Understanding biopolymer clusters formation in microalgae systems for wastewater: application to membrane photo-bioreactors

微藻由于具有生产诸如药品、营养保健品、动物饲料、化妆品和生物柴油等增值产品的潜力而引起了相当大的兴趣。虽然高价值产品的应用是可行的，但由于藻类生物质生产的高成本，生物柴油生产的应用价值较低，但市场潜力大得多，在经济上仍然不可行。生长介质和藻类生物量的收集费用已被特别确定为对生产总费用的主要贡献，必须大大减少，以便能够广泛应用。然而，利用废水作为藻类生长介质已被证明是一种可持续的低成本选择，因为它提供了藻类生长所需的营养物质，同时提供了废水修复。应用于废水处理，微藻被证明可以有效地去除营养物质（磷和氮）到非常低的水平，也显示出去除有害化学物质如重金属和有机微污染物的潜力。从本质上讲，微藻可以用于废水污染修复，同时通过生产藻类生物量提供附加价值。然而，藻类收集仍然是主要的限制，解决这个问题将是发挥这些技术真正潜力的关键。膜光生物反应器是将藻类光生物反应器与用于直接分离藻类生物质的膜相结合的集成系统，已被认为是更传统藻类系统的有前途的替代品，因为它们通常具有与典型藻类光生物反应器相同的优点，但集成膜提供了完全保留藻类细胞和解耦的生物量和水力保留时间。这可以增加生物量浓度，从而在显著缩短接触时间的情况下加强这一过程。虽然膜有利于藻类生物量的收集，但对于所有的膜系统来说，污染成为主要的限制。藻类生物量和水中存在的任何有机和无机化合物在膜上的积累将影响其水力性能，并导致能源需求和成本的增加。膜污染是不可避免的，因此通过实施缓解措施来控制膜污染的形成，以获得可持续的运行，并使该技术在经济上可行是至关重要的。以往关于微藻对膜污染的研究强调了藻源有机物的高污染性，特别是微藻分泌的可溶性生物聚合物。重要的是，生物聚合物已被证明是造成不可逆污染的主要原因，因为它们可以渗透到膜的毛孔中并阻塞通道，从而显著影响膜的过滤性能和清洁要求。有趣的是，生物聚合物也被证明在某些系统中自然聚集。在这种情况下，形成的生物聚合物团簇不能再进入膜的孔隙，只会在膜表面形成饼层，这基本上是可逆的。这将减少对过滤性能的影响，降低操作成本，从而使该技术在经济上可行。本研究的目的是通过建立在膜光生物反应器中处理废水的可控生物聚合物簇形成的基础，并展示颗粒生物聚合物组合对膜污染可逆性的有益影响，从而开发一种可持续的、经济上可行的基于藻类的废水处理和藻类生物质生产技术，以回收资源。

Microalgae attract considerable interest due to their potential for the production of value-added products such as pharmaceuticals, nutraceuticals, animal feed, cosmetics and biodiesel. Although the applications for high value products are viable, applications for biodiesel production, with a lower value but much greater market potential, are still not economically viable due to the high cost of algal biomass production. The costs of growth medium and algal biomass harvesting have specifically been identified as the major contributions to the total cost of production and will have to be significantly reduced to enable widespread application. The use of wastewater as an algae growing medium has however been shown to be a sustainable low cost option as it provides the nutrients needed for algae growth while simultaneously delivering wastewater remediation. Applied to wastewater treatment, microalgae are proven to efficiently remove nutrients (phosphorus and nitrogen) to very low levels and also demonstrated potential to remove hazardous chemicals such as heavy metals and organic micro-pollutants. Essentially, microalgae can be used for wastewater pollution remediation whilst providing added value through the production of algal biomass. However, algae harvesting remains the major limitation and solving this problem will be the key to deliver the true potential of these technologies.Membrane photobioreactors, which are integrated systems combining an algal photobioreactors with a membrane for direct separation of the algal biomass have been identified as promising alternatives to more conventional algae systems as they generally have the same advantages as typical algal photobioreactors but the integrated membrane provides complete retention of algae cells and decoupled biomass and hydraulic retention times. This enables increased biomass concentrations and consequently intensification of the process with significantly shorter contact times. While the membrane facilitates algal biomass harvesting, as for all membrane systems, fouling becomes the main limitation. The accumulation on the membrane of the algal biomass and any organic and inorganic compounds present in the water will affect its hydraulic performance and contribute to an increase in energy demand and costs. Membrane fouling is inevitable so it will be critical to control its formation through the implementation of mitigation measures to obtain sustainable operation and to make the technology economically viable.Previous studies on membrane fouling by microalgae have highlighted the highly fouling nature of algogenic organic matter and more specifically the soluble biopolymers excreted by microalgae. Importantly, biopolymers have been shown to be the main contributors to irreversible fouling as they can penetrate in the pores of the membrane and block the channels, significantly affecting membrane filtration performance and cleaning requirements. Interestingly, biopolymers have also been shown to naturally aggregate in some systems. Promoting clustering would then enable to transfer the highly fouling compounds from the soluble to the particulate fraction in which case the biopolymer clusters formed can no longer enter the pores of the membrane and will only contribute to the formation of cake layer on the surface of the membrane, which is essentially reversible. This will then lead to a reduced impact on the filtration performance and decreased costs of operation therefore making the technology economically viable. The aim of this research is then to develop a sustainable and economically viable algae based technology for wastewater treatment and algal biomass production for resource recovery by establishing the basis for controlled biopolymer clusters formation in membrane photo-bioreactors treating wastewater and demonstrating the beneficial impact of the particulate biopolymer assemblages on the reversibility of membrane fouling.