Dissolved organics in tannery wastewaters and their alteration by a combined anaerobic and aerobic treatment

The dissolved organic load of tannery wastewater and of the effluents of an anaerobic and aerobic treatment were investigated over a 2 yr period. The average dissolved organic carbon content (DOC) of raw wastewater is 900 mg 1−1, corresponding to a discharge of 23 kg DOC t−1 raw hide. The two step biological treatment removes an average 85% of the DOC and 50% of the UV254-absorbance. The high initial toxicity of tannery wastewater, as determined in the luminescence inhibition test (EC50: 5 ml 1−1) is diminished by two orders of magnitude. GC-MS screening analyses of fractionated wastewater extracts determined about 200 individual components in the various wastewater qualities. Identified substances were grouped into 12 substance classes, among them aliphatic and aromatic carboxylic acids, alcohols, phenols, indoles, cyclohexanes and ethoxylates, and their contents were quantitatively assessed. Anaerobic and aerobic treatment exhibit significantly different impacts onto the dissolved organic matter: reproduction of raw wastewater constituents and formation/release of new components are important processes under anaerobic conditions; aromatic compounds are poorly removed. Aerobic processes aim at mineralization and are, thus, effective against all substance classes. Despite the substantial improvement of the tannery wastewater quality by the two step biological treatment, some luminescence inhibition remained (EC50: 400 ml 1−1) and persistent substances of environmental concern were identified in the fully treated effluents.