Synergistic interplay of photobioreactor mixing and static mixer configuration on Rhodoseupdomonas palustris’s growth and biohydrogen production

Abstract

• Mixing influenced R. palustris ’s biomass and biohydrogen productivity. • Low L/D cycling frequencies are sufficient to alleviate the mutual shading effect. • Regarding biomass productivity a 70 % increase was recorded with static mixers. • Static mixers increased cumulative and specific hydrogen production 11.2-fold and 114 % at 0.15 L/min. Inducing light/dark (L/D) cycles through mixing or recirculating the media in a photobioreactor improves light utilization for biohydrogen production during photoferentation. However, the mutual shading effect is a key challenge for photofermentation using purple non-sulphur bacteria (PNSB). In this study, mixing was explored as a mitigating methodology through the investigation of three different static mixer configurations: concentric, spiral, and half-moon, across four laminar flowrates: 1.2 L/min, 0.71 L/min, 0.31 L/min and 0.15 L/min. At 0.15 L/min, the half-moon and spiral static mixers significantly improved specific and cumulative hydrogen production by 11.2-fold and 114 %, respectively. Also, recirculating the fluid via a peristaltic pump resulted in a significant shearing effect, impacting cell viability. Inducing L/D cycles via a static mixer was thus beneficial for hydrogen production over an increased flow rate. Additionally, biomass productivity increased by up to 70 %, reaching a cell dry weight of 7.0 g/L with the spiral static mixer configuration at 0.15 L/min flow rate. Interestingly, this study found that the static mixers only significantly increased specific hydrogen production and light conversion efficiency at the lowest flow rate of 0.15 L/min, beyond which they showed no significant improvement in biohydrogen production. This indicates that high L/D cycling frequencies are unnecessary to alleviate the mutual shading effect for R. palustris, which is advantageous for upscaling. These findings emphasise the potential of static mixers as an alternative to increase light conversion efficiency and industrial hydrogen production.