The
Alternative to Pump and Treat
Skimming
devices, especially belt skimmers, should be given serious consideration for
groundwater remediation.
Sometimes,
an important technological advance begins with a small step – back! Such is
the case with remediation of hydrocarbons from groundwater utilizing existing
recovery and monitoring wells.
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 bio-remediation 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.
Bailers
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.
Slurpers
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.
Passive Bailers
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 Pumps
Electric
product pumps are an active means to remediate through monitoring wells. That is
to say, these pumps 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
groundwater 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.
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 that 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