Cleaner Biofuel Production via Process Parametric Optimization of Nonedible Feedstock in a Membrane Reactor Using a Titania-Based Heterogeneous Nanocatalyst: An Aid to Sustainable Energy Development

Membrane technology has been embraced as a feasible and suitable substitute for conventional time- and energy-intensive biodiesel synthesis processes. It is ecofriendly, easier to run and regulate, and requires less energy than conventional approaches, with excellent stability. Therefore, the present study involved the synthesis and application of a highly reactive and recyclable Titania-based heterogeneous nanocatalyst (TiO2) for biodiesel production from nonedible Azadhiracta indica seed oil via a membrane reactor, since Azadhiracta indica is easily and widely accessible and has a rich oil content (39% w/w). The high free fatty acids content (6.52 mg/g KOH) of the nonedible oil was decreased to less than 1% via two-step esterification. Following the esterification, transesterification was performed using a heterogeneous TiO2 nanocatalyst under optimum conditions, such as a 9:1 methanol-oil molar ratio, 90 °C reaction temperature, 2 wt.% catalyst loading, and an agitation rate of 600 rpm, and the biodiesel yield was optimized through response surface methodology (RSM). Azadhiracta indica seed oil contains 68.98% unsaturated (61.01% oleic acid, 8.97% linoleic acid) and 31.02% saturated fatty acids (15.91% palmitic acid, 15.11% stearic acid). These fatty acids transformed into respective methyl esters, with a total yield up to 95% achieved. The biodiesel was analyzed via advanced characterization techniques like gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (NMR), whereas the catalyst was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and Fourier transform infrared spectroscopy (FT-IR). Due to its physicochemical properties, Azadirachta indica seed oil is a highly recommended feedstock for biodiesel production. Moreover, it is concluded that the Titania-based heterogeneous nanocatalyst (TiO2) is effective for high-quality liquid fuel synthesis from nonedible Azadirachta indica seed oil in a membrane reactor, which could be an optional green route to cleaner production of bioenergy, eventually leading to sustenance, robustness, and resilience that will aid in developing a holistic framework for integrated waste management.

Microalgal cultivation characteristics using commercially available air-cushion packaging material as a photobioreactor

Air-cushion (AC) packaging has become widely used worldwide. ACs are air-filled, dual plastic packaging solutions commonly found surrounding and protecting items of value within shipping enclosures during transit. Herein, we report on a laboratory assessment employing ACs as a microalgal photobioreactor (PBR). Such a PBR inherently addresses many of the operational issues typically encountered with open raceway ponds and closed photobioreactors, such as evaporative water loss, external contamination, and predation. Using half-filled ACs, the performance of microalgal species Chlorella vulgaris, Nannochloropsis oculata, and Cyclotella cryptica (diatom) was examined and the ash-free dry cell weight and overall biomass productivity determined to be 2.39 g/L and 298.55 mg/L/day for N. oculata, 0.85 g/L and 141.36 mg/L/day for C. vulgaris, and 0.67 g/L and 96.08 mg/L/day for C. cryptica. Furthermore, maximum lipid productivity of 25.54 mg/L/day AFDCW and carbohydrate productivity of 53.69 mg/L/day AFDCW were achieved by C. cryptica, while maximum protein productivity of 247.42 mg/L/day AFDCW was attained by N. oculata. Data from this work will be useful in determining the applicability and life-cycle profile of repurposed and reused ACs as potential microalgal photobioreactors depending upon the end product of interest, scale utilized, and production costs.

Seawater supplemented with bicarbonate for efficient marine microalgae production in floating photobioreactor on ocean: A case study of Chlorella sp

Cultivation of microalgae on ocean provides a promising way to produce massive biomass without utilizing limited land space, and using seawater as culture medium can avoid consumption of valuable fresh water. Bicarbonate is proved as a better approach for carbon supply in microalgae cultivation, but Ca²⁺ and Mg²⁺ in seawater is subjected to precipitate with carbonate derived from it. In this study, cultivation with this medium for a marine Chlorella sp. resulted in productivity of 0.470 g L^-1 day^-1, despite of continual precipitation caused by increased pH due to bicarbonate consumption. Actually, this precipitation is favorable, since it can work as a flocculation harvesting method for microalgae. The highest flocculation efficiency of 98.9 ± 0.0% was observed in cultures with 7.0 g L^-1 NaHCO₃, which was higher than that of cultures without bicarbonate (44.1 ± 0.2%). Additionally, the spent medium after flocculation supported better growth (1.60 ± 0.0 g L^-1) than the fresh medium (1.26 ± 0.0 g L^-1). Outdoor cultivation with floating photobioreactor on ocean resulted in the productivity of 0.190 g L^-1 day^-1, which was higher than that in land-based culture systems. The floating system also benefited from better temperature control with range from 20.6 to 37.2 °C, due to solar heating and surrounding water cooling. These results showed feasibility of efficient microalgae biomass production with fully utilizing of ocean resources, including culture medium preparation and temperature control with seawater, as well as wave energy for mixing, holding great potential to produce massive biomass to support sustainable development of human society.

New Surface Modification Method To Develop a PET-Based Membrane with Enhanced Ion Permeability and Organic Fouling Resistance for Efficient Production of Marine Microalgae

This paper presents a new surface modification strategy to develop a poly(ethylene terephthalate) (PET)-based membrane having a hydrophilic surface, high nutrient ion permeability, sufficient mechanical strength, and organic fouling resistance, using an anthracene (ANT)-attached polyethylene glycol (PEG) surface modification agent (SMA) synthesized in this work. During the modification process, the ANT parts of the SMAs poke through and anchor to the surface of a commercial PET woven fabric via physical interactions and mechanical locking. The PEG chain parts coat the surface in the brush and arch forms, which generates a hydration layer on the fabric surface. The consequently obtained surface property and unique structure of the modified PET-based membrane result in higher nitrate ion permeability, organic fouling resistance, and microalgae production compared to those of the unmodified one. These are also affected by the molecular weight of the PEG and the number density of the anchored SMAs. The study demonstrates that this new surface modification method has the potential to allow the development of a desirable PET-based membrane for the efficient massive production of marine microalgae.

Hydrodynamic performance of floating photobioreactors driven by wave energy

BACKGROUND

Unlike conventional cultivation systems, liquid mixing in floating photobioreactors (PBRs) is solely induced by their hydrodynamic movement in response to waves, and this movement is affected by the wave conditions (wave height and wave period), the PBR configuration and the culture depth. However, to the best of our knowledge, a practical study of the hydrodynamic movements of PBRs has not been previously conducted.

RESULTS

This study aims to investigate the hydrodynamic performance of floating PBRs in response to wave conditions. First, the effects of the experimental wave height (2-10 cm) and wave period (0.8-1.8 s) on movement was investigated using two 1.0 m2 PBR models: a square PBR (1.0 m/1.0 m; length/width) and a rectangular PBR (1.7 m/0.6 m). The results indicated that wave movement became not only more intense with increasing wave height, but also less intense when the wave period decreased. However, the square PBR experienced more intense movement than the rectangular PBR, but also little mooring force. The effects of culture depth (0.5, 1.0 and 2.0 cm) were investigated and the results showed that the culture depth significantly affected the hydrodynamic movements of the PBRs; however, the mooring forces were unaffected. Finally, the movement and mooring-line forces of PBRs equipped with different mooring systems were investigated. The use of two different mooring systems had little effect on PBR movement; however, a mooring system with floaters was able to significantly reduce the mooring line forces compared to a system without floaters. During this study, the greatest force (10.5 N) was found for the rectangular PBR using a mooring system without floaters, whereas the lowest force (0.67 N) was observed for a rectangular PBR using a mooring system with floaters.

CONCLUSIONS

These studies have provided basic data describing the fluid dynamics of floating PBRs; as well as their structural design and scale up. These results also provide guidance for the selection of ocean fields with suitable wave conditions; as well as a proper mooring methods to ensure safe operation.

Theoretical Calculations on the Feasibility of Microalgal Biofuels: Utilization of Marine Resources Could Help Realizing the Potential of Microalgae

Microalgae have long been considered as one of most promising feedstocks with better characteristics for biofuels production over conventional energy crops. There have been a wide range of estimations on the feasibility of microalgal biofuels based on various productivity assumptions and data from different scales. The theoretical maximum algal biofuel productivity, however, can be calculated by the amount of solar irradiance and photosynthetic efficiency (PE), assuming other conditions are within the optimal range. Using the actual surface solar irradiance data around the world and PE of algal culture systems, maximum algal biomass and biofuel productivities were calculated, and feasibility of algal biofuel were assessed with the estimation. The results revealed that biofuel production would not easily meet the economic break‐even point and may not be sustainable at a large‐scale with the current algal biotechnology. Substantial reductions in the production cost, improvements in lipid productivity, recycling of resources, and utilization of non‐conventional resources will be necessary for feasible mass production of algal biofuel. Among the emerging technologies, cultivation of microalgae in the ocean shows great potentials to meet the resource requirements and economic feasibility in algal biofuel production by utilizing various marine resources.

Membrane technology in microalgae cultivation and harvesting: a review

Membrane processes have long been applied in different stages of microalgae cultivation and processing. These processes include microfiltration, ultrafiltration, dialysis, forward osmosis, membrane contactors and membrane spargers. They are implemented in many combinations, both as a standalone and as a coupled system (in membrane biomass retention photobioreactors (BR-MPBRs) or membrane carbonation photobioreactors (C-MPBRs). To provide sufficient background on these applications, an overview of membrane materials and membrane processes of interest in microalgae cultivation and processing is provided in this work first. Afterwards, discussion about specific aspects of membrane applications in microbial cultivation and harvesting is provided, including membrane fouling. Many of the membrane processes were shown to be promising options in microalgae cultivation. Yet, significant process optimizations are still required when they are applied to enable microalgae biomass bulk production to become competitive as a raw material for biofuel production. Recent developments of the coupled systems (BR-MPBR and C-MPBR) bring significant promises to improve the volumetric productivity of a cultivation system and the efficiency of inorganic carbon capture, respectively.

Interactions of marine mammals and birds with offshore membrane enclosures for growing algae (OMEGA)

BACKGROUND

OMEGA is an integrated aquatic system to produce biofuels, treat and recycle wastewater, capture CO2, and expand aquaculture production. This system includes floating photobioreactors (PBRs) that will cover hundreds of hectares in marine bays. To assess the interactions of marine mammals and birds with PBRs, 9 × 1.3 m flat panel and 9.5 × 0.2 m tubular PBRs were deployed in a harbor and monitored day and night from October 10, 2011 to Janurary 22, 2012 using infrared video. To observe interactions with pinnipeds, two trained sea lions (Zalophus californianus) and one trained harbor seal (Phoca vitulina richardii) were observed and directed to interact with PBRs in tanks. To determine the forces required to puncture PBR plastic and the effects of weathering, Instron measurements were made with a sea otter (Enhydra lutris) tooth and bird beaks.

RESULTS

A total of 1,445 interactions of marine mammals and birds with PBRs were observed in the 2,424 hours of video recorded. The 95 marine mammal interactions, 94 by sea otters and one by a sea lion had average durations of three minutes (max 44 min) and represented about 1% of total recording time. The 1,350 bird interactions, primarily coots (Fulica americana) and gulls (Larus occidentalis and L. californicus) had average durations of six minutes (max. 170) and represented 5% of recording time. Interactive behaviors were characterized as passive (feeding, walking, resting, grooming, and social activity) or proactive (biting, pecking, investigating, and unspecified manipulating). Mammal interactions were predominantly proactive, whereas birds were passive. All interactions occurred primarily during the day. Ninety-six percent of otter interactions occurred in winter, whereas 73% of bird interactions in fall, correlating to their abundance in the harbor. Trained pinnipeds followed most commands to bite, drag, and haul-out onto PBRs, made no overt undirected interactions with the PBRs, but showed avoidance behavior to PBR tethers. Instron measurements indicated that sea-otter teeth and gull beaks can penetrate weathered plastic more easily than new plastic.

CONCLUSIONS

Otter and bird interactions with experimental PBRs were benign. Large-scale OMEGA systems are predicted to have both positive and negative environmental consequences.

Potential impact of biofouling on the photobioreactors of the Offshore Membrane Enclosures for Growing Algae (OMEGA) system

The influence of PBR composition [clear polyurethane (PolyU) vs. clear linear low-density polyethylene (LLDPE) (top) and black opaque high-density polyethylene (bottom)] and shape (rectangular vs. tubular) on biofouling and the influence of biofouling on algae productivity were investigated. In 9-week experiments, PBR biofouling was dominated by pennate diatoms and clear plastics developed macroalgae. LLDPE exhibited lower photosynthetic-active-radiation (PAR) light transmittance than PolyU before biofouling, but higher transmittance afterwards. Both rectangular and tubular LLDPE PBRs accumulated biofouling predominantly along their wetted edges. For a tubular LLDPE PBR after 12 weeks of biofouling, the correlation between biomass, percent surface coverage, and PAR transmittance was complex, but in general biomass inversely correlated with transmittance. Wrapping segments of this biofouled LLDPE around an algae culture reduced CO2 and NH3-N utilization, indicating that external biofouling must be controlled.