A novel horizontal photobioreactor for high-density cultivation of microalgae

•A novel low-cost floating horizontal photobioreactor (HBR) for algae was designed.•Successful cultivation of the microalgae Nannochloris atomus in the HBR.•High algae biomass concentration was achieved without contamination issues.•Biomass productivity doubled, when light intensity tripled.•High productivity achieved during semi-continuous outdoor operation over 165days. Microalgae are a promising source of biofuels and bioproducts, as they consume CO2 to grow, multiply quickly, and can be cultivated in wastewater and on marginal land. Development of low-cost and high-efficiency microalgal cultivation systems is important to the cost-competitiveness of algae. A floating horizontal photobioreactor (HBR) was developed that is inexpensive and scalable, as it is manufactured from inexpensive plastic film and is modular. Its performance was successfully tested using the marine microalgae Nannochloris atomus Butcher CCAP 251/4A in a 65-L prototype unit. High biomass concentration of 4.0gL−1 and productivity of 12.9gm−2d−1 was achieved indoors under artificial illumination of 31.3klux (435μmolm−2s−1). Outdoors, during semi-continuous operation in Florida, the HBR achieved over the course of 165days a maximum biomass concentration of 4.3gL−1 and an average biomass productivity of 18.2gm−2d−1 without any contamination issues.