Abstract
Continuous biodegradation of air pollutants i.e., volatile organic compounds (VOC), which are released to the atmosphere during many industrial operations, is one of the fastest-growing areas of bio-technology. Substantive bacterial growth, immobilization and VOC-degradation reaction, accompanied by typical fluctuations of other variables like pH, can significantly alter the general corrosion rate as well as potential for localized corrosion. Results from on-line corrosion measurements performed utilizing multiple electrochemical techniques in a large-scale, continuous bio-degradation of VOC mixture are presented in this paper. The VOC mixture containing styrene, alcohol and a sulfur compound, was decomposed involving a mixture of Pseudomonas and Thiobacillus bacteria immobilized on polypropylene rings. Corrosion measurements were obtained by using industrial-type, electrochemical probes with electrodes made of carbon and stainless steels (UNS G10180, UNS S31603; UNS S30400). Four corrosion variables – general corrosion rate, localized corrosion potential (Pitting Factor), Stern-Geary Co-efficient (B Value) and capacitance (CMI) were continuously recorded concomitant with other process parameters such as solution pH, oxygen concentration, flow rate, temperature etc. Observed fluctuations of corrosion variables were in line with bacteria growth showing the build-up of the bio-film. Significant potential for localized corrosion (Pitting Factor) was observed to be accompanied by fluctuations in bio-process parameters.
Introduction
Environmental regulations are continuously pushing industry for emission reduction of NOx, CO, H2S and other poisonous low-molecular weight gases. Volatile organic compounds (VOC) represented by carbon-based chemicals like vinyl acetate, ethyl acetate, styrene or dimethylsulphide (DMS), however less known to public, play significant role in air poisoning. Industrial emission of those substances is much lower in comparison to gases like NOx, but their high toxicity, reactivity and consequent severe interference with living organisms puts them at the top of a list of industrial pollutants. Effective removal of VOC from off-gases is a challenging process due to very low concentration of these toxic compounds.
Among many chemical and physical VOC cleaning/removal processes such as absorption, oxidation or chemisorption, biological techniques play a special role due to their low cost and high efficiency in VOC reduction. Trickle-bed bioreactor (TBB) system utilizing immobilized bacteria species is one of most promising solution for biological removal of VOC. Industrial bioreactors, mostly constructed from stainless steels and operated with the presence of living bacteria are continuously exposed to the threat from Microbiologically Influenced Corrosion (MIC). Degradation of different VOC usually requires specific bacteria strains and hence, there is no single approach to the MIC problem in these bio-reactors.