Step One: Understand the Basics. First, understand that while designs vary, all oil skimmers rely on the fluid properties of specific gravity and surface tension. Most use a moving medium to remove floating oil from the fluid's surface (the exception is a floating suction skimmer). Floating oil and grease cling to skimming media more readily than water. This allows media in the shape of a belt, disk, drum, etc. to pass through the fluid surface and pick up floating oil and grease while rejecting most of the water. The oily material is subsequently removed from the media with wiper blades or pinch rollers.
In addition, realize that the kind of water contaminant being removed affects the kind of skimmer you should choose. For example, grease skimming involves higher viscosity hydrocarbons. In that case, an oil skimmer must be operated at temperatures high enough to keep the grease fluid. This may require heating elements in the fluid reservoir and skimmer unit to keep the grease in a liquid state for easier pick-up and discharge. If floating grease forms into solid clumps or mats in the reservoir, a spray bar, aerator or other mechanical apparatus can be used to break up the grease and facilitate skimming.
Finally, recognize the power of a skimmer. Often oil skimming by itself can achieve the desired level of water purity. In more demanding situations, skimming is a cost-effective means of removing most of the oil before using more complicated and costly treatments, such as coalescers, membrane filters and chemical processes. This is often the case in bioremediation of groundwater, where an oil skimmer is used as a first step to remove most of the oil that's contaminating the water.
Step Two: Make Sure You Have a Suitable Application. When properly selected and applied, oil skimmers are highly economical solutions for separating oil from water to allow their reclamation and reuse or safe disposal. However, there are certain applications that are more suitable than others, which allow skimmers to achieve optimum results. Typical applications best suited for oil skimmers encompass:
· Wastewater sumps where removing floating hydrocarbons can reduce the cost of disposal and lower the contingent liabilities of wastewater discharge.
· Coolants and cutting fluids where skimming tramp oils extends coolant life, improves the quality of machined parts, reduces irritating smoke that forms during machining, lessens the chance of dermatitis and helps prevent the fluid from developing a "rotten egg" odor.
· Heat treating operations where trench oils must be removed from heat-treated parts, and can be captured with a skimmer for reuse or disposal. This reduces oil purchases, prolongs wash water life and lowers disposal costs.
· Parts washers where removing floating oils from a wash tank prevents re-contamination of the parts as they are removed from the fluid and extends fluid life.
· Food processing facilities where the removal of vegetable oils, greases and animal fats from a plant's wastewater stream reduces processing and disposal costs.
· Parking lots, garages and service facilities where waste oil from leaks, spills and other sources must be retrieved from sumps before water can be discharged to storm or sanitary sewers.
· Outdoor ponds, lakes and basins where floating oils are present, skimmers provide inexpensive and effective removal, solving a serious environmental problem.
· Oil recovery/monitoring wells where a belt skimmer can be used instead of a down well pump to remove oil, fuel and other hydrocarbon fluids. Generally, this is more cost-effective and reduces maintenance headaches. Skimmers can reach depths of 200 feet or more and remove floating hydrocarbon products despite fluctuating water tables.
· Remediation processes where skimmers can be used effectively in tandem with other types of separators, or additional technologies to reduce the overall cost and time required for cleanup. For example, after removing most of the oil with a skimmer, biological agents can be used to remove the remaining water contaminants.
Step Three: Match the Oil Skimmer Design to Application Characteristics. There are several types of industrial oil skimmers. Choosing one best suited for your application will maximize oil removal while minimizing capital outlay and skimmer operating costs. Pay particular attention to the following oil skimmer/application characteristics:
· Operating conditions - The performance and life of the pick-up medium, wiper blades, pulleys, etc. are affected by operating conditions. These include temperatures in and out of the liquid, the pH of the solution and the presence of solvents or other reactive chemicals. The oil skimmer and its components must be designed for these operating variables.
· Hazardous materials - Applications involving flammable materials or explosive vapors require the use of explosion-proof (or air-driven) motors and controls.
· Temperature/Viscosity - All oil skimmers require floating hydrocarbon material to be in a liquid, free-flowing state. (An exception to the floating oil caveat is dense non-aqueous phase liquids, which can be skimmed when a polymer belt skimmer medium is used to reach to the bottom of a tank where the contaminant has settled.) If floating oil or grease congeals or solidifies at ambient temperatures, the reservoir and/or skimmer will require heaters to maintain fluid flow.
· Removal Rate - Oil skimmer removal rates, expressed in gph, vary with oil viscosity. Typically, manufacturers rate skimmers using SAE 30 weight motor oil at 65°F (18°C). It's wise to ask for test data, especially if your application involves a much different viscosity. Your oil skimmer selection should be based on the maximum amount of oil to be removed within the shortest available time. (See Removal Rate Guidelines below.)
· Skimmed Water Content - All oil skimmers pick up some water with the oil; suction skimmers pick up more water than other types. High water content increases the cost to recycle or dispose of the oil. Generally, the ratio of water to oil decreases with thicker films of floating oil and slower moving pick-up media. A concentrator or decanter installed at the skimmer discharge port provides secondary oil/water separation that can reduce water content to nearly zero.
· Residual Oil - A skimmer removes oil as long as it is present. Depending on the oil influx rate and the oil skimmer's removal rate, residual oil in the water may be as low as a few parts per million. When further reduction is required, a secondary removal stage such as membrane filtration may be needed.
· Portability - In some plants, mobile equipment service shops, and remediation sites, a portable skimmer can sometimes service multiple machines, sumps, or wells.
· Tank or Sump Characteristics - The location, shape and capacity of a tank or water impoundment are major factors in choosing the right skimmer. Also consider fluctuations in water level, turbulence and possible emulsions. Although skimmers do not cause emulsions, they may have trouble removing certain types.
· Size/Design - Oil and water can emulsify when subjected to turbulence and other mechanical agitation. Avoid this by having water return to the tank below the liquid surface at as low a velocity as practical. Make sure the tank or sump provides quiet areas, weirs and sufficient volume to allow adequate time for oil/water separation.
· Shape - Tanks without nooks and crannies for oil to accumulate in are best. If you have an irregular shape, put the skimmer where the largest amount of oil accumulates. Consider a means of directing oil toward the skimmer such as a floating boom or baffle plate.
· Location/Installation - Questions to ask about the physical location and characteristics of the tank and collection container:
- Does skimmed material need to be pumped from the skimmer to the container?
- Will skimmer access for periodic maintenance be a problem?
- How much mounting space is available?
- Are tank or container modifications required? (Total skimmer system costs may involve additional components, tank modifications, and skimmer maintenance.)
Oil Skimmer Removal Rate Guidelines: Consider an example where the total oil influx is 200 gallons a day. The calculated average would be 200/24 = 8.3gph. However, if most of it comes during a single 8-hour plant shift, you probably need a removal rate three times that average, especially if you need to prevent the discharge of contaminated water to a sewer system. As a rule of thumb, specify approximately twice the maximum capacity you anticipate needing for normal conditions.
Part II of this article will cover different types of oil skimmers, their designs, and optional features.
