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Gasification residues/chars (GR) and activated carbon (AC) are added to wastewater treatment processes mainly as a fourth purification stage, e.g., to adsorb heavy metals or pharmaceutical residues. However, the effects of GR or AC, which are transferred to the anaerobic digestion (AD) via the sludge, are not yet fully understood. Although, the positive effect of char addition on AD has been demonstrated in several investigations, systematic studies with chemically well described chars are still missing. Therefore, in this study, different chars were characterized in detail, subjected to AD in different concentrations, and their effect on methane production investigated. GR of a gasification plant with a floating fixed bed technology, carbon made by chemical impregnation with ZnCl2 from waste-wood, carbon produced by thermochemical activation with CO2 from GR and commercial powdered AC were used for the experiments. Among others, thermogravimetric analysis, physisorption, pH, and conductivity analysis were used to characterize the chars. Mesophilic AD batch tests with different concentrations (0.025, 0.05, 0.5, 1.0, 7.0, 14.0 gL−1) of all chars (GR and ACs, respectively) were performed with digester sludge from a wastewater treatment plant for a period of 47 d. Volatile fatty acids (VFA) as well as biogas production and CH4 concentrations were monitored. It could be shown, that concentrations below 1.0 g char L−1 did not result in significant effects on CH4 and/or VFA production, whereas high concentrations of GR and AC influenced both, the CH4 yield and kinetics. Depending on the production process and the characteristics of the chars, the effect on AD varied, whereby both, positive and negative effects on biogas yield and methane production were observed. This study provides the first systematic evaluation of char application to AD processes, and therefore allows for better predictions of char applicability and effect.

In order to substitute fossil resources in activated carbon (AC) production, recent efforts have focused on the utilization of renewable raw materials. Regions with important wood industries offer two potentially underestimated resource types: forestry residue biomass (FRB) and waste wood (WW). Although these materials are widely available (approx. 130 mio. m3a-1 FRB, approx. 50 mio. ta-1 WW in the EU), they are mostly valorised through energy production, as they are high in ashes and may be contaminated with organics and heavy metals. In this study, both FRB and WW were treated via one-step pyrolysis for AC production. ZnCl2 was applied as activating agent at pyrolysis temperatures varying from 400 to 600 °C and residence times between 1 and 3 h. Overall, 76 samples were prepared and characterized thoroughly via elemental analysis, N2/CO2 ad/-desorption, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and infrared spectroscopy (FTIR). The produced carbons showed specific surface areas of up to 1430 m2g−1 and a pore size distribution with a micropore share of up to 80 %. The presence of oxygen-containing functional groups was confirmed via FTIR. Potential feedstock contamination can be mitigated, as minerals and heavy metals could be leached out (up to –99.15 %) by an additional wash step and organic contamination undergoes thermal cracking during pyrolysis. The presented study could be a next step in upcycling considerable waste streams from the wood sector through localised and non-fossil-based AC production.

As the use of medicine increases worldwide, more and more drugs are found in wastewaters. It is now known that wastewater is a main pathway to enter the environment. Even state-of-the-art WWTPs are not able to remove these organic micropollutants (OMPs), or only to a limited extent. Therefore, an additional treatment step can be required and activated powdered carbon (APC) could be a possible solution. In this work, phosphoric acid is used for chemical impregnation and further activation. Two reactor approaches were used, a standard tubular (TR) and a fluidized bed reactor (FBR). Reaction times (RT) vary between 0.5 and 2/1 h at 700 to 1000 °C. For comparison, various parameters were carried out, such as specific surface area (SSA), total carbon (TC), yield and pore size distribution. The new FBR achieved higher SSA (1354.19 m2g-1) and a better pore distribution while using less impregnation agent (IA), lower temperatures and a shorter RT.