Part I: Organoclays Improve Performance of Pump and Treat Remediation Systems
The so-called pump and treat method has been used quite extensively for the remediation of contaminated ground water. Now it is perceived by some in the field as too expensive when compared to newer in-situ methods for the removal of organic compounds. Examples of these are bio-remediation, in-situ oxidation and surfactant injection. It has been found, however, that pump and treat can be economically acceptable and competitive when organically modified clay is combined with the activated carbon used in the conventional version of the system.
Water supply shortages exist in many regions of the world, including in parts of the U.S., where western states such as Nevada and California are particularly affected. The situation generally is going to get worse and conservation and water quality protection measures are becoming increasingly important.
A Contaminated Aquifer is a Source of Useful Water
Based on this observation we can establish the premise that every aquifer contaminated with potentially hazardous substances should be saved and re-developed. Each can be viewed as a source of drinking water, irrigation water, or at least of industrial water. Since no single recovery method is perfect for every situation, an engineering design team planning a remediation project has to be aware of what systems are available.
The pump and treat approach is one of them. In its simplest form, the system consists of a well and a pump. Ground water is pumped out of the aquifer and passed through an oil/water separator (if needed), a bag filter (if needed), and a filter vessel filled with granular activated carbon (GAC). This sequence is followed in line by an air stripper and another tank filled with vapor phase carbon.
Oil/water separators are key components of a pump and treat contaminated water remediation system.
The efficiency of the activated carbon for liquid phase extraction of hydrocarbons depends, in part, on the size of the molecule compared to the size of the pores on the carbon surface. Large, sparingly soluble hydrocarbons such as oil, grease, polynuclear aromatics (PNAs), natural organic material (NOM), and others can quickly foul the carbon by blinding the pores, resulting in frequent change outs that drastically increase the cost of the operation.
Organoclays, or organically modified clays, in granular form can remove 50% or more of their weight of oil and other large, soluble, chlorinated hydrocarbons. This is seven times more effective than activated carbon (which can remove 5%-10% of its own weight of oil), and can amount to a 50% cost reduction to the operator. In addition organoclays, which are natural ion exchange resins, will remove small amounts of heavy metals, to provide a further benefit. This methodology has been in use for a few years. Evidence now suggests that if a vessel of organoclay is placed in front of a carbon vessel in a remediation process train, costs can be cut sufficiently to make the simple pump and treat approach an attractive option once again.
Pump and treat systems may be more expensive than in-situ methods such as those mentioned earlier if they are not designed properly. But bio-remediation is a slow process, requires much attention, and appropriate bacteria have yet to be developed to handle many contaminating substances, for example PCBs (polychlorinated biphenyls). Certain bacteria can plug up pores in silty clay aquifers, and if a large variety of organic contaminants are present, there may not be enough types of bacteria to address all of them. Bio-remediation also may not be practical during periods of cold weather. And processes using surfactants tend to be expensive and difficult to manage in terms of dosage requirements.
The presence of iron can be a serious problem, and pump and treat may be the method of choice for removing iron contamination. Heavy metals in general need to be removed by a pump and treat system, regardless of whether the source is natural, such as a dolomitic aquifer containing lead or zinc, or whether it is a manmade landfill. Also, metallic compounds may be toxic to the bacteria used in bio-remediation arrangements. If the end use of the water is for irrigation or for drinking water delivery, where acceptable concentrations of PCBs and dioxenes are so small they are in the parts-per-billion (ppb) or parts-per-trillion (ppt) range, a process employing activated carbon in a pump and treat scheme may be the only way to achieve those limits.
Organoclays: What They Are and How They Behave
Organoclays are bentonite clays modified with quaternary amines. Bentonite is a volcanic rock whose main constituent is the mineral montmorillonite. This gives the bentonite a significant ion exchange capacity. By transferring the nitrogen end of a quaternary amine onto the surface of clay platelets by cation exchange (exchanging sodium or calcium ions for positively charged nitrogen), the bentonite is organically modified and becomes organophilic, which also means hydrophobic. The clay platelets are stacked on top of each other in a layered structure. When these platelets are placed in water the amine molecular chains are activated and stand up like dry hair. This causes "pillaring" of the platelets, allowing the end of the amine chains to stand or dangle in the water and react with organic compounds that pass by. The chains dissolve or partition into large organic compounds such as sparingly soluble chlorinated hydrocarbons.
Oils are the most prominent of these. These same compounds, on the other hand, will blind the pores of activated carbon. Since the chemical reaction takes place on the outside of the clay platelet, rather then internally as in carbon, blinding or coating of the surface is not a factor. The organoclay is loaded into the same type of filter vessels used for the activated carbon. It is mixed with anthracite to prevent early plugging of the interstitial pores, and in this condition it removes 50% of its weight or more of free and mechanically emulsified oil, and some soluble organics such as benzene.
Factors Important in System Design
The design of a cost-effective organoclay-modified GAC-based pump and treat system must take into consideration certain properties of the activated carbon. For instance, the pH of the water should be maintained on the acidic side. In acidic conditions, hydrocarbons are less soluble and oils are de-emulsified. Empty bed contact time (EBCT) and mass transfer zone (MTZ) are parameters that must be addressed to achieve maximum efficiency. They are important also for the organoclay itself, particularly in industrial wastewater applications where several types of oily organics may be present in the wastewater stream.
An understanding of chemical oxygen demand (COD) also is necessary. Oily contaminants can account for a significant part of the COD value, and removing them with organoclay can assist in bringing the COD into regulatory compliance, a factor important in vehicle washing applications.
In Part II of this article, the author will provide more technical details about the modified pump and treat system, and offer observations based on several case studies. This conclusion will appear on Water Online next week.
About the Author:
George Alther, founder of Biomin Inc., which manufacturers 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. Alther holds a Master's of Science in Geology from the University of Toledo and a Bachelor's Degree in Business Administration.
For more information on this topic, contact Biomin Inc. at: phone248-544-2552, fax 248-544-3733, or email biomin@aol.com.