• Energy Research

Abstract Anaerobic thermochemical carbonization can be used to treat sewage sludge and facilitate its reuse. The aim of this study was to compare and evaluate chars produced by pyrolysis and hydrothermal carbonization (HTC) of sewage sludge, so-called sewchars, under varying process conditions in terms of their soil amelioration potential. Three different sewage sludges, two pyrolysis and four HTC sewchars were evaluated with respect to their stability in soil, perspective for nutrient recycling and potential use as soil amendments. The sewchars were produced at different temperatures (pyrolytic low temperature conversion LTC 400 °C; PYREG pyrolysis 700 °C; HTC 180–200 °C) and for varying treatment times (LTC and PYREG 1 h; HTC: 4–8 h). O/C and H/C ratios of about 0.7 and 1.8 respectively, indicated that HTC sewchars will tend to be less recalcitrant in soils than pyrolysis sewchars. The duration and temperature of HTC correlated positively with the chars’ contents of adsorbed labile carbon and nitrogen compounds. However, HTC failed to increase the chars’ resin-extractable phosphorus contents. The cation exchange capacity of sewage sludge was reduced by HTC but stayed on a higher level than after pyrolysis. With increasing treatment temperature, the pH, ash (content) as well as macro- and micronutrient contents increased. Pyrolysis sewchars also accumulated heavy metals; however their contents were lower than those reported previously. An enrichment of heavy metals was also observed for the HTC sewchars, albeit to a much lesser extent.

Abstract Anaerobic thermochemical carbonization can be used to treat sewage sludge and facilitate its reuse. The aim of this study was to compare and evaluate chars produced by pyrolysis and hydrothermal carbonization (HTC) of sewage sludge, so-called sewchars, under varying process conditions in terms of their soil amelioration potential. Three different sewage sludges, two pyrolysis and four HTC sewchars were evaluated with respect to their stability in soil, perspective for nutrient recycling and potential use as soil amendments. The sewchars were produced at different temperatures (pyrolytic low temperature conversion LTC 400 °C; PYREG pyrolysis 700 °C; HTC 180–200 °C) and for varying treatment times (LTC and PYREG 1 h; HTC: 4–8 h). O/C and H/C ratios of about 0.7 and 1.8 respectively, indicated that HTC sewchars will tend to be less recalcitrant in soils than pyrolysis sewchars. The duration and temperature of HTC correlated positively with the chars’ contents of adsorbed labile carbon and nitrogen compounds. However, HTC failed to increase the chars’ resin-extractable phosphorus contents. The cation exchange capacity of sewage sludge was reduced by HTC but stayed on a higher level than after pyrolysis. With increasing treatment temperature, the pH, ash (content) as well as macro- and micronutrient contents increased. Pyrolysis sewchars also accumulated heavy metals; however their contents were lower than those reported previously. An enrichment of heavy metals was also observed for the HTC sewchars, albeit to a much lesser extent.

AbstractCombing hydrothermal carbonization (HTC) of sewage sludge with wet oxidation of the resulting process waters (PWs) is a favorable economic process leading to a wastewater that is better amenable to biological treatments. Five PWs from HTC of sewage sludge and straw were oxidized with molecular oxygen. A good removal of chemical oxygen demand and dissolved organic carbon was shown. The oxidized waters proved good substrates for a subsequent anaerobic digestion. However, the oxidized PWs were identified as more toxic in germination tests with cress (Lepidium sativum L.) compared to the untreated PWs.

AbstractCombing hydrothermal carbonization (HTC) of sewage sludge with wet oxidation of the resulting process waters (PWs) is a favorable economic process leading to a wastewater that is better amenable to biological treatments. Five PWs from HTC of sewage sludge and straw were oxidized with molecular oxygen. A good removal of chemical oxygen demand and dissolved organic carbon was shown. The oxidized waters proved good substrates for a subsequent anaerobic digestion. However, the oxidized PWs were identified as more toxic in germination tests with cress (Lepidium sativum L.) compared to the untreated PWs.

The carbonization of biomass residuals to char has strong potential to become an environmentally sound conversion process for the production of a wide variety of products. In addition to its traditional use for the production of charcoal and other energy vectors, pyrolysis can produce products for environmental, catalytic, electronic and agricultural applications. As an alternative to dry pyrolysis, the wet pyrolysis process, also known as hydrothermal carbonization, opens up the field of potential feedstocks for char production to a range of nontraditional renewable and plentiful wet agricultural residues and municipal wastes. Its chemistry offers huge potential to influence product characteristics on demand, and produce designer carbon materials. Future uses of these hydrochars may range from innovative materials to soil amelioration, nutrient conservation via intelligent waste stream management and the increase of carbon stock in degraded soils.

The carbonization of biomass residuals to char has strong potential to become an environmentally sound conversion process for the production of a wide variety of products. In addition to its traditional use for the production of charcoal and other energy vectors, pyrolysis can produce products for environmental, catalytic, electronic and agricultural applications. As an alternative to dry pyrolysis, the wet pyrolysis process, also known as hydrothermal carbonization, opens up the field of potential feedstocks for char production to a range of nontraditional renewable and plentiful wet agricultural residues and municipal wastes. Its chemistry offers huge potential to influence product characteristics on demand, and produce designer carbon materials. Future uses of these hydrochars may range from innovative materials to soil amelioration, nutrient conservation via intelligent waste stream management and the increase of carbon stock in degraded soils.