Thesis submitted in fulfilment of the requirements for the degree
Master of Technology: Chemical Engineering
in the Faculty of
Engineering
at the
Cape Peninsula University of Technology
2014 / Environmental legislation focusing on wastewater disposal in industries that utilise cyanide
and/or cyanide-related compounds has become increasingly stringent worldwide, with many
companies that utilise cyanide products required to abide by the Cyanide International Code
associated with the approval of process certifications and management of industries which
utilise cyanide. This code enforces the treatment or recycling of cyanide-contaminated
wastewater. Industries such as those involved in mineral processing, photo finishing, metal
plating, coal processing, synthetic fibre production, and extraction of precious metals, that is,
gold and silver, contribute significantly to cyanide contamination in the environment through
wastewater. As fresh water reserves throughout the world are low, cyanide contamination in
water reserves threatens not only the economy, but also endangers the lives of living
organisms that feed from these sources, including humans. In the mining industry, dilute
cyanide solutions are utilised for the recovery of base (e.g. Cu, Zn, Ni, etc.) and precious
metals (e.g. Au, Ag, etc.). However, for technical reasons, the water utilised for these
processes cannot be recycled upstream of the mineral bioleaching circuit as the
microorganisms employed in mineral bioleaching are sensitive to cyanide and its complexes,
and thus the presence of such compounds would inhibit microbial activity, resulting in poor
mineral oxidation. The inability to recycle the water has negative implications for water
conservation and re-use, especially in arid regions. A number of treatment methods have
been developed to remediate cyanide containing wastewaters. However, these chemical and
physical methods are capital intensive and produce excess sludge which requires additional
treatment. Furthermore, the by-products that are produced through these methods are
hazardous. Therefore, there is a need for the development of alternative methods that are
robust and economically viable for the bioremediation of cyanide-contaminated wastewater.
Biological treatment of free cyanide in industrial wastewaters has been proved a viable and
robust method for treatment of wastewaters containing cyanide. Several bacterial species,
including Bacillus sp., can degrade cyanide to less toxic products, as these microorganisms
are able to use the cyanide as a nitrogen source, producing ammonia and carbon dioxide.
These bacterial species secrete enzymes that catalyse the degradation of cyanide into
several end-products. The end-products of biodegradation can then be utilised by the
microorganisms as nutrient sources.
This study focused on the isolation and identification of bacterial species in wastewater
containing elevated concentrations of cyanide, and the assessment of the cyanide
biodegradation ability of the isolates. Thirteen bacterial isolates were isolated from
electroplating wastewater by suppressing the growth of fungal organisms and these species
were identified as species belonging to the Bacillus genus using the 16S rDNA gene. A
mixed culture of the isolates was cultured in nutrient broth for 48 hours at 37°C, to which FCN
as KCN was added to evaluate the species‟ ability to tolerate and biodegrade cyanide in
batch bioreactors. Subsequently, cultures were supplemented solely with agro-waste
extracts, that is, Ananas comosus extract (1% v/v), Beta vulgaris extract (1% v/v), Ipomea
batatas extract (1% v/v), spent brewer‟s yeast (1% v/v) and whey (0.5% w/v), as the primary
carbon sources. Owing to the formation of high ammonium concentration from the cyanide
biodegradation process, the nitrification and aerobic denitrification ability of the isolates,
classified as cyanide-degrading bacteria (CDB) was evaluated in a batch and pneumatic
bioreactor in comparison with ammonia-oxidising bacteria (AOB). Furthermore, the effects of
F-CN on the nitrification and aerobic denitrification was evaluated assess the impact of F-CN
presence on nitrification. Additionally, optimisation of culture conditions with reference to
temperature, pH and substrate concentration was evaluated using response surface
methodology. Using the optimised data, a continuous biodegradation process was carried
out in a dual-stage packed- bed reactor combined with a pneumatic bioreactor for the
biodegradation of F-CN and subsequent nitrification and aerobic denitrification of the formed
ammonium and nitrates.
The isolated bacterial species were found to be gram positive and were able to produce
endospores that were centrally located; using the 16S rDNA gene, the species were found to
belong to the Bacillus genus. The species were able to degrade high cyanide concentration
in nutrient broth with degradation efficiencies of 87.6%, 65.4%, 57.0% and 43.6% from 100
mg F-CN/L, 200 mg F-CN/L, 300 mg F-CN/L, 400 mg F-CN/L and 500 mg F-CN/L
respectively over a period of 8 days. Additionally, the isolates were able to degrade cyanide
in an agro-waste supported medium, especially in a medium that was supplemented with
whey which achieved a degradation efficiency of 90% and 60% from 200 mg F-CN/L and 400
mg F-CN/L, respectively over a period of 5 days. The nitrification ability of the isolates was
evaluated and the removal of NH4
+/NO3
- by the CDB and AOB in both shake flasks and
pneumatic bioreactor was determined to be pH dependent. The maximum NH4
+/NO3
-
removal evaluated over a period of 8 days for CDB and 15 days for AOB, observed at pH 7.7
in shake flasks, was 75% and 88%, respectively, in the absence of F-CN. Similarly, the
removal of NH4
+/NO3
- in a pneumatic bioreactor was found to be 97.31% for CDB and 92%
for AOB, thus demonstrating the importance of aeration in the designed process. The
nitrification by CDB was not inhibited by cyanide loading up to a concentration of 8 mg FCN/
L, while the AOB were inhibited at cyanide loading concentration of 1 mg F-CN/L. The
CDB removed the NH4
+/NO3
- in PBSs operated in a fed-batch mode, obtaining efficiencies
>99% (NH4
+) and 76 to 98% (NO3
-) in repeated cycles (n = 3) under F-CN (≤8 mg F-CN/L).
The input variables, that is, pH, temperature and whey-waste concentration, were optimised
using a numerical optimisation technique where the optimum conditions were found to be: pH
9.88, temperature 33.60 °C and whey-waste concentration 14.27 g/L, under which 206.53
mg CN-/L in 96 h can be biodegraded by the microbial species from an initial cyanide
concentration of 500 mg F-CN/L. Furthermore, using the optimised data, cyanide
biodegradation in a continuous mode was evaluated in a dual-stage packed-bed bioreactor
connected in series to a pneumatic bioreactor system used for simultaneous nitrification
including aerobic denitrification. The whey-supported Bacillus sp. culture was not inhibited by
the free cyanide concentration of up to 500 mg F-CN/L, with an overall degradation efficiency
of ≥99% with subsequent nitrification and aerobic denitrification of the formed ammoniu and
nitrates over a period of 80 days.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:cput/oai:localhost:20.500.11838/868 |
| Date | January 2014 |
| Creators | Mekuto, Lukhanyo |
| Publisher | Cape Peninsula University of Technology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | http://creativecommons.org/licenses/by-nc-sa/3.0/za/ |
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