Production of energy (biodiesel) and recovery of materials (biochar) from pyrolysis of urban waste sludge

Abstract

Safe disposal of sewage sludge is one of the most pressing issues in the wastewater treatment cycle: at the European Union level, sludge production is expected to reach 13 Mt by year 2020. Sludge disposal costs may constitute up to, and sometimes above, 50% of the total cost of operation of a WWTP, and contribute to over 40% of its GHGs emissions. The most common disposal options at the moment are landfilling, disposal in agriculture (about 40% EU-wide), incineration or co-incineration, and use in the industrial production of bricks, asphalts and concrete. Sewage sludge, however, still contains beneficial resources such as nutrients, that can be recovered through specific processes (e.g. precipitation as struvite) and energy, recoverable through a variety of approaches. Microwave-assisted pyrolysis of urban waste sludge was applied for the production of oil, (Syn)gas, and biochar that were afterwards characterized and compared to mainstream alternative fuels (biodiesels) and other material recovery options. Sustainability issues related to the production of biodiesel/biochars from urban wastewater treatment sludge are also discussed. The paper shows that waste urban sludge can indeed be a full component of the urban circular economy by allowing, if properly processed, recovery of energy resources at multiple levels: bio-oils (biodiesel), syngas and bio-char, all having definite advantages for final residues use and disposal. Biodiesel, in particular, allowing energy recovery as liquid fuel, offers a much more flexible and efficient utilization.

Induced changes of pyrolysis temperature on the physicochemical traits of sewage sludge and on the potential ecological risks

“…Biochar produced from organic residues such as SS has the potential to present high concentrations of PTE, and their content increases with pyrolysis temperature as they form inorganic salts, hydroxides, oxides, and/or sulfides 18 , 71 . Similar findings were obtained in this study (Tables 4 and 5 ), in which the concentration and MEI values of Cu, Zn, Pb, Mn, and Fe increased in line with pyrolysis temperature due to the loss of volatile materials and moisture 3 , and the high boiling points of PTEs 72 .…”

“…The geochemical forms of PTEs in the environment determine their bioavailability, ecotoxicity, diffusion in mobile forms, and consequently their fate in the environment 71 . Metal fractionation is the term used to identify and quantify the different operationally defined species forms or phases in which an element occurs 78 .…”

Energy Issues in Sustainable Urban Wastewater Management: Use, Demand Reduction and Recovery in the Urban Water Cycle

“…While incineration can recover energy only as heat (it may be used for centralized district heating of private and public buildings), other thermal processes (e.g., pyrolysis, hydrothermal carbonization) can extract energy from sludge in solid or liquid form, with more ample application possibilities [89]. Pyrolysis may be driven by conventional thermal energy (e.g., electric or from primary fuels), by convection or by microwave-assisted devices, which are more energetically efficient [90].…”

Biochar production from sewage sludge and microalgae mixtures: properties, sustainability and possible role in circular economy

“…Therefore, researchers' interest has switched to non-combustion, more environmentally sustainable technologies such as gasification and pyrolysis. Pyrolysis is the thermal degradation of biomass in the absence of oxygen, resulting in the production of liquid (biooil) and solid (biochar) residues, and gaseous products (py-gas), effectively transforming wastes into valuable products [11,12,13,14]. These show different possible applications, in particular biochar has proven multiple uses as solid fuel, soil conditioner for agricultural land, and industrial applications in flue gas cleaning, as building material, or aid in contaminated sites remediation [15].…”