Dear Advice and Counsel,

For five years, our company (a small job shop electroplating facility) has attempted to operate a wastewater treatment system using "air flotation," with little success. Our local POTW has just notified us that, if we cant comply within six months, we will have to shut down the plant. I am a new hire and dont know much about this flotation technology. The company that manufactured the equipment has gone out of business. Can you provide any help?

Signed, Floating Down the River

Dear Floating,

Dissolved-air flotation solids separation systems have been on the market for some time, but have not "caught on" as a generally accepted means of solids separation for metal finishing wastewaters. This technology has been used often for separating oil and water, and has been successful in some shops that have a consistent inflow composition in their wastewater. Lets take a detailed look at these clarifiers.

Flotation Clarifiers

Flotation is a gravity separation process based on the attachment of air, or gas, bubbles to solid (or liquid) particles, that are then carried to the liquid surface where they accumulate as floating sludge (much like an up-set gravity clarifier). The floating sludge can be skimmed off. The process consists of two stages: (1) The production of suitably small bubbles, and (2) their attachment to the particles. Depending on the method of bubble production, flotation is classified as dissolved air, electrolytic, induced-air, nozzle-air (aspiration) or dispersed-air. The air flotation system used most commonly for metal finishing wastes is the dissolved-air system. We will cover only that system in detail, but the generalizations given also apply to the other systems.

Dissolved-Air Flotation

The main advantage of these systems is they occupy less floor space than either a conventional clarifier or a "lamella" system. Operating costs, however, are higher with flotation.

A typical dissolved-air flotation system uses a pressure pump, air supply, retention tank, and flotation tank. The system is normally operated with a portion of the effluent recycled through the system, in an effort to obtain uniform solids loading in the inflow. Operating variables include air pressure, recycle ratio, retention time, air:solids ratio, solid loading rate, polymer feed rate (type, too) and hydraulic loading rate.

Dissolved-air flotation produces fine bubbles of less than 100 µm dia.(1) The process is based on the higher solubility of air in water, at elevated pressure. Part, or all, of the feed is saturated with air. The pressure is reduced in the flotation clarifier, so that fine air bubbles appear and are available for flotation. The fine bubbles act as tiny "balloons," lifting the sludge upward, while the "clarified" water travels downward.

There are three ways of creating the flotation: (1) Saturation at atmospheric pressure and flotation under vacuum; (2) saturation under static head with flow upward resulting in bubble formation (micro-flotation); and, most commonly, (3) saturation at pressures higher than atmospheric (200-700 kPa) and then flotation under atmospheric conditions.

A portion of the effluent is usually air-saturated and recycled at rates of 25-50 percent, or even 100 percent. The recycling approach avoids clogging problems, flow variations in the influent, and affords less danger of flog breakup. The feed, often treated with flocculating agents, enters the inlet mixing chamber, as does recycled material that is nearly saturated with air. The amount of air required is determined by a mass ratio of air to dry solids of 0.015 to 0.030. The now pressurized feed enters the flotation zone. Solids that accumulate at the top of the water-flow are typically skimmed off by the sludge removal mechanism. The effluent exits at the other end of the unit, with a portion being recycled.

As with gravity settling tanks, an important design variable is in the area of the unit. The area must be matched with the feed flow-rate, so that outlet velocity is not greater than particle rising velocity. The overflow-rate/unit-area of flotation surface (the hydraulic loading) of the system normally is established by trial and error.

There are several problems with this technology for use by a job shop electroplating facility. One is that the mass ratio of air to dry metal hydroxide solids is critical to the solids separation/flotation efficiency. If the mass of air to solids ratio is too small, there wont be enough bubbles. If the ratio is too large, the bubbles will be too large. Consequently, in either case, the flotation efficiency will be significantly reduced.

A job shop electroplating facility generates wastewater that fluctuates significantly in solids content from one operating hour to the next, depending on the work load. The solids content in the wastewater is far from consistent because job shops do not always have the same workload, part geometry, part size, etc. Unless there is a way to adjust the amount of air that is introduced to the wastewater, based on the solids content, the ratio of air to solids in the wastewater from this facility will fluctuate greatly, causing inefficient removal of solids.

Use of Flocculating Agents With Air Flotation

According to theory, the greater the contact angle between the metal hydroxide particle and the air bubble, the greater the float-ability of the particle. The particles can be hydrophilic or hydrophobic, affecting this contact angle. The job of the polymer is to overcome these conditions and make the particles uniformly floatable. The type of polymer, the manufacturer of the polymer, and the concentration of polymer used will, therefore, also have some effect on the performance of the air flotation system.

The effects of flocculating agents on the efficiency of solids separation with air flotation technology was investigated by Gopalratnam, Bennett, and Peters 2. The study found that a single dosage rate is not effective for all metals. They also found that the dosage (concentration) of polymer is critical to the efficiency of the system. In quite a few experiments, too little or too much polymer increased concentration of certain metals. Polymer dosage must, therefore, be carefully measured to within the best concentration range, based on jar testing.

Also, even under the best conditions, metal hydroxide removal efficiencies ranging from 79-97 percent were achieved. Such removal efficiencies are not high enough to meet regulatory standards unless additional equipment (polishing filtration, for example) is installed.

Combined with the fact that different metals have different solubilities at different pH values, this all adds up to a system that is troublesome to operate and control in most job shops.

Its a good guess that the problems "Floating" is experiencing originate from attempting to use this technology on an influent that contains highly variant solids concentrations, and that the number of variables that must be controlled on this system are too great to accomplish the objective on a consistent basis.

Our advice would be for "Floating" to first conduct extensive jar testing to determine the optimum pH for rendering the metals in the mixture of raw wastewater insoluble. If jar testing indicates that metal hydroxide formation is not adequate for obtaining compliant soluble metal concentrations, additional precipitation using sulfides (dithiocarbamates or ferrous sulfide) may be required. The jar testing must be performed on enough raw wastewater samples to assure that the optimum pH is on wastewater that represents the full spectrum of in-coming concentrations. Until an optimum pH is found that yields soluble metal concentrations below regulated values, there is no hope for any solids-liquid separation technology to work effectively.

Once an optimum pH has been found, the next step is to obtain samples from as many suppliers of flocculating agents as possible. Be sure to mention the need for a flocculating agent (sometimes called a "collector") for an air flotation system. Another series of jar tests then will be performed to determine the floccing agent that does the best job of floating off solids. This will be a more difficult laboratory simulation, requiring a lab scale air flotation simulator. Gravity settling can be used as a crude alternate for weeding out the least effective flocculants, but there is a danger with substituting one technology for another in any comparison.

A laboratory scale air flotation system was described by Gopalratnam 2.

When an optimum pH and flocculating agent are found, the air flotation system will need some fine tuning. Start with a 25 percent recycle flow and take hourly samples of the wastewater mixture entering the air flotation system for at least two working days. Analyze the hourly samples for suspended solids content, so that you can set the proper air to solids ratio. Adjust the air:solids ratio by varying the pressure of the air-feed line. By trial and error, you will need to adjust the air pressure, recycle rate, and flocculent feed rate to a total optimum state.

A large equalization tank may improve the efficiency of the air flotation system by maintaining steady in-coming concentrations. Even the best data reported in the referenced literature, however, indicates that obtaining a compliant effluent directly from an air flotation system is highly unlikely.

If the above optimization steps do not succeed in yielding a compliant effluent, "Floating" should consider replacing or augmenting the air flotation technology with methods of solid/liquid separation that have a wider experience base, such as gravity clarification or micro-filtration.

To balance the discussion, we are aware that there are some installations utilizing air flotation successfully. In most of these cases, the incoming waste water has highly predictable content and flow rate. In the case of oil/water separations, air flotation systems have been highly effective. In applications of simultaneous oil and metal hydroxide separation, experimental systems using air flotation have demonstrated oil-removal efficiencies of 86-96 percent, while, at the same time, removing 71 -97 percent of the heavy metals in the inflow.

References

1. Svarovsky, Ladislav, Chemical Engineering, 103(July 197)

2. Gopalratnam, Bennett, and Peters, Proc., 9th AESF/EPA Conference on Environmental Control for the Metal Finishing Industry(1988)