Abstract
Hydrothermal carbonization (HTC) of microalgae biomass for the production of triacylglycerides is a potentially valuable enabling technology for a waste water treatment-based integrated biorefinery. Here, HTC was used to treat Phaeodactylum tricornutum lipid-rich biomass producing a solid hydrochar from the surface of which adsorbed lipids were removed by hexane extraction following filtration of the solid hydrochar from the process liquid product. Approximately 7% of the input biomass was recovered and transesterified for qualitative and quantitative GCMS analysis for fatty acid methyl esters. Transesterifiable lipids accounted for 94% of the material recovered by solvent extraction. Of the transesterified fatty acids (FA) analysed, the majority was monounsaturated (40.4%) and saturated (37%) C-16 FA. Other FA detected included saturated and monounsaturated C-18 (7.7% and 1.9%) and saturated C-14 (5.3%) and C-25 (1.5%). Thermal analysis (TGA/DSC) of the hydrochar in air showed calorific values of 10.6 MJ kg-1 (delipidated hydrochar) and 3.1 MJ kg-1 (non-delipidated hydrochar) with the latter exhibiting the presence of volatalisable components. Germination trials were conducted to assess the potential phytotoxic effects of these hydrochars. Delipidated hydrochar showed a germination index of 73% suggesting the presence of some phytotoxicity. Non-delipidated hydrochar showed high germination index results of 102% (unground) and 126 % (ground). Taken together with the observation of reduced root hair proliferation in these two test conditions, this suggests the operation of a second phytotoxic effect that is removed by delipidation.
Highlights
As climate change and other serious environmental challenges intensify, there is a growing need for circular-economic approaches to resource recovery and pollution prevention; the integrated biorefinery is a promising concept in this respect
The co-produced hydrochar is of potential utility as a fuel and/or as a biochar: this study investigates the fuel properties and potential phytotoxicity of the coproduced hydrochar, with a view to investigating potential moderation of hydrochar phytotoxicity during the delipidation process
Over the three replicate Hydrothermal carbonization (HTC) experiments 7.48 ± 0.08% of input algal biomass was recovered as non-evaporable material following hexane extraction of the solid hydrochar product
Summary
As climate change and other serious environmental challenges intensify, there is a growing need for circular-economic approaches to resource recovery and pollution prevention; the integrated biorefinery is a promising concept in this respect. 11 million tons of waste water are processed each day in UK, coming from a range of commercial, industrial, and public sources (Department for Environment, Food and Rural Affairs , 2012). The treatment of such masses to remove organic, inorganic, and anthropogenic compounds is costly, the recycling of such byproducts, for example ammonia, nitrogen, and phosphorus, is being explored as a potential route to add value into the water treatment process. The presence of inorganic nitrogen and phosphorus in the wastewater stimulate microalgae growth, resulting in a dense bloom which can subsequently be used as a biofuels feedstock (Abdel-Raouf et al, 2012).
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