Part II: Organoclays Improve Performance of Pump and Treat Remediation Systems
By George R. Alther
Editor's Note: In Part I of this series, the author discussed the potential cost-effectiveness of a pump-and-treat groundwater remediation system where an organically modified clay is added to the granular activated carbon of the traditional method used in recent years. Part II looks at the process in specific terms, illustrates results by referring to several case applications, and gives details of possible applications of organoclays.
The use of organoclay as a pre-polisher in the remediation process can produce cost savings even if an organic contaminant such as oil is present in the water at a level as low as one part per million (ppm). This concentration amounts to 0.0000084 lb of oil per gallon of water.
Let's suppose someone installs a vessel with 1,000 lb of granular activated carbon (GAC) to remove 3 ppm of benzene at a water flow rate of 100 gallons per minute (gpm). But the 1 ppm of oil that is present is not taken into account since it is perceived to make little difference to the effectiveness of the process.
In 24 hours the GAC removes 1.2 lb of oil. At this rate the 1,000 lb of carbon, with an absorbent capacity of 5% of its weight for oil, will be exhausted by that 1 ppm of oil within 42 days. Considering that the system was intended also to capture the 3 ppm of benzene, the accumulative effect of removing the oil and benzene will be that the GAC is spent within two weeks. This would turn out to be a rather costly approach. The following case illustrates how the incorporation of organoclay significantly improves the performance of the system.
Use of Organoclay Adds to Life of GAC Bed, Lowers Costs
Assume a remediation system is operating at a water flow of 30 gpm, with 10 ppm of contained oil, and running 24 hours per day, seven days per week. The objective is to remove 5 ppm of BTEX compounds (benzene, toluene, ethylbenzene and xylene) so that this water is treated to a level of quality that falls within regulatory limits. The presence of the oil results in the activated carbon having to be changed out seven times per year.
The quantity of GAC used is approximately 1,070 lb. Regenerated GAC is required to re-fill the process vessel and costs about $0.85/lb. Labor for the change out of the material is about $0.90/lb. So the total combined cost of material plus labor for seven change outs of this quantity of GAC in one year is over $13,500.
If a second vessel is added to the system and filled with 2,000 lb of organoclay the cost benefit is clear. Assuming that the typical price of organoclay is $1.40/lb, its cost is $2,800 for the fill. Also assume that only one change out per year is required, and the labor cost of the change out is $0.90/lb, or $1,800 total. Disposal in an approved landfill of 3,200 lb of organoclay, which now includes the weight of the oil and possibly other low-volatility contaminants that have been removed in the 12-month period, could cost around $500. So the total cost is $2,800 $1,800 $500—or $5,100. Changing the activated carbon in the first tank just once at the end of the year adds about $2,000. So the approximate total one-year cost of the combined system is $7,100, versus $13,500 when only the GAC-loaded vessel is operated alone.
The owner/operator has saved almost 50% in operating costs by adding a tank of organoclay to the processing arrangement. And since such a system requires little maintenance, the pump-and-treat approach to the removal of these organic contaminants is clearly more affordable.
Examples Illustrate Effectiveness of System
In one actual remediation project the groundwater below a U.S. Army site was found to be contaminated with diesel fuel. Three vessels were loaded with activated carbon and set up in series to remove the contaminant. The concentration of the diesel fuel was so high that the carbon beds experienced breakthrough after only two days. The carbon was replaced, and three additional tanks filled with organoclay were added in front of the GAC. Cleanup was accomplished in three months, and neither the organoclay nor the carbon beds had experienced breakthrough at that point.
At another site the groundwater was contaminated with 100 ppm of fuel oil, 10 ppm of BTEX compounds, and 90 ppm of heavier PNA's including anthrazine, napthalene, paraffin and waxes. Originally two activated carbon tanks were installed, but the GAC had to be changed out every few days. Additionally, the treated water discharged from the vessels still had a surface sheen of oily material, probably paraffin and waxes, that the process did not remove. A decision was made to install another vessel ahead of the two GAC tanks and load it with organoclay. The performance of the combined system has been such that total organic carbon (TOC) analyses have not detected any of the contaminants, and the life of the carbon beds has been extended to the point where change outs are required every four to six months.
New Process Combines Three Phases Into One
A new pump-and-treat method is called Bio-Slurping. This system combines vacuum enhanced recovery and bio-venting into a single step. The procedure can be used for removal of residual contamination from soil in the vadose zone, and recovery of free (non-emulsified) oil. This is oil that has not been mechanically emulsified by the natural pressure that exists within the aquifer.
A single pump extracts soil gas, free oil and groundwater simultaneously. On the surface the water is then treated conventionally with oil/water separators (to recover the free product), activated carbon treatment and air stripping, as well as vapor phase carbon to clean up the soil gas. This process is applicable when the objective is to remove light non-aqueous phase liquids (LNAPLs) from an aquifer. It has been shown to be more economical than the standard pump and treat method because three steps are performed in one.
A small system of this type was installed at a U.S. Air Force base where groundwater contamination had occurred. After extraction and removal of most of the free oil the water, containing about 1,000 ppm of oil, was allowed to stand in a settling tank. After about four hours more oil had risen to the surface and was skimmed off. The water, still showing a concentration of some 200 ppm of emulsified oil, was passed through a 55-gal drum containing 250 lb of organoclay. The outflow analysis indicated an oil residual of around 5 ppm, which was low enough for compliance with local discharge requirements. If a second drum of organoclay had been added to the system, the water would have been of potable quality.
Many Potential Applications Exist for Organoclay/GAC Concept
Free oils are found in many water sources, including ground water, landfill leachate, industrial wastewater discharges, storm water runoff, and bilge water from marine sources. Applications for organoclay/activated carbon systems include among others the recovery and clean up of bilge water, boiler steam condensate, refinery wastewater, vehicle wash water, and water used in oil well operations. Free oil sources include jet fuel, gasoline, diesel fuel, kerosene, crude oils, tars, cutting fluids, lubricants, and fats.
It is known that phenols, BTEX compounds and PCBs can partition into oil phases. In some cases organoclay has been found to be so effective at removing these contaminants from water that activated carbon is used only in a backup mode, ensuring that any contaminants left in the effluent are in trace quantities that are non-detectable. Organoclays also have been used successfully as pre-polishers for ion exchange resins, membrane filters and air strippers, all of which are susceptible to fouling by oil and natural organic matter.
Disposal of spent non-hazardous organoclay can be accomplished in a number of ways, for instance by landfilling, land farming, as an ingredient of asphalt, or as part of the blended fuel used to fire cement kilns. If the contained contaminant is known to be hazardous the spent organoclay must be incinerated.
Another pump-and-treat method involves the use of fluidized bed filters. In this approach the activated carbon is inoculated with bacteria that continuously remove organic compounds near the surface (opening) of the pores,which in effect produce an in-situ re-generation. This process also can be engineered to incorporate organoclays, resulting in a cost-competitive, self-cleansing system.
Metallic contaminants also can be captured and removed with organoclay-based systems. At a site in Oklahoma the ground water contained colloidal nickel. With the use of organoclay nickel compounds were removed to the point where their concentration in the effluent was lower than the local discharge limits.
In another industrial case where metallic contaminants were a concern, at a refinery in Alberta, Canada, groundwater is used as boiler feed water. Its iron content is regularly 3–5 ppm, which created problems in the equipment. With the groundwater pH averaging 7.8, ferric iron would precipitate out, and co-precipitation of other heavy metals present also would occur. However the organoclay was shown to be unaffected by the iron and continued to perform satisfactorily, producing boiler feed water of suitable quality.
Pump-and-Treat Again is a Viable Remediation Method
Experience with small scale and larger scale installations has shown that a variety of pump-and-treat methods are available and perfectly viable for recovering contaminated water when they are properly designed. The use of organically modified clays in a pre-polishing role to the granular activated carbon used in the basic method, or as a post-polisher to oil/water separators, can cut operational costs by 50% or more. They can help pump-and-treat systems take their place alongside other processes as an effective and economical way to accomplish ground water remediation.
The first installment of this two-part series, Part I: Organoclays Improve Performance of Pump and Treat Remediation Systems, posted on Water Online on Feb. 11.
About the Author: George Alther, founder of Biomin Inc., Ferndale, Michigan, which manufactures organoclays used in water filtration media, has worked in the environmental field for 25 years. He has authored 80 scientific papers, has three patents and four others pending. For more information on this topic, contact Biomin Inc. at: phone248-544-2552, fax 248-544-3733, or email biomin@aol.com.