Sustainable dewatering and drying of self-flocculating microalgae and study of cake properties

Efficient dewatering and drying at lower cost have been a requirement in reducing the cost of microalgal biomass production. The self-flocculation capacity of Scenedesmus obliquus and the hydrophobic nature of polypropylene non-woven fabric membrane (PNM) are explored in this study. Formation of ‘algal bio-filter layer’ due to natural forces showed the efficiency of the process. Compared to the conventional methods, the cost of harvesting reduced substantially. The physico-chemical and mechanical properties of the harvested cake were studied. To the best of our knowledge, this is the first report on PNM usage for dewatering and drying as well as algal cake properties. The membrane showed high biomass recovery efficiency (94%) for Scenedesmus sp. but very low (58%) for Chlorella sorokiniana (non-self-flocculating). Dewatering on sand-bed followed by drying on wire-gauge (10.5% m.c) found to be the best arrangement. Solar drying of 1–2 cm thick slurry was achieved within 34 ± 2 h. Based on drying period, true density, bulk density, inter-cake porosity, breaking strength, grinding and selective physical properties, obtaining a 2-mm-thick cake was comparatively more energy efficient for post-harvest operations. The estimated cost of pilot-scale dewatering-drying was $ 0.048 per kg of dry algal biomass. Dehydration using PMN followed by natural drying was found to be energy efficient and comparatively very cost effective for pilot scale harvesting.