Possibilities of reduction of recipient loading of tannery wastewater in Slovenia

Abstract

The leather industry is well known as a high consumer of water (30 to 80 m³ for 1 ton of processed raw skins). At the same time this industry is known for the high specific pollution of wastewater. The main characteristics of tannery wastewater are: high salinity, high organic loading (COD, BOD₅), high content of ammonia and organic nitrogen, and presence of specific pollutants (sulphide, chromium).

The largest Slovenian tannery (IUV Vrhnika) processing 40 to 50 t of pig skins daily has a good system of physico-chemical pre-treatment of their wastewater (coagulation and flocculation with Al-sulfate and anionic polyelectrolyte). In pre-treatment, about 60% of organic substances are removed (COD and BOD₅) and above 95% of sulfide and chromium. Concerning Slovenian regulations and taking into consideration the fact that this tannery is located on the relatively small and slowly running river Ljubljanica such pre-treatment is not sufficient. The company would like to supplement pre-treatment with biological treatment.

Laboratory and pilot scale feasibility experiments including conventional biological treatment with activated sludge and combined anaerobic-anoxic-aerobic treatment with fixed biomass using mechanical pretreated wastewater and pretreated wastewater with flotation were performed. It became clear that effective biological treatment requires long retention times (several days) for applicable effluent.