2.3. Gas-Lift Reactor Inoculation and Operation

A glass gas-lift reactor with a working volume of 4 L (Figure ​Figure11) was used for a larger scale experiment on hyperthermoacidophilic sulfur reduction with H₂ as the electron donor and CO₂ as the carbon source. The temperature of the reactor was maintained at 80 °C with a water jacket heated with a Julabo F32-HL heater (Julabo, Seelbach, Germany), and the pH in the reactor was controlled at 3.5 using 1 M sulfuric acid. The reactor was inoculated with 15 g·L^–1 of wet Emmen sludge, and elemental sulfur (41 grams, dry weight) was suspended in 50 mL anaerobic demineralized water and supplied to the reactor through a feed port. The reactor was operated in a semibatch mode by replacing 1 L of reactor solution by fresh medium every 8 days to ensure sufficient supply of nutrients. Additional sulfur was supplied regularly, aiming to maintain a sulfur concentration of at least 10 g·L^–1. Sulfur has a tendency to accumulate as a foam layer above the water–gas interface on the top of the reactor. The accumulated sulfur was regularly manually pushed back to the reactor solution with a piece of tubing. Hydrogen and carbon dioxide were supplied as the electron donor and as the carbon source, using two mass flow controllers (type El-FLOW, model FG-201CV-RAD-22-V-DA, Bronkhorst) (Veenendaal, the Netherlands) at a rate of 1.5 and 0.5 L·h^–1 (volume at standard conditions), respectively. Enhanced gas transfer and mixing was achieved by recirculating the gas at a rate between 50–80 L·h^–1 using a vacuum pump KNF Neuberger type N820.3FT.18 (Freiburg, Germany). Gas and liquid flow tubing and connections were made from PTFE (Schott A.G, Mainz, Germany and Serto A.G, Fuldabrück, Germany). The redox potential and pH were continuously measured with a Prosense QP181X-TJ/Ag/AgCl/12 × 250 mm glass electrode.

Scheme of the gas-lift bioreactor with H₂S stripping in the outlet gas.

During the first 67 days of operation, a H₂S absorption solution was installed in the recycle gas line. However, this led to inefficient scrubbing and an underestimation of sulfide production. Therefore, on day 67 the configuration was changed as shown in Figure ​Figure11, with absorption of H₂S from the effluent gas in a 2 M NaOH solution, after which sulfide production could be monitored effectively.