Harvesting Microalgal Biomass from a Cultured Algae-Based Wastewater Pond System

Wastewater stabilization ponds (WSPs) are a popular and economical method to treat wastewater. In the current study, algae-treated effluent from WSPs was used to compare microalgal biomass harvesting methods (aluminum sulfate coagulation and chitosan flocculation) and microalgal biomass drying methods (sun drying, oven drying, and freeze drying). The physicochemical characteristics of the supernatants resulting from microalgal biomass harvesting were determined and the microbial composition of the harvested microalgal biomass was investigated using next-generation sequencing. The optimal chitosan and alum concentrations to harvest microalgal biomass were 0.368 and 9.96 g/L, respectively. The calculated optimal exposure period was 2 h. The harvest microalgal biomass with alum caused a decrease in the sulfate concentration (68.44 mg/L) and nitrogen (below detection limit), while the orthophosphate (14.93 mg/L), iron (0.12 mg/L), and total carbon concentration (327.78 mg/L) increased in comparison to the positive control. Furthermore, harvesting the microalgal biomass with chitosan resulted in a decreased sodium (155.17 mg/L) and magnesium concentration (12.40 mg/L), while an increase in ammonia (27.57 mg/L), total carbon (182.04 mg/L), and iron concentration (0.16 mg/L) was observed. The dominant microbial phyla in the positive control were Bacteroidetes (0.242) and Proteobacteria (0.340). Cyanobacteria (0.725) was dominant in the alum-harvested microalgal biomass and Proteobacteria (0.504) was dominant in the chitosan-harvested microalgal biomass. Drying the microalgal biomass to 0.1 NTU using the sun- and oven-drying methods would require 9 days and 25 h, respectively. The freeze-drying method was not able to dehydrate the microalgal biomass to 0.1 NTU. Depending on the application of the dried microalgal biomass, in this case as potential animal feed or soil fertilizer, the sun-drying method may be the superior method because higher-quality nutrients of the microalgal biomass would be retained, while the oven-drying method would require less time but could potentially destroy most of the nutrients.