Hoffland Environmental, Inc. (HEI) dissolved air flotation systems (DAF) efficiently separate total suspended solids (TSS) and fats, oils, and grease (FOG) particles from industrial effluents. The system generates air bubbles ranging from molecular to micro bubbles. Air is dissolved in the treated water by the addition of compressed air to a pressurized stream of recycled effluent. The pressurized water then passes through a pressure relief valve reducing the pressure to atmospheric. This depressurized water is then mixed with the conditioned influent. When the pressure is reduced the super-saturated air begins to precipitate from solution. The molecules of air affix themselves or seed to the flocculated solids. The precipitated molecules coalesce into a micro bubble. The bubble with the suspended solids and/or minute oil droplets are buoyant and float to the surface. This simple process combined with optimum conditioning chemistry and properly designed equipment provides a sophisticated solids and FOG separation system.

HEI has performed extensive laboratory evaluation on the size of the air bubble and effective liquid-solid separation. The data overwhelming demonstrate that the smaller the air bubble, the more efficient the separation. By comparison, the effluent from an induced air flotation system (IAF) typically contains twice as much TSS and FOG as the effluent from a DAF. In the few applications where the reduced separation of the IAF is satisfactory, HEI is able to provide induced air flotation as an alternative.

System Design Criteria

There are five important factors to be considered in properly sizing your DAF. Air to Solids Ratio is the quantity of air required to float a given amount of solids and FOG’s. It usually is expressed as lbs. of air/lb. of solids with the normal ration being between 0.01 and 0.10 lbs air/lb solids. HEI recommends a 0.03 ratio. The amount of air that dissolves in water is determined by the temperature of the recycled water and imposed pressure. Under normal HEI design conditions (75 psig at 70°F) the average amount of air dissolved is 9% by weight. The TSS (total suspended solids) must be analyzed and the appropriate percent recycle rate established. A waste stream of 1000 mg/l TSS will require a recycle rate of 50%, and 2000 mg/l TSS will require 100% recycle. Hydraulic Loading (gpm/ft2) is the relationship between available surface area of the flotation cell and the total flow rate through the DAF influent plus recycle. Hydraulic loading is total flow rate divided by the effective surface separation area. Design hydraulic loading varies from 1.5 to 5.0 gpm/ft2. HEI recommends 2.5 gpm/ft2. Launder Weir Ring Overflow Rate in gpm per linear foot is a convenient-to-calculate value that can be assumed to reflect eddy current formation that would carry solids or FOG under the underflow baffle. It must not be so rapid as to entrain precipitated solids. The overflow must be completely uniform across the launder weir (and thus under the baffle). HEI prefers the circular DAF design to reduce the launder weir overflow rate. The DAF influent enters through a central mixing well ("distributor") and flow out horizontally through ports. The bubbles attached to the TSS and FOG float to the surface. The cleaned water flows down under a baffle and over the launder weir. This radical outflow design dramatically reduces internal velocity and eddy currents, which assures complete separation of the buoyant flocculated particles. HEI does not normally recommend the rectangular designed DAF unit because the high velocity at the inlet is never reduced. Hence, the expected separation is not as efficient in the rectangular design as can be expected in the circular design DAF’s. HEI utilizes the rectangular design for customers with limited available space. Solids Loading is the relationship between the effective surface area of the DAF cell and the total amount of TSS & FOG entering the system. The average design load is 1.0 to 3.5 lbs./hr/sq. ft. HEI recommends not exceeding 2.0 lb./hr/sq. ft. The calculation should include not only the influent TSS but also all chemicals added to condition and flocculate the waste stream. Solids Removal of both float and sediment (if any) must also be considered in the general design. In addition to the skimming rake to remove the float, HEI also has a bottom rake to sweep any sediment to the center bottom outlet. HEI dissolved air flotation systems are very effective in the removal of suspended solids and free FOG. Emulsified oil and grease must be chemically conditioned before removal. Due to "van der Waals" charges, particles in industrial wastewater usually exist as a colloidal suspension. These charges must be neutralized before the solids can be flocculated and floated. The small, destabilized colloids collect and agglomerate with the long chained polyelectrolyte into large floccules which can be rapidly floated to the surface.

Influent Mixing Tubes

Central Mixing/Distribution

Float Removal and Sediment Removal

DAF Cell

Outlet Chamber

Pressurization Unit

Materials of Construction

Industrial Applications

Aircraft Maintenance and Manufacture Animal Slaughter Houses Automotive Maintenance Ballast Water Treatment Biological Sludge Chemical Manufacture Cosmetics Manufacture Feed Lot Runoff Food Processors Industrial Laundries Meat Packers Metal Removal		Oilfield Produced Water Oil Refinery Paint Manufacture Power Plants Pulp and Paper Railroad Maintenance Rendering Plants Seafood Processors Tank Truck Cleaning Tanning Textile Manufacture

Turn-Key Design