Challenges and knowledge gaps in corrosion and materials degradation of bioreactors for waste feedstocks.

Industrial and environmental biotechnologies (IB/EnvB) are able to transform wastes and agro-industrial side-streams into bio-based products such as biomethane, biohydrogen, volatile fatty acids, lactic acid, succinic acid, ethanol, butanol, biosurfactants, enzymes and polyhydroxyalkanoates. Common corrosion drivers in the microbially-mediated production of these products are water, chlorides, hydrogen sulphide, acids, alcohols, biofilms, and concentration gradients or 'dead' zones within the bioreactor. The demand for cheaper bioreactor vessels, combined with the increasing use of these more challenging feedstocks, creates uncertainty about the durability of bioreactor materials. Current structural materials such as stainless steel, coated metals and composites offer good resistance to a wide range of exposure conditions; but may be more vulnerable to microbially-influenced corrosion and abrasion in novel bioprocesses utilising waste-type feedstocks. Existing standard test methods are insufficient, as they do not adequately characterise the potential for bioreactor materials degradation in realistic IB/EnvB conditions under long-term operation with combined corrosion-abrasion phenomena. Future research must prioritise identification and application of suitable testing strategies to ensure that the bioreactor manufacturing industry continues to provide reliable, cost-effective bioprocessing infrastructure for the new sustainable bio-economy.