Wastewater treatment high rate algal pond (WWT HRAP) biomass for low-cost liquid biofuel production

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dc.contributor.advisor Craggs, R en
dc.contributor.advisor Farid, M en
dc.contributor.author Mehrabadi, Abbas en
dc.date.accessioned 2017-02-02T22:49:29Z en
dc.date.issued 2017 en
dc.identifier.uri http://hdl.handle.net/2292/31723 en
dc.description.abstract Currently despite intense efforts algal biofuel production is still not economically competitive with fossil fuel. To lower algal biofuel production costs, replacement of pure algal biomass with free gravity-harvested algal-bacterial biomass produced in wastewater treatment high rate algal ponds (WWT HRAPs) has been suggested as a niche opportunity. While most WWT HRAP studies have focused on optimization of treatment performance, the biomass energy productivity and its suitability for quality biofuel production have not been previously investigated in detail. Hence, the objectives of this study are to:  Examine the biomass energy yield potential of WWT HRAP,  Evaluate different strategies to improve biomass energy yield and quality of without impacting pond treatment performance, and  Examine the suitability of HRAP biomass for conversion to biodiesel, pyrolytic biooil and bio-crude production. The average biomass production, energy content and energy yield of two identical pilotscale WWT HRAP (monitored weekly over one year) were 21.5 ton VSS/ha/year, 19.2 GJ/ton and 413 GJ/ha/year respectively. Biomass energy yield is dependent on several factors increasing with warmer climate, lower grazing pressure, higher biomass algal proportion and higher lipid content. Since at full-scale climate conditions are not controllable, and there is little opportunity to increase biomass lipid content without impacting on pond treatment performance, biomass energy yield can only be increased by controlling zooplankton grazing or improving algal biomass productivity. HRAP tend to select of colonial algal species due to the pond mixing that maintains them in suspension. Colonial algae are therefore more easily harvested by simple gravity settling compared to unicellular algal species and are due to their larger size are unable to be grazed by the majority of zooplankton. Hence, an investigation was made of the treatment performance and biomass energy production of the most dominant WWT HRAP colonial algal species. Of the colonial species tested, Mucidosphaerium pulchellum and Micractinium pusillum cultures had the highest nutrient removal and the highest biomass energy yield under simulated New Zealand summer and winter conditions. However, due to much better settleability, Micractinium pusillum, had the greatest potential for both wastewater treatment and biomass energy yield. Two outdoor mesocosm-scale (HRAM) experiments using different air:CO2 mixtures (up to 10% CO2) conducted in summer and winter showed that the biomass energy yield could be improved with CO2 addition. In the summer experiment, compared to the aerated cultures (the control), the highest improvements of the biomass energy yield and its gravity harvestable proportion (43.8% and 102%, respectively) were achieved in the 5% CO2 cultures (pH 6-7). While in the winter experiment, the greatest improvements (̴ 14% for the biomass energy yield and ̴ 33% for the harvestable fraction) occurred in the 0.5% CO2 cultures (pH 7-8). These experiments indicate maintaining a pond pH of 7-8 in winter and 6-7 in summer with CO2 addition would be most beneficial. To assess the quality of WWT HRAP biomass for biodiesel production, the biomass lipids were extracted and profiled during both the annual monitoring of the pilot-scale HRAP and the two CO2 addition HRAM experiments. The biomass lipid profiles were highly complex which led to production of low-quality biodiesel. CO2 addition did not affect biodiesel quality and only enhanced biodiesel productivity by up to 20% due to increased biomass productivity. Overall, less than 30% of the biomass energy yield (413 GJ/ha/year) was recovered in the form of low-quality biodiesel. The low lipid content and high lipid complexity of the WWT HRAP biomass together with the technical limitations of lipid extraction such as drying, cell disruption and solvent extraction make energy recovery from the whole biomass more attractive. Thus, biomass energy recovery via conversion of the whole biomass through pyrolysis and hydrothermal liquefaction (HTL) was investigated at different temperatures. Overall, temperature had a positive effect on the yields of the target products but negatively affected its quality so that the maximum yield (7 wt% pyrolytic bio-oil and 24.9 wt% bio-crude) had the lowest quality (highest complexity and nitrogen content) and was produced at the highest temperatures (500 ºC in pyrolysis and 300 ºC in HTL). The maximum % of the energy content of the biomass recovered in the biofuel products, were 15% and 47.4% for the pyrolytic bio-oil and biocrude respectively. While, HTL is a more favourable conversion process to maximise energy recovery from such a complex biomass, it was not feasible from an energy yield point of view at the tested conditions. Further investigations on the use of the by-products are required to improve biomass energy recovery and consequently the economic viability. Overall based on the results WWT HRAP biomass is not a promising feedstock for lowcost quality liquid biofuel production due to its high complexity which leads to low-quality biofuel production. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.relation.isreferencedby UoA99264930509002091 en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights.uri https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm en
dc.rights.uri http://creativecommons.org/licenses/by-nc-sa/3.0/nz/ en
dc.title Wastewater treatment high rate algal pond (WWT HRAP) biomass for low-cost liquid biofuel production en
dc.type Thesis en
thesis.degree.discipline Chemical and Materials Engineering en
thesis.degree.grantor The University of Auckland en
thesis.degree.level Doctoral en
thesis.degree.name PhD en
dc.rights.holder Copyright: The author en
dc.rights.accessrights http://purl.org/eprint/accessRights/OpenAccess en
pubs.elements-id 611561 en
pubs.record-created-at-source-date 2017-02-03 en

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