Isolation and evaluation of autochthonous microalgae strains for biodiesel production and wastewater treatment.
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Date
2018
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Abstract
The current depletion of global fossil fuel reserves and increasing CO2 emission are generating global
climate change concerns hence, the increasing calls for the development of renewable biofuels with low
CO2 emission. Microalgae has been touted as the most suitable feedstock of renewable fuels due to a
number of reasons including high biomass productivity, fast growth rates, cost-effective cultivation
systems and alleviation of food security concerns associated with use of other feedstock for biofuel
production. Bioprospecting could yet reveal microalgae with high lipid contents for biodiesel
production and novel characteristics such as production of unique bioactive compounds. With an
attractive climate and plenty of sunlight all year round, South Africa is in a unique position to take
advantage of these organisms to guarantee her future energy needs. This research thus aims to bioprospect
for indigenous strains of microalgae in aquatic habitats and wastewater treatment plants in
KwaZulu-Natal province of South Africa and evaluate their potential for biofuel production and
wastewater remediation. DGGE and T-RFLP fingerprinting techniques were adapted to give an
overview of the diversity of microalgae in various aquatic habitats including wastewater treatment
plants (WWTP), rivers, estuaries and marine ecosystems in order to identify areas for potential
bioprospecting. Ten (10) microalgae species from these aquatic environments were isolated, identified
and evaluated for lipid, total carbohydrate and protein accumulation using well defined procedures. The
lipid profile, biodiesel characteristics as well as effect of starvation on lipid accumulation were also
determined. The biomass production and simultaneous phycoremediation potential of two
autochthonous isolates Asterarcys quadricellulare and Neochloris aquatica were also investigated.
Microalgae classes; Trebouxiophyceae and Chlorophyceae were observed to be dominant in fresh water
habitat, while those belonging to the class Ulvophycea (Oltmannseillopsis spp.) dominated the salt
water environment. At the brackish water habitat, microalgae belonging to the classes
Trebouxiophyceae (Chlorella minutissima) and Chlorophyceae (M. sturmi) were dominant. A shift in
community structure was observed at the maturation ponds of WWTPs and in rivers over time. The
growth rates of 10 isolated microalgae ranged from 0.219 ± 0.003 to 0.175 ± 0.023 gL-1day-1 while dry
weight ranged from 0.433 ± 0.208 to 1.167 ± 0.153 gL-1. Chlorococcum LM1 showed high
accumulation of lipid (11.93 ± 0.76 mg/L). The microalgae isolated in this study accumulated high
carbohydrate content ranging from 25 to 61 % of their dry weight while protein content ranged from
1.06 ± 0.1 mg/L to 1.39 ± 0.1 mg/L accounting for 21 to 28% dry weight. Lipid accumulation also
varied under nutrient limitation condition. Lipid accumulation was enhanced in some of the isolates
such as Chlorococcum sp. LM1 (17.2%) and C. sorokiniana NWS5 (24%) while a decrease was
observed in others such as C. minutissima TS9 (16%), N. aquatica Toti4 (3.5%) and Chlorococcum sp.
LM2 (6.5%). Response to enhanced lipid accumulation via starvation seem to be unique to each algal
strain irrespective of species. The lipid profile consisted mainly of saturated fatty acid such as oleic acid
(C18:1), palmitic acid (C16:0) and stearic acid (C18:0) with low amounts of polyunsaturated fatty acids
such as linoleic acid (C18:2 n-6). Characteristic of the biodiesel based on the lipid profile reveal a low
viscosity and density. The biodiesel was determined to be of good quality with high oxidation stability,
low viscosity and conformed to the ASTM guidelines. Low total phosphorus concentration in the
wastewater resulted in an unbalanced N:P ratio of 44 at the Northern wastewater treatment works
(NWWTW) and 4 at the Umbilo wastewater treatment works (UWWTW). Asterarcys quadricellulare
utilized the wastewater for growth and reduced the COD of the wastewater effluent from the UWWTW
by 12.4% in contrast to Neochloris aquatica which did not show any growth. Asterarcys quadricellulare
was able to utilise the wastewater achieving a growth rate up to 0.18 day−1 in sterilized wastewater from
the NWWTW and 0.17 day−1 in the unsterilized wastewater from UWWTW. Asterarcys quadricellulare
accumulated high biomass of 460 mg/L compared to 180 mg/L in Neochloris aquatica. Total nitrogen
(TN) and Phosphorus (TP) were reduced by 48% and 50% respectively by Asterarcys quadricellulare
cultivated in sterile wastewater from NWWTW while, Neochloris reduced the TP by 37% and TN by
29%. At the UWWTP, TP and TN were reduced by 32% and 44% respectively by Asterarcys
quadricellulare cultivated in sterile wastewater while 29% and 19% reduction were recorded in
Neochloris aquatica. The study showed the diversity and community structure of microalgae in aquatic
ecosystems in the study area. Autochthonous microalgae were rich in lipid, carbohydrate and protein
and could be applied for biofuel production. Wastewater effluent can be used to generate biomass for
biodiesel production while treating wastewater. However, optimization of the N:P ratio and carbon
source are necessary to improve remediation and biomass productivity for future commercial scale
production.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.