passive treatment of acid mine drainage in bioreactors - École ...

OttawaGeo2007/OttawaGéo2007
PASSIVE TREATMENT OF ACID MINE DRAINAGE IN
BIOREACTORS: SHORT REVIEW, APPLICATIONS,
AND RESEARCH NEEDS
G.J. Zagury and C. Neculita
Department of Civil, Geological, and Mining Engineering - École Polytechnique
de Montréal, Montreal, QC, Canada
B. Bussière
Department of Applied Sciences - Université du Québec en Abitibi-
Témiscamingue, Rouyn-Noranda, QC, Canada & NSERC Polytechnique/UQAT
Industrial Chair in Environment and Mine Waste Management and Canada
Research Chair on Abandoned Mine Site Rehabilitation
ABSTRACT
Sulphate-reducing passive bioreactors have received much attention lately as alternating technologies for acid mine
drainage (AMD) treatment. The long-term efficiency is sometimes limited because they rely on the activity of sulphatereducing
bacteria (SRB) which is primarily controlled by the reactive mixture composition. Our review shows that the
most important mixture component is the organic carbon source. Several studies conducted to find the best mixture of
natural organic substrates for SRB showed that a combination of organic carbon sources is preferable than a unique
source. In addition to metal sulphide precipitation, which is the main metal removal mechanism, other mechanisms
including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The more
successful designs allow the flow to be reversed between upward and downward modes. Pilot and field-scale passive
bioreactors filled with mixtures of organic and cellulosic wastes were installed in Canada and USA and efficiently
removed sulphate and metals for periods up to 5 years. Additional work needs to be done to properly assess the longterm
efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, toxicity assessment of treated
effluent from on-site passive bioreactors has yet to be performed on a regular basis.
RÉSUMÉ
Les bioréacteurs passifs sulfato-réducteurs ont reçu dernièrement beaucoup d’attention comme biotechnologies
prometteuses pour le traitement du drainage minier acide (DMA). L’efficacité des bioréacteurs passifs sulfatoréducteurs
est parfois limitée parce qu’elle dépend de l’activité des bactéries sulfatoréductrices (BSR), qui est principalement
contrôlée par la composition du mélange réactif. Notre revue montre que la source de matière organique est le
composant le plus important du mélange réactif. Plusieurs études menées dans le but de trouver le meilleur mélange de
substrats pour les BSR montrent que les qu’une combinaison des sources de carbone organique est préférable à une
seule source. A part la précipitation des sulfures métalliques, qui est le mécanisme principal d’enlèvement des métaux,
plusieurs d’autres mécanismes y compris l’adsorption et la précipitation des carbonates et des hydroxydes métalliques
peuvent intervenir dans un bioréacteur passif. La plus efficace conception est celle dans laquelle le changement entre
les écoulements vertical et horizontal est possible. Plusieurs bioréacteurs passifs à l’échelle pilot et de terrain, remplis
de mélanges des déchets organiques et cellulosiques ont été installés au Canada et en Etats-Unis et qui enlèvent d’une
manière efficace les sulfates et les métaux. Il reste encore du travail afin de bien évaluer l’efficacité des mélanges
réactifs à long terme et les mécanismes d’enlèvement des métaux. De plus, l’évaluation du potentiel écotoxique de
l’effluent traité par les bioréacteurs passifs ex-situ doit être approfondie.
1 INTRODUCTION
Mine wastes, generated from active and inactive mining,
present a potential impact on human health and the
environment and constitute an important environmental
problem for government entities, the mining industry, and
the general public (Aubertin and Bussière, 2001; Blowes
et al., 2003). The main problem related to mine wastes
occurs when they contain sulfide minerals that can
oxidize and generate acid mine drainage (AMD). Waters
contaminated by AMD have low pH and alkalinity, and
high concentrations of heavy metals and sulphates. To
avoid any significant impact, they must be collected and
treated before being discharged into the environment
(Neculita et al., 2007).
Passive bioreactors were used over the past 20 years for
AMD treatment on remote mine sites, without power, and
with extreme winter conditions (Dvorak et al., 1992,
Gusek et al., 1999; Reisinger et al., 2000; Reisman et al,
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2003; Tassé et al., 2003; Kuyucak et al., 2006). They
offer advantages such as high metal removal at low pH,
stable sludge, very low operation costs, and minimal
energy consumption. Additionally, construction materials
are readily available, common, and generally inexpensive
(Zaluski et al., 1999). They rely on sulphate-reducing
bacteria (SRB) that can be found in natural environments
where anoxic conditions prevail. SRB oxidize organic
matter, produce bicarbonates (HCO 3 - ) that raises pH and
alkalinity of water, and reduce sulphates present in AMD
to sulfides under anaerobic conditions. Sulfides then
combine with metals to form insoluble metal sulfides.
In passive bioreactors, AMD is fed horizontally or
vertically, flows through a solid reactive mixture
(contained into a pond or a tank), and is released treated
into the environment (Neculita et al., 2007). Reactive
mixture composition is crucial for the efficiency of the
treatment process (Cocos et al., 2002). Efficient reactive
mixtures generally contain an organic carbon source, a
bacterial source or SRB inoculum, a solid porous
medium, a nitrogen source and a neutralizing agent
(Waybrant et al., 1998 and 2002; Cocos et al., 2002;
Zagury et al., 2006). The most important mixture
component is the organic carbon source.
Generally, SRB use the best short-chain organic carbon
molecules (e.g. methanol, ethanol, and lactate)
(Postgate, 1984). However, these substrates are
expensive and can make this biotechnology prohibitive.
Due to their availability, natural organic materials such as
wastes from agricultural and food processing industry
have been assessed for their potential to promote and
sustain sulphate-reduction.
Experience shows that a mixture of several wastes
performs better than a single waste (Waybrant et al.,
1998 and 2002; Zagury et al., 2006). However,
efficiencies obtained in laboratory bioreactors are better
than in pilot or full-scale bioreactors, which none have
remained operational without significant overhaul or
modification for more than 3 to 4 years (URS Report,
2003). In long-term operation passive bioreactors
efficiency can be limited by organic carbon availability to
SRB.
This paper focuses on organic matter importance for
passive bioreactors performance during long-term
operation. Several studies conducted to find the best
mixture of natural organic substrates for SRB are briefly
reviewed. Particular emphasis was given to field
applications. Moreover, critical parameters for design and
long-term operation are discussed. Finally, some
research needs are underlined.
2 ORGANIC CARBON SOURCES USED IN PASSIVE
BIOREACTORS
AMD contaminated waters contain relatively low
concentrations of dissolved organic carbon (

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carbon with no preferential flow paths over 32 months
(Zaluski et al., 2003).
The extent of a study is another important parameter. In
short-term studies, higher proportion of poultry manure
were essential for promoting higher sulphate-reduction
rates (Cocos et al., 2002), while higher proportions of
coniferous bark and/or sawdust have been associated
with sluggish sulphate-reduction rates (Tassé and
Germain, 2002). In long-term studies, the efficiency of
cellulosic substrates alone for the biological treatment of
AMD has been confirmed (Tuttle et al., 1969; Chang et
al., 2000; Tassé et Germain, 2002; Johnson and
Hallberg, 2005). Moreover, a mixture containing a high
content of cellulosic wastes (40% sawdust, 10% wood
chips, and 10% alfalfa hay) and 10% organic wates (cow
manure) gave the best efficiency in a long-term field study
(Reisman et al., 2003). However, cellulosic wastes alone
were also reported to entail carbon limiting conditions
(Béchard et al., 1994).
These results stress the importance of a well established
microflora in the presence of mixtures of cellulosic and
other complex natural organic carbon sources.
Nevertheless, after an acclimatization period, better
results in terms of metal and sulphate removal were
reported with sawdust alone (Tuttle et al., 1969; Johnson
and Hallberg, 2005) than with compost alone (Gibert et
al., 2003 and 2004; Zagury et al., 2006). Sawdust is
already integrated in the new generation of passive
bioreactors because it provides a significantly greater
hydraulic conductivity and appears to be a better energy
source for bacterial community (URS Report, 2003).
2.3 Configuration of organic substrates
In the most used configuration design of passive
bioreactors, natural organic materials serve as substrate
and as support for microbial attachment and metal
precipitation (Tsukamoto et al., 2004). In this case, the
most effective design is typically when the substrate is
sandwiched between pipes set in inert gravel at the top
and bottom of the bioreactor (URS Report, 2003).
However, in such configuration, the lifetime of bioreactors
is limited by the organic carbon available to SRB that
affect the extent of microbial activity and treatment
efficiency (Gibert et al., 2004; Tsukamoto et al., 2004). In
other configurations, bioreactors are filled with a
combination of organic matter, crushed limestone, and
cobbles placed in discrete chambers (Zaluski et al.,
2003). Also, site-specific passive systems that
incorporate anaerobic/ aerobic cells and limestone/ rock
filters have been proposed (Johnson and Hallberg, 2005;
Kuyucak et al., 2006).
2.4 Long-term biodegradability
Evaluation of substrate longevity based on fixed amounts
of organic carbon and limestone, and the relative
consumption rates observed is, however, difficult to
assess due to high variability in kinetics of sulphatereduction
during the treatment (Reisman et al., 2003). In
order to extend the long-term performance of a
bioreactor, few solutions were proposed.
Regular addition to a depleted matrix of organic
compounds such as methanol (Tsukamoto et al., 2004),
sucrose (Béchard et al., 1994), lactate (Tsukamoto et al.,
2004), and acetate (Gibert et al., 2004) were successfully
tested. However, in this case the reactor is no more
passive.
Bioactivation of bacterial consortia with an easily
available organic source (e.g. lactate) and then replacing
it with a less expensive source such as ethanol
(Kaksonen et al., 2003) or a mixture of wood chips, leaf
compost, and poultry manure (Beaulieu et al., 2000) was
also tested. However, partial degradation of ethanol to
acetate increased residual organic carbon in the effluent
(Kaksonen et al., 2003) or longer lag period was
necessary for SRB to get acclimated to the more complex
organic carbon sources (Beaulieu et al., 2000). New
formulations of suitable organic carbon sources, such as
patented mixtures of organic materials (methonak,
molasses, methanol and wood chips) commercialized by
ARCADIS treatment systems or hydrogen release
compounds (HRC) by REGENESIS technologies are also
available. Commercialized reactive mixtures can be used
for in-situ treatment of AMD contaminated waters in pit
lakes, smelter ponds, and flooded and underground
workings (e.g. Gilt Edge Mine, Anchor Hill Pit Lake,
Hollister Mine, and Sweetwater Mine). HRC is an electron
donor organic material, designed to produce controlled
release of lactic acid, when hydrated. HRC can be directly
injected in the contamination source area or used in
permeable reactive barriers (PRB) applications.
3 METAL REMOVAL MECHANISMS IN PASSIVE
BIOREACTORS
Precipitation in the form of sulfides (Pb 2+ , Co 2+ , Cd 2+ ,
Cu 2+ , Ni 2+ , Fe 2+ , Zn 2+ ), hydroxides (Fe 3+ , Cr 3+ , and Al 3+ ),
and carbonates (Fe 2+ , Mn 2+ ) represents the main metal
removal mechanism in passive bioreactors. Adsorption,
surface precipitation, and polymerization on inorganic
support, solid organic matter, bacteria, and metal
precipitates are sorption mechanisms that can occur. Coprecipitation
with (or adsorption onto) Fe and Mn oxides
and bacterially produced metal sulfides (Jong and Parry,
2004) is also responsible for metal removal from AMD.
Finally, AMD quality is improved by filtration of the
suspended and colloid materials (Wildeman and
Updergraff, 1997).
3.1 Adsorption/ absorption
Adsorption of dissolved metals onto organic sites in the
substrate material is an important process upon the start
of a passive bioreactor (Machemer and Wildeman, 1992;
Gibert et al., 2005). pH is a very important variable
because SRB retain metals via biosorption due to the
neutral and/or deprotonated state of binding ligands on
cell walls. At pH 3.0, a biomass content > 6 g/L increased
the efficiency of metal removal, favouring sedimentation
of the iron precipitate, and rates of filtration (Santos et al.,
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2004), whereas at pH 7.0, biosorption capacity was
constant regardless of the experimental conditions (e.g.
stirring and biomass type) (El Bayoumy et al., 1997).
These contradictory results may be explained by a more
or less active microbial population in the biomass used as
well as to a possible competition between metals.
Competition among Fe, Cu, Zn, and Mn for organic
adsorption sites was confirmed by laboratory tests and
field-tests in wetlands (Machemer and Wildeman, 1992).
Therefore, at pH slightly acidic to neutral (as in
bioreactors), adsorption of dissolved metals on the
substrate material is an important metal removal
mechanism. Over time however, the adsorption sites
become saturated. This saturation may take from 3-8
weeks (Waybrant et al., 1998), to 4-8 months (Zaluski et
al., 2003).
3.2 Metal precipitation
Once sulphate-reducing conditions develop, sulfide
precipitation becomes the main metal removal
mechanism (Machemer and Wildeman, 1992; Béchard et
al., 1994). Sulfide precipitation is the desired mechanism
of metal and sulphate removal because metal sulfides are
highly insoluble and less bioavailable compared to other
metal species (Wildeman and Updergraff, 1997).
Sulphate reduction is confirmed by lower concentrations
of sulphates in the effluent waters than in the affluent and
lower redox potentials (Johnson and Hallberg, 2005).
Manganese and arsenic are less efficiently removed as
sulfides in passive bioreactors (Wildeman and
Updergraff, 1997; Zaluski et al., 2003). The relatively high
solubility of MnS, which forms only when the Mn
concentrations are very high compared with others
metals, can explain the relatively low removal efficiency
as manganese sulfide. A novel enhanced bioremediation
system that consists of a passively aerated subsurface
gravel bed was reported (Johnson and Younger, 2005).
The exact process responsible for arsenic removal during
the early stage treatment is not clear but adsorption or
concomitant co-precipitation with other metal-sulfides or
with ferrihydrite has been suggested (Jong and Parry,
2003; Zaluski et al., 2003). When reducing conditions are
established, formation of insoluble arsenic sulfide may
occur. However, pilot and field-scale bioreactors that
efficiently removed arsenic or arsenic along with other
metals for a period of almost two years or more were
reported (Reisman et al., 2003; Tassé et al., 2003).
3.3 Metal removal mechanisms evaluation
Recent studies about metal removal mechanisms are
based on data obtained from the effluent water chemistry
during the operation and from the solid phase analysis of
the bioreactor spent mixture (Neculita et al., 2007).
Water chemistry is used in thermodynamic chemical
equilibrium models such as WATEQ4F (Amos and
Younger, 2003) and VMINTEQ (Waybrant et al., 1998,
2002; Zagury et al., 2006) to explain metal removal during
the early stage of bioreactors operation. Results generally
suggest that the initial decrease (particularly in shortterm;
see Neculita et al. 2007, this conference) in metal
concentration can be attributed to adsorption or
precipitation of (oxy)hydroxides and carbonates.
However, these models do not take into account the
bacterial activity that entails the precipitation of metal
sulfides (Zagury et al., 2006).
Solid phase analyses include sequential extraction
procedures complemented with determination of acid
volatile sulfides and simultaneously extracted metals
(Jong and Parry, 2004). Additionally, mineralogical
analyses can identify the chemical form of metals in the
solid phase. Generally, the number of techniques for
collecting mineralogical data is limited by the poor
cristallinity of the precipitates and the relatively low
concentrations of metal sulfides (Machemer et al., 1993;
Gibert et al., 2005). Scanning electron microscopy has
been the most successful technique, whereas X-ray
diffraction or Mossbauer analyses have been less
effective in detecting amorphous metal sulfides
(Machemer et al., 1993). Additional work is needed to
accurately assess the various metal removal mechanisms
occurring in passive bioreactors.
4 DESIGN OF PASSIVE BIOREACTORS
Passive bioreactors are designed to be operated in
“downflow” mode (inflow is through the reactor top) or in
“upflow” mode (inflow is through the reactor bottom). The
most successful design allows the flow to be reversed
between modes (URS Report, 2003).
Vertical flow bioreactors have been used in numerous
laboratory and field studies (Dvorak et al., 1992;
Tsukamoto and Miller, 1999; Chang et al., 2000; Tassé et
al., 2003; Tsukamoto et al., 2004; Johnson and Hallberg,
2005b; Kuyucak et al., 2006).
Recently, flow in a horizontal plane was reported in a field
study (Zaluski et al., 2003).
A three-step system separating SRB activity from metal
precipitation units and from a pH control system was also
proposed at the laboratory scale (Prasad et al., 1999).
The flow pattern can affect both the transport of metals
and their interaction with the substrate. Bioreactors with
vertical flow may show preferential channels of influent
AMD percolating through the reactive mixture. The
upward flow bioreactors tend to last longer because
upward flow limits compaction and preferential flow paths
(URS Report, 2003). However, release of metals is a
potential problem especially in upflow bioreactors
because the treated water will flush metals released
during oxidation in the upper substrate as it leaves the
bioreactor (URS Report, 2003). A horizontally oriented
bioreactor using a mixture of cow manure and cut straw
did not show preferential flow patterns during a 32-month
field operation period (Zaluski et al., 2003). This
configuration seems more promising, whereas the threestep
process requires higher maintenance costs.
The more successful designs allow the flow to be
reversed between modes (URS Report, 2003).
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5 EXISTING PILOT AND FIELD-SCALE PASSIVE
BIOREACTORS
First generation bioreactors generally use substrates
consisting of composted animal manure or mushroom
compost because they provide significant alkalinity
(Dvorak et al., 1992, URS Report 2003). New generation
of bioreactors use a combination of limestone, sawdust,
and alfalfa instead of animal manure because it provides
alkalinity, a significantly higher hydraulic conductivity, and
appears to be a better energy source for bacterial
community (URS Report, 2003). Nevertheless, efficient
pilot and field-scale passive bioreactors usually contain
mixtures of both organic and cellulosic wastes (Gusek et
al., 1999; Reisinger et al., 2001; Resiman et al., 2003;
Tassé et al., 2003; Zaluski et al., 2003; Kuyucak et al.,
2006). They are briefly reviewed below.
5.1 Bioreactors in Northern Québec, Canada
Two field bioreactors were installed and successfully
operated on mine sites in Northern Québec, Canada. The
first was set up in 1999-2000 at Wood Cadillac, and the
second was built in 2004 at Cadillac Molybdenite.
Wood Cadillac biofilter for arsenic control
At the Wood Cadillac mine site, the tailings (450 000 t)
contain a low proportion of sulphide minerals (1% S) and
arsenic (a mean proportion of 3600 mg/kg) in the form of
arsenopyrite and arsenous pyrite (Tassé et al., 2003).
Even if the effluent from the site is not acidic (pH usually
> 5), the water has to be treated to removed metals
(mainly As). The biofilter (50x57 of area x1m deep) was
built in 1999-2000, with a substrate consisting of yellow
birch barks and with a design HRT (hydraulic retention
time) of 25 h (Isabel et al., 2000; Germain and Cyr, 2003).
Since spring 2000, a monitoring of affluent and effluent
quality was started. During the early stage of operation,
As removal was 90-95%, that corresponds to final
concentrations that respect maximal admissible values
(14µg/L), and sulphate concentrations was reduced to
values < 250mg/L at a pH of 5.5-6.5 (Germain and Cyr,
2003). The last data available show that the biofilter is still
efficient for alkalinity rising and for As removal (Libero,
2007).
Cadillac Molybdenite passive bioreactor
At Cadillac Molybdenite mine site, tailings cover an area
of about 600x400 and the depth varies between 4 and
15m (Kuyucak et al., 2006). A preliminary chemical
analysis of seepage indicated a low pH, high acidity, and
elevated metal concentrations. Consequently, during the
fall 2004, two bioreactors were set up for the collection
and treatment of contaminated waters at two locations.
The bioreactors used a substrate consisting of wood
chips, limestone, hay, and manure (Kuyucak et al., 2006).
The targeted HRT at the design stage was 5 days. The
operation was started during the fall 2004. After a very
cold winter the quality of the treated effluent respected
maximal admissible concentrations (Directive 019) for pH
and metals (Al, Cu, Fe, Ni, and Zn), while Mn removal
was less efficient (3-5.7 mg/L in the effluent compared to
5.8 mg/L in the affluent). Sulphates concentrations
decreased from 887 mg/L to values as low as 360 mg/L.
5.2 Bioreactors in USA
Little or no power, as well as infrequent maintenance
requirements are the preferred treatment technologies for
the remediation of mine tailings and AMD leachate in
USA, especially for abandoned mine sites, and for
extreme winter conditions (Reisman et al., 2003). As a
result, several passive bioreactors were built and
successfully operated for a few years (Gusek et al., 1999;
Reisinger et al., 2001; Resiman et al., 2003; Zaluski et
al., 2003).
Two anaerobic cells of 1930 m 3 each were filled with
reactive mixture made of composted cow manure,
sawdust, inert limestone, and alfalfa (Gusek et al., 1999).
They proved efficient for the treatment of mildly
contaminated neutral mine drainage from a former
underground lead-zinc mine. Thus, affluent
concentrations of Pb (0.4 mg/L) and of Zn (0.36 mg/L)
were reduced in treated effluent to less than 0.05 mg/L
and 0.088 mg/L, respectively.
Three other passive bioreactors were constructed in 1998
at an abandoned mine site (Butte, Montana) to treat AMD
(Zaluski et al., 2003). The substrate used consisted of
cow manure and cut straw; the design HRT is between
4.5 and 5.5 days. Performance of filled-bioreactors was
monitored over 32 months. Resulted reported indicate
that Zn, Cu, and Cd ions were transformed as sulfides,
due to SRB activity. Concentrations of 11.1 mg/L (Zn), 3
mg/L (Cu), and 0.05 mg/L (Cd) were removed by SRB
processes to a final threshold of

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conclusions indicated that precipitation of (oxy)hydroxides
carbonates minerals was responsible for early metal
removal. After 14 weeks of operation, sulphate reduction
was occurring at varying degrees and metal (Fe, Cu, Pb,
Ni, and Cd) and precipitation by this mechanism
appeared to predominate.
Affluent concentrations varied from 0.6 mg/L (Pb) to 20
mg/L (Fe and Zn), while effluent concentrations ranged
from 0.15 mg/L (Pb) to 10 mg/L (Fe). For Zn, after 6
weeks of operation effluent released up to 80 mg/L Zn.
Moreover, these four bioreactors successfully removed
concentrations of As up to2.5 mg/L to less than 0.5 mg/L.
However, SRB do not efficiently removed manganese.
A pilot-scale cylindrical cell (4.6x1.2m) was constructed in
1997 to treat two basic streams of mine water
contaminated by copper (from the former Ferris-Haggarty
mine, Wyoming). Bioreactor was filled with a mixture of
cattle manure, sawdust, limestone, hay, and alfalfa
(Reinsinger et al., 2000). The system was effective for
over two years and suggests the viability of removing
copper (from up to 25 mg/L to less than 1 mg/L) using
passive treatment techniques at near freezingtemperatures.
6 MAIN RESEARCH NEEDS
Some of the main research needs are presented in the
following. It is not, however, an exhaustive list. More
information on this aspect can be found in the review
presented by Neculita et al. (2007).
6.1 Long-term biodegradability of natural organic carbon
Different aspects need to be further investigated for a
better design and operation of on-site passive treatment
systems (Neculita et al., 2007). The depletion rate of
organic matter is a key problem. An improved methodical
analysis of natural organic substrates is warranted to
assess their ability to promote sulphate-reduction and
metal removal. Anaerobic degradation of complex organic
carbon compounds to simpler molecules by other
microflora may limit the rate at which substrates become
available to SRB. More work must be conducted to
understand and differentiate the fundamental biochemical
and microbiological reactions that occur in anaerobic
bioreactors with complex natural organic substrates.
Bioreactors are recommended to be allowed to ”mature”
before fed with AMD, especially when recalcitrant
materials are included in the substrate to provide longterm
provision of organic carbon. After maturation
however, the amount of colloids and DOM in pore water
and within the effluent should be assessed. Metals bound
to DOM and colloids are highly mobile and can flow out of
the treatment system.
6.2 Ecotoxicological potential of treated water
Limited work has been done on the direct assessment of
the ecotoxicological potential of biologically treated AMD
waters (Neculita et al., 2007).
The first study that performed whole effluent toxicity
assays on undiluted wetland effluent was performed by
Song et al. (2001). In this study, a laboratory-scale
wetland was used to treat slightly alkaline (8.0-8.5)
synthetic lead mine drainage and synthetic lead smelter
wastewater. A significant toxicity decrease in wetland
effluent for all organisms studied and 100% survival of
fathead minnows and Daphnia magna was observed.
However, lethality of Ceriodaphnia dubia was 100% in an
undiluted effluent. Dilution of effluent to half strength
increased survival to 75-100%. Wetlands thus offer
encouraging promise for decreasing the toxicity of leadcontaminated
wastewater.
A second work (Riesen et al., 2005) focused on the
ecotoxicity of aqueous effluents of mining and
metallurgical operations. This study presents
comparatively results on effluents from chemical (lime
precipitation and sulphides precipitation) and biological
treatment (anaerobic bioreactor). Toxicity tests results
showed that effluent quality from biological treatment was
better that chemical treatment for water flea Daphnia
magna and green algae Pseudokirchneriella sucapitata
(formerly Selenastrum capricornutum).
Very recently, a third study (Libéro, 2007) performed an
ecotoxicological evaluation of treated effluent from a fieldscale
passive bioreactor (Wood Cadillac, Northern
Quebec). Two acute tests (mortality on rainbow trout and
Daphnia Magna) and four sublethal tests (growth and
survival on fathead minnows, reproduction and survival
on Ceriodaphnia dubia, and growth inhibition on algae
Pseudokirchneriella sucapitata and on plant Lemna
minor) were conducted. The biologically treated effluent
was not acutely toxic for any organisms tested. A low sub
lethal toxicity was found for Ceriodaphnia dubia for which
an IC25 of 17.4% (inhibition of reproduction) was found.
However, more tests need to be done in order to clearly
assess the toxicity of treated effluents by passive
bioreactors on a regular basis.
7 CONCLUSIONS
Sulphate-reducing passive bioreactors are reasonable
alternative technologies for AMD treatment on remote
sites, without power, and with extreme winter conditions.
The efficiency of sulphate-reducing passive bioreactors is
sometimes limited because they rely on SRB, which is
primarily controlled by the reactive mixture composition.
Our review shows that the most important mixture
component is the organic carbon source. Several studies
conducted to find the best mixture of natural organic
substrates for SRB showed that a combination of organic
sources is preferable than a unique source.
In addition to metal sulphide precipitation, which is the
main metal removal mechanism, other mechanisms
including adsorption and precipitation of metal carbonates
and hydroxides occur in passive bioreactors.
The more successful designs allow the flow to be
reversed between upward and downward modes.
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Pilot and field-scale passive bioreactors filled with a
mixture of organic and cellulosic wastes were installed in
Canada (Northern Québec) and USA and efficiently
removed sulphate and metals for periods up to 5 years.
Additional work needs to be done to properly assess the
long-term efficiency of reactive mixtures and the metal
removal mechanisms. Furthermore, toxicity assessment
of treated effluent from on-site passive bioreactors has
yet to be performed on a regular basis.
AKNOWLEDGEMENTS
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC)
through the industrial NSERC Polytechnique/ UQAT Chair
in environment and mine waste management, and the
chair’s industrial and governmental partners.
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