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AbstractThe Water Company of the North Alentejo (Águas do Norte Alentejano) (Portugal) 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 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 these 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 biodegradability degree. This assay was performed according to the methodology indicated in the test 301E of OECD [1], taking into account the 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 stabilization 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 chemical and biological parameters as it was 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 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 did not affect significantly the wastewater biodegradability. The Cr (III) was easily removed by a chemical coagulation-flocculation process.
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