Dissolved Air Flotation Systems (DAFs) for Bakeries

Prepared by:
Roy E. Carawan
Extension Food Science Specialist
North Carolina State University

Edd G. Valentine
Research Scientist
Georgia Tech Research Institute

Published by: North Carolina Cooperative Extension Service

Publication Number: CD-43

Last Electronic Revision: March 1996 (JWM)

DAF systems are often suggested for pretreating wastewater - will a DAF work for your bakery?

Bakeries are facing increasingly stringent restrictions on their wastewater discharges. To help reduce the concentration of contarninants in their wastewater and to avoid sewer surcharges, some plant managers have installed systems to pretreat the wastewater before it is discharged to municipal sewers or other disposal outlets. Many other managers are considering pretreatment to reduce surcharges or have been asked by their publicly owned treatment works (POTW) to consider waste reduction. One way to pretreat wastewater is with a dissolved air flotation system (DAE;), which can remove insoluble materials such as fats, oils, and grease (Figure 1).

Figure 1. A dissolved air flotation (DAF) system.

At present, there is only limited experience with using DAF systems for bakeries. This publication describes the DAF system and its operation and shows you how to decide if a DAF is the appropriate choice for your bakery.

How a DAF Works

A DAF is a gravity separation system that uses air bubbles in a wastewater holding tank to help float insoluble materials to the surface so they can be removed. Some materials that are heavier than water can also be removed if chemical flocculents are used. The flocculents cause these materials to join together in clusters that are lighter than water and therefore float. Pollutants are concentrated in the material that accumulates on the surface, called the float. Other names for the float include DAF float, skimmings, or sludge.

Note that soluble contaminants such as sugar are not usually removed by the system, although they are occasionally trapped in the float along with other particles.

Similarly, bakery wastewater often contains a large amount of settleable solids—materials such as dough pieces, bits of fruit, nuts, raisins, and product. These materials will settle because they are heavier than water. Thus, a DAF tends to negate the settling of these materials. Most heavier materials will settle if given enough time, but some will be trapped with the float. Therefore, the nature of these materials must be considered in DAF designs for bakeries.

Questions to Ask

These are some of the questions to ask when considering the implementation of a DAF system:

System Components and Operation

The three major parts of a DAF system are:

Types of Systems

Two types of DAF systems are commonly used:

Variations in the commercially available systems include features such as tank design (compact systems utilize inclined plate packs) and skimmer design.

Selecting a System of the Proper Size

DAFs are designed on the basis of the peak flow rate expected unless the concentration of insoluble solids exceeds 3,000 milligrams per liter (mg/l). The flow usually ranges from about 1 to 6 gallons per minute per square foot of surface area (GPM/ft2). The diagram in Figure 2 shows how a DAF operates.

Figure 2. Operation of a DAF system.

Observed Performance

Many food processing plants, such as poultry and meat processors, use DAFs as pretreatment systems. The experience of these plants may be useful for bakery managers as they consider DAFs for pretreatment. However, wastewater characteristics vary widely, depending on the products produced and the time of day. Also, bakeries use considerably less water than large meat and poultry plants.

Several bakeries are currently using DAFs as pretreatment systems. These systems often have removal efficiencies similar to those in Figure 3. Note that these DAF removal efficiencies were obtained from a multiproduct bakery. Bread bakeries have substantially less biochemical oxygen demand (BOD^) and fats, oils, and grease (FOG) contents. Therefore, opportunities for a DAF in bread plants would be more limited.

Figure 3. DAF removal for multiproduct bakery.

DAF Chemical Use

Decisions about the use of chemical flocculents in DAFs are based on cost, system efficiency, and the intended use of the DAF float. Chemicals commonly used include the trivalent metallic salts of iron, such as FeCI2 or FeSO4 or aluminum, such as AISO4.

Concentrations of chemical flocculents used normally range from 100 to 500 mg/l. (One mg/l in 1 million gallons per day is 8.34 pounds of material.) The pH of the wastewater must be adjusted to fall between 4.5 and 5.5 for the ferric compounds or between 5.5 and 6.5 for the aluminum compounds using an acid such as H2SO4 or a base such as NaOH. In many applications, after the DAF, a pH control system utilizing a base such as NaOH may need to be added to assure that the discharge pH is within the limits specified by the POTW.

Organic and inorganic polymers (anionic or cationic) are often used to enhance the DAF process. The most commonly used inorganic polymers are the polyacrylamides.

Organic compounds such as carrageenan, chitosan, and lignosulfonic acid, or their derivatives can also be used. When the float is to be fed to animals intended for human consumption, only materials that have been approved by the United States Food and Drug Administration (FDA) Office of Veterinary Medicine should be used.

Disposal of the Float

The pollutants removed by the DAF system are concentrated in the float and must be used or disposed of properly to avoid environmental damage. DAF float can be disposed of by:

DAF Float Dewatering Systems

Because the DAF float often contains only 2 to l0 percent solids, it must often be dewatered before utilization or disposal. The float is commonly dewatered using one of the following methods:

Making Your Decision

In-plant changes that reduce wastes are the best and most economical way to reduce pollution and should always precede treatment considerations. Bakery managers need to carefully evaluate any decision to pretreat their wastewater. A DAF is only one of a number of available wastewater treatment technologies. Limited evidence is available to show that DAFs will work for bakeries. Consider the pros and cons listed above. If a DAF is the best alternative, keep in mind that most POTWs (and some states) require engineers to design pretreatment systems. These systems must be designed to meet the effluent limitations. Remember that the pollutants removed and concentrated by a DAF must still be disposed of properly if pollution is to be avoided.

Dissolved Air Flotation Systems (DAFs)

PROS

  • • Reduce grease (FOGs)
  • • Reduce suspended solids (TSS) and settleable solids (but only if equipped with a bottom sweep).
  • • May help bakeries meet POTW permit restrictions
  • • Do not require excessive maintenance or management
  • • May reduce surcharges, especially for larger bakeries CONS

  • • Do not remove soluble materials such as sugars
  • • Do not remove the BODs associated with soluble materials
  • • Only concentrate the pollutants; the float must still be disposed of properly.
  • • Are costly to buy and are expensive to operate. (If high chemical use is required, operational costs will be high.)
  • • They rarely cost less than surcharges, especially for smaller bakeries.

    Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
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