By Jim Larsen
Small system operators affected by the recent passage of new water regulations should consider diatomaceous earth (DE) precoat filtration to meet stricter standards.
Diatomaceous earth (DE) precoat filtration for the production of potable water has a long history, dating back to the development of portable filter units by the U.S. Army during World War II. Over the last 50+ years, the use of diatomaceous earth filters has evolved primarily as a technology for small systems serving less than 10,000 people. More than 100 water systems in the United States use DE filtration. These systems range in size from 0.03 to 30 mgd.
The Long Term 1 Surface Water Treatment Rule (LT1SWTR) and the Stage 1 Disinfectant/Disinfection Byproduct Rule (D/DBPR) appear to have competing goals. This conflict was termed by Alan Roberson, AWWA director of environmental affairs, "the small system train wreck." The surface water treatment rule seeks to lower pathogen levels, particularly Cryptosporidium parvum. This rule mandates lower turbidity standards in order to ensure removal of protozoan pathogens. It is estimated that 50 percent of small systems will be required to make some process modification in order to meet this new rule.
The D/DBPR seeks to limit the formation of disinfection byproducts, and this would seem to encourage the decreased use of disinfectants or conversion to alternative disinfection technologies. Small systems previously have not been required to meet such standards and a sizable impact with huge costs is forecast.
Research carried out under the auspices of the EPA demonstrated the effectiveness of DE filtration for Giardia removal. More recent research, conducted by the Independent Diatomite Producers Association, demonstrated a six-log capability for the removal of Cryptosporidium. The EPA grants log removal credits to DE systems meeting an effluent turbidity standard of 0.3 NTU. This double standard recognizes the superior separation capability of DE filtration.
DE filter media amended with strong-base anionic resin achieved significant removals of Ultraviolet (UV) absorbency and dissolved organic carbon with turbidity reduction and acceptable headloss development. This ability to amend DE precuts and bodyfeed demonstrates that this technology has the potential to lower the formation potential for disinfection byproduct precursors. The benefit to small systems could be the ability to retain current disinfection practices rather than switch to more expensive alternatives.
Together, these statistics speak of filtration performance exceeding current and anticipated standards for potable water production. How much does this kind of performance cost? Typically, DE plants require only a small footprint so land acquisition costs are minimized. Operating and maintenance costs for DE filtration are comparable to slow sand filtration, while at the same time offering particle separation at the microfiltration level.
Several filter manufacturers offer products and systems for potable water production. Small systems can be custom designed or they can be purchased as turnkey package installations. Fully instrumented and automated plants are available with production capacity up to 1 MGD for less than $650,000. Capital costs of filters for larger plants are typically much lower than for membrane systems of equal size, and operation and maintenance costs for larger plants are typically under $0.25 per thousand gallons, well below alternative technologies.
DE filtration offers the small system operator flexibility in meeting changing source water conditions. Commonly, DE filters are fed raw water with turbidity below 5 NTU; however turbidity spikes as high as 20 NTU can be handled by adjusting the media dosage called bodyfeed to accommodate the rise in turbidity. Commercially available monitoring and control systems make this adjustment automatically so system operation may be left unattended. Flexibility also is available in the selection of several grades of DE precoat media that can be matched to source and finished water requirements.
Finally, DE filtration offers the small system operator the ability to remove protozoan pathogens and lower the formation potential for disinfection byproduct precursors.
About the Author: James R. Larsen is general manager of product and applications support for World Minerals Inc., in Lompoc, Calif.
