Optimization of hydrogen production using a coculture of Chlamydomonas reinhardtii and activated sludge bacteria
Abdelsalam Zidan, Muhammad Asad Javed, Ashraf Aly Hassan
Abstract
The production of biophotolytic hydrogen (H 2 ) relies on the effective management of oxygen (O 2 ) levels. Coculturing bacteria with microalgae helps mitigate the excess O 2 produced by algal cells. After depleting O 2 , the bacteria activate the enzyme hydrogenase in microalgae, leading to H 2 production. In this study, Chlamydomonas reinhardtii was cocultured with indigenous bacteria from activated sludge at varying algae-to-bacteria ratios (1:1, 1:1.5, 1:2, 1:2.5, and 1:3 v/v), with an illumination intensity of 2.8 mmol/m 2 /s (31 × 10 3 lux). The 1:1.5 v/v ratio yielded the highest H 2 volume (1162 mL/L) and the highest O 2 concentration (153.2 mL/L) over a 6-day period. Production of all gaseous components ceased for all ratios as the pH dropped below 4 due to acetate accumulation, and the concentration of acetate reached approximately 1 g/L by the end of each experiment. Gas composition analysis after the first day of coculture revealed that H 2 , CO 2 , N 2 , and O 2 constituted 25%–46%, 20%–40%, 5%–30%, and 1%–10% of the total gas volume, respectively. Glucose (10 g/L) was introduced as an external carbon source for all cultures. After 6 days, the coculture maintained a high total organic carbon (TOC) level of 3.1 g/L, whereas the initial TOC ranged between 3.9 and 4.3 g/L. The findings illustrated a significant correlation between H 2 production, acetate accumulation levels, and O 2 consumption. The algae–activated sludge coculture method substantially enhanced H 2 production compared with previously published methods employing only one or two types of bacterial cultures, underscoring its potential for more efficient biophotolytic H 2 production. • Coculture of C. reinhardtii and activated sludge improves H 2 production. • The coculture at the 1:1.5 ratio achieved the highest H₂ yield of 1162 mL/L. • Oxygen consumption by bacteria enhances hydrogenase activity. • Acetate accumulation impacts pH, affecting gas production efficiency. • High illumination intensity enhances biohydrogen production.