Selecting the right waste treatment option
August 14, 2003
Are you having problems with wastewater discharges from your metalworking facility? Have you received a violation notice from your sewer use authority? Do your environmental experts speak a language you don't understand or continually reject new fluids you would like to use? Or perhaps you would like to use new or improved lubricants, cleaners, rust preventives, or detergents, but the products you've tried have failed the waste treatment tests.
A variety of technologies for treatment of industrial waste fluids produced by metalworking processes are available. Some of these technologies can treat waste from complex waste streams, even commingled, and allow you to reuse the water in your forming processes.
Contract Hauling. The first option is not to treat your waste and have it hauled away. Consider the location of the facility generating the waste stream, volume, waste type, and concentration. If you do decide to haul waste, make sure your waste hauler complies with applicable laws.
In this top view of a continuous-flow chemical treatment system for processing oily wastewater, the first tank uses sulfuric acid and aluminum sulfate to destabilize the emulsion. The second tank uses sodium hydroxide to react with the aluminum sulfate to form a floc to coagulate the oil in the wastewater.
Chemical Treatment. Two chemical treatment methods are available: batch processing and continuous flow (see Figure 1). Additionally, within each category are more options. For example, you can use:
One significant benefit of chemical treatment is it allows you to process silicone, high solids, very diluted, and concentrated wastes with oil content up to 15 percent simply by adjusting the chemical feed ratios.
A limitation of chemical treatment is that you lose flexibility in product usage. Some products are based on simple formulas and are easy to waste-treat, while others are difficult to waste-treat. Thus, you lose the flexibility to switch products easily. Also, treatment results vary from product to product. Cleaners with high amounts of chelates can interfere with chemical waste treatment.
All of these variables make chemical treatment labor-intensive. Every significant chemical change in your plant will require a new chemical balance. If you use chemical treatment, you will need to conduct bench trials for continued treatment optimization for the life of the system.
This ultrafilter system uses 2,000 square feet of spiral-configured membranes.
Membrane Treatment—Ultrafiltration. In ultrafiltration, an ultrafilter with small openings, or pores, from 0.1 to 0.001 micron rejects oil droplets that are too big to pass through the membrane pores (see Figure 2). System pressures vary from 30 to 50 pounds per square inch (PSI).
An advantage of ultrafilters is that they can handle many metalworking chemistries and cleaners. Unlike chemical treatment, differing strengths of emulsifiers have minimal impact on performance, which provides more product flexibility and the need for fewer bench trials. The oil phase in an ultrafilter can be concentrated to about 40 percent.
However, membrane surfaces can foul, causing a rapid decline in the treatment system's flow rate. Common foulants are silicone defoamers, silicates found in cleaners, graphite, and suspended solids.
Certain paint solvents can permanently damage membranes quickly, and pure oil can smear over membrane pores, inhibiting flow rate.
Many times the flow can recover with proper detergent cleaning, but some foulants, such as graphite, can cause permanent, nonrecoverable loss of membrane flow.
Another limitation of membrane treatment is that certain organic chemicals, such as those found in synthetic metalworking fluids and some cleaners, can pass through an ultrafilter essentially untreated.
Evaporation. The evaporation method is simple in principle: You apply heat to the waste fluid, and the water and some volatile components evaporate into the atmosphere.
This compact industrial evaporator is designed specifically for the treatment of oily wastewater. It can process about 500 gallons per day.
The advantage of this treatment method is that it discharges into the air rather than into the local sewer (see Figure 3), eliminating wastewater. Unlike chemical treatment equipment, evaporators are unaffected by variations in emulsifiers, silicones, and silicates. In addition, evaporators can tolerate high levels of solids and oil. The oil phase in an evaporator can concentrate to 60 percent or more.
A disadvantage of the evaporation method is that it's an energy-intensive process as compared to other methods. Because many communities require an air permit for the evaporator stack, some evaporator users may risk exchanging a potential water/sewer violation for a clean air violation. Evaporators also sometimes release objectionable odors.
Biological Treatment. Although biological treatment is relatively new in the evolution of industrial waste treatment, it has been used to treat sanitary wastewater for decades. Every sewage plant uses some form of biological treatment to digest sanitary wastewater, so why not apply it to wastewaters that contain spent stamping fluids?
Some companies specialize in developing bacteria for different strains of industrial wastes, and these products can work effectively. Biological treatment also consumes minimal energy.
However, biological treatment has its own unique challenges. Bacterial digestion of industrial waste can be difficult to control. Large changes in chemical composition, temperature, concentration, and toxicity of the waste solution can affect the performance of the system.
A compounding problem is that today's advanced industrial fluids are formulated to resist biodegradation in the working environment primarily to inhibit the formation of objectionable odors. This makes these fluids resistant to biological treatment, so they may require a different treatment method. Also, some recent studies suggest that fluids with high bacteria counts may pose a health risk to workers.
Mechanical Vapor Recompression (MVR). This is an advanced treatment option that uses a combination of methods previously discussed.
Consider a pot of water boiling on a stove. Assume your goal is to create distilled water by collecting the steam vapors. Based on the laws of thermodynamics, it requires one British thermal unit (Btu) to raise 1 pound of water 1 degree F. This is referred to as sensible heat, and it is defined as the heat energy stored in a substance as a result of an increase in its temperature.
Further, it requires 970 Btus to turn 1 pound of water into steam at 212 degrees F. This is referred to as the latent heat of vaporization. Conversely, when a pound of steam is condensed to form a pound of water, it gives up 970 Btus of heat. Thus, the heat is recoverable.
The essence of making evaporation cost-effective is to reuse the heat. This is done by adding a blower to raise the temperature 8 to 10 degrees higher than the temperature in the vessel. The sum of the boiling energy and the blower energy can be recovered by using a simple plate and frame heat exchanger. A smaller heat exchanger can be used to capture further a portion of the remaining sensible heat. Typical discharge temperature is approximately 110 degrees F.
So what does this mean? It takes almost 1,000 times more energy to turn water into steam than it does to raise its temperature 1 degree F. Basic evaporation is energy-intensive.
When you are boiling water on a stove, both forms of energy are used: raising the liquid to near boiling (sensible) and then boiling the liquid to create steam (latent). Unfortunately, all this heat energy is lost into the air above the pot along with the vapor. According to the laws of thermodynamics, this heat is recoverable, but how?
Assume you are boiling water on a stove in a vessel. Now capture the vapor above the vessel with a cover. Route this captured vapor via a duct to a blower. The blower compresses the vapor, which increases the density of the vapor and raises its temperature. The temperature inside the vessel is lower than the temperature outside after the blower compresses the vapor. With a heat exchanger, the energy of the latent heat can be recovered and returned back to the inside of the vessel (see Figure 4), recovering approximately 90 to 95 percent of the energy.
Some advantages of this treatment are that it can concentrate up to 55 percent oil continually with no chemicals, has low energy consumption, and produces no metals in the effluent (see Figure 5).
The main disadvantages with this process are that it costs about 10 percent more than current technologies and has a noisy blower, which may require separate noise reduction depending on site conditions.
This MVR system processes 10,000 gallons per day. It is 20 feet long by 10 feet wide by 20 feet high.
The Environmental Protection Agency has issued new effluent limitation guidelines and pretreatment standards for the Metal Products and Machinery (MP&M) category. These new regulations are likely to have an impact on many users of metalworking fluids.