Dissolved Air Flotation, Part 2

DAFT....Vacaville, California

Some Design and Operational Ideas
The first, (and in my NOT so humble opinion!), consideration in design is the method in which the dilute sludge (or influent water) is combined with the recycle water. I have had to date, 11 years of experience in two varying different methods. I, like many other operators, have found that this IS EXTREMELY CRITICAL to the efficient operation of a DAFT.

In the first system the dilute sludge is piped in from the top of the tank. A separate pipe carrying the recycle water, (containing the supersaturated air) comes in from the bottom of the tank. (See Figure 1) The recycle water pressure goes to tank depth pressure after it passes the back-pressure valve, releasing the air from solution, and forming minute air bubbles. The theory is ok, but in reality there are far too many solids that are NOT attached to air bubbles rising from below. I have heard this type of blending of the recycle water and solids as an "area release" method. The result is a lower solids capture rate when compared to the next manner in which the solids and recycle water are mixed together.

In the second system (Figure 2) the dilute sludge is combined with the recycle water in a common pipe upstream of the back-pressure valve. This combined flow is then expelled into the DAFT tank where the resulting supersaturated air solution is released "within the particle environment." I have heard of this type of blending referred to as the "pipe blending method." This system, at least as far as I have been able to see, is far superior to the "area release" method.

Generally speaking, for redundancy, there are two (2) recycle pumps, and two (2) thickened sludge pumps dedicated to serve one (1) DAFT. If there are two or more DAFT units, then shared back up (redundant) pumps may be used.

Polymer can be added to each DAFT's recycle flow or to the sludge being fed to the unit for thickening to improve thickening performance. In most cases, the thickener design is based on not having to use polymer under normal conditions. Many installations, (and ours is included) have found that the best polymer application point is "upstream" and as close to the back pressure valve as possible. It seems that the addition of the polymer at greater distances allows for coagulation-flocculation of the solids/particles in the sludge source to be thickened, and then the floc is "sheared" as it passes thru the back-pressure valve, resulting in a decrease in the particle capture rate. If the polymer is injected at a point just upstream of the back pressure valve, it appears that it allows for, and may even assist, in coagulation-flocculation with resulting higher particle capture rates.

"Polymer feed systems" are usually disabled upon loss of sludge feed flow. Polymers are often used to increase the solids capture rate in dissolved air flotation process. The most effective chemicals used are moderate weight polyamines or very high molecular weight cationic polyacrylamide polymers. Polymers can increase the solid's recovery in "the float" from 85% to 98% and can also reduce the suspended solids in the subnatant (the water leaving the daft and returning for treatment.)

The use of chlorinated water for making up solutions, and as a dilution water source, of some polymers may in-activate some of the "active sites" of the polymer chain, resulting in a slightly higher polymer use. There appear to be some polymers, where there exists a relationship of the "higher the chlorine concentration the greater the polymer in-activation."

If the recycle water solids concentration becomes unacceptably high, many treatment facilities have the capability of using un-chlorinated secondary effluent as a standby source of water for the recycle pumps.

Thickened sludge skimmed into the DAFT float box may be pumped out by the thickened sludge pumps operating under on/off control based on float box level.

Each thickened sludge pump usually has a loss-of-prime sensor and a high discharge pressure switch, to cut out the pump on loss of suction or a closed discharge valve respectively. The cutouts should operate even in HAND mode.

The skimmer drive should have a torque overload cutout.

All pumps and the two skimmer drives (but not the polymer units) have motor failure cutouts which must be reset manually.

Alarms
Special attention must be paid to "loss of air pressure". In most installations, dilute sludge to be thickened will still be admitted to the DAFT under this condition, and the recycle pump will still run, but the retention tank will soon become "waterlogged" and thickening will cease. To recover from this condition, before starting again on recycle, most DAFT tanks must be drained to remove that quantity of settled and non-thickened sludge. The tank can be refilled with secondary clarifier effluent water to start up the process again.

Logs and Records
Dilute sludge feed and thickened sludge flow rates are usually monitored by the facilities instrumentation system. These flow rates are usually indicated, recorded, and totalized.

Lab Requirements
Dilute sludge feed, thickened sludge flows, and recycle water suspended solids are analyzed by the lab on a regular basis to support calculations of mass wasted, and to check DAFT performance efficiency.

Routine Operation
To optimize operation of the DAFTs, the skimmer mechanism speed and the back-pressure valve position may have to be adjusted periodically. Routinely check the air to solids ratio; air pressure; and recycle flow rate. Optimize the polymer addition, if polymer is used. Keep the weirs clean, and free of solids, to minimize any short-circuiting (this can not be emphasized enough during design, and in operation). Monitor the amount of solids not captured in the float and have to be cleaned from the bottom of the tank. The dilute sludge flow rate to each DAFT must be set so as not to exceed the solids loading limit.

Troubleshooting

Condition: Thin "Float" Sludge
Possible Causes:
Insufficient Air
Excessive air/solids ratio(float appears very frothy)
Skimmer moving too quickly
Improper Chemical Feed

Corrective Action:
Check air compressor, air flow, and eductor
Adjust speed
Adjust dosage

Condition: High Effluent TSS
Possible Causes:
Insufficient Air
Improper Chemical Addition Point
Skimmer moving too slowly
Unit Overloaded

Corrective Action:
Check air compressor, air flow, and eductor
Check mixing, dosage, and addition points
Adjust speed
Check flow vs. design and TSS loading

Math Review Problem:
Calculate the solids loading rate, in lbs/day/ft2 for a dissolved air flotation thickener that is 25 feet in diameter, has a 400 GPM recycle rate, operates with an air to solids ratio of 0.04, and is thickening 8,800 pounds of dry solids per day loading rate.

Solution:
First, look at all of the "given information" and realize that you will need to ignore all of the information that has nothing to do with this calculation! Then, look at the units desired, and we find that we will need to place in the numerator the pounds per day of solids being fed to the unit. In the denominator we will need to place the surface area of the tank floor, in ft2 units. The UNITS tell us, in many cases, what we need to do!

The "units" tell us, in this case, that we need to determine the number of "pounds per day" and then divide that value by the surface area, in ft2 of the DAFT.

Go to Past Issues of Operator Notebook

Return to Wright's Training Homepage