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Co-cultures of different microorganisms are considered promising inocula for treating palm oil mill effluents (POME) and producing value-added bio-products (e.g., biofuels and fatty acid-derived materials). However, the efficiency of yeast-bacteria co-culture for microbial lipid production through bioremediation of wastewater remains a bottleneck. In this study, the performance of a co-culture for lipid accumulation through POME bioremediation was investigated using a yeast (Lipomyces starkeyi) and a bacterium (Bacillus cereus). A maximum biomass of 8.89 ± 0.33 g/L and lipid production of 2.27 ± 0.10 g/L were achieved by the co-culture inoculum, which were substantially higher than those of the monocultures. Besides, the co-culture inoculum attained a maximum chemical oxygen demand (COD) removal of 83.66 ± 1.9%, while the individual cultures of B. cereus and L. starkeyi obtained 74.35 ± 1.7% and 69.01 ± 2.3%, respectively. The bioremediation efficiency was confirmed by the seed germination index (GI) of Vigna radiata (Mung bean). It was observed that the co-culture inoculum had a higher GI compared to the untreated POME and even the monoculture-treated POME. We argue that the symbiotic association of a yeast-bacteria co-culture in POME could be an attractive approach for achieving maximum biomass as well as lipid production and simultaneous bioremediation of POME.

The wastewater burden and increased energy demands have spurred interest in finding possible solutions that could concomitantly address both issues. Microbial lipid production, together with the bioremediation of wastewater, could be a promising solution to address both problems. In this context, the present study was designed to evaluate the potential of Bacillus cereus (B. cereus) for accumulating microbial lipids through the bioremediation of palm oil mill effluent (POME) in batch mode fermentation. Different dilutions of POME media (25%, 50%, 75%, and 100%) were used to determine the optimum POME concentration for enabling the maximum yield of biomass and maximum lipid production. The 50% (v/v) POME was found to have potential for the highest biomass growth (8.09 g/L) and lipid accumulation (1.46 g/L), with a lipid content of 18.04% (dry weight basis). B. cereus accumulated a higher biomass and lipid content than other bacterial strains, such as Rhodococcus opacus and Pseudomonas aeruginosa, under similar conditions. On the other hand, the degree of bioremediation was assessed by evaluating several wastewater parameters and determining the seed germination index (GI) of Vigna radiata. POME treated with B. cereus displayed higher GI values than the untreated samples due to the significant remediation of detrimental organics present in the POME. This finding was further confirmed by the substantial reduction in pollution load, particularly in chemical oxygen demand (COD) and biochemical oxygen demand (BOD) for 50% POME, thus demonstrating removal efficiencies of 79.35% and 72.65%, respectively. Therefore, the results of this study suggest that B. cereus cultivation in POME could be a promising technique for attaining higher biomass growth and lipid production in conjunction with the bioremediation of POME. The approach of achieving dual objectives that is utilized in the present study provides a novel strategy for palm oil millers.

Microalgal biofuels are considered as one of the most promising substitutes to the nonrenewable fossil fuels. However, the efficient recovery of microalgal lipids is emergent for the overall process economics and sustainability. The conventional lipid extraction technique entails direct use of organic solvents to liberate the intracellular lipids, thus making the extraction process more time-consuming and less eco-friendly. Furthermore, the efficiency of extraction process is affected by several problems, such as the rigid structure and composition of microalgal cell walls, the water content of biomass, the limited accessibility of lipids, the reduced mass transfer, and the formation of stable emulsions. Various modifications (e.g., use of green solvents, direct transesterification, pretreatment for disrupting microalgal cells, etc.) to conventional lipid extraction have been proposed to mitigate the problems associated with lipid extraction. Among them, the pretreatment approach (i.e., mechanical, chemical, biological cell disruption) in combination with solvent-based extraction would be a promising approach to address the problems during lipid extraction. Therefore, intensive research, review, and analysis are required to understand the challenges in lipid extraction and to develop a useful extraction process in terms of biomass status (dry/wet), solvent use, lipid yield, extraction time, and scalability. In this chapter, we have reviewed the composition and possible applications of microalgal lipids with the main challenges in their extraction process. Furthermore, the advantages and limitations of several pretreatment methods for cell disruption have been comprehensively discussed.

The depletion of nonrenewable energy sources drives the exploration for renewable energy sources as an alternation route of energy generation. Hence, in the past decade, research efforts have been employed to explore potential renewable energy sources utilizing waste biomass. Anaerobic digestion (AD) is considered as an efficient technique to treat different kinds of wastes such as agricultural, industrial, and food wastes to produce renewable energy. Until now, two predominant fermentation processes, namely submerge or liquid-state anaerobic fermentation (LSAF) and dry or solid-state anaerobic fermentation (SSAF) have been implemented to extract energy from waste biomass. Nowadays, SSAF is claimed to be more advantageous than LSAF for a number of reasons including its comparatively smaller reactor volume, lower energy requirements for heating, minimal material handling, and lower total parasitic energy loss. Due to its low moisture content, the digestate of SSAF can be used as fertilizer or pelletized fuel, which is much easier to handle than the effluent from LSAF. However, for enhancing the performance of dry AD processes, a suitable reactor needs to be designed so that the maximum amount of substrate could be fermented, to make the technology more economically viable. Therefore this chapter is aimed at discussing the different kinds of bioreactors along with their potential benefits, challenges, and limitations, which have been explored recently for biogas production through dry fermentation.

The aim of the study was to investigate the feasibility of using irreversible electroporation (EP) as a microbial cell disruption technique to extract intracellular lipid within short time and in an eco-friendly manner. An EP circuit was designed and fabricated to obtain 4 kV with frequency of 100 Hz of square waves. The yeast cells of Lipomyces starkeyi (L. starkeyi) were treated by EP for 2-10 min where the distance between electrodes was maintained at 2, 4, and 6 cm. Colony forming units (CFU) were counted to observe the cell viability under the high voltage electric field. The forces of the pulsing electric field caused significant damage to the cell wall of L. starkeyi and the disruption of microbial cells was visualized by field emission scanning electron microscopic (FESEM) image. After breaking the cell wall, lipid was extracted and measured to assess the efficiency of EP over other techniques. The extent of cell inactivation was up to 95% when the electrodes were placed at the distance of 2 cm, which provided high treatment intensity (36.7 kWh m-3 ). At this condition, maximum lipid (63 mg g-1 ) was extracted when the biomass was treated for 10 min. During the comparison, EP could extract 31.88% lipid while the amount was 11.89% for ultrasonic and 16.8% for Fenton's reagent. The results recommend that the EP is a promising technique for lowering the time and solvent usage for lipid extraction from microbial biomass. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:838-845, 2018.

The discharge of palm oil mill effluent (POME) on arable land causes large amounts of environmental distress due to its high concentration of phenolic compounds and chemical oxygen demand (COD). The approach of simultaneous microbial oil production and wastewater treatment is an attractive option to combine renewable energy production and environmental resilience. This study aims to produce cost effective microbial lipids using the oleaginous yeast Lipomyces starkeyi through the bioremediation of POME. A moderately dilute solution (50%) of POME showed higher microbial growth and lipid accumulation and offered a significantly higher degree of bioremediation. A lipid content of 21.32% was achieved with 50% POME, whereas the value was 15.14% for 25% POME. Three different techniques including ultrasonic treatment, Fenton's reagent and Fenton's + ultrasonic were employed to extract lipids from microbial biomass, and the maximum lipid concentration was obtained using the Fenton's + ultrasonic treatment. The degree of bioremediation was evaluated by the calculating seed germination index (GI) values. Higher GI values were observed for the 25% and 50% dilutions compared to undiluted (100%) POME. This combined approach can be a potential alternative technology that integrates bioremediation of POME with microbial lipid production.

Biohydrogen production from different types of waste biomass using mixed culture inoculum has been an active research area in recent years. Several emerging techniques has been studied targeting to achieve higher efficiency and competitive hydrogen production through the dark fermentation. However, the production of biohydrogen from complex waste biomass meets the setback of lower final product yield due to the low substrate degradation rate. Besides, hydrogen consuming microbes in the mixed culture impede the rate of biohydrogen production. Thus, the waste biomass substrate and microbial inoculum are highly required to pretreat for enhancing system performance. With the aim to gain deeper insight into waste biomass and inoculum pretreatment techniques, this review compiles recently practiced pretreatment techniques with their limitations. The major challenges of these methods and few strategies to overcome these limitations with future directions are discussed in this review. Moreover, the efficiency of a potential nonconventional pretreatment technique such as electroporation (EP) over traditional techniques to degrade the complex waste biomass and reduce the load of hydrogen consuming microbes in the mixed culture consortium was reviewed. This review argued to provide a deeper insight to develop an efficient pretreatment technique by combining traditional techniques and EP for enhancing mixed culture driven biomass-based hydrogen production.