Energy demand reduction through TMP-based backwashing control in a pilot-scale vertical upflow multi-column membrane photobioreactor for wastewater reclamation

Wastewater reclamation via microalgal-bacterial membrane bioreactors are a recent approach to promote carbon neutrality and a circular water economy. Although membrane fouling control is a critical issue, there is a lack of knowledge on the application of automatic control cleaning strategies. This study proposes a feedback control system for backwashing initiation based on transmembrane pressure ( TMP ) monitoring. A pilot-scale, vertical upflow multi-column membrane photobioreactor (VUC-MPBR) treating secondary effluent was used. The system successfully operated outdoors for over 100 days, achieving robust organic matter removal (58.3%), moderate nutrient recovery (21.3% and 27.8% for nitrogen and phosphorus, respectively), and efficient biomass separation (0.6 g/L in the recirculating suspension and 10.3 g/L of concentrated biomass at the bottom of the columns). Direct-flow filtration with the automatic backwashing strategy enhanced fouling control at high permeate fluxes (42.5 L/h m 2 ), reducing the specific energy demand for membrane scouring by 55% compared with the conventional backwashing strategy. Residual fouling analysis indicated that backwashing flux played a dominant role, whereas duration had a comparatively minor effect. TMP set-point value to be established resulted to be crucial to avoid a large pressure drop across the cake layer inducing cake compression. Under sustainable fouling conditions, the control system provided high net permeate fluxes (35.6–36.1 L/h m 2 ) with low specific energy demand for membrane scouring (0.028–0.040 kWh/m 3 ) for over 500 h, representing part of a total process specific energy demand of approximately 0.23–0.25 kWh/m³ (including liquid recirculation and permeate filtration/backwashing). Therefore, a VUC-MPBR operated with TMP sp -controlled backwashing represents a low-energy demand process configuration for microalgae-based wastewater reclamation.\