Ferrous step-change method for kLa measurement in an airlift bioreactor containing iron sulfate broth for energy storage

Abstract

The transition to renewable energy will require significant grid level energy storage capacity to maintain electrical supply and demand balance. A biological hydrogen-based energy storage technology using ferric sulfate as an oxidant in a proton exchange membrane fuel cell, termed the BioGenerator, is studied in this work. Operation of the fuel cell generates ferrous sulfate which must be oxidized through use of iron oxidizing bacteria and oxygen in an airlift reactor. The volumetric oxygen mass transfer coefficient (k_La) in the airlift bioreactor is investigated for use in design optimizations to minimize energy expenditure in aeration within the system. A novel high-precision technique of determining k_La profiles in bioreactors using aerobic chemolithotrophs in ferric sulfate broths is explored to meet this goal. The new method makes use of measurable ferrous iron bio-oxidation with a redox probe along with bioreaction stoichiometry to solve for oxygen uptake rate. The procedure, deemed the static ferrous step-change method, is compared against the dynamic biological in-situ method and proves to be superior in terms of scalability, time savings, ease of use, and data quality. Aeration mixing effect on dissolved oxygen homogeneity within the bulk liquid in the downcomer packed bed of semi-pilot and pilot scale airlift bioreactors used in the study are investigated. The developed static ferrous-step change method may be applicable for practical use in any operation where aerobic iron-oxidizing acidophilic microbes are present in iron sulfate based liquor or leachate, potentially useful in processes such as copper or uranium extraction.