Friday, March 14, 2014

Sulphate Reducing Bacteria (SRB) as harmful inhabitants of process waters

SRB is an interesting group of bacteria which can use sulphate as an electron acceptor for the respiration. Despite the chances of certain other bacteria to use sulphate as the sulphur source for their S-containing cell components, the "real" sulphate reducers transform SO4(2-) to S(-2) in their energy metabolism and oxygen actually inhibits their growth - they are therefore obligate anaerobes. Certain yield of energy may be achieved through fermentation by SRB's but this type of metabolism is regarded as relatively insignificant one for them.

These bacteria have first detected in waste waters of sulphite pulp mills but modern paper machine processes can also induce their growth if certain sulphur-containing compounds are available.

In addition to H2S production (which is a hazardous gas), colour problems can arise because the metal sulphides. FeS is an indicator compound in the analytical detection of SRB's but also a harmful agent of discolorization of paper and paperboard. Last but not least, SRB's have been shown to be conneceted to a certain type of iron corrosion and it is all possible to find those problems still today whenever technical structures with poor steel quality and certain types of organic deposits on their surfaces are combined.

SRB's have also other, peculiar features like the tendency to follow non-exponential growth curve. They have been the subject of firm microbiological research only since the middle of 20th century because their need of anaerobiosis was not understood earlier.

A new point of view shows the negative effects of high sulphate concentrations in lakes and rivers which receive waste water of bioleaching mining. If sediments of hypolimnion (lower water column of the lake) will be rich of sulphate and oxygen levels will simultaneously drop, the production of poisonous H2S is obvious. An article about the effects of sulphate in natural waters will be published soon by my fellow microbiologist and me. I will inform You when it happens.
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