Evaluation of photosynthetic light integration by microalgae in a pilot-scale raceway reactor

Abstract

The improvement of photosynthetic efficiency in a 100 m 2 raceway reactor by enhancement of light regime to which the cells are exposed is here reported. From Computational Fluid Dynamics it was calculated that the light exposure times ranged from 0.4 to 3.6 s while the exposure times to darkness were much longer, from 6 to 21 s. It was demonstrated that these times are too long for light integration, the cells fully adapting to local irradiances. This phenomenon was validated in the real outdoor raceway at different seasons. Simulations allows to confirm that if total light integration is achieved biomass productivity can increase up to 40 g/m 2 •day compared to 29 g/ m 2 •day obtained considering local adaptation, which is close to the experimental value of 25 g/m 2 •day. This paper provides clear evidence of microalgae cell adaptation to local irradiance because of the unfavourable cell movement pattern in raceway reactors.

Continuous 3-year outdoor operation of a flat-panel membrane photobioreactor to treat effluent from an anaerobic membrane bioreactor

“…move rapidly from the lit parts of the reactor to darker zones, which improves biomass productivity (Barceló-Villalobos et al, 2019). Light path is a key parameter in the light available to the reactor, since the radiated light decreases with depth due to the light absorbed by the water and biomass(Cho et al, 2019).Other important variables in MPBR performance were OD680, VSS, P, HRT, TEC, Chl, NH 4 , N t , NOxR and BRT (Figure 6b).…”

Dataset to assess the shadow effect of an outdoor microalgae culture

“…1 ). The microalgae close to the surface in a photobioreactor (PBR) absorb most of the photons, restricting the light received in the inner part of the PBR [2] . This is known as shadow effect or self-shading [3] , [4] .…”

Effect of light intensity, light duration and photoperiods in the performance of an outdoor photobioreactor for urban wastewater treatment

“…Photoinhibition can be reduced by combining periods of high light irradiance with periods of darkness [27]. Since algae have been reported to respond to light intensity almost instantaneously [28], the temporary lack of light is considered to allow the dark reactions of photosynthesis, which are slower than the light reactions [30], to use the stored energy from light reactions [25] without the addition of extra photons that cannot be used for photosynthesis. In fact, the excess of photons absorbed by microalgae is emitted as heat or fluorescence and reduce photosynthetic efficiency [11,22,31].…”

“…Also, the cells near the surface absorb most of the applied light irradiance, causing a dark zone where photosynthesis is limited [30,43], known as the shadow effect or selfshading [28,33,41]. The volume of the dark zone depends on the microalgae biomass concentration, microalgae pigments, light intensity, light path, culture turbidity and PBR opacity [22,25,28,44].…”

“…Mixing of the microalgae culture can help to mitigate this shadow effect since it involves the movement of algae from the highly illuminated areas of the reactor to dark zones [30], therefore reducing photoinhibition [46] and applying a random FLE to the culture [47]. In contrast, mixing is usually poor within open systems [30].…”