Implementation of flocculation and sand filtration in medium recirculation in a closed microalgae production system

Recycling growth medium is a necessity to reduce production cost and ecological foot print of large scale microalgae production systems. To prevent contamination and/or enrichment with particulate matter, medium recycling requires pre-treatment of the centrifuge supernatant (centrate), prior to medium replenishment and re-use. In this study, we investigated the applicability of high pH induced flocculation and/or sand filtration to interface with an existing microfiltration setup in order to prepare recycled growth medium for the mass cultivation of marine microalgae. Sand filtration partly alleviated the burden on subsequent microfiltration, but proved to be particularly useful to remove high pH induced flocs from the centrate. Combination of both techniques resulted in a removal of 78±18% of particles, resulting in an improvement of ‘modified fouling indices’ by 75±19%. Despite a partial to complete removal of remaining nutrients such as phosphate, calcium and magnesium during treatment, a cost saving of 72% compared to a scenario with fresh medium can be achieved. •Characterisation of flocculation and sand filtration of centrifuge supernatant•Optimal combination of flocculation and sand filtration was established•Pre-treatments enable sustainable recirculation of spent microalgae growth medium