Applying membrane separation technology for integrating high strength cow manure digestate treatment and microalgae cultivation in an open reactor system

Direct discharge of anaerobic digestate, which is the effluent that comes out from the anaerobic digestion process, can cause several environmental problems due to the high concentration of phosphorus and nitrogen. This study aims to propose a system that can allow cow manure digestate treatment through microalgae cultivation with no digestate dilution. A novel open reactor system with microfiltration membranes that separate the digestate tank and the cultivation tank was proposed. Two configurations of reactors were tested. First a circular reactor equipped with one membrane that could allow vertical transfer of nutrients was tested. The system was scaled up with a new rectangular reactor that was equipped with three membranes that could allow horizontal nutrients transfer. Nutrients transfer from the digestate tank into the cultivation tank was confirmed. Light transmittance of 99.9% of the culture solution after 7 days of continuous equilibration with the digestate could be maintained. Microalgae could be successfully cultivated using undiluted digestate; where microalgae growth productivity and nitrogen removal of around 45 mg/L/day and 32.1% could be achieved, respectively. Our proposed microfiltration membrane open reactor system was shown to be promising for integrating microalgae cultivation with digestate treatment without the need for digestate dilution.