Environmental Engineering Reference
In-Depth Information
metal contamination is that these organisms employ metabolic processes to create
detoxifying chemical intermediates from simple nutrients. This can greatly reduce
cost while maintaining and even enhancing the rate of remediation over that of
physicochemical processing [104]. Summaries of the few metal bioremediation
systems that exist are provided below. These provide a basis of comparison with
the anaerobic metal-sulfide bioremediation technology proposed at the end of this
section.
3.8.1 Packed-Bed Bioreactor
Wagner-Dobler [105] developed a packed-bed bioreactor in which elemental mer-
cury produced by bacterial mercury reductase enzymes collects outside of the
bacterial cells. Mercury is a liquid at 22
◦
C and virtually insoluble in water. These
researchers prevented its contact with air to remove any potential volatilization of
Hg(0) so that droplets of mercury form [106]. Their packed-bed bioreactor accu-
mulates mercury from waste water as it is converted into Hg(0) inside the reactor
[105-107]. A biofilm of mercury reducing bacteria formed on the packed bed com-
posed of inert carrier material (e.g., siran or pumice granules). Wastewater amended
with nutrients to feed the bacteria was passed through the bed in an up-flow mode
such that there was a hydraulic retention time of 15-60 min. This bioreactor was
effective on both synthetic mercury chloride solutions and chlor-alkali cell waste
water [106, 108, 109]. Recovery values were between 93 and 100% [105].
The major drawbacks of this system is that the treatment rate is severely limited
by (1) the limited flow rates needed to prevent volatilization of Hg(0), (2) by efflu-
ent mercury concentration from the source, and (3) the mercury collected must be
removed by distillation, a dangerous and expensive process.
3.8.2 Other Metal Bioremediation Systems
At present very few other bioreactor systems have been developed beyond the exper-
imental stage for heavy metal bioremediation. These include (1) the Homestake
Mine (Lead, South Dakota) in which zinc and copper are adsorbed to microbial
biomass, (2) the sulfur reducing bacterial systems such as Thipaq at the Budelco zinc
refinery in the Netherlands, (3) Metex anaerobic sludge reactor in Linde, Germany,
and (4) the Bio-Substrat anaerobic micro-carrier reactor, also in Germany [105].
Absorption to microbial biomass has been implemented by the Homestake Mine
(Lead, South Dakota) for the removal of zinc and copper from mining runoff.
This process exploits the ability of biomass to absorb metals [110]; i.e. inactivated
biomass acts as a matrix to which ionic metals adhere. After the matrix has cap-
tured its capacity of metals, these can then be washed off by changing the liquid
conditions such as lowering of the pH to remove the metal ions. The process can
