For the past few years, state government has based decisions on the amount of clean up that they require by the risk posed to the environment. Risk Based Corrective Action, commonly called 'Rebecca' (RBCA), is a process that utilizes the principles of exposure assessment, toxicity and mobility to make corrective action decisions on sites that are cost effective while still protecting human health as well as that of the environment. Many times all they require is removal of the free-phase product, allowing any emulsified contaminant to degrade with time. RBCA has saved many millions of dollars both for taxpayers and for businesses. With state regulatory agencies taking this approach, oil skimming devices have emerged as one of the most cost effective means of remediation.
Wastewater engineers in industrial settings have, for many years, understood the value of skimmers in the removal of hydrocarbons from water. Food processing plants, the metals industry, machining firms and utilities have all used skimmers with great success for wastewater treatment. Recently, skimmer manufacturers have modified their product for remediation of groundwater. The skimmer’s ability to get into tight spaces and remove relatively large amounts of hydrocarbons lends itself perfectly to groundwater remediation.
Since most oils, fuels and other hydrocarbon liquids have the tendency to float on water, skimmers are designed to remove only the top, free-phase, product layer. With only product being removed, the cost and maintenance of other down-well and water treatment equipment can be eliminated. Another cost advantage to skimming is that in many cases the product can be salvaged for reuse - further reducing the overall price by eliminating the disposal cost.
Picking the Most Appropriate Skimmer
The options for remediation through recovery wells are practically unlimited since these wells come in a variety of sizes; any of the available technologies such as pump and treat of bioremediation can be used in the correct size recovery well. Monitoring wells, however, are small, typically less than 4 inches in diameter. Initially installed for the monitoring of groundwater they are cheaper to construct and just large enough to allow a baling device or oil/water interface detector to pass through. As a cost savings measure, these small diameter wells are increasingly being used for product removal. With the increase in this new use, it is only natural that a number of devices are showing up claiming the ability to remove product through monitoring wells.
The typical bailer is a clear or semi-opaque plastic tube, open at the bottom, with a string or rope attached at the top to lower and retrieve the unit. The device is lowered through the oil/water interface and fills with liquid from the well. A low-pressure check valve allows liquid to enter as it is inserted, but closes as the bailer is removed to retain the collected liquid. Bailers were initially used to detect the presence and quantity of contamination in wells.
The best way to describe slurping devices is to compare them to a large drinking straw. A hose or tube is inserted into the well until it reaches the product and then a vacuum is applied to lift the liquid to the surface where it is collected.
A passive bailer is a tube that is normally filled with absorbent material. It is lowered into the well, secured with a rope or string and left to perform its function. Typically, a hydrophobic screen near the top of the unit allows the product to enter and be collected by the absorbent material but keeps the water out. Once installed at a set level, the passive bailer is able to collect product only within a set range.
Electric product pumps are an active means to remediate through monitoring wells. That is to say, they are not a 'sit and wait' device. Utilizing hydrophobic screens to keep water out, these units pump product to the surface where it is collected. Many styles are coupled with float sensors and some with buoyancy devices to assist them in keeping water out of the pump.
Using external pressure as the drive mechanism, peristaltic pumps collect product in a pliable tube within a hard outer shell. Positive pressure, either hydraulic or pneumatic, within the outer shell collapses the flexible tube and pushes the product out of the tube. A series of check valves keeps the product flowing in one direction only.
While not new to oil removal, continuous loop belt skimmers are relatively new to well remediation. The belt, which can have a vertical drop of 100 feet is lowered into a well to a point below the water/product interface. The belt, driven by an above ground motor, passes through a wiper mechanism which removes the product for collection. The cleaned belt is then returned back down the well to collect more of the free-phase oil.
Advantages and Disadvantages
Product recovery methods work in varying degrees, the problem is to find the one which is best suited to the site-specific problems, government requirements and the client’s wishes. The factors, which must be weighed, are efficiency, initial cost and operating cost.
Bailing or slurping a well may be a short term solution, but would probably not be the method of choice for long term projects - those which last several years. The initial cost of material is minimal but, the operating cost is driven very high due to the labor-intensive requirements of these methods. Operating cost is further compounded by the amount of water recovered that must then be treated or disposed of as hazardous waste.
Passive bailers are also relatively inexpensive initially, but here again maintenance can drive up the operating cost. Setting the unit at the proper depth can be tricky when the water table fluctuates; efficiency drops off dramatically as the viscosity increases and the product has a harder time passing through the hydrophobic screen. While not as labor intensive as some methods, frequent adjustment of the depth and purging the unit of collected product may be required.
Pumping systems designed for product-only recovery have a higher initial cost but are also considerably more efficient. The wide variety of sizes gives them the ability to fit many applications, including monitoring wells, but they can be difficult to position and keep at the correct depth. Floating units which 'ride' the water table can make the problems associated with setting the depth easier, but caution should be taken to ensure this does not also affect removal efficiency. Some units will leave a layer of free product as thick as 1 to 2 millimeters. It should also be considered that as with any system that relies on hydrophobic screens to separate water and product, efficiency is decreased as viscosity is increased. Viscosity can also be a drawback when it comes to magnetically driven electric units. The high viscosity can cause the magnets to 'de-couple' while pumping and decrease efficiency. Peristaltic units must rely on proper operation of the check valves to perform at efficient levels. Anytime pressure is lost, due to one or more of the check valves sticking, the unit becomes inoperative. Should water enter the unit, most product pumps will emulsify the mixture and make recovery much more difficult. When the product viscosity is relatively low and these units are properly set, they can be an efficient means of removing oil from a well.
Continuous loop belt skimmers have an initial cost less than product pumping systems, but higher than bailers and slurpers. With all the system’s equipment located at the wellhead, maintenance is relatively easy and inexpensive. Viscosity however, has an inverse effect on belt skimmers. As the viscosity drops, the skimmer actually has a more difficult time collecting product. This effect is countered by the continuous nature of the unit. It may take two or three passes but the belt will eventually get all of the oil out of the well. A properly sized unit will also remove product until a light sheen or less remains. By varying belt materials and speed, depending on the product being recovered, efficiency can be maximized on a site by site basis. When used in conjunction with other equipment and technologies (i.e. bio remediation, filters or oil/water separators) a much more efficient system is realized for a reasonable increase in cost.
With the requirements for site remediation becoming more reasonable, the use of skimming devices in lieu of pump and treat systems is increasing. The enormous expense involved with treating millions of gallons of water including the equipment, monitoring and related maintenance is being replaced with a much more common sense attitude. Skimmers, especially belt skimmers, as a means of remediation, not only meet the challenge but, most times exceed. Pump and treat still has a place in this industry, but the small step 'backward' to time proven skimming, a more reasonable and cost effective method, can not be overlooked.