Microalgae dewatering is a major obstruction to industrial-scale processing of microalgae for biofuel prodn. The dil. nature of harvested microalgal cultures creates a huge operational cost during dewatering, thereby, rendering algae-based fuels less economically attractive. Currently there is no superior method of dewatering microalgae. A technique that may result in a greater algal biomass may have drawbacks such as a high capital cost or high energy consumption. The choice of which harvesting technique to apply will depend on the species of microalgae and the final product desired. Algal properties such as a large cell size and the capability of the microalgae to autoflocculate can simplify the dewatering process. This article reviews and addresses the various technologies currently used for dewatering microalgal cultures along with a comparative study of the performances of the different technologies.

Dewatering of microalgal cultures : a major bottleneck to algae-based fuels

Microalgae has been identified as a potential feedstock for biodiesel production since its cultivation requires less cropland compared to conventional oil crops and the high growth rate of microalgae. Research on microalgae oils often are focused on microalgae oil extraction and biomass harvesting techniques. However, energy intensive and costly lipid extraction methods are the major obstacles hampering microalgae biodiesel commercialisation. Direct biodiesel synthesis avoids such problems as it combines lipid extraction techniques and transesterification into a single step. In this review, the potential of direct biodiesel synthesis from microalgae biomass was comprehensively analysed. The various species of microalgae commonly used as biodiesel feedstock was critically assessed, particularly on high lipid content species. The production of microalgae biodiesel via direct conversion from biomass was systematically discussed, covering major enhancements such as heterogeneous catalysts, the use of ultrasonic and microwave- techniques and supercritical alcohols that focus on the overall improvement of biodiesel production. In addition, this review illustrates the cultivation conditions for biomass growth and lipid productivity improvement, the available harvesting and lipid extraction technologies, as well as the key challenges and future prospect of microalgae biodiesel production. This review serves as a basis for future research on direct biodiesel synthesis from modified microalgae biomass to improve profitability of microalgae biodiesel.

Sustainability of direct biodiesel synthesis from microalgae biomass: a critical review.

Microalgae have been widely explored because of the diverse number of their worthwhile applications and potential as a source biomass for the production of biofuels and value-added materials. However, downstream techniques have yet to be fully developed to overcome techno-economic barriers. Flocculation is a superior method for harvesting microalgae from growth medium because of its harvesting efficiency, economic feasibility. Various kind of bio-flocculation harvesting methods are consider as attractive low cost and environmentally friendly options and able to harvest >90% biomass. Lipid recovery from microalgal cells is a major barrier for the biofuel industry because of process complexity and algae cell structure. Thus, the pretreatment method is necessary to disrupt the cell walls of microalgae and enhance lipid extraction. Many techniques, including dry methods of extraction, are already being implemented but found out that they are not efficient and cost-effective. Various new wet harvesting strategies have been claimed to extract major lipids in cost-efficient (30% less than conventional) way as wet technologies can eliminate the cost of cell drying and associated instruments. It is necessary to develop new methods which are energy and cost-effective, and environmentally friendlier for the commercialization of biofuels. Therefore, this review presents the advances in the progress of various flocculation harvesting methods with special emphasis on innovative bio-flocculation, the underlying mechanism of microalgae and flocculation. In this study also summarize the recent progress on microalgal oil extraction processes, and comparison was made between the processes in terms of sustainability, technology readiness, and applications in larger scales.

Modern developmental aspects in the field of economical harvesting and biodiesel production from microalgae biomass

Biodiesel production from algal biomass is widely considered a sustainable alternative to petroleum fuels, especially in the transportation sector. However, the high energy consumption associated with the harvesting phase poses a significant impediment to the substantial commercialization of algal biodiesel. Centrifugation is among those harvesting methods that have a high efficiency close to 100% but can consume more energy than it produces. This review aims to address possible solutions for the issue of the high energy cost of centrifugation as an efficient harvesting technique. In addition, a detailed overview of the potential types of centrifugation that can be used to harvest microalgae with their pros and cons is provided. The most prevalent centrifugal devices that have been identified to be effectively useful to harvest algal biomass include disc-stack centrifuge, solid-bowl centrifuge, hydrocyclone, tubular centrifuge, and multi-chamber centrifuge. Overall, the analysis showed that centrifugation alone is not suitable for the harvesting of microalgae for biodiesel production. Nevertheless, coupling centrifugation with other harvesting technologies, e.g. with flocculation, as a primary pre-concentration step, can increase the energy output significantly thus, reducing the production cost.

Harvesting of microalgae by centrifugation for biodiesel production: A review

Energy is indispensable to human existence and sustains economic and individual activities globally. The rising environmental challenges of global warming and other climate changes due to increasing dependence on fossil fuels (coal, petroleum, natural gas) as the primary energy source requires innovation away from carbon-based fossil fuel sources. Hence, the development of renewable and low-emission energy sources becomes imperative to guarantee sustainable economic and human population growth and fuel security. In this context, microalgae biofuels are increasingly appreciated as viable alternative to fossil-based fuels because their biomass can be transformed into various clean fuel commodities such as biodiesel, biogas, and bioethanol in an environmentally-friendly and sustainable manner. However, high energy and financial costs of harvesting microalgal biomass for biofuel production constitute a substantial bottleneck in the industry. Bio-based flocculation methods have recently received increased research attention due to their high efficiency, sustainability, and environmentally-friendly attributes. In this review, the feasibility and performance of bio-based technologies for large-scale, cost-effective and sustainable harvesting of microalgal biomass for biofuel production were evaluated. The review shows that the harvesting efficiencies of bio-based processes are not significantly different from those achieved with energy-intensive traditional biomass separation methods such as centrifugation and membrane filtration. Harvested biomass met the moisture content metrics for biofuel feedstocks. The utilization of natural coagulants in algal biomass recovery is associated with low energy consumption, low environmental impacts and reduced effect on the chemistry of recycled medium. Production of recombinant bioflocculants are emerging but the application in algal biomass harvesting is yet to be reported. Bioflocculation approaches provide opportunities for the integration of photosynthetic biorefineries with bioremediation of wastewaters, and this enhances the potential of microalgae biotechnology for biofuel production.

Bio-based flocculants for sustainable harvesting of microalgae for biofuel production. A review

Mass harvesting of marine microalgae using different techniques

Harvesting, a challenging step of microalgae processing appears to be promising using flocculation method by the addition of tannin based natural biopolymer, AFlok-BP1. In this study, marine microalgae Nannochloropsis sp. was flocculated using AFlok-BP1 to evaluate its effectiveness in terms of flocculation efficiency, pH and dosages used, zeta potential, floc morphology characterization with comparison to the uses of aluminium sulphate and alkaline-induced pH. Oleaginous marine microalgae Nannochloropsis sp. was found to work effectively using AFlok-BP1 at an acidic pH 5 with more than 90% efficiency obtained at 160 mg/L. The finding was further supported with great instability of microalgae surface charge of -0.33 ± 0.27 mV, hence maximum destabilization of microalgae was believed to occur. The use of alum also effective to harvest Nannochloropsis sp. at neutral pH 7 with comparable efficiency as AFlok-BP1, but high dosage needed for more than 300 mg/L was at major concerns. Similarly, improved efficiency was observed for alkaline-induced pH for almost 100% efficiency. This would provide a better alternative for marine microalgae flocculation however, high concentration of minerals in the harvested biomass due to pH increase in need of further evaluation. To summarize, AFlok-BP1 would be a viable option to harvest marine microalgae Nannochloropsis sp. for the production of valuable compounds as it is an environmental friendly flocculant and provide no potential toxicity to the harvested biomass.

Effective use of tannin based natural biopolymer, AFlok-BP1 to harvest marine microalgae Nannochloropsis sp.

The production of high-value bioproducts from microalgae biomass has been widely investigated However, their 
production is hindered by the expensive harvesting process To date, flocculation followed by DAF process has been 
accepted as one of the affordable harvesting approaches In this study, the use of DAF technique was attempted to 
harvest marine microalgae Nannochloropsis sp Batch DAF harvesting was carried out using fabricated DAF unit 
equipped with several compartments including separation column, product collecting vessel and rotary skimmer 
Tannin-based biopolymer flocculant, AFlok-BP1 at pH 5 with a concentration of 160 mg/L was used to facilitate the 
flocculation of particles The effects of different saturator pressure at 18, 2, and 22 bar were then evaluated at a 
constant volume of 6 L microalgae culture The effects of different microalgae culture volumes (6, 8 and 10 L) were 
also evaluated at a fixed saturator pressure of 22 bar The highest pressure at 22 bar yielded the best result with the 
highest total solid of 319 ± 001% and a maximum yield of 170 ± 005 g/g (wet basis) The microalgae concentration 
was the lowest (0027 g/L) when 6 L of culture volume was used However, the values were significantly higher when 
the culture volume was increased to 8 and 10 L to approximately 0035 and 0050 g/L, respectively As a conclusion, 
the study provided evidence for the feasibility of DAF technique in harvesting marine microalgae Nannochloropsis sp

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Nurafifah Fuad

Papers

Mass harvesting of marine microalgae using different techniques

Effective use of tannin based natural biopolymer, AFlok-BP1 to harvest marine microalgae Nannochloropsis sp.

Harvesting marine microalgae Nannochloropsis sp. using Dissolved Air Flotation (DAF) technique