Waste-to-Carbon: Is the Torrefied Sewage Sludge with High Ash Content a Better Fuel or Fertilizer?

Abstract

Sewage sludge (SS) recycling is an important part of the proposed ‘circular economy’ concept. SS can be valorized via torrefaction (also known as ‘low-temperature pyrolysis’ or ‘roasting’). SS can, therefore, be considered a low-quality fuel or a source of nutrients essential for plant growth. Biochar produced by torrefaction of SS is a form of carbonized fuel or fertilizer. In this research, for the first time, we tested the feasibility of torrefaction of SS with high ash content for either fuel or organic fertilizer production. The research was conducted in 18 variants (six torrefaction temperatures between 200~300 °C, and three process residence times of 20, 40, 60 min) in 5 repetitions. Fuel and fertilizer properties and multiple regression analysis of produced biochar were conducted. The higher heating value (HHV) of raw SS was 21.2 MJ·kg−1. Produced biochar was characterized by HHV up to 12.85 MJ·kg−1 and lower H/C and O/C molar ratio. Therefore, torrefaction of SS with high ash content should not be considered as a method for improving the fuel properties. Instead, the production of fertilizer appears to be favorable. The torrefaction increased C, N, Mg, Ca, K, Na concentration in relation to raw SS. No significant (p < 0.05) influence of the increase of temperature and residence time on the increase of biogenic elements in biochar was found, however the highest biogenic element content, were found in biochar produced for 60 min, under the temperature ranging from 200 to 240 °C. Obtained biochars met the Polish regulatory criteria for mineral-organic fertilizer. Therefore SS torrefaction may be considered a feasible waste recycling technology. The calculation of torrefaction energy and the mass balance shows energy demand <2.5 GJ∙Mg−1 w.m., and the expected mass yield of the product, organic fertilizer, is ~178 kg∙Mg−1 w.m of SS. Further investigation should consider the scaling-up of the SS torrefaction process, with the application of other types of SSs.

Soil Nutrients and Carbon Dynamics in the Presence of Biochar-swine Manure Mixture Under Controlled Leaching Experiment Using a Midwestern USA Soil

“…Biochar applications in crop agriculture are often limited by the heavy metals content associated with some types of feedstock and sources (Pulka et al, 2020). Biochar is capable of sorbing heavy metals and making them less available to soil exchange sites for the plant to uptake (Zhang et al, 2013).…”

Is Biochar from the Torrefaction of Sewage Sludge Hazardous Waste?

“…Data presenting the fuel properties of SS and biochars (lost on the ignition, ash content, C, H, N, O, higher heating value, lower heating values) and fertilizer properties of SS and biochars (Ca, Mg, K, Na content) obtained in this experiment were published by [6]. Raw data of solid content, mass yield, HM contents in SS and biochars, hydrophobicity, toxicity, and HMs' and other pollutants' concentrations in water extracts from SS and biochars are included as Supplementary Materials Excel S1.…”

“…Biochar is generated from the thermal treatment (torrefaction, pyrolysis) of biowaste [1]. Recent studies indicated that sewage sludge (SS) is a suitable feedstock for torrefaction [2][3][4][5][6]. Torrefaction (also known as "roasting" or "high-temperature drying") could densify carbon and stabilize SS, thereby offering a sustainable resource recovery solution.…”

“…As the SS torrefaction technology is developing, researchers are focusing on the optimization of the process parameters and the improvement of the fuel properties of the biochar [7,8]. Previous studies indicated that both raw and torrefied SS have average fuel properties [4,6]. To date, the main raw SS disposal methods include landfilling, landscaping, incineration, and agricultural applications.…”

“…To date, the main raw SS disposal methods include landfilling, landscaping, incineration, and agricultural applications. Therefore, the agricultural application of torrefied SS has been proposed as an alternative solution for its energetic utilization [6]. However, the aspects of the potential influence of torrefied SS on the environment have not been intensely studied, yet.…”