Promising Pathway for Algal Biofuels through Wastewater Cultivation and Hydrothermal Conversion

Abstract

The purpose of this study is to demonstrate feasibility of an integrated wastewater algae-to-biocrude process that can sustainably cultivate algal biomass for biofuel production. This process used pilot-scale algal cultivation ponds fed with municipal wastewater as the nutrient source. The open ponds were self-inoculated from the wastewater source, resulting in a mixed-culture microalgal community with distinct differences compared to laboratory-maintained and fertilized monocultures: 29.0% dry weight (dw) ash, 48.9% ash-free dry weight (afdw) carbon, 37.5% afdw oxygen, and 14.0% afdw lipid. The harvested algae was processed using hydrothermal liquefaction at 350 °C (autogenous pressures up to 2000 psig) for 1 h using 3 g of freeze-dried algae and 50 mL of water. The yield of biocrude was 44.5 ± 4.7% afdw, with an elemental weight percent composition of 78.7% carbon, 10.1% hydrogen, 4.4% nitrogen, and 5.5% oxygen and an energy content of 39 MJ/kg. Hydrothermal processing also resulted in the formation of 18.4 ± 4.6% afdw aqueous co-products (ACPs) and 45.0 ± 5.9% dw solid biochar. The ACPs contained 4550 ± 460 mg L −1 organic carbon, 1640 ± 250 mg L −1 total nitrogen, and 3.5 mg L −1 total phosphorus. The solid biochar product contained >20% dw carbon with an energy density between 8 and 10 MJ kg −1 . This study is the first hydrothermal liquefaction paper of wastewater-derived microalgae. The municipal wastewater matrix and resultant mixed-culture biomass significantly influenced liquefaction product distribution, yielding a higher proportion of biochar, which may be a valuable co-product. This paper explores the potential for wastewater-fed algal systems integrated with hydrothermal liquefaction, which together overcome challenges identified by the 2012 National Research Council's report on algal biofuel sustainability.

Simultaneous removal of inorganic and organic compounds in wastewater by freshwater green microalgae

“…[15][16][17] This study also has environmental implications in terms of interaction between various contaminants and algae as these green algae are widely present in different aquatic environments. Algae could play an important role in the degradation of some contaminants in the environment.…”

“…11,[13][14][15][16][17][18] In this study, we aimed to further assess the capability of these four green microalgae species in the simultaneous removal of various contaminants (nitrogen, phosphorus, metals, organic compounds) in real wastewater. Estrogenic EDCs such as nonylphenol possess the ability to disrupt the endocrine systems of higher organisms by interacting with the estrogen receptor.…”

Assessing the potential of polyculture to accelerate algal biofuel production

“…HTL uses heated, compressed water to dissolve organic compounds, and its elevated ion production can accelerate the acid-catalyzed, hydrolytic decomposition of algal biomacromolecules [143,144]. Output from HTL typically consists four separable fractions [144][145][146][147][148][149][150][151][152][153][154][155][156][157][158][159]: (1) an oily biocrude that can be upgraded using refining techniques to produce hydrocarbon fuels, (2) an aqueous co-product (ACP) phase, (3) a solid residue that may be suitable for use in soil amendments, absorbents, catalysts, and asphalt [145][146][147][148][149][150], and (4) a possible gaseous phase, depending on feedstock and processing conditions [152]. The ACP can be further processed to produce additional energy products and process heat [150,152,153], hydrogen for co-processing [152,153,160], and the capture and recycling of water, nutrients (N, P, etc.)…”

“…of polyculture biomass. This has made it possible to re-consider the use of polycultures as a viable option for biofuel production, regardless of the neutral lipid content of the biomass [145,161,162]. Elevated non-organic ash content in the wet algal biomass feedstock will be an issue for HTL, in that its presence adds ma terial loading to the processing stream that does not contribute to fuel production, requires additional system processing capacity (and associated capital and operating costs) to accommodate, and reduces the effective fuel product yield per mass unit of material processed [140,[163][164][165][166][167][168].…”

“…Roberts et al [145], Chen et al [147], and Pate et al [164,168] used polyculture algae biomass produced from wastewater and contaminated surface water as HTL feedstock and demonstrated that the bio-crude oil yields and energy recovery were comparable to those converted from monoculture microalgae [145,147,164,168]. In typical one-step direct hydrothermal liquefaction process some portion of valuable polysaccharides, reducing sugar, and protein are decomposed into bio-gas and biochar through hydrolysis, dehydration, and polymerization.…”