• Energy Research

Landfilling of municipal waste, an environmental challenge worldwide, results in the continuous formation of significant amounts of leachate, which poses a severe contamination threat to ground and surface water resources. Landfill leachate (LL) is generated by rainwater percolating through disposed waste materials and must be treated effectively before safe discharge into the environment. LL contains numerous pollutants and toxic substances, such as dissolved organic matter, inorganic chemicals, heavy metals, and anthropogenic organic compounds. Currently, LL treatment is carried out by a combination of physical, chemical, and microbial technologies. Microalgae are now viewed as a promising sustainable addition to the repertoire of technologies for treating LL. Photosynthetic algae have been shown to grow in LL under laboratory conditions, while some species have also been employed in larger-scale LL treatments. Treating leachate with algae can contribute to sustainable waste management at existing landfills by remediating low-quality water for recycling and reuse and generating large amounts of algal biomass for cost-effective manufacturing of biofuels and bioproducts. In this review, we will examine LL composition, traditional leachate treatment technologies, LL toxicity to algae, and the potential of employing algae at LL treatment facilities. Emphasis is placed on how algae can be integrated with existing technologies for biological treatment of LL, turning leachate from an environmental liability to an asset that can produce value-added biofuels and bioproducts for the bioeconomy.

Microalgae are a promising source of biofuels and bioproducts, as they consume CO2 to grow, multiply quickly, and can be cultivated in wastewater and on marginal land. Development of low-cost and high-efficiency microalgal cultivation systems is important to the cost-competitiveness of algae. A floating horizontal photobioreactor (HBR) was developed that is inexpensive and scalable, as it is manufactured from inexpensive plastic film and is modular. Its performance was successfully tested using the marine microalgae Nannochloris atomus Butcher CCAP 251/4A in a 65-L prototype unit. High biomass concentration of 4.0 g L(-1) and productivity of 12.9 g m(-2)d(-1) was achieved indoors under artificial illumination of 31.3 klux (435 μmol m(-2)s(-1)). Outdoors, during semi-continuous operation in Florida, the HBR achieved over the course of 165 days a maximum biomass concentration of 4.3 g L(-1) and an average biomass productivity of 18.2 g m(-2)d(-1) without any contamination issues.

Abstract Microalgae are considered a promising source of renewable diesel and jet fuel. Currently, large-scale microalgae cultivations are performed in open ponds because of their low capital and operating costs, but they generally suffer from low cell mass yield and high risk of contamination. A novel, cost-effective, and modular horizontal bioreactor (HBR) for algae cultivation was developed, as described in the present study. The HBR was designed to keep costs low and was engineered to minimize water and energy use while enhancing CO 2 and nutrient uptake. The selected marine microalgal strain, Picochlorum oculatum (Nannochloris oculata) , has shown potential for biofuel production. A series of controlled indoor growth experiments was first performed to identify the appropriate P. oculatum growth conditions before demonstrating the HBR performance. Supplying CO 2 continuously or by pH-control (pulsed) did not affect culture progression. Growth on urea and nitrate yielded comparable results, while ammonium was less effective. Varying inoculum size from 10% to 15% or 20% had no significant effect on lag time and final cell concentration and comparable growth was measured in the 7–8 pH range. The 150-L HBR's performance was successfully demonstrated outdoors by growing P. oculatum at the identified growth conditions selected to reduce operating costs (pH-controlled CO 2 , pH 7.5, 10% inoculum, and nitrate). High-density growth was achieved without any contamination issues in outdoor HBR cultivations over 68 days in central Florida during two consecutive growth cycles.

Microbial production of ethanol might be a potential route to replace oil and chemical feedstocks. Bioethanol is by far the most common biofuel in use worldwide. Lignocellulosic biomass is the most promising renewable resource for fuel bioethanol production. Bioconversion of lignocellulosics to ethanol consists of four major unit operations: pretreatment, hydrolysis, fermentation, and product separation/distillation. Conventional bioethanol processes for lignocellulosics apply commercial fungal cellulase enzymes for biomass hydrolysis, followed by yeast fermentation of resulting glucose to ethanol. The fungus Neurospora crassa has been used extensively for genetic, biochemical, and molecular studies as a model organism. However, the strain's potential in biotechnological applications has not been widely investigated and discussed. The fungus N. crassa has the ability to synthesize and secrete all three enzyme types involved in cellulose hydrolysis as well as various enzymes for hemicellulose degradation. In addition, N. crassa has been reported to convert to ethanol hexose and pentose sugars, cellulose polymers, and agro-industrial residues. The combination of these characteristics makes N. crassa a promising alternative candidate for biotechnological production of ethanol from renewable resources. This review consists of an overview of the ethanol process from lignocellulosic biomass, followed by cellulases and hemicellulases production, ethanol fermentations of sugars and lignocellulosics, and industrial application potential of N. crassa.

Bioethanol production from sweet sorghum bagasse (SB), the lignocellulosic solid residue obtained after extraction of sugars from sorghum stalks, can further improve the energy yield of the crop. The aim of the present work was to evaluate a cost-efficient bioconversion of SB to ethanol at high solids loadings (16 % at pretreatment and 8 % at fermentation), low cellulase activities (1-7 FPU/g SB) and co-fermentation of hexoses and pentoses. The fungus Neurospora crassa DSM 1129 was used, which exhibits both depolymerase and co-fermentative ability, as well as mixed cultures with Saccharomyces cerevisiae 2541. A dilute-acid pretreatment (sulfuric acid 2 g/100 g SB; 210 °C; 10 min) was implemented, with high hemicellulose decomposition and low inhibitor formation. The bioconversion efficiency of N. crassa was superior to S. cerevisiae, while their mixed cultures had negative effect on ethanol production. Supplementing the in situ produced N. crassa cellulolytic system (1.0 FPU/g SB) with commercial cellulase and β-glucosidase mixture at low activity (6.0 FPU/g SB) increased ethanol production to 27.6 g/l or 84.7 % of theoretical yield (based on SB cellulose and hemicellulose sugar content). The combined dilute-acid pretreatment and bioconversion led to maximum cellulose and hemicellulose hydrolysis 73.3 % and 89.6 %, respectively.

High water demand is a major challenge for the algae industry, so cultivating algae in wastewater can have the double benefit of biomass production and water remediation. The use of landfill leachate (LL), which is wastewater generated in landfills, was investigated to grow the microalga Picochlorum oculatum in a novel horizontal bioreactor (HBR), a low-cost modular cultivation system that reduces water evaporation and contamination risk thanks to its enclosed design. Pilot-scale (150 L) and commercial-scale (2000 L) HBRs that were operated outdoors in Florida using LL in batch and semi-continuous modes generated high cell density cultures (1.7·109 cells mL-1) and reached up to 1.9 g L-1 of dry biomass suitable for biofuel production. Demonstrating the ability of ample non-potable water sources, such as LL, to support algae cultivation is essential for improving the sustainability and cost-effectiveness of commercial algal biofuels and bioproducts, as freshwater resources become increasingly scarce.