Wrede, Digby (2019) Utilisation of High Rate Algal Ponds to Treat Secondary Lagoon Effluent and Enhance Biomass Production. PhD thesis, Victoria University.

Abstract

High rate algal ponds (HRAPs) can be utilised as an efficient and economical wastewater treatment method while also producing algal biomass. This study focused on the use of HRAPs to assimilate nutrients from secondary lagoon effluent and investigated various methods in which to enhance the algal biomass productivity of the HRAPs. The natural operation, productive potential, biomass production, nutrient removal capacity and environmental conditions were observed. From these findings, three experiments were proposed to enhance biomass production and in turn, the nutrient removal of the HRAPs. The first experiment was the addition of three separate algal cultures to the HRAPs during winter. Two of the algal species enhanced biomass production, however, there was no significant difference in nutrient removal during any of these experiments. The second set of experiments controlled the pH of the HRAPs utilising an inorganic and organic acid to determine if it was solely the control of pH which enhanced biomass production or if the addition of carbon that played a significant role. It was found that under high algal productivity conditions utilising inorganic acid to control pH negatively impacted algal growth whereas utilising organic acid significantly enhanced algal growth. The third experiment compared secondary lagoon effluent and primary lagoon effluent as the media sources. Secondary lagoon effluent was found to have higher biomass productivity by 106mg/L. This was thought to be a result of the primary lagoon effluents high colloidal turbidity. The results from the biomass enhancement experiments alongside the natural operation of the HRAPs were utilised to develop a simple and accurate algal growth model which utilised readily available data. The model aims to determine the biomass production of HRAPs in the south-eastern Australian climate which operated under elevated pH levels. The model was validated against the use of both secondary and primary lagoon effluent in the HRAPs and returned an R-squared value 0.98, suggesting a high accuracy. Following this work, two algal harvesting methods were investigated; membrane filtration and fungal flocculation. Three different membrane filtration systems were trialled and compared; ceramic crossflow system, polytetrafluoroethylene (PTFE) submerged system and a metal crossflow system. The PTFE membrane was found to be the most effective of the membranes tested for harvesting algae due to its low fouling tendency, low cost and relatively constant flux. The flocculation capability of fungi to flocculate algae was examined. Aspergillus oryzae was found to be the most effective fungi species trialled for monoculture flocculation with over 95% removal for all algal species tested. The fungal flocculation of mixed algal communities in wastewater samples was also investigated and removal values of 70-100% were achieved. Overall, the work conducted provides valuable information on the operation and enhancement of HRAPs. Furthermore, the simple model developed can be utilised to help identify the potential of an area for algal biomass production and the feasibility of incorporating HRAP systems into an existing wastewater treatment facility. The two harvesting techniques trialled offer new and vital insight into the often-difficult process of algal harvesting.

Search Google Scholar

Repository staff login