Sustainable Harvesting of Microalgae by Coupling Chitosan Flocculation and Electro-Floatation

In this study, we proposed a new method for edible microalgae harvesting by coupling chitosan flocculation and electro-floatation using carbon electrodes, and tested it at different operation conditions. The surface charge of microalgae cells was measured to explore the underlying mechanisms. The responses of growth medium to microalgae harvesting were also investigated to evaluate the feasibility of sustainable utilization of culture medium.

(1) Microalgae harvesting efficiency

The microalgae harvesting efficiency of the proposed method were tested using Chlorella vulgaris (C. vulgaris). C. vulgariscells (FACHB-24) were purchased from the Institute of Hydrobiology, Chinese Academy of Sciences, and cultured in the BG11 medium, which consists of 500 mg L^-1 Bicin, 100 mg L^-1 KNO₃, 100 mg L^-1 b-C₃H₇O₆PNa₂, 50 mg L^-1 NaNO₃, 50 mg L^-1Ca(NO₃)₂•4H₂O, 50 mg L^-1 MgCl₂•6H₂O, 40 mg L^-1 Na₂SO₄, 20 mg L^-1 H₃BO₃, 5 mg L^-1 Na₂EDTA, 5 mg L^-1 MnCl₂•4H₂O, 5 mg L^-1 CoCl₂•6H₂O and 0.8 mg L^-1 Na₂MoO₄•2H₂O, 0.5 mg L^-1 FeCl₃•6H₂O and 0.5 mg L^-1 ZnCl₂. The batch cultures were conducted in an illuminating incubator (LRH-250-G, Guangdong Medical Apparatus Co., Ltd., China) with continuous cool white fluorescent light of 2500 ± 500 lux on a 12 h light and 12 h darkness regime at the temperature of 30 ± 1°C.

The microalgae harvesting efficiency system consists of a flat stir paddle (Zhongrun Water Industry Technology Development Co., Ltd., China) for mixing during chitosan flocculation and two round carbon electrode plates (Jinjia Metal Co., Ltd., China) for electro-floatation. The carbon electrode plate has a surface area of 55.4 cm² and a thickness of 0.2 cm, which was horizontally installed at the bottom with a gap of 2 cm between the two plates. There are 85 small round holes on each carbon electrode plate to allow gas bubbles freely pass it during electrolysis, such that the effective surface area was 38.7 cm². The electric current was supplied by a direct current power supply (DF1730SL5A, Ningbo Zhongce Dftek Electronics Co., Ltd., China).

C. vulgaris culture at the exponential growth phase was used in the microalgae harvesting test. The initial cell concentration was set to 3.63 × 10¹⁰ cells L^-1. 0.5 L of readily prepared C. vulgaris solution was transferred to the harvesting cell. Water-soluble chitosan was purchased from Qingdao Yunzhou Bioengineering Co. Ltd., China. Prior to the test, a chitosan stock solution (2 g L^-1) was prepared as follows: 1 g chitosan was added to 0.5 L distilled water and completely diluted by stirring. After chitosan was added, the microalgae solution was stirred at 200 rpm for 2 min and 40 rpm for another 10 min; electro-floatation was started in the last 5 min during chitosan flocculation. The microalgae solution was allowed to stand for 10 min, and then water samples were carefully collected from an outlet 2 cm above the carbon electrode plate to enumerate the cell number using an Axioskop 2 mot plus microscope (Carl ZEISS, Germany). The microalgae harvesting efficiency was calculated as (initial cell concentration-sample cell concentration)/initial cell concentration × 100%. In the test, the chitosan dosage was set to 0, 2, 4, 6, 8, 10, 12 and 15 mg L^-1, and the current density was set to 0, 0.2, 0.4 and 0.6 A. All the tests were conducted in triplicate at the raw microalgae solution pH of 8.6.

(2) Surface charges of chitosan, microalgae cells and microalgae flocs

The surface charges of chitosan, microalgae cells and microalgae flocs were characterized using a Zetasizer 2000 (Malvern Co. United Kingdom).

(3) Sustainable utilization of microalgae culture medium

Before and after microalgae harvesting, medium nutrients (phosphate, ammonium and nitrate) were measured according to Chinese Monitoring Analysis Method of Water and Wastewater; medium pH and temperature were measured using a Yellow Springs Instruments (Yellow Springs, Ohio, USA)

(4) Cost evaluation

The cost of microalgae harvesting was estimated by summing flocculants and energy costs per unit of harvested microalgae biomass. The chitosan and electric power are 0.03 USD g^-1 and 0.08 USD (kWh)^-1, respectively.