Please use this identifier to cite or link to this item: https://hdl.handle.net/10321/3817
Title: Optimization of biomass and lipids production from microalgae using wastewater in a pilot scale raceway pond
Authors: Rawat, Ismail 
Issue Date: 2021
Abstract: 
Microalgae provide a sustainable renewable solution for the production of commodity products
such as liquid biofuels. There are numerous benefits to using algae for the production of biofuels,
however, the cost of production is a major hurdle to commercial-scale development. Major
factors influencing the production of algae are the cost of nutrients, availability of water,
contamination, and grazers. Research into algal biomass for biofuels production at laboratory
scale does not translate directly to cultivation at large scale due to the change in cultivation
conditions and the constant flux of environmental factors. This study focuses on the upstream
processes of cultivation of biomass in a ~ 1146 m2 raceway pond. It demonstrates biomass
productivity under different climatic conditions and utilisation of post-chlorinated wastewater as
a water and nutrient source. The study further elucidates the population dynamics of the system
and provides insight into the challenges faced during the cultivation of algae at large scale.
An indigenous Scenedesmus sp. gave biomass productivity of 31.23 g/m2
/d with lipid production of
29.6 % lipid/g DCW in a 10 m2
raceway pond in a greenhouse using BG11. Biomass productivity
was reduced to 13.09 g/m2
/d with a lipid content of 22.9 % lipid/g DCW under 3-fold higher
irradiance. Biomass productivity of circular 3000L ponds at the large scale site resulted in the
highest biomass and acceptable lipid content using 250mg/L NaNO3 although significantly lower
than the 10 m2
raceway ponds. Wastewater has shown potential to replace conventional media.
Post-chlorinated wastewater was found to have low levels of nitrogen and phosphorus but
contained metals that act as micronutrients for algae. Supplemented wastewater proved to be an
effective growth. Six individual runs of a covered 1146 m2
raceway pond driven by paddlewheel were conducted
over 15 months. The average water temperature ranged from 20.61±0.68°C during mid-winter
to 31.03±2.22°C in late summer. Daylight ranges from 10.25 to 14 hours in winter and summer
respectively. The highest average light intensity was 359.00±212.71 µmol/m2
/s from Mid-winter
to early spring and 645.44±330.58 µmol/m2
/s in late summer. Biomass productivities were low
ranging from 2.7 to 7.34 g/m2
/d for most runs of the raceway pond, mainly due to the long periods
of cultivation. Average productivity at day 7 for all raceway runs was 7.25 g/m2
/d. Adaptive
Neuro-Fuzzy Inference System (ANFIS) modelling of the system elicited that the major factors
affecting biomass productivity in the raceway pond were light intensity, pH, and depth for the
raceway pond. The model showed that maximum biomass productivity is possible at a depth
between 20 and 22 cm at light intensities between 200 and 400 µmol/m2
/s. pH in the range of 9
to 9.5 correlated positively with light intensity ranging from 200 to 1000 µmol/m2
/s with maximum
biomass expected in the region of 400 to 500 µmol/m2
/s.
The main algal constituents for the raceway ponds were Scenedesmus obliquus, Scenedesmus
dimorphus, Chlorella, Keratococcus, and species of unidentified cyanobacteria. Either Scenedesmus
or Chlorella was dominant for extended periods. Bacteria in open systems can have a positive or
negative effect on the growth of microalgae but is dependent on the strains of microalgae and
bacteria as well as prevailing conditions making these systems highly complex. Rhodobacteraceae,
Plactomycetaceae, Xanthomonadaceae, Flavobacteriaceae, Phycisphaeraceae, Comamonadaceae, and
Cyclobacteriaceae were found to be the major families of bacteria that proliferate at different levels
during the cultivation period in the circular ponds and the raceway pond. These families of bacteria have several beneficial traits to algae cultivation however further investigation is
required.
Modelling the system revealed that pH, depth, and light intensity were factors having a substantial
effect on biomass productivity. As the system was carbon limited addition of CO2 (preferably a
waste stream) could significantly enhance the overall biomass productivity. A major factor
negatively affecting biomass productivity was the size of the pond. Inadequate mixing impacts
biomass productivity in terms of access to nutrients and gaseous exchange. Shorter periods of
cultivation resulted in higher productivities. For the scale of the system, semi-continuous
harvesting would be required to achieve shorter residence time. This must be balanced against
the energy utilization and cost of harvesting potentially lower culture densities
Description: 
Submitted in fulfilment of the requirements of the degree of Doctor of Philosophy: Biotechnology in the Faculty of Applied Sciences at the Durban University of Technology, 2021.
URI: https://hdl.handle.net/10321/3817
DOI: https://doi.org/10.51415/10321/3817
Appears in Collections:Theses and dissertations (Applied Sciences)

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