air is added to the DAC where it dissolves in the water under a pressure of 400–600 kPa. The pressure is maintained in the DAC by a back pressure regulating valve. The pressure drop across this valve provides a high level of shear, which assists the full precipitation of the micro-fine air bubbles directly into the incoming waste stream.

At this point, the air bubbles attach to the pollutant particles through a combination of surface tension, surface charge attraction and encapsulation. The size of the air bubbles is extremely important — too large a bubble and the bubble will rise so rapidly that it is likely to break away from the pollutant. Bubbles of a size below 120 μm are ideal as they have a natural surface attraction to solid surfaces.

While the principle of DAF is relatively simple, it should be understood that DAF is basically a physical separation process and the performance of such equipment is strongly dependent on the nature of the incoming stream. Pretreatment of effluent can be crucial to the effectiveness of the DAF process. A DAF system will only remove those particles that the air bubbles can attach to. If the pollutants are initially in an emulsion or a fine colloidal dispersion, to enable any reasonable degree of treatment coagulation and flocculation processes are required. A properly designed sludge removal system is also essential.

With all the elements in place, DAF is one of the most reliable of all solids liquids separation processes. They are usually fully automated requiring minimal operator attendance.

Operating costs of DAF systems are favourable when comparing with most other process technology. Power and air consumption is relatively low while maintenance is infrequent. The volume of sludge generated is considerably reduced due to the sludge-thickening nature of a DAF. Dry solids concentrations in the sludge can reach 10% in certain applications compared to fractions of 1% for settled sludge. The sludge produced from DAF systems is generally not spadable so, depending on volumes, further mechanical dewatering should be a consideration.

DAF sizing

The DAF process is a continuous operation. Overall capacities of the system are therefore largely dependent on hours of operation. The smallest commercially available DAF systems generally rate at a few thousand litres per hour, although factors such as solids loading are critical to final sizing. As factory-built units, DAF systems can handle in excess of 0.5 ML/h. Larger units are generally custom designed and flotation tanks are constructed on site in concrete.

A common myth is that clarification is less expensive than DAF. It is true that the mechanics of operation are not quite as involved as DAF; however, when taking into account space requirements, operating costs and performance, DAF systems generally emerge as winners.

AJM-Eimco Water Technologies
www.ajmenviro.com.au