Potential of sustainable bioenergy production from Synechocystis sp. cultivated in wastewater at large scale – A low cost biorefinery approach

Currently, the most important barrier in algal cultivation is growth nutrients cost. In the present study, the isolated alga Synechocystis PCC6803 was acclimatized under laboratory conditions using municipal wastewater and taken for large-scale cultivation in an open raceway pond. At the semi-continuous mode, the alga produced the highest biomass yield of 0.21 g L⁻¹ d⁻¹ and it was estimated that the algae could be able to produce 94.5 tones dry biomass ha⁻¹ year⁻¹. A low-cost study was conducted to harvest the maximum biomass recovery. The result shows that a combined use of iron (III) chloride and biopolymer enhanced the harvesting process and the maximum biomass recovery was achieved up to 98.7%. In addition, the biomass production cost was thoroughly explored, and it was estimated that the microalgae Synechocystis biomass production cost was approximately 2–3 US$ per kg of biomass. Meanwhile, to reduce the lipid extraction cost, a direct transesterification was performed using tungstated zirconia as a heterogeneous catalyst and the highest biodiesel yield of 90.5% was obtained. In addition, the biodiesel fuel properties were analyzed, and the results revealed that most of the fuel properties found within ASTM D6751 limits. Furthermore, the lipid extracted residues was explored for bioethanol extraction, and 0.186 g bioethanol/g residue was obtained. This is the first study to demonstrate the alga Synechocystis cultivation in an open raceway pond using municipal wastewater successfully and produced two potential biofuels.