February 2008, Vol. 20, No.2
Study Examines Impacts of Membrane Residuals
Membrane treatment of water and wastewater has been hailed for its ability in removing pathogens, disinfectant byproduct precursors, and other constituents, according to consulting, engineering, and construction firm Black & Veatch (Kansas City, Mo.). However, as membrane technology grows in popularity, so does the residual waste produced during membrane filtration.
Until recently, the focus on membrane systems has been on the benefits of the product water (permeate) side of the system. The system sidestreams (concentrates) bring their own special set of issues that need examination, according to Black & Veatch.
Black & Veatch recently completed a research study it led for the WateReuse Foundation (Alexandria, Va.) to find out what impacts these rejected pollutants have on wastewater treatment plants (WWTPs) and collection systems, and how system operators might assess these impacts. The research team consisted of Black & Veatch’s director of water reuse, Alan Rimer, and senior wastewater treatment process specialists Ed Kobylinski and Gary Hunter. Also on the team were University of North Carolina (UNC; Chapel Hill) environmental sciences and engineering professor Francis A. DiGiano, and private consultant Barnes Bierck.
The research team developed two models — one created by UNC to predict the concentration of specific chemicals at specific connection points in the collection system, and the other an overarching, systemwide mass balance model from Black & Veatch to predict the concentration of components in the effluent at the WWTP and in the waste solid streams. At press time, The WateReuse Foundation was scheduled to release a guidance manual in early 2008 that will provide an overview of this work and detailed overview on the use of the models, including tips on how to collect necessary input data and how to structure the mass balance model for a specific community’s collection and treatment system(s).
Residuals generated during membrane treatment typically are discharged to a utility’s wastewater collection system and ultimately end up at the local WWTP. The materials carried in the brine streams are generally not encountered in such high concentrations, which prompts the question: At such elevated concentrations, do these residuals adversely impact wastewater treatment plants and collection systems?
The research team conducted a literature survey and a survey of various utilities to learn about their experiences with membrane residuals discharges; problems (in collection or treatment systems, with planning, or otherwise) with discharges and solutions applied; control procedures in place or suggested for addressing membrane residual discharges; and potential further research needs.
One of the challenges in treating poor-quality water supplies, according to Black & Veatch, is elevated total dissolved solids (TDS) concentration. Among the many constituents of a typical water supply, TDS has always been one of the more difficult and expensive to remove, states Black & Veatch.
The research revealed dangers posed by TDS and specific ions to WWTPs and collection systems. For example, common concrete structures including concrete-lined pipe can be attacked by chloride or sulfate at concentrations in excess of 1000 mg/L (1000 ppm). High chloride concentrations — in the 7000 mg/L (7000 ppm) range — can inhibit nitrification. TDS can be acutely toxic to freshwater aquatic species at concentrations as low as 1500 mg/L (1500 ppm). Disinfection also can be adversely affected by the accumulation of nonbiodegradable COD.
In addition, elevated TDS can affect the density of water, and in turn, the settling of solids in primary and final clarifiers; and rapid changes in TDS concentrations can kill aeration basin microorganisms, researchers found.
The models, literature review, survey results, and other project documentation will be provided in the guidance manual. For more information contact Water Reuse Foundation director of research, Josh Dickinson, at (703) 548-0880 or firstname.lastname@example.org.
Fast-Growing Bacteria Speeds Up Water Quality Tests
A Kansas State University (KSU; Manhattan) professor spent much of a recent visit to Hawaii in a laboratory making sure that a trip to one of the islands’ beaches will be more enjoyable.
Daniel Y.C. Fung, professor of food science at KSU, has developed a water quality test that can tell Hawaiian authorities in a matter of hours, rather than days, whether a beach should be closed because of contamination, according to a KSU news release. Fung’s research appears in the September issue of the Journal of Rapid Methods Automation in Microbiology, of which Fung is founder and editor.
Whereas Escherichia coli is the bacterium that closes many beaches in the mainland United States, Fung said Hawaii’s beaches face a problem with Clostridium perfringens, a bacterium that is better acclimated to the temperature and latitude of the South Pacific. Clostridium perfringens is important, Fung said, because it is an indicator of fecal contamination, and its prevalence determines whether beaches in Hawaii will be open or closed.
The problem with testing for Clostridium perfringens, Fung said, is that the test can take as long as 48 hours. By that time, visitors may already have been in contact with contaminated water, or the threat may be gone.
“At that point, the results have no meaning,” Fung said.
That’s why Fung, whose research centers on developing real-time testing methods, has come up with a test that can indicate the prevalence of Clostridium perfringens in just 6 hours, according to the news release.
In lieu of a $10,000 anaerobic chamber, the test uses a “Fung double tube” consisting of a ready-made small test tube inserted into a larger one.
“That’s what’s exciting — it costs about a dollar per test, and you can use it anywhere,” Fung said.
A sample of ocean water or another substance is cultured at 42ºC (108ºF) in an agar specialized for Clostridium perfringens. Six hours into the test, colonies appear, and scientists can count the number of Clostridium perfringens and determine its prevalence in the sample. “It’s the fastest-growing bacteria we know,” Fung said.
Fung is suggesting that Hawaiian authorities start tests at about 11 p.m. so that by the following morning they can know whether they need to close a beach because of contamination.
“With this method, you can have real-time data to benefit public health and safety in Hawaii,” Fung said.
For more information, contact Fung at email@example.com.