For the cultivation of Dunaliella salina (a green unicellular eukaryote photosynthetic microalga), a 20 L indoor helical‐tubular photobioreactor was designed. The inner diameter and the thickness of the PU (polyurethane) tube were 12 and 2 mm, respectively, and its length was 75 m. An open pond was located on the top of the PBR structure and a pump circulated the culture medium from the pond to the tubes. Another part of the tube was connected to an airlift column (which was connected to the bottom of the pond), and the culture medium completed its circulation by moving from the airlift column that connected the closed system (tubular) to the open system (open pond). Eight LED lamps with 10 000 lx were set around the tube and a 2000 lx LED was adjusted on the top of the pond. The culture salinity within the PBR was 1 mol L−1 and four intermittent steps of 0.5 mol L−1 salt stresses were injected into the culture medium. The highest beta‐carotene production within this hybrid helical‐tubular PBR was 4.85 µg of beta‐carotene per mg of dry weight of microalgae at 2.5 mol L−1 salinity.

Light is one of the most critical factors for the growth of microalgae; therefore, optimization and accessibility of light improve productivity and wastewater nutrient removal is important. The flashing light effect allows microalgae to use light effectively through intermittent exposure. Nutrient assimilation and membrane fouling were considered in a reciprocal membrane photobioreactor (RMPBR) and an LED flashing light was provided as a source of illumination. The objective of this study was to bridge the gap between light delivery and RMPBR performance. In this connection, we examined flashing lights with frequencies of 1 Hz and 1000 Hz at a constant duty cycle of 60% along with continuous light. Regarding the effect of the flashing light on RMPBR, the cells which acclimated to flashing light of 1000 Hz resulted in maximum removal of nitrate (97%) and phosphate (70%). However, the culture with the low frequency of 1 Hz was only able to remove nitrate and phosphate up to 68% and 47%, respectively. Furthermore, the reactor with 1-Hz flashing light frequency was fouled earlier within 84 h thereby reaching the highest pore-blocking. Proteins and carbohydrates were recognized as a major cause of fouling in the lowfrequency flashing light. We conclude that high-frequency flashing light (1000 Hz) could be an effective light condition for nutrient uptake in wastewater treatment.

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The membrane photobioreactor (MPBR) is a well‐operated system concerning microalgae cultivation and nutrients assimilation from wastewater effluent. In the present paper, a sample of pulp and paper wastewater was primarily treated by activated sludge system (ACS), and the concentration of nitrate and phosphate decreased as about 26 and 10%, respectively. Then, it was transferred into six flat plate MPBR systems with 5 L capacity and 0.45 μm membrane pore size for the secondary treatment process (nitrate and phosphate assimilation) by Chlorella vulgaris (green microalgae) species during six cultivation periods. In this section, the effects of different light intensities (100 and 300 μmol m−2 s−1) and light–dark cycles (24–0, 16–8, and 12–12) on nitrate and phosphate uptake through the treated pulp and paper wastewater effluent after 24 hr were investigated. The maximum photosynthetic productivity and nitrate‐phosphate removals after 24 hr (nitrate: 57% and phosphate: 43%) were recorded for the culture under 300 μmol m−2 s−1 and 24–0 light–dark regime within the MPBR system.

BACKGROUNDThe membrane photobioreactor is a combination of membrane technology and the photobioreactor system. It is an economic and efficient system for microalgae cultivation. Light cycles and its intensity are significant parameters in the biomass production and synthesis of metabolites, such as protein and carbohydrates, within the microalgae cells. These parameters can, therefore, have a significant impact on the rate and characteristics of fouling in membrane photobioreactor systems.RESULTSIn this work, Chlorella vulgaris (green microalgae) was cultivated within the designed reciprocal membrane photobioreactor (RMPBR) under 24–0, 16–8, and 12–12 light–dark (L/D) cycles. Also, the transmembrane pressure (TMP) and different hydraulic membrane resistances (cake layer, pore‐blocking, and fluid resistances) were investigated. The 16–8 L/D cycle had the fastest fouling time (96 h), highest pore‐blocking resistance (Rp), and the maximum concentrations of protein and carbohydrate in the supernatant and permeate flux. However, the 12–12 L/D recorded the maximum cake layer resistance (Rc), the highest content of protein within the cake layer, and the longest fouling time (226 h). Additionally, the highest carbohydrate concentration within the cake layer was for the 24–0 L/D regime, and it was fouled after 162 h.CONCLUSIONThe Rc for each cycle had the highest values compared to the other calculated resistances. When the membrane is fouled, the mechanical membrane cleaning method employed in the RMPBR, which is in the form of an automatic spongy blade, cleans the cake layer on the membrane and improves its operation up to 80% via decreasing the Rt of the system. © 2020 Society of Chemical Industry (SCI)