Innovative treatment of tannery sludge (2008)

The Italian tan line of industrial activity is characterized by 2,400 companies and 30,300 workers. The companies are concentrated in four main industrial poles: S. Croce sull'Arno in Toscana, Arzignano and Valle del Chiampo in Veneto, Solofra in Campania and Turbigo Castano in Lombradia. The Italian tan activity is leader in the European Union and in the World too. The wastewater produced during the tan activity is collected to centralized industrial wastewater treatment plants. The most common depuration treatment of this wastewater is biological activated sludge. The excess sludge produced during the treatment process contents all the pollutants removed from water and it is still a waste to treat and dispose off. The tannery sludges are currently a big ecological and environmental problem in Europe and in Italy, too. The production in Italy is around 280.000 tons every year. No process carried out up to now, has been successful for a reuse of the sludge and the only possible destination remains the landfill. The aim of the current research work is to analyze the capacity of some biological treatments to be applied in the management system of tannery sludge. The target is to reach the social and environmental sustainability of management system of tannery sludge. The research work has regarded four topics: minimization of sludge and energy saving, aerobic stabilization of dry tannery sludge, biological hydrogen production from tannery sludge and environmental sustainability of tannery sludge landfill. Energy saving can be achieved evaluating the optimal conditions for the thermal drying system of tannery sludge. The optimization of the thermal drying system aims to characterize the minimum value of total solids content of the sludge, that inhibits or that stops the biological activity of degradation of organic substances in the sludge. The minimum total solid content emerged in the research work seems to be 75%. Consequently an amount of moisture of 25% is not sufficient to support microbial activity, and the dry sludge does not show appreciable biological degradation processes, independently by he biodegradability of organic substances in the sludge. Aerobic stabilization of mechanically dried tannery sludge before landfill, has the targets of stabilization of organic substances and reduction of water content. The advantages of a preliminary aerobic treatment are the reduction of environmental impacts of landfilling, better disposal conditions and low costs for treating plant investment and management. The results seem to be encouraging for the development of an aerobic treatment technology for the stabilization of tannery sludge. The treatment seems to work though the negative characteristics of the tannery sludge. Volatile solid content has been reduced, moisture content has been reduced depending on aeration rate, leachability of metals has been reduce. Even the biological activity, measured by respiromentric test and fermentative test, has reached the values suggested in the international literature as stable waste conditions. Biological hydrogen production is an interesting opportunity to produce hydrogen from renewable source, at low costs and sustainable environmental impacts. The biological processes are not only environmental friendly, but also they lead to open a new avenue for the utilization of renewable energy sources which are inexhaustible. For the investigation of the biological production of hydrogen by dark fermentation, laboratory tests have been conducted at a mesophilic process temperature of 35°C in batch operation. The results show a good production of hydrogen from glucose and also from Kitchen Waste. The hydrogen production from tannery sludge has been comparable with the results from kitchen waste but with slower production rate and long lag phase. The methane production has not been avoided. Probably the low biodegradability of tannery sludge and the fact that this substrate contents different types of biomass allows methanogenic bacterial to maintain or establish again their activity. The last part of present research work has been set up to understand the behaviour of tannery sludge dispose of in sanitary landfill, especially when the barrier systems will fail and it has regarded the evaluation of the sustainability of landfill disposal of dry tannery sludge. The emission of biogas and quality of leachate indicate that the degradation of organic substances start quickly and the thermal drying treatment does not sterilize the sludge. The biogas emissions and the leachate quality do not indicate a sustainable condition. Comparing these conditions with lysimeters contatining aerobically stabilize tannery sludge, the good effects of pre-treatment can be observed. The biogas production has been very low. Comparing the concentration of metals of lysimeters containing pre-treated sludge and not pre-treated sludge, it is possible to understand the effects of biological stabilization of sludge and the advantages to reach the environmental sustainability of landfill.