Effect of substratum topography on algal turf colonization and productivity under different nutrient conditions

Abstract

Algae can act as a promising source for biofuel production, pollution recovery from natural waters and nutrient recovery from wastewaters. Typical algae cultivation involves algae in the suspended form, and separation methods including flocculation, filtration, and centrifugation contribute to high cultivation costs. Benthic algae, which grow attached to a growth substratum, is a good alternative to suspended algae for cultivation, as algal biomass can be harvested using mechanical scraping and vacuuming. This approach, called algae turf scrubbers (ATS), have been used for benthic algae cultivation at a large scale in outdoor algae cultivation for pollutant recovery from natural waters and wastewaters. There is little control, however, over the environmental conditions (temperature, light intensity, nutrients and pH) in outdoor ATS systems, and design of the reactor components, such as the growth substratum topography characteristics, can be key to determining the quantity and quality of the biomass produced. The characteristics of the substratum topography can be altered to control the colonization of algae, maximize algal biomass densities, and determine species selectivity to affect the quality and quantity of biomass. The objective of this research is to test the effect of substratum surface topography, using additive manufacturing (AM) technology to prototype, on the biomass density (biomass per unit area) and species selectivity under varying nutrient concentrations (low, medium and high). Substratum test samples were designed using hemispheres of 500µm, 1000µm and 2000µm radius with AM technology and a plain surface was kept as control. Replicates of each of surface topography were made using clay. Four Algal species (Oedogonium crassum, Sirogonium sticticum, Microspora floccose and Mougeotia scalaris) were seeded into a laminar flow lane reactor, and cultivated under different nutrient treatments (low, medium, and high). Repeated harvests of algal biomass were analyzed for biomass density, ash content, and species abundance, and correlations between these parameters, surface topography, and nutrient treatment were investigated. Results demonstrated that nutrient concentration has a primary effect on algal biomass density. The highest nutrient concentration had 186% more biomass density than the lowest concentration (control) and 136% more than the medium concentration. Substratum topography had a secondary effect on the biomass density, and different surface topographies had different biomass densities under each nutrient concentration. The surface topography with 2000 µm radius hemispheres has the highest average biomass density (1.06 ± 0.53 mg/cm2) followed by the surface with 500 µm radius hemispheres (0.92 ± 0.41 mg/cm2) for seven day harvest period. Biomass from the medium nutrient concentration had the highest ash content (17.16% ± 0.71%), whereas the highest nutrient concentration had lowest ash content percent (14.11% ± 0.32%). Nutrient concentration also has a primary effect on the abundance of algal species in the system. At the lowest concentration, Microspora floccose was in abundance (40.00% ± 1%), and at medium nutrient concentration Microspora floccose (45.68% ± 0.76%) and Mougeotia Scalaris (43.50% ± 0.84%) were in abundance. Oedogonium crassum (34.14% ± 1.25%) and Sirogonium scalaris (39.14% ± 1.19%) were most abundant at the highest nutrient concentrations. Substratum characteristics affect the species abundance only at the lowest nutrient concentrations, where Microspora floccose was the only species out of the four affected by substratum characteristics, where it was observed to be more abundant on 500 µm radius hemispheres and 2000 µm radius hemispheres. These results demonstrate the efficacy of using substratum design to control biomass characteristics and quantity in attached growth algae cultivation systems.