Introduction in Dissolved Air Flotation

Introduction [1]

                    Flotation may be used in lieu of the normal clarification by solids-downward-flow sedimentation basins as well as thickening the sludge in lieu of the normal sludge gravity thickening. Water containing solids is clarified and sludges are thickened because of the solids adhering to the rising bubbles of air. The breaking of the bubbles as they emerge at the surface leaves the sludge in a thickened condition.

                  In a flotation system for solid/liquid separation, there are at least two methods by which gas bubbles can be used to increase the buoyancy of suspended solids: (a) entrapment of the bubbles in the particle structure; and (b) adhesion of the bubbles to the particle surface (see Fig. 1). In the former case, as the gas bubbles rise toward the surface, the controlled turbulence in the inlet compartment causes contact between the solids.

The floc, formed by the natural floc-forming properties of the materials or by the chemicals

that have been added, increases in size because of more contact with other solids.

Eventually, a structure is formed that does not permit rising gas bubbles to pass through or around it. [2]

 

What is a DAF?

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by removing suspended solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it is then removed by a skimming device. Chemicals can be added to the feed water to improve solids removal.

Where and why are DAFs used?

DAFs are widely used in treating industrial wastewater effluents from oil refineries, chemical plants and paper mills to the food & beverage industries. DAFs are used to removed suspended solids, such as  Total Suspended Solids (TSS), Fats, Oil & Grease (FOG) and other pollutants from wastewater so that the water is suitable for re-use or discharge to a wastewater treatment facility.

How does a DAF work?

Wastewater is fed into a DAF system and hit with a stream of “whitewater”, which is recirculated clarified water from the DAF that’s super saturated with dissolved air. As these two mixtures blend together, microscopic bubbles attach to solid particulates, giving them enough buoyancy to surface in the DAF tank.

As solids accumulate in a floating layer on the top of the DAF tank, a skimmer gently nudges the sludge toward a discharge hopper.

Any solids that don’t float will sink to the “V” bottom of the DAF tank. Settled solids are concentrated and discharged by an automatically controlled pneumatic drain valve.

The clarified water flows out via an under-over weir on either side of the DAF unit. Some of this water is used in the recirculation loop while the rest flows out of the vessel.

What are Common Types of DAFs?

  • Open Tank

Open Tank DAFs are ideal for treating wastewater that is heavily loaded with solids.  This type of wastewater requires significant amounts of free surface area for flotation and separation.   Open tank DAFs are normally rectanglar in shape and are built wider and longer as the free separation area requirement increases.

  • Plate Pack

Plate Pack DAFs are characterized by high-built (tall) tanks with inclined, corrugated plate packs.  Water is introduced into the plate packs in a cross-flow configuration, reducing the distance solids have to float to be effectively separated. As they collide with an angled plate, light solids accelerate upward and heavy particles settle. Wastewater enters the plate pack heavily laden with flocculated contaminants and exits devoid of suspended and colloidal solids.

Which DAF Type is Right for My Application?

Generally speaking, a plate pack DAF unit is suited for high hydraulic and low solids loading rates. Open style DAF tanks are better suited for high solids loading rates.

But before we go and place plate pack DAFs and open tank DAFs into specific applications or industries, let’s understand one concept: anywhere you can use a plate pack DAF unit, you can also use an open tank DAF unit, but the opposite does not hold true. Similar to the rule that says, “a square is a rectangle, but a rectangle is not a square.” The trade off for always going with an open style tank is the amount of floor space they occupy, and their overall cost, especially as flow rates rise above a few hundred gallons per minute.

 

 

 

What are Common Components of a DAF?

Flotation Tank

The Flotation Tank is where the water separates from pollutants.  The water is flowing through the flotation tank at a slow speed, giving time for heavy objects to settle down and lighter objects to float up to the top with help from bubbles and flocculators.  The flotation tank can vary in depth, width and length based on the application and the time needed for pollutants to be removed.

The Dewatering Grid is a rectangular framework of angular steel plates that lock sludge in place as it rises to the surface. Only when sludge has thickened enough to rise above the top edge of the grid can it be skimmed and pushed to the float hopper.

The Dewatering Grid helps:

  • Operators control sludge thickness
  • Eliminate pre-mature removal of solids
  • Reduce build-up and/or re-entrainment of sludge
  • Generate drier sludge

Skimmers

While many DAF system designs push sludge across the entire length of the tank in the same direction as the wastewater flow called a Co-Current Skimmer, another design uses a skimmer assembly that rotates against the hydraulic flow of the water called a Counter-Current Skimmer.  The Counter-Current Skimmer design shortens the sludge skimming distance and eliminates solids carry-over.

Flocculators are designed to provide the mixing action and retention time required to adequately coagulate and flocculate solids in wastewater to improve solids removal.

DAF pumps are a key component of all DAF Systems. On it rides some of the largest capital, operations and maintenance expenses involved in wastewater pre-treatment systems.

DAF manufacturer’s approach recycle pumps in two different manners.

The first way is to provide a specialty white water pump.  This pump not only pumps the water but also dissolves the air into the water.  These types of pumps are often more difficult to find and are more expensive.  Also, with putting air in the pump, there is always a risk of cavitation, which causes internal damage and results in more-frequent-than-desired parts replacement.

The second way is a more efficient and cost effective approach.  It is to combine a standard ANSI pump with an angled air dissolving tube.  In this second case, the pump doesn’t do any air dissolving – it just pumps water.  No air in the pump, means very little risk for cavitation.  In this way, standard ANSI recycle pumps don’t do any air dissolving – it just pumps water. That means we can provide higher solids tolerances, use stronger pump materials, operate at lower pressures, and do it at a cost much lower than possible with a specialty whitewater pump.

Angled Air Dissolving Tube

Where many DAF system manufacturers use a mechanical means to dissolve air into water, i.e. a specialty “whitewater pump,” there is a more efficient and cost-effective approach.

The air dissolving tube is where whitewater is generated.  This short expansion in the recirculation piping allows clarified effluent and a small volume of compressed air to mix until saturation is achieved. The angled configuration allows for increased water and air interface so saturation occurs almost instantly.

This design works so well that often a customer will change from their specialty whitewater pump to an ANSI pump and angled air dissolving tube.

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science.

References

[1]   Nicholas P. Cheremisinoff, Handbook of Water and Wastewater   Treatment Technologies, Butterworth-Heinemann,2002,p 62.

[2]Lawrence K. Wang,  Physicochemical treatment processes, Humana Press Inc, Totowa, New Jersey 0751,2004.

 

Factors affect Dissolved Air Floatation (DAF) process performance

Flotation [1]

         Flotation may be used in lieu of the normal clarification by solids-downward-flow sedimentation basins as well as thickening the sludge in lieu of the normal sludge gravity thickening. Water containing solids is clarified and sludges are thickened because of the solids adhering to the rising bubbles of air. The breaking of the bubbles as they emerge at the surface leaves the sludge in a thickened condition.

         Figure.1 shows the flow sheet of a flotation plant. The recycled effluent is pressurized with air inside the air saturation tank. The pressurized effluent is then released into the flotation tank where minute bubbles are formed. The solids in the sludge feed then stick to the rising bubbles, thereby concentrating the sludge upon the bubbles reaching the surface and breaking. The concentrated sludge is then skimmed off as a thickened sludge.

The effluent from the flotation plant are normally recycled back to the influent of the whole treatment plant for further treatment along the influent raw wastewater. Figure 2 shows an elevational section of a flotation unit.

Dissolved Air Floatation Sludge Thickening

        In the dissolved air flotation (DAF) thickening process, air is introduced to the sludge at a pressure in excess of atmospheric pressure. When the pressure is reduced to atmospheric pressure and turbulence is created, air in excess of that required for saturation leaves the solution as fine bubbles 50 to 100 μm in diameter. These bubbles attach to the suspended solids or become enmeshed in the solids matrix. Since the average density of solids–air aggregates is less than that of water (0.6 to 0.7), they rise to the surface. Good solids flotation occurs with a solids–air aggregate specific gravity of 0.6 to 0.7. The floating solids are collected by a skimming mechanism similar to a scum skimming system.

         DAF thickening is used most efficiently for waste activated sludge.

Although other types of sludge, such as primary sludge and trickling filter

sludge, have been flotation thickened, gravity thickening of the sludge is

more economical than DAF thickening.

        The schematic of a typical DAF thickening rectangle tank is presented in Figure 3. The major components of a DAF system are the pressurization system with an air saturation tank, a recycle pump, an air compressor and pressure release valve, and a DAF tank equipped with surface skimmer and bottom solids removal mechanism. Figure 4 illustrates two models of flotation thickeners that are in use in Russia and Ukraine. There are three ways in which the pressurization system can be operated.

         In the method called total pressurization, the entire sludge flow is pumped through the pressurization tank and the air-saturated sludge is then passed through a reduction valve before entering the flotation tank. In the second method, called partial pressurization, only a part of the sludge flow is pumped through the pressurization tank. After pressurization, the pressurized and unpressurized streams are combined and mixed before they enter the flotation tank. In the third method, called the recycle pressurization, a portion of the subnatant is saturated with air in the pressurization tank and then combined and mixed with the sludge feed before it is discharged into the flotation tank.

         The major advantage of the recycle pressurization system is that it minimizes high-shear conditions, an important parameter when dealing with flocculant-type sludge. The recycle pressurization system also eliminates clogging problems with the pressurization pump, air saturation tank, and pressure release valve from the stringy material in the feed sludge. For these reasons, recycle pressurization is the most commonly used. The recycle flow can also be obtained from the secondary effluent, which has the advantage of lower suspended solids and a lower grease content than the subnatant from the DAF tank.

The flotation tank can be circular or rectangular and made of steel or concrete. Smaller tanks are usually steel and come completely assembled. For large installations requiring multiple tanks or large tanks, concrete tanks are more economical. Rectangular tanks have several advantages over circular units. In rectangular tanks, skimmers skim the entire surface and the flights can be closely spaced, allowing more efficient skimming. In a rectangular tank, bottom sludge flights are usually driven by a separate unit and hence can be operated independent of the skimmer flights.

         The main advantage of circular units is their lower cost in terms of structural concrete and mechanical equipment. However, shipping problems limit a completely assembled steel circular unit to about 9 m2 (100 ft2) or less.

Process Design Considerations [2]

Numerous factors affect DAF process performance, including the following:

  • Type and characteristics of feed sludge
  • Solids loading rate
  • Hydraulic loading rate
  • Air-to-solids ratio
  • Polymer addition

       Although sufficient data are available from operating units in more than 40 years to size DAF systems, bench- and pilot-scale testing can provide valuable information. Most manufacturers of DAF systems have designed and built bench-scale units for evaluations. These manufacturers have scale-up criteria for their equipment to predict full-scale operational parameters. Consideration should also be given to renting a pilot unit available from most manufacturers to test and evaluate the effect of such parameters as solids and hydraulic loading, recycle ratio, air-to-solids ratio, and polymer type and dosage. If sludge is not available for testing, a detailed review must be made of experience at installations where a similar type of sludge is being thickened by DAF thickeners. [3]

Type and Characteristics of Feed Sludge a variety of sludge can be thickened effectively by flotation. These include conventional WAS, extended aeration sludge, pure-oxygen activated sludge, and aerobically digested sludge.

The first step in designing a DAF system is to evaluate the characteristic of the feed sludge. Information is needed about the range of solids concentration that can be expected. If WAS is to be thickened, the mixed liquor sludge volume index (SVI) must be determined because SVI can significantly affect the DAF thickening performance. The SVI should be less than 200 if a float concentration of 4% is required with nominal polymer dosage.

Solids Loading Rate The solids loading rate is expressed as the weight of

solids per hour per unit effective flotation area. Typical solids loading rate are given in Table 1. The loading rates shown will normally result in a minimum of 4% concentration in the float. As can be seen from the table, the solids loading rate can generally be increased up to 100% with polymer addition.

Hydraulic Loading Rate The hydraulic loading rate is expressed as combined flow rates of feed sludge and recycle per unit effective flotation area (m3/m2·d or gpm/ft2). When like units are canceled, it becomes a velocity gradient to the average downward velocity of water as it flows through the flotation tank. The maximum hydraulic loading rate must always be less than the minimum rise rate of the sludge–air particles to ensure that all the particles will reach the surface before they reach the effluent end of the tank. Suggested hydraulic loading rates range from 30 to 120 m3/m2·d (0.5 to 2 gpm/ft2).

Air-to-Solids Ratio The air-to-solids ratio is perhaps the single most important factor affecting DAF performance. It is defined as the weight ratio of air to the solids in the feed stream. The ratio for a particular application is a function of the characteristics of the sludge, principally the SVI, the air dissolving efficiency of the pressurization system, and distribution of the air–solids mixture into the flotation tank. For domestic wastewater sludge, reported values of air-to-solids ratios range from 0.01 to 0.4, with most systems operating at a value under 0.06.

Polymer Addition Chemical conditioning with polymer has a marked effect on DAF thickener performance. The particles in a given sludge may not be amenable to the flotation process because their small size will not allow proper air bubble attachment. The surface properties of the particles may also have to be altered before effective flotation can occur. Sludge particles can be surrounded by electrically charged layers that disperse the particles in the liquid phase. Polymers can neutralize the charge, causing the particles to coagulate so that the air bubbles can attach to them for effective flotation.

Bench- or pilot-scale testing is the most effective method to determine the optimal amount of polymer required and the point of addition (in the feed stream or the recycle stream) for a particular installation. Typical polymer dosages range from 2 to 5 g polymer per kilogram of dry solids (4 to

10 lb/ton).

In the lower ranges of solids and hydraulic loading rates, polymer addition typically is not necessary. Polymer conditioning usually affects solids capture to a greater extent than float solids concentration. With polymer addition, float solids can be increased by about 0.5%; however, the solids capture efficiency can be increased from about 90% to better than 95%.

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Technical Manager Louts for Water Treatment

References

[1]      Physical–chemical treatment of water and wastewater / Arcadio Pacquiao Sincero, Sr.,Gregoria Alivio Sincero, IWA publishing ,2003.

[2]   Wastewater sludge processing / Izrail S. Turovskiy, P. K. Mathai, John Wiley & Sons, Inc., Hoboken, New Jersey,2006.

[3]   Gulas, V., Benefi eld, L., and Randall, C. (1978), Factors Affecting the Design of Dissolved

Air Flotation Systems, Journal of the Water Pollution Control Federation, Vol. 50, p. 1835.

[4]   (1979), Process Design Manual for Sludge Treatment and Disposal, EPA 625/1-79/011