Ultrafiltration Harvesting of Microalgae Culture Cultivated in a WRRF: Long-Term Performance and Techno-Economic and Carbon Footprint Assessment

Abstract

A cross-flow ultrafiltration harvesting system for a pre-concentrated microalgae culture was tested in an innovative anaerobic-based WRRF. The microalgae culture was cultivated in a membrane photobioreactor fed with effluent from an anaerobic membrane bioreactor treating sewage. These harvested microalgae biomasses were then anaerobically co-digested with primary and secondary sludge from the water line. Depending on the needs of this anaerobic co-digestion, the filtration harvesting process was evaluated intermittently over a period of 212 days for different operating conditions, mainly the total amount of microalgae biomass harvested and the desired final total solids concentration (up to 15.9 g·L−1 with an average of 9.7 g·L−1). Concentration ratios of 15–27 were obtained with average transmembrane fluxes ranging from 5 to 28 L·m−2·h−1. Regarding membrane cleaning, both backflushing and chemical cleaning resulted in transmembrane flux recoveries that were, on average, 21% higher than those achieved with backflushing alone. The carbon footprint assessment shows promising results, as the GHG emissions associated with the cross-flow ultrafiltration harvesting process could be less than the emissions savings associated with the energy recovered from biogas production from the anaerobic valorisation of the harvested microalgae.

Towards Optimisation of Microalgae Cultivation through Monitoring and Control in Membrane Photobioreactor Systems

“…For the MPBR unit, GHG total was correlated with electrical energy consumption and the CO 2 uptake by microalgae biomass according to [42,43]. Concerning the cross-flow ultrafiltration harvesting unit, GHG total was correlated with electrical energy consumption and the potential increase in methane yield for the subsequent digestion unit, due to the additional pre-treatment of microalgae induced by the cross-flow ultrafiltration, as observed under mesophilic conditions [52].…”

“…However, the operating period with fixed SRT and HRT results in emission savings 4% lower than those of the controlled SRT period. This was attributed to the higher biomass concentration in the PBR in the experiment from [43], leading to a lower concentration ratio in the harvesting plant and, consequently, lower biodegradability of the harvested biomass [52].…”

“…The final anaerobic digestion unit was defined to operate under mesophilic conditions as outlined in [52], and its thermal and electrical energy consumption, along with biogas production, were considered in the calculation of GHG total .…”