Biologically-mediated, simultaneous removal of nitrate and arsenic from drinking water sources

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Proposal Title: Biologically-mediated, simultaneous removal of nitrate and arsenic from drinking water sources Principal Investigators: Raskin, Lutgarde Institutions: University of Michigan Proposal No: CBET-0967707 Contamination of groundwater with various oxy-anionic pollutants has been a major concern in the context of providing safe drinking water throughout the world. Regulatory pressures have resulted in the development of technologies suitable for the treatment of individual contaminants. However, the co-existence of multiple contaminants makes it imperative to develop treatment systems that provide simultaneous removal of as many contaminants as possible. The proposed research will develop a novel technology for the biologically-mediated, simultaneous removal of two contaminants that frequently co-occur in groundwater, i.e., nitrate and arsenic. Biological treatment of drinking water is gaining in popularity as multiple contaminants often can be converted to innocuous compounds in a single reactor. The proposed research will develop a system consisting of two fixed-bed biologically active carbon (BAC) bioreactors operated in series for the simultaneous removal of nitrate and arsenic (arsenate, As(V)). These contaminants can serve as electron acceptors for microorganisms when an electron donor (e.g., acetate) is provided. Denitrifying bacteria convert nitrate to dinitrogen gas and arsenate can be converted by arsenate reducing bacteria to arsenite (As(III)), which can be removed by sorption to iron sulfides. Iron sulfides are generated by the reduction of sulfate by sulfate reducing bacteria in the presence of ferrous iron (Fe(II)). Iron and sulfate are present in groundwater thus providing all necessary components to make these reactions take place. If ferric iron (Fe(III)) is the predominant form of iron, iron reducing bacteria can reduce Fe(III) to Fe(II), thus ensuring the Fe(II) form exists to interact with the sulfides produced. Iron sulfide sequesters arsenite very efficiently, and protects against reductive mobilization, which is possible when other forms of adsorbed arsenic are disposed in landfills. The PIs hypothesize that the microbial communities that will develop in the fixed-bed BAC bioreactors will be capable of reducing dissolved oxygen, ferric iron, nitrate, arsenate, and sulfate in a sequential manner. Regular backwashing of the reactors will remove excess biomass and occasional backwashing will allow collection of arsenic solids (sulfides laden with arsenic) deposited in the reactor. Three objectives with associated tasks were developed: (i) a bench-scale, fixed bed BAC bioreactor system will be operated to study the mechanisms responsible for contaminant removal in detail. The microbial community and the produced solids will be characterized and the relationship between microbial community activity and the characteristics of the deposited solids will be studied. (ii) The system will be optimized for efficient and sustained removal of nitrate and arsenic to below detection. (iii) Post-treatment of the effluent and stability of produced solids will be evaluated. The development of a one-step treatment system with a small footprint that can remove multiple contaminants, is affordable and simple to operate, and produces limited and safely disposable waste is highly desirable for developed and developing countries trying to expand drinking water sources. The link with a U.S. engineering firm ensures that the results will be applied in practice in developed countries. Researchers of the Institute for Social Research at the University of Michigan, who have extensive experience working with rural communities in Nepal, will help them explore the practical difficulties and willingness of arsenic-affected rural people of Nepal to adopt the proposed treatment technology. Additional broader impacts from this work include the integration of research results into existing courses taught by the PIs and into activities associated with existing K-12 outreach programs that will be continued with this project.