Pyrolysis of Dried Wastewater Biosolids Can Be Energy Positive

Abstract

Pyrolysis is a thermal process that converts biosolids into biochar (a soil amendment), py-oil and py-gas, which can be energy sources. The objectives of this research were to determine the product yield of dried biosolids during pyrolysis and the energy requirements of pyrolysis. Bench-scale experiments revealed that temperature increases up to 500 °C substantially decreased the fraction of biochar and increased the fraction of py-oil. Py-gas yield increased above 500 °C. The energy required for pyrolysis was approximately 5-fold less than the energy required to dry biosolids (depending on biosolids moisture content), indicating that, if a utility already uses energy to dry biosolids, then pyrolysis does not require a substantial amount of energy. However, if a utility produces wet biosolids, then implementing pyrolysis may be costly because of the energy required to dry the biosolids. The energy content of py-gas and py-oil was always greater than the energy required for pyrolysis.

Transformation of biosolids to biochar: A case study

“…Till date, there exists one study where energy balance of pyrolysis process for the production of biochar from biosolids has been performed. The study confirmed that pyrolysis of biosolids can be energy positive if energy required for pyrolysis can be sufficed by py‐gas and py‐oil . However, the study is limited to dry biosolids (~5% moisture content).…”

Comment on “Pyrolysis of dried wastewater biosolids can be energy positive”

“…contest the claim that pyrolysis of heat-dried biosolids can be energy positive (Rollinson & Oladejo, 2019a). We write here to add clarity to this topic, as well as to address specific points raised on our paper (McNamara, Koch, Liu, & Zitomer, 2016) in recent correspondence (Rollinson & Oladejo, 2019b).…”

Toxicity of Various Pyrolysis Liquids From Biosolids on Methane Production Yield

“…Primary and waste activated sludge volumes produced from municipal water reclamation processes are increasing due to population increase and more strict effluent requirements (Agrafioti et al, 2013;Bravo et al, 2017). Conventional solids management including landfilling and biosolids land application for agriculture are employed (Agrafioti et al, 2013;McNamara et al, 2016). However, there are health and environmental concerns related to these approaches due to pathogens, micropollutants, and excess nutrients in the environment as well as a desire for energy and resource recovery from wastewater solids (Bridle and Skrypski-Mantele, 2004;Tsai et al, 2009;Hamidi et al, 2017;Khazaei et al, 2017Khazaei et al, , 2018.…”

“…Pyrolysis is one thermochemical technology that converts wastewater solids to biochar, pyrolysis gas (py-gas), and pyrolysis liquid. In pyrolysis, sludges or solids are heated in the absence of oxygen to temperatures >400 • C (McNamara et al, 2016;Liu et al, 2017Liu et al, , 2018. Biochar is a solid product similar to charcoal that can be used as a beneficial soil amendment because of its high capacity to hold moisture and nutrients for plants (Laird et al, 2009;McNamara et al, 2016).…”

“…In pyrolysis, sludges or solids are heated in the absence of oxygen to temperatures >400 • C (McNamara et al, 2016;Liu et al, 2017Liu et al, , 2018. Biochar is a solid product similar to charcoal that can be used as a beneficial soil amendment because of its high capacity to hold moisture and nutrients for plants (Laird et al, 2009;McNamara et al, 2016). In addition, pyrolysis reduces or eliminates pathogens and micropollutant mass when transforming sludge to biochar (McNamara et al, 2016;Liu et al, 2017).…”