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Removal of Cationic Heavy Metals from Drinking Water Supplies through the Ion Exchange Membrane Bioreactor

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Title: Removal of Cationic Heavy Metals from Drinking Water Supplies through the Ion Exchange Membrane Bioreactor


1
Removal of Cationic Heavy Metals from Drinking
Water Supplies through the Ion Exchange Membrane
Bioreactor
  • Adrian Oehmen
  • Universidade Nova de Lisboa
  • Portugal

2
Heavy Metal Pollution in Waterways
  • Mercury is the heavy metal with the highest known
    toxicity.
  • Mercury is a bioaccumulative toxin that attacks
    the central nervous and endocrine systems.
  • Can cause brain damage
  • Mercury pollution enters water systems mainly
    through rainfall and effluents from industrial
    processes.
  • In water supplies, mercury exists primarily in
    the cationic form (Hg2).
  • Maximum contaminant level for mercury in drinking
    water
  • 1-2 ppb (WHO, US EPA)

3
Ion Exchange Membrane Bioreactor (IEMB)
  • Combines the transport of an ionic pollutant
    (e.g. Hg2) with its simultaneous bioconversion.
  • Hg2 is transported through a cation exchange
    membrane at the expense of a harmless counterion
    (Na).
  • Hg2 is then converted to Hg0 via biological
    reduction, stripped from the liquid phase and
    recovered in the gas phase.
  • The IEMB concept has successfully been applied
    for the removal of anionic pollutants such as
    nitrate, perchlorate and arsenate from drinking
    water.

4
Hg2 Transport and Bioreduction through the IEMB
Stripping to Gas phase
Hg0
Oxygen, carbon source, nutrients
Biocompartment
Water compartment
Cation Exchange Membrane
Biofilm
5
Advantages of the IEMB System
  • Promotes selective and efficient pollutant
    removal
  • The associated brine solution from membrane
    transport is treated
  • Provides a physical barrier between the polluted
    water and microbial cells, carbon sources. The
    initial water matrix is otherwise maintained
    largely intact.
  • Thus, prevents secondary contamination of
    drinking water
  • The treated water production rate does not depend
    on the hydraulic retention time of the
    biocompartment

6
Objectives of this Study
  • Selection of a suitable cation exchange membrane
  • Evaluate Hg2 transport through 11 different
    commercially available cation exchange membranes
    by Donnan dialysis.
  • Biological Hg2 removal using mixed microbial
    cultures
  • Investigate the effect of carbon source on the
    process performance and Hg2 reduction kinetics.
  • Integrate these 2 processes in the ion exchange
    membrane bioreactor to achieve Hg2 removal from
    drinking water

7
Cation Exchange Membrane Selection Hg2 Flux
  • Fumatech FKE membrane exhibited high flux, good
    mechanical stability and reasonable price

8
Microbial Hg Resistance Mechanism
(Wagner-Döbler et al., 2003)
  • Hg2 is converted to Hg0, via the MerA enzyme
  • Hg0 is then stripped from the liquid to the gas
    phase, and recovered through e.g. adsorption onto
    various materials

9
Mercury Measurement Methodology
Carbon source, nutrients, biomass, Hg2
Gas-phase measurement
N2 gas
Gas Flow (with Hg0)
Liquid-phase measurement
10
Biological mercury reduction
Glucose culture
Acetate culture
  • Two mixed cultures were enriched with Hg2
    reducing organisms using different carbon sources
  • Glucose
  • Acetate
  • Most of the Hg2 was reduced to Hg0 and stripped
    to the gas phase

11
Biomass Growth of the Mixed Cultures
Glucose culture
Acetate culture
  • Hg2 bioreduction and biomass growth were not
    simultaneous
  • Biomass growth commenced only after Hg was
    completely removed
  • Delay in growth of acetate culture
  • Glucose was partially converted to acetate and
    tended to accumulate

12
Carbon Source Swap
Glucose culture
Acetate culture
  • Glucose culture
  • Hg2 reduction rate much slower with acetate as
    carbon source
  • Acetate culture
  • Half-saturation coefficient (KHg) is
    substantially higher with acetate
  • Mercuric acetate complexes may be more difficult
    to biodegrade

13
Comparison of Culture/Carbon Source
  • Glucose was a more effective substrate for
  • Culture selection
  • Bioreactor operation
  • Good Hg mass balance recovery was achieved (gt85)

14
IEMB Setup
Membrane
Water Effluent
Air Pump
Bio-Medium
pH Meter
Bio-Effluent
Dissolved Oxygen Meter
Water Feed
Magnetic Stirrer
Gold Trap
Filters
Water Compartment
Bio Compartment
Gas Flow (with Hg0)
15
IEMB operation
  • IEMB operated using the glucose mixed culture
  • Hg2 removal gt98 at an F/A ratio of 1.5 l/(m2h)
  • Hg also removed in the biocompartment (lt10 ppb)

F/A 15 l/(m2h)
F/A 1.5 l/(m2h)
16
IEMB Effect of Membrane Pre-Treatment
  • Membrane pre-treatment in HgCl2 increased Hg flux
    through the membrane

17
Conclusions
  • A suitable cation exchange membrane (Fumatech
    FKE) was selected for IEMB operation
  • Glucose was found to be a more favourable carbon
    source for the operation of mixed microbial
    cultures, in terms of
  • Enriching an effective microbial community for
    Hg2 bioreduction
  • Maximising the rate of Hg2 bioreduction,
    minimsing the mercury half-saturation coefficient
    (KHg)
  • The integrated IEMB system was shown to be very
    effective in removing a high level of Hg (gt98)
  • Experimental study is currently ongoing to
    evaluate its potential at low Hg concentrations
  • Process applicable for the removal of other heavy
    metals with optimisation of the biocompartment
    (e.g. biosorption)

18
Acknowledgements
  • Co-authors D. Vergel, J. Fradinho, J. L. Capelo,
    S. Velizarov, J. G. Crespo, M. A. M. Reis
  • The financial support by Fundação para a Ciência
    e a Tecnologia (FCT), Portugal through Project
    No. PPCDT/AMB/57356/2004 and postdoctoral
    research grant SFRH/BPD/20862/2004.
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