Author: Ngai Ning Cheng

Cheng, Ngai Ning, 2017 EXPLOITATION OF WASTEWATER GROWN MICROALGAE FOR THE PRODUCTION OF BIOGAS, Flinders University, School of the Environment

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There is a current need to question if CO2 addition enhances algal production in all types of wastewater, due to the conflicting views exist within the literature, and limited comparative investigations have been undertaken. In addition, while other wastes are commonly co-digested e.g. industrial organic wastes, fruit and vegetable solid waste, olive wastes and farm wastes, there are limited studies on the digestion of algal biomass either as a sole substrate or co-digested with other wastes, significantly for this proposal, only limited studies are considered co-digestion with pig slurry.

In this current research, a laboratory approach was utilised to examine the effect of the addition of CO2 on the growth of microalgae in wastewaters of three different BOD5 strengths. Somewhat uniquely in this area of wastewater research and algal biomass production (ALBAZOD) a comparison was also made, between the outcomes for biomass production and treatment, of pH stasis using acid rather than CO2.

Results of the research demonstrated that the addition of CO2 did not increase biomass production since the native organic carbon pool, following bacterial mineralisation, within both wastewaters was sufficient to support optimal biomass production under the prevailing conditions of light and temperature. The corresponding statistically significance also suggested that the maintenance of pH stasis in the absence of carbon addition implies that the forcing of the carbonate bicarbonate equilibrium in favour of free CO2 was of more likely importance to productivity than external carbon addition. The differential response of wastewaters to CO2 addition, in terms of biomass production, reported here suggests that careful consideration is required before investing capital in infrastructure to support CO2 addition to large scale systems. The results suggest that wastewaters with low BOD5 content or a low available organic carbon pool or which have been extensively pretreated resulting in a recalcitrant organic carbon pool resistant to mineralisation are most likely to respond positively to CO2 addition. In contrast, wastewaters which have not been extensively treated and which contain a large, readily mineralisable organic carbon pool are unlikely to respond positively to CO2 addition.

Co-digestion of pig slurry and ALBAZOD resulted in a slightly higher methane yield under psychrophilic temperatures than pig slurry alone, however, the increase was not significantly different statistically and the results also suggested that the ratio should be carefully considered as the biodegradability of ALBAZOD was lower than the biodegradability of pig slurry. One of the challenges of this research was the low VS loading rate in low concentration of microalgae biomass present in large volume of water sample. However, this was considered a typical ALBAZOD substrate obtained following dissolved air flotation; a common and relatively low cost separation technology suitable for on-farm operation that is without the adoption of high energy – high capital cost concentrating systems such as centrifugation. Similar observations were also recorded when co-digested with waste activated sludge. It was observed that a much longer solid retention time was required for solo ALBAZOD anaerobic digestion. It was concluded overall conclusion, that the low biodegradability of algae cell wall which caused the extended period of digestion.

The research presented here provides a better understanding of how to achieve integration of algae and wastewater treatment by determining, whether it is necessary to supply external CO2, and evaluating the outcome of anaerobic co-digestion of algal biomass with either pig slurry or waste activated sludge.

Keywords: High Rate Algal Pond, HRAP, algae, microalgae, biomass, wastewater, carbon dioxide, CO2, total organic carbon, particulate organic carbon, anaerobic co-digestion, pig slurry, algal biomass, albazod, psychrophilic, waste activated sludge, WAS

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of the Environment
Supervisor: Howard Fallowfield