Effect of pyrolysis conditions on sewage sludge derived biochars for high value composites applications

The economy of the whole wastewater treatment system is significantly burdened by the increasing amounts of sewage sludge due to the progressive implementation of the Urban Waste Water Treatment Directive 91/271/EEC and by the complexity of the treatments required for guaranteeing a safe handling and a proper end-of-life of the sludge. For this reason, thermal treatments of sewage sludge have been studied in the past for their efficient valorization in terms of energy and/or matter recovery. Among them, pyrolysis represents a viable route aiming at the recycling of resources without production of harmful substances to the humans or the environment. A lot of work has been done on the use of sludge-derived char as a fertilizer and soil conditioner showing its safer application with respect to the untreated sludge. The nutrients were intensified with the temperature rising (except nitrogen) and the bioavailability and the leaching of heavy metals was reduced [1]. However, the physical and chemical characteristics of biochar can be exploited also for the production of high value-added materials. Carbon materials such as nanotubes received a great attention due to their ability to enhance mechanical, electrical and thermal properties of polymer composites [2], but high costs and low reproducibility have discouraged their use. In this study sludge-derived char (SCHAR) is studied as a possible alternative to other high cost carbon fillers. Sewage sludge from a civil wastewater treatment plant was pyrolyzed both in slow [3] and fast [4] pyrolysis conditions at three different temperatures, 500, 600 and 700 °C. A lignocellulosic biomass was also processed in the same experimental conditions for comparing the SCHARs with typical biochars (BCHARs). The influence of the temperature and the heating rate on the char yields and properties was evaluated with a particular attention to those physico-chemical characteristics relevant for the determination of the electrical and mechanical properties of the polymer composite. The produced chars were characterized in terms of elemental analysis, ash content and speciation and electrical conductivity. Morphology of the chars was studied through Scanning Electron Microscopy and adsorption porosimetry. The retention of the heavy metals in the chars during pyrolysis was also monitored. Chars having the highest values of electrical conductivity were used for preparing composites based on epoxy resins [5]. The complex permittivity and the mechanical properties of the composites were measured and an attempt to correlate the obtained values to the physical and chemical characteristics of the chars was made. As expected, the pyrolysis temperature affected positively the electrical conductivity of chars (c.f. Fig. 1), but in the case of SCHARs this is not trivially correlated to the increase of the BET surface that is significantly lower than the values obtained for the BCHARs.