oa Water Recycling And Harvesting Of Algal Biomass By Membrane Filtration

Micro algae are a single celled biomass with a very high potential in the biobased economy. In algal biomass cultivation, the harvesting step (i.e., separation of the dilute single cell algae from the growth medium) is a considerable part of the capital and operational cost. These processes typically require the use of high amounts of chemicals and/or a significant energy input. A second important point of attention in algae cultivation is water recycle. Due to the low concentration of the algae in the cultivation (ca. 0,2 g/l dry algae in open ponds and 2 g/l in photobioreactors), large amounts of water need to be processed to produce algae paste. For example in open pond cultivation a production installation of 1000 ton dry algae per year requires about 700 m³/h water to be processed. Thus for large scale installation medium recycle is a sine qua non. A solution that tackles both issues simultaneously is the submerged flat panel membrane system. The membrane system is used as the first dewatering step in a hybrid system of algae harvesting with centrifugation as final concentration. This technology has the potential to lower energy and investment costs compared to centrifugation alone. The technology furthermore has major advantages on water recycling as > 95 % of the water needs to be removed to produce a 20 % paste of algae. As the membranes don't add any chemicals and remove all suspended solids and bacteria, the technology is very promising toward medium recycle. Submerged membrane filtration is preferred over other membrane filtration technologies (e.g. crossflow filtration), due to its low energy demand and low shearing forces. Membrane fouling is controlled by the cleaning effect of coarse bubbling aeration. Additionally, the flat panel membranes used in this study are backwashable (patented flat sheet membrane envelopes with an integrated permeate channel, IPC). The algae filtration experiments were performed on both lab and pilot-scale submerged reactors with different membranes (MF and UF), algae species (Nannochloropsis, Pavlova, Isochrysis, Phaeodactylum), algae concentration, filtration regimes and filtration cycle times and aeration flows. The results show that membrane operation with backwashing results in higher stable fluxes than operation with only relaxation which is the normal operating mode for submerged membranes. Moreover the flux of UF membranes is more stable than MF membranes and that shorter filtration times result in higher stable fluxes (in the backwashing regime). Aeration flow turned out to be critical towards achievable flux levels and the characteristics of the algae suspension. VITO will further develop and optimize the technology for harvesting and water recycle on pilot scale and demonstration scale. The latest information on the developments of this technology will be presented.