Challenges in microalgal biofuel production: A perspective on techno economic feasibility under biorefinery stratagem.

Rapidly exhausting fossil fuels combined with the ever-increasing demand for energy led to an ongoing search for alternative energy sources to meet the transportation, manufacturing, domestic and other energy demands of the grown population. Microalgae are at the forefront of alternative energy research due to their significant potential as a renewable feedstock for biofuels. However, microalgae platforms have not found a way into industrial-scale bioenergy production due to various technical and economic constraints. The present review provides a detailed overview of the challenges in microalgae production processes for bioenergy purposes with supporting techno-economic assessments related to microalgae cultivation, harvesting and downstream processes required for crude oil or biofuel production. In addition, biorefinery approaches that can valorize the by-products or co-products in microalgae production and enhance the techno-economics of the production process are discussed.

Carbon streaming in microalgae: extraction and analysis methods for high value compounds.

There is a growing recognition that carbon-neutral biofuels and microalgae are eco-friendly options because of their high CO2 sequestering capability and ability to grow in wastewater/sea water and non-arable land. Also the intrinsic properties of microalgal systems can be exploited for high value compounds such as carbohydrates, lipids, pigments and proteins. This article provides a comprehensive review of various microalgae cultivation practices utilizing organic and inorganic carbon sources. The merits and demerits of the various extraction and analytical procedures have also been discussed in detail.

Closed circuitry operation influence on microbial electrofermentation: Proton/electron effluxes on electro-fuels productivity.

A novel biocatalyzed electrofermentor (BEF) was designed which uncovers the intricate role of biocatalyst involved in cogeneration of electro-fuels (hydrogen and electricity). The specific role of external resistance (Rext, electrical load) on the performance of BEF was evaluated. Four BEFs were operated separately with different resistances (25, 50, 100 and 200 Ω) at an organic load of 5 g/L. Among the tested conditions, external resistance (R3) with 100 Ω revealed maximum power and cumulative H2 production (148 mW and 450 mL, respectively). The competence of closed circuitry comparatively excelled because it facilitates congenial ambiance for the enriched EAB (electroactive bacteria) resulting high rate of metabolic activity that paves way for higher substrate degradation and electro-fuel productivity. Probing of electron kinetics was studied using voltammetric analyses wherein electron transfer by redox proteins was noticed. The designed BEF is found to be sustainable system for harnessing renewable energy through wastewater treatment.

Temperature induced stress influence on biodiesel productivity during mixotrophic microalgae cultivation with wastewater.

The role of operating temperature as a physical stress factor for enhancing lipid induction during microalgae cultivation with domestic wastewater was evaluated. Experiments were designed with dual mode microalgae cultivation viz., growth phase (GP) and temperature induced stress phase (25 °C, 30 °C and 35 °C). GP showed enhancement in biomass growth and carbohydrate accumulation while stress phase (SP) operation at 30 °C showed noticeable improvement in lipid productivities (total/neutral lipid, 24.5/10.2%). Maximum carbohydrate utilization was observed during SP at 30 °C operation (57.8%) compared to 25 °C (50.6%) and 35 °C (26.9%) correlating well with the lipid synthesis. Interestingly the neutral lipid content documented five-fold increment illustrating feasibility towards good biodiesel properties. Biodiesel profile at 30 °C temperature is well supported by higher saturated fatty acids (SFA) to unsaturated fatty acids (USFA) ratio. GP operation showed good COD and nutrient removal concomitant to the biomass growth.

Relative effect of different inorganic acids on selective enrichment of acidogenic biocatalyst for fermentative biohydrogen production from wastewater.

The effect of different inorganic acids viz., HNO3, HCl, H2SO4 and H3PO4 on inoculum pretreatment to selectively enrich hydrogen (H2) producing acidogenic bacteria was evaluated in anaerobic sequencing batch bioreactors. Relative positive efficiency of HNO3 pretreated consortia in enhancing H2 production (11.85 mol H2/kg CODR) was noticed compared to other acids (HCl, 5.64 mol H2/kg CODR; H2SO4, 7.65 mol H2/kg CODR; H3PO4, 6.90 mol H2/kg CODR) and untreated-parent consortia (control, 6.80 mol H2/kg CODR). On the contrary, substrate degradation (COD removal) was higher with the control operation (ξCOD, 66.3%; substrate degradation rate (SDR), 1.42 kg CODR/m(3)-day) compared to pre-treated culture. HNO3 pre-treatment resulted in a shift in the fermentation pathway towards more acetic acid production, while other acid pretreatment and untreated culture showed mixed type fermentation (acetic, butyric, propionic acids). The bio-electrochemical analysis and dehydrogenase activity supported the biocatalyst performance after HNO3 pretreatment with specific enrichment of Firmicutes and Bacillus.

Microalgae mediated bio-electrocatalytic fuel cell facilitates bioelectricity generation through oxygenic photomixotrophic mechanism.

Electrogenic activity of oxygenic photo-bioelectrocatalytic fuel cell (PhFCOX) under mixotrophic mode was evaluated using atmospheric CO2 and domestic wastewater as carbon sources for harnessing bioelectricity with mixed microalgae as anodic biocatalyst. PhFCOX operation showed good electrogenic activity (3.55 µW/m(2)) associated with higher biomass growth (2.87 g/l) and chlorophyll content (5.12 mg/l). Electrogenic activity was relatively higher during the day time (46 mV; 0.6 mA) compared to the night (6 mV; 0.01 mA). Performance of PhFCOX undergoing oxygenic photosynthesis (DO; 3.5 mg/l) was compared with the mixotrophic fuel cell (PhFCAX) with photosynthetic bacteria as biocatalyst under anoxygenic conditions (DO; 0.45 mg/l). The dissolved oxygen produced during photolysis of water in oxygenic photosynthesis is a major limiting factor affecting the electrogenic activity. Voltammetric and amperometric analysis along with electron transfer kinetics (Tafel analysis) supported the bio-electrochemical behavior of PhFCOX and PhFCAX.

Mixotrophic operation of photo-bioelectrocatalytic fuel cell under anoxygenic microenvironment enhances the light dependent bioelectrogenic activity.

Electrogenic activity of photo-bioelectrocatalytic /photo-biological fuel cell (PhFC) was evaluated in a mixotrophic mode under anoxygenic microenvironment using photosynthetic consortia as biocatalyst. An acetate rich wastewater was used as anolyte for harnessing energy along with additional treatment. Mixotrophic operation facilitated good electrogenic activity and wastewater treatment associated with biomass growth. PhFC operation documented feasible microenvironment for the growth of photosynthetic bacteria compared to algae which was supported by pigment (total chlorophyll and bacteriochlorophyll) and diversity analysis. Pigment data also illustrated the association between bacterial and algal species. The synergistic interaction between anoxygenic and oxygenic photosynthesis was found to be suitable for PhFC operation. Light dependent deposition of electrons at electrode was relatively higher compared to dark dependent electron deposition under anoxygenic condition. PhFC documented for good volatile fatty acids removal by utilizing them as electron donor. Bioelectrochemical behavior of PhFC was evaluated by voltammetric and chronoamperometry analysis.

Biodiesel production from isolated oleaginous fungi Aspergillus sp. using corncob waste liquor as a substrate.

The study documented the potential of isolated filamentous fungus Aspergillus sp. as whole cell biocatalyst for biodiesel production using Sabourauds dextrose broth medium (SDBM) and corncob waste liquor (CWL) as substrates. SDBM showed improvement in both biomass production (13.6 g dry weight/1000 ml) and lipid productivity (23.3%) with time. Lipid extraction was performed by direct (DTE) and indirect (IDTE) transesterification methods. DTE showed higher transesterification efficiency with broad spectrum of fatty acids profile over IDTE. CWL as substrate showed good lipid productivity (22.1%; 2g dry biomass; 48 h) along with efficient substrate degradation. Lipids derived from both substrates depicted high fraction of saturated fatty acids than unsaturated ones. Physical characteristics of fungal based biodiesel correlated well with prescribed standards. CWL derived biodiesel showed relatively good fuel properties (acid number, 0.40 mg KOH/g of acid; iodine value, 11 g I2/100 g oil; density, 0.8342 g/cm³) than SDBM derived biodiesel.