FAQs for Dissolved Air Flotation (DAF)

Based on webinar: Best Practices For Design and Application

FAQs For Webinar: DAF - Best Practices For Design And Application

How is the A/S ratio determined?

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.

For a poultry application, how effective is a DAF in terms of percent of total suspended solids (TSS) removal without using coagulants and flocs for a poultry application?

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.

Would a DAF system be applicable to a system used to separate chicken blood from water and what coagulant would you use?

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.

Can you share about ups and downs of DAFs for metals removal? Specifically zinc both total and dissolved. Our limit is 0.8mg/l we seem to hover 1-30.

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.

For the refinery biological removal process, did you have any issues with re-introducing return activated sludge (RAS) that had been previously dosed with polymer/coagulant? Can this adversely impact the biological process after reintroduction?

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.

For air to solids ratio (A/S), you mentioned floc robustness. Is there a quantitative measurement for determining the "robustness" of floc and how do you adjust for that in the air/solids ratio?

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.

What is the best method for determining the solids loading rate on highly variable flows and concentrations for a 24/7 facility?

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.

Please go over the Big 4 one more time. What is the ideal design for food plants based upon on your experience for each of the 4? For example, Hydraulic Load of 3 gpm / ft2 surface area.

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.

What biological system did you use for the bakery example?

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.

In aquaculture recirculated saltwater systems we use aquarium skimmers with ozone. Could DAF technologies be modified to get an industrial size skimming of particles? Our water must be low in TSS and we need to get rid of fine particles below 60 micron and skim them off as foam.

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.

Any issues with FOG floating in the EQ tank since aeration used to mix?

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.

Are solids blown down manually or automatically?

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.

Is there a link to the webinar: Excellence In Wastewater Series: DAF - Application And Design Best Practices?

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