AbstractsBiology & Animal Science

Enhancing the performance of wastewater microalgae through chemical and physical modifications in High Rate Algal Ponds

by Donna Lee Sutherland

Institution: University of Canterbury
Department: School of Biological Sciences
Year: 2015
Keywords: wastewater treatment; photosynthesis; microalgae; high rate algal ponds
Record ID: 1302544
Full text PDF: http://hdl.handle.net/10092/10255


High rate algal ponds (HRAPs) are an advanced pond that provide efficient and cost-effective wastewater treatment, as well as the ability to recover nutrients in the form of microalgal biomass. Microalgal photosynthesis, nutrient uptake and subsequent growth, coupled with aerobic bacteria degradation of organic compounds, are fundamental to the process of wastewater treatment in HRAPs, yet are often limited in these ponds and, in particular, microalgal photosynthesis is well below the reported theoretical maximum. Understanding how the physico-chemical environment affects microalgal performance is therefore critical to improved wastewater treatment and nutrient recovery, yet has been the subject to few studies to date. This research focused on the enhancement of microalgal photo-physiology, growth and nutrient removal efficiency (NRE) through modification to the physical and chemical environment in wastewater HRAPs. In this study, I first examined the seasonal dynamics of microalgal performance in full-scale wastewater HRAPs. While both retention-time corrected chlorophyll biomass and photosynthetic potential increased from winter to summer, the summer-time performance was considered to be constrained, as indicated by the decreased light absorption, light conversion efficiency and NRE. The physico-chemical environment in the full-scale HRAPs were characterised by high day-time pH, high light attenuation and long, straight channels with low turbulence. This led to questions regarding 1) effects of nutrient supply, in particular carbon and 2) the role of the HRAP light climate on microalgal performance. I addressed these questions using a series of experiments that involved either changing the nutrient concentration and its supply or by modifying the light environment, through changes in pond operational parameters including CO2 addition, influent dilution, pond depth, hydraulic retention time (HRT), mixing speed and frequency. The overall results from these experiments showed that carbon was the primary and light the secondary limiting factors of microalgal performance. These limitations negatively affected light absorption, photosynthesis, productivity and NRE. While each operational parameter tested impacted on microalgal performance, to some degree, CO2 addition had the greatest influence on light absorption, photosynthetic efficiency and productivity, while continuous mixing had the greatest effect on NRE. Adding CO2 increased light absorption by 110% and 128%, maximum rate of photosynthesis by 185% and 218% and microalgal biovolume by between 150 – 256% and 260 – 660% (species specific), when cultures were maintained at pH 8 and 6.5, respectively. Providing sufficient mixing to achieve continuous turbulence enhanced NRE by between 300 – 425% (species specific), increased biomass concentrations between 150% and 4000% (species specific) compared to intermittent and no mixing, respectively, and increased harvest-ability of colonial species. However, at present, both CO2 addition and mechanical mixing attract high…