Available at: https://digitalcommons.calpoly.edu/theses/1896
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
To be environmentally sustainable, algae biofuel production requires extensive recycling of water and nutrients during algae cultivation. Such recycling can be in the form of reuse of external wastewaters, such as from municipal sewers, but, to achieve significant biofuels production, internal recycling at the algae farms is also needed. A potential difficulty with internal recycling is the accumulation of auto-inhibitory compounds in recycled algae pond water. An additional concern is that nutrients in the residuals from algae biofuel manufacturing will be only partly bioavailable. Both these factors can limit the extent of internal recycling. Ultimately, combining external and internal recycling might be the most sustainable as long as these limiting factors are not excessive. Another important consideration in algae biofuel feedstock production is harvesting, which can be energy intensive and expensive. Bioflocculation of algae cells followed by sedimentation is a low input harvesting method, but one that might be affected by resource recycling. In the present 7-month study, algae productivity and settleability were measured for several types of water and nutrient recycling, using pilot raceways and tubes settler tanks fed municipal wastewater.
Two control raceway sets and two types of water and nutrient recycling raceway sets were operated concurrently. Each set was comprised of duplicate raceways (33-m2 surface area each, 0.3-m deep, 4-day hydraulic residence time with CO2 supplementation). One control set was fed primary clarifier effluent while the other control set was fed nitrified, filtered reclaimed wastewater. The two experimental raceway sets were fed different nutrients. One set received anaerobically-digested algal biomass (algae digestate) and the other set chemical fertilizer. Both recycling raceway sets were monitored to determine if water and nutrient recycling had any detrimental impacts on algal biomass growth.
Productivity was reported as gross and net in terms of volatile suspended solids (VSS). Gross productivity was based on raceway effluent biomass flow, which could be harvested for biofuel production. Net productivity was based on influent minus effluent biomass flows, which better represents algae growth. As a complement to the productivity data, measurements were made of indictors (salt, soluble chemical oxygen demand, and soluble UV absorbance) of potentially inhibitory substances and of nitrogen and phosphorus concentrations, which would have limited growth if they were too low.
After 7 months of continuous operation and 50 cycles of water reuse, the algae productivities of the water and nutrient recycling raceways matched or exceeded the productivities of the control raceways. Net productivities of the primary effluent-fed and reclaimed water-fed control ponds averaged 15.4 and 18.0 g-VSS/m2-day, respectively, while the digestate-fed and fertilizer-fed recycling ponds net productivities averaged 19.6 and 18.8 g-VSS/m2-day, respectively. The average standard deviation of the various duplicate sets ranged from 0.3 – 4.0 g-VSS/m2-day. These results do not indicate that algae were inhibited during the 7 months of continuous water recycling and digestate fertilization. In fact, the results suggest that recycling enhances productivity, although the statistical power of this experiment was low.
Indicators of potentially inhibitory substances were monitored during the experiment (salinity, soluble chemical oxygen demand, and UV absorbance). Only salt concentrations showed significant accumulation in the ponds, but not to concentrations expected to be inhibitory. A greater number of replicates and a longer experimental period are recommended in future experiments.