Abstract

The essay presents the research on the impact of chromium compounds on the effectiveness of biological treatment of effluents by activated sludge, the structure of activated sludge, the composition of microfauna and sedimentation characteristics of activated sludge. The determinations of the indexes of pollution of wastewater have been carried out according to Polish standards, with the exception of CODCr of biologically treated effluents containing hexavalent chromium. For such effluents a modified method for marking CODCr has been developed, eliminating the impact of hexavalent chromium added to effluents.

In the research it has been established that oxygen indicators were not a reliable criterion for the assessment of impact of chromium compounds on activated sludge. Their value did not reflect the level of activated sludge degradation. The effluents treated with strongly degraded activated sludge, with a completely fibrous structure and very little of microfauna had low values of oxygen indicators, testifying to effective treatment. With the increase of chromium concentration in the effluents undergoing treatment there was an increase in the count of filiform bacteria, most probably that is why; notwithstanding the high concentrations of chromium compounds, activated sludge retained metabolic activity. In this situation it was impossible to determine the allowable concentrations of chromium for activated sludge, due to the fact that it was impossible to establish which part of organic substances was metabolized by heterotrophic bacteria characteristic of activated sludge, and which part by filiform bacteria.

It was also misleading to analyze the sedimentation characteristics of activated sludge on the basis of sludge volume index (SVI). Strongly degraded sludges with completely fibrous structure were characterized by low values of volume indexes testifying to good sedimentation characteristics. In fact, such fibrous sludge sedimented very well, but the reasons for this were adsorbed chromium compounds.

In the research it has been established that oxygen indicators were not a reliable criterion for the assessment of the impact of chromium compounds on activated sludge. The microscopic structure research and microfauna composition examination of the activated sludge proved necessary.