Maximize water cleanup performance

• By Douglas Reed
• Feb 01, 2000

Organically modified clays have gained in popularity during the past few years, due to their ability to remove small amounts of oil, grease, tannins and other large organic molecules with low solubility. They have found widespread use in the cleanup of groundwater, and, most recently, industrial wastewater.

Activated carbon is the most popular media for the removal of organic compounds, such as solvents, from industrial wastewater. It is usually used in conjunction with other treatment methods, such as ultra-filtration, as a polisher. Powdered activated carbon is used in batch treatment systems for the same purpose.

Organoclays, which are organically modified clays, act as pre-polishers for granular activated carbon, and post-polishers for oil/water separators. They are helpful because separators lower the oil content down to only 10-20 parts per million (ppm), while carbon experiences plugging of its pores due to free oil, grease and tannins. This plugging phenomenon is due to the physical structure of carbon, which is a network of pores that can adsorb organics. If these organics are of a similar or larger diameter than the pores, the pores become plugged and the carbon is rendered useless for wastewater treatment.

Field results have shown that organoclays are also effective in the removal of such organics as polynuclear aromatics, benzene, toluene, ethylbenzene and xylene (BTEX), chlorinated phenols and other chlorinated organics. Furthermore, the data suggest that when water contains compounds, such as BTEX, pre-polishing with organoclay helps the carbon to focus on removing benzene, while the organoclays remove most of the toluene and xylene. This makes the entire system last much longer, saving the end user in change-out costs and down time. Even though organoclay is not regenerated, it is much more economical to change out a system once a year, rather than six times.

To test these observations, a laboratory experiment was designed using micro-column tests.

Description of organoclays

Organoclays consist of bentonite modified with quaternary amines. The major constituent of the bentonite, a chemically altered volcanic ash, is the clay mineral montmorillonite. It has a cation exchange capacity of 70-95 millequivalents per 100 grams (meq/gm). The nitrogen end of the quaternary amine, which has a positive charge, is exchanged onto the clay surface for sodium and calcium. The replaced sodium and calcium ions ionically bond with the free chlorine ions from the amines, forming salts that are washed out during backwash. The clay becomes a solid based non-ionic surfactant. The amine chains extend into the water, removing the oils and other non-polar or slightly polar organics by partition.

Laboratory methods

The micro-column technique consists of pumping a solution through a small sorbent sample, for example 1 gram, through a small column into which the sorbent is packed, until influent concentration of the contaminant equals that of the effluent. The solid is a fine powder, in this case organoclay and activated carbon powder (PAC). In this test, synthetic water solutions spiked with organics, including benzene, toluene, xylene and naphtalene, were prepared and passed through the mini columns to determine the effectiveness of each sorbent, organoclay and PAC. The advantage of this technique is that the equilibrium concentration is the same as the influent concentration, and therefore under ready control. This technique has more real-world applications than isotherms, even though the two sorbents, when used in the field in flow-through adsorption vessels, are of granular nature, which tend to have higher removal capacities than powders.

Results

Figure 1 is a graphic illustration of the mini-column test results. When testing the removal capacity of the sorbents for benzene, toluene, o-xylene and naphtalene, the organoclay performs nearly as well as carbon, improving in performance as the solubility of the compounds decreases. The organoclay outperforms carbon with the less soluble polychlorinated biphenyl 1260, and motor oil, which is nearly insoluble. Surprisingly, organoclay removes methylene chloride much more effectively than does carbon. The same had been observed in an earlier study with vinyl chloride. It is theorized that compounds such as methylene chlorides have a high electro-negativity due to the presence of large amounts of halogens such as chlorides. The organoclay possesses some positive charges on the surface, which would then chemically bond with these negative charges and remove the compounds from the water. Therefore, two removal mechanisms account for the organoclay's effectiveness, partition and ionic bonding.

These tests were followed by a set of tests using a ternary effluent, which is a wastewater containing three different organics. The effluent was spiked with benzene, toluene and naphtalene. This allowed observation of the competition amongst the contaminants for sorption sites, and determination if the less soluble compounds keep the more soluble ones, such as benzene, off the sorption sites. Nine hundred milligrams per liter (mg/L) of each compound were added to water. It was possible to add that much naphtalene because the benzene and toluene helped dissolve it. Usually its solubility is 10 mg/L. This concentrate was pumped separately through an organoclay and a carbon column. The effluent was tested for each solvent to determine the breakthrough, i.e., which compound broke through earlier and at what levels. The last column was filled first with 0.5 grams of organoclay, followed by activated carbon, to determine if the organoclay/carbon combination (not mixture) was indeed more effective then each of t hem alone.

Figure 2 illustrates the results. Benzene breaks through first, followed by toluene and naphtalene last, as expected based on their solubility. This scenario is similar to three people wanting to enter a train at the same time. The skinniest one slips in first, the heaviest one last.

This competition, however, is not 100 percent proof. The total adsorbed amounts are higher than the individually adsorbed amounts of the three solvents at breakthrough, probably because the geometrically arranged packing of solvents of different sizes, either in the carbon pores or around the amine chains, favors a higher packing density.

The most important result is shown in the "total combined" graph. By placing the organoclay in front of the carbon, the removal capacity is doubled when compared to the graphs for either organoclay or carbon individually. This is also shown in a standard permeation column experiment with gasoline. Again, organoclay followed by carbon is much more effective then either sorbent alone, even though the amount of sorbent in each case is double than that in the combined column. Similar results were obtained from a real world scenario, a groundwater clean up project. The organoclay not only removes the oil completely, but also a significant amount of other solvents, increasing the activated carbon's effectiveness for the removal of the volatile organic compounds (VOCs).

Case histories

A 200 gallon per minute petroleum wastewater treatment system in Norfolk, Va., uses organoclay as a pretreatment to activated carbon. The system treats bilgewater and tank washwater. Wastewater is transported by barge or truck to the facility, where it is pretreated in holding tanks, breaking down emulsions. Following pretreatment, the water is pumped into a dissolved air flotation tank, then pumped through a vessel with 7,000 pounds of organoclay, followed by polishing with 5,000 pounds of activated carbon. The organoclay removes not only oil and grease, but also naphtalene and other polynuclear aromatics (PNAs), while the carbon removes benzene, toluene and xylenes (BTX) at maximum efficiency, resulting in a long-lasting, highly efficient system.

A New York manufacturer of pharmaceutical products such as creams and ointments is required to clean its equipment daily with soap and hot water. The wastewater is collected in a grease trap separator, from which it passes into a sump pit, where a surface oil skimmer removes any floating oil and grease. This wastewater then passes through an oil magnet filter for discharge into the New York City sewer. New York City regulations stipulate a maximum limit of 50 ppm petroleum hydrocarbons, a level frequently exceeded by the manufacturing plant. The oil phase consists of white petroleum, stearic acid, fatty acid stearates, fatty alcohols, mineral oils, oxyethylene ethers and stearates, all of which are emulsified. The average concentration of organics is 5 to 400 ppm. Average daily discharge is 3,110 gallons.

Two parallel trains of carbon canisters were set up. Each lead unit contains 300 pounds of organoclay/anthracite blend, followed by 190 pounds of a blend of coconut and bituminous carbon. The organoclay removes all the organics of low solubility, including xylene and toluene, while the carbon polishes the water. After 250,000 gallons, no hydrocarbons have been detected in the effluent. This system cost $8,000, while the originally planned ultrafiltration system would have cost $40,000.

Conclusions

The results from these tests and real life situations show that the cost of water treatment, whether it be industrial wastewater, or "pump and treat" groundwater cleanup systems, can be significantly lowered by using the organoclay/carbon approach. Pre-polishing with organoclay, first of all, lowers activated carbon consumption by 700 percent. Secondly, the organoclay removes a substantial amount of less soluble solvents, further improving the effectiveness of activated carbon for more soluble solvents.

These data and case histories show that organoclays can be used to improve efficiency, and thus lower the costs affiliated with carbon, even if there is no oil, or only a very small amount (1 ppm), present in the water.

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This article appeared in the February 2000 Environmental Protection magazine, Vol. 11, No. 2, p. 37.

This article originally appeared in the 02/01/2000 issue of Environmental Protection.