Phycoremediation of industrial and municipal domestic wastewaters with concomitant biomass propagation for bioenergy production.

Abstract

The utilization of microalgae has been endorsed as a great source of biofuel generation and wastewater reclamation without any adverse effects. Microalgae have high growth rates, efficient photosynthesis process and biomass productivity which serve as an economic advantage. Microalgae can be used for the dual purpose of biodiesel production and wastewater treatment due to their ability to sequester organic pollutants such as nitrogen and phosphorus in wastewater. Thus, the aim of this study was to bioprospect for indigenous hyper lipid producing indigenous microalgal strain for biofuel production and wastewater treatment. Different water samples were collected from diverse aquatic habitats, including freshwater, brackish and marine water in KwaZulu-Natal, South Africa. Eight indigenous microalgal strains were isolated and screened for biomass accumulation and lipid yield using Nile red fluorescence microscopy screening and gravimetric analysis. The strains were identified based on their morphological characteristics and 18S rRNA gene sequence analysis to belong to the genera Chlorella, Neochloris and Chlamydomonas. They showed high lipid yield ranging from 14 ± 6.2 to 38 ± 8.8% dcw, proving to be a good feedstock for biodiesel production. Of the eight isolated microalgae, Chlorella sp. T4 was selected for further analysis based on the growth kinetic, lipid productivity and fatty acid profiles. The strain was subjected to different cultivation conditions to enhance lipid productivity by varying nitrogen and phosphorus concentration. A significant decrease in biomass accumulation and low quantum efficiency of photosystem (Fv/Fm) value was observed under nitrogen and phosphorus limiting conditions. The lowest biomass yield of 0.58 ± 0.03 g L-1 was found in nitrogen limiting medium (0.75 g L-1). High lipid productivity of 15.54 ± 0.7 mg L-1 d-1 was obtained under nitrogen limiting condition which was 1.37- fold higher than phosphorus limiting (0.02 g L-1) condition after 21 days. Nutrient stress caused an increase in the expression of Acetyl-coenzyme A carboxylase carboxyl transferase subunit beta (accD), ketoacylACP synthase-1 (KAS-1), omega-6 desaturase (ω-6 FAD) and omega-3 desaturase (ω-3 FAD) genes responsible for lipid biosynthesis. Whereas a decrease in Ribulose bisphosphate carboxylase large chain (rbcL) gene expression level was noted due to nutrient stress lowering the photosynthetic rate. Fatty acid methyl esters produced - under nutrient limiting conditions were found to be suitable for the production of high-quality biodiesel with enhanced oxidative stability and cold flow properties. The ability of Chlorella sp. T4 to utilize the different nutrient-rich environments and remove nutrients from poultry and dairy wastewater was investigated to ascertain its possible use for the sustainable and low-cost treatment of wastewater. Chlorella sp. T4 showed high nitrogen and phosphorus removal efficiency of 85 to 95% and 35 to 93%, respectively. This was followed by a high biomass yield of 1.28 g L-1 and 0.85 g L1 obtained using raw poultry and dairy sludge wastewater, respectively. The biomass contained significant iv amounts of lipids (16.2–25.7 % dry wt.), carbohydrates (20.7–33.1 % dry wt.), and proteins (24.5–34.6 % dry wt.), regardless of the wastewater type. Biodiesel properties of lipids extracted from the cell grown in poultry and dairy wastewater complied with most of the international standards by ASTM D6751 and EN 14,214. Based on the lipid productivity and fatty acid profile, sludge dairy wastewater was used for biomass propagation for simultaneous bioethanol and biodiesel production. Optimization of cell disruption and extraction techniques resulted into high lipid and sugar recovery efficiency. Through acid hydrolysis using sulphuric acid, 2.14 g L-1 of sugar was recover from Chlorella sp. T4 biomass and fermented to ethanol (0.81 g L-1) using Saccharomyces cerevisiae. High lipid content of 21.7 ± 0.5% was recovered from the residual biomass after hydrolysis and converted into biodiesel via transesterification. The biodiesel produced from the residual biomass meets most of the standards specified by ASTM D6751 and EN 14214. In conclusion, hyper lipid producing microalgal strain Chlorella sp. T4 isolated from KwaZulu-Natal, South Africa showed potential for biofuel production after proper optimization of growth conditions. The potential of Chlorella sp. T4 to utilize different wastewater high in nutrient concentration confirm potential application during large scale cultivation for biofuel production to address energy crisis and water shortage.