An integrated approach to assess the biodegradability of a waste water containing chromium by using chemical and biological methods

The Águas do Norte Alentejano (Water and Wastewater Service Company belonging to the Águas de Portugal Company) has decided to implement a treatment plant to treat an urban wastewater with a significant influence of tannery effluents. The design of the wastewater treatment plant was attributed to a consortium coordinated by Sisaqua Company. Since the levels of the biodegradability and toxicity of the wastewater to be treated were unknown, Sisaqua Company asked the UBiA/New University of Lisboa to develop and execute a monitoring plan to define the chemical composition and the levels of the biodegradability and toxicity of the wastewater. A composite sampling procedure of the wastewater was performed during a daily working cycle of the tanneries. The composite sample was characterized for 19 chemical parameters. Five of those parameters were total Cr, Cr (VI), Zn, Cu and Al, which were determined in the dissolved and suspended fractions. The composite sample was also submitted to a biological test to assess its degree of biodegradability. This assay was performed according to the methodology referred in the test 301E of OECD [1], taking into account its amendment of 1992. A respirometric assay was carried out to assess the toxic effect of the composite sample on the biological populations involved in the biological oxidation of organic compounds. This assay followed the "Sapromat" methodology developed by Brabander and Vandeputte [2]. Finally, the composite sample was submitted to a coagulation-flocculation process, according to the methodologies proposed by Eckenfelder [3, 4] and Pereira [5]. The supernatants were characterized for the same parameters as it was previously described for the composite sample. It was possible to conclude that the high levels of Cr present in the composite wastewater were mainly in the trivalent oxidation state. Since this oxidation state is less toxic for biological populations than the Cr (VI) and this metal was mainly in the suspended fraction, it was possible to obtain a biodegradability degree of the organic compounds quite similar to those observed for domestic wastewaters. The toxicity determined by the "Sapromat" method was slightly higher than those determined for domestic wastewaters. Nevertheless, this toxicity level did not affect significantly fee wastewater biodegradability. The Cr (III) was easily removed by a chemical coagulation-flocculation process.