By Peter Cartwright, P.E., CWS-VI
Apesticide, as defined by Webster, is “any chemical used for killing insects, weeds, etc.” It’s generally an organic compound and used primarily in the food processing and agricultural industry, but pesticides are also used to fumigate homes and in the industrial environment.
Correspondingly, Webster defines herbicide as “any substance used to destroy plants, especially weeds, or to slow down their growth.”
The U.S. Environmental Protection Agency (USEPA) appears to be using the term “pesticide” to include both of these products.
Whereas the use of pesticides and herbicides has significantly reduced the incidence of infected food supplies and had a marked effect on protecting the population, many of these compounds are identified as suspected carcinogens and overuse of them has resulted in contamination of our water supplies. For example, atrazine, the most widely used herbicide in the United States, has been linked to cardiovascular damage, congestion of the heart, lungs and kidneys, and is also a suspected carcinogen. The USEPA has set a maximum contaminant level for atrazine in drinking water of 3 parts per billion (ppb) or 0.003 milligrams per liter (mg/L). Other compounds of concern include acetochlor, cyana-zine, alachlor, prometon, metolachlor and simazine.
Another potential health concern is that some scientists suspect certain organic compounds that are chemically similar to many pesticides and herbicides may be endocrine disrupters. The endocrine system—also referred to as the hormone system—is made up of glands located throughout the body which produce hormones secreted into the bloodstream, including receptors in the various target organs and tissues which recognize and respond to the hormones.
A variety of chemicals are known to disrupt the endocrine systems of animals in laboratory studies and compelling evidence has been accumulated indicating endocrine systems of certain fish and wildlife have been affected by chemical contaminants, resulting in developmental abnormalities and reproductive impairment. However, the relationship of human diseases of the endocrine system to exposure to environmental contaminants is poorly understood and scientifically controversial.
When water supplies are contaminated, it’s virtually impossible in most cases to identify the source of chemical contamination as it usually results from runoff from agricultural land, resulting in “non-point source pollution.” Another significant contributor found to be a source in recent years is herbicides used on residential and other lawns, as well as golf courses.
A further problem is that many pests and weeds have become more resistant to treatment chemicals resulting in increased application rates and the development of new chemicals whose effect on humans may be unknown or little understood.
So, as is often the case, in an effort to protect the health of the population in one area, we have created a threat in another.
So what can be done about it?
This technology is based on the chemical attraction of organic compounds to certain “media”—activated carbon and certain other adsorptive resins with high surface areas and specific properties, which cause the compounds to adsorb onto the surface of the resin.
The most popular media is a family of materials known as activated carbon. These media are manufactured from carbon sources (petroleum-based, wood-based, nut shell-based, etc.) which have been heated at high temperatures in the absence of oxygen to produce a carbon material with an extremely high surface area. There are a number of manufacturers, each with several types of activated carbon with their own specific properties. As a result, there are a number of activated carbon products that exhibit excellent pesticide and herbicide removal characteristics.
Adsorptive resins manufactured from specific organic polymers are another media that exhibit high surface area characteristics and are capable of adsorbing these contaminants. They’re generally available from manufacturers of ion exchange resins.
Two classes of membrane products are available today which are effective for the removal of pesticides and herbicides:
- UF membranes
- NF membranes
UF membranes are available in many different “molecular weight cutoff” constructions—the minimum molecular weight of the compound that will not pass through the membrane. The smallest molecular weight cutoff currently available for ultrafiltration membranes is in the range of 5,000 daltons, although values this low are extremely difficult to measure with accuracy. A dalton is a unit of mass equal to approximately 1.65 x 10-24 grams.
NF membranes are characterized by their ability to remove both ionic and non-ionic contaminants, but to a lesser degree than RO membranes. NF membranes exhibit significantly lower molecular cutoffs than UF membranes—typically in the range of 400-to-1,500 daltons.
Distillation is the process by which water is boiled and the resulting steam is condensed into liquid water. Most waterborne contaminants have boiling points above the temperature of water, so they’re left behind and the resulting condensate (distillate) is purified water. Some pesticide and herbicide compounds are relatively volatile, meaning the temperature at which they’ll boil is below that of water. In this case, the distiller is fitted with an activated carbon filter, through which the distillate passes before being collected.
Drinking water supplies have been contaminated with numerous pesticide and herbicide compounds, each with its own chemical formula and molecular weight; however, they can all be effectively reduced with the proper application of the correct technology.
The practical application of treatment technologies for pesticide and herbicide removal is a function of system size. Today, for larger sized systems, such as those serving municipalities, the economics favor membrane technologies with continuous processing characteristics and backwashing capabilities of capillary fiber membrane devices. Adsorptive media are batch systems in that when the media has adsorbed a certain level of contaminants, the system has to be shut down and the media either regenerated or replaced. This is more labor intensive than the membrane processing alternative. The high energy costs associated with distillation make this technology less attractive for large applications.
On the other hand, for point-of-use and point-of-entry (POU/POE) applications, adsorptive media—particularly activated carbon—are more cost effective. The fact that filter cartridges containing activated carbon are readily used in POU applications for chlorine removal and use of carbon in filter tanks is not that uncommon for POE applications, favors this approach. Obviously, strict adherence must be paid to system maintenance, including media replacement. Distillation is also attractive for POU applications.
In-line TOC (total organic carbon) analyzers can be used as an indication of the presence of pesticides and herbicides, but this equipment can only be justified on very large applications because of cost. For small applications, a laboratory-run chemical analysis is required to determine the effectiveness of the removal system.
As more data are developed on the toxicity of the plethora of pesticide and herbicide compounds and analytical sensitivity increases, there’s little doubt new regulations affecting the kind and extent of drinking water treatment will come into force. There’s every reason to believe the technologies of adsorption, membrane separation and distillation will continue to be the technologies of choice, thereby creating an increasing demand for systems of all sizes.
Because the presence of most pesticides and herbicides in drinking water constitutes a health hazard, strict adherence to regulatory requirements must be followed. It’s imperative all analytical data are obtained from qualified, USEPA-approved laboratories and that the specific treatment system be approved by the appropriate regulatory agency.
In the case of drinking water systems defined by the USEPA as “small systems,” state health department approval is typically required. To assist regulators in determining the efficacy of a particular removal system, a USEPA initiative know as ETV (Environmental Technology Verification) has been developed. This is a set of nine comprehensive protocols addressing various health-related drinking water contaminants, each associated with one or more test plans written around a specific removal technology. In the case of pesticide and herbicide contaminated drinking water, the appropriate protocol is SOC (synthetic organic chemicals), and the appropriate test plans are those addressing adsorptive media and membrane technologies. None has been developed for distillation to date.
For POU/POE applications, the appropriate testing protocols are those provided by NSF International, and the pertinent tests would be NSF 53 for adsorptive media, NSF 58 for membrane processing and NSF 62 for distillation. NSF 58 is actually written around RO rather than UF or NF membranes; however, it does contain a section that could apply to SOC reduction.
To make a pesticide/herbicide reduction claim, even if a particular water supply isn’t considered at risk, it’s important that all pesticide and herbicide removal claims be verified by testing such as that required to meet NSF 53, 58 or 62.
Now that the USEPA has adopted the position that POU/POE technologies may be considered candidates for BAT (best available technologies) to assist small systems in meeting federal drinking water guidelines, there will be more and more opportunities arising. In those applications where the technologies meet the performance and economic criteria, those companies that can offer the most appropriate technology-based system with the necessary performance verification data will be in an excellent position to benefit from the treatment prospects afforded by drinking water containing pesticide and herbicide contaminants.
About the author
Peter S. Cartwright, president of Cartwright Consulting Co. and a principal in Cartwright, Olsen & Associates Inc., Minneapolis, Minn., is a registered professional engineer in several states. He has been in the water treatment industry since 1974 and has published more than 100 papers and articles on related issues. Cartwright is chairman of the Water Quality Association’s Ozone Task Force and holds a Certified Water Specialist, Level VI designation. A member of the WC&P Technical Review Committee since 1996, his expertise includes such high technology separation processes as reverse osmosis, ultrafiltration, microfiltration, electrodialysis, deionization, carbon adsorption, ozonation and distillation. He can be reached at (612) 854-4911, (612) 854-6964 (fax) or email: CartwrightConsul@cs.com