FAQs for Dissolved Air Flotation (DAF)

The pounds of air being injected into the whitewater system is divided by the dry pounds of solids loading to the DAF. A typical range is 0.005 to 0.01 for the A:S ratio although the range can vary from 1.

DAF systems are highly dependent on a good chemical program. In our experience coagulants and flocculants are needed for removal of fine total suspended solids (TSS) and emulsified fats, oils & greases (FOG). If the TSS particle is large enough the micron-sized air bubbles will attach and float it to the surface, but without a good chemical program, the removal efficiency declines as particle size decreases.

Yes, a DAF system is frequently used in poultry operations to separate blood and other constituents in the wastewater. Typically, a ferric based chemistry is most effective.

DAF systems are used to float material using micron sized bubbles. Key for metal removal is to precipitate a particle large enough and with low enough composite density to float. Other constituents in the wastewater can affect the efficiency of the precipitation chemistry. In our experience DAF systems are capable of removing most dissolved metals to concentrations to as low as about 1 mg/L. For lower concentrations supplementation by ion exchange and/or use of a fine filter is needed for capturing residual dissolved metals or very fine particles.

We have not seen any problems in most other applications where chemically flocculated biological solids from a DAF system were returned to process. In most cases, the chemical dosages were fairly low and did not affect biological activity. In rare cases, some foaming can occur which is either intermittent and/or easily mitigated by using addition of a small concentration of defoamer.

Floc robustness is a qualitative description of how well the floc holds together especially when it is subjected to whitewater injection in the DAF system. Generally, there is no adjustment of air flow needed in the whitewater system unless it is found that excess air injection is causing the floc to break up.

The best way of dealing with highly variable flows and concentrations is to use an equalization tank upstream of the DAF system to buffer the variations in loading on the DAF. If equalization is not possible, collect hourly samples and analyze for TSS (and O&G) and calculate the mass loading (lbs TSS/hr) based on the peak sustained flowrates during each hourly sampling period.

There really is no single ideal design for food plants. For example, the solids and hydraulic loading rates used for a poultry plant application may be different than those used for a vegetable processor. Even in the case of a poultry plant, there may be different treatment goals (e.g., product recovery vs. meeting sewer O&G discharge permit concentrations) that will affect the DAF design. It’s important to consider all of these factors when designing a DAF system.

The Phase I system was a primary DAF for the removal of TSS, O&G, and insoluble BOD. The primary DAF effluent was treated by a Phase II system consisting of an aerobic MBBR (moving bed bioreactor) followed by a secondary DAF to clarify biological solids prior to discharge to sewer. The combined (Phases I and II) system (primary DAF + MBBR + secondary DAF) provided BOD, TSS and O&G removals of 97, 99, and 99%, respectively.

DAF has been effectively used for removing solids from aquaculture effluents. In this case, it may depend less on particle size and more on particle charges and specific gravity. Treatability testing would be recommended to determine if DAF or another treatment technology would be appropriate.

You will get some FOG build up in the EQ with just aeration for mixing, but we have used other forms of mechanical mixing in conjunction with aeration to alleviate this issue. There is no single solution for FOG build up. You would generally drain and empty the tank to remove the buildup, which can be done annually when the tank is inspected for corrosion and leaks.

Either is available based on customer preference. An automatic blowdown system on the DAF is often preferred to reduce maintenance labor and failure to blowdown the system on a regular basis could have a negative effect on the process caused by solids build up in the bottom of the DAF unit.

The link to that 38 minute webinar is on our Evoqua YouTube channel in the Webinar Playlist: https://youtu.be/rfdlR6B8Jig