Is Biochar from the Torrefaction of Sewage Sludge Hazardous Waste?

Abstract

Improved technologies are needed for sustainable management of sewage sludge (SS). The torrefaction (also known as biomass “roasting”) is considered a pretreatment of SS before use in agriculture. However, it is not known whether the torrefaction has the potential to decrease heavy metals’ (HMs) leachability and the SS toxicity. Thus, the aim of the study was to evaluate the influences of the SS torrefaction parameters (temperature and process time) on HM contents in biochar, HM leachability, and biochar toxicity, and compare them with raw SS. The experiments were designed in 18 combinations (six temperatures, 200, 220, 240, 260, 280, and 300 °C; and three process times—20, 40, 60 min). Standard tests were used to determine HMs content, leachability, and toxicity. Results indicated that the torrefaction did not increase (p < 0.05) the HM content in comparison to the raw SS. The leachability of Zn, Ni, Cu, Cr, and Mn from SS biochars was similar to raw SS. However, the degree of leachability varied significantly (p < 0.05) from as low as 0.1% for Cu to high as 16.7% for Cd. The leachability of Cd (<16.7%) and Pb (<11.9%) from biochars was higher than from raw SS (<6.1% and <2.4%, respectively). The leachability of Cd from SS biochar, in five torrefaction combinations, was higher than the threshold value for hazardous waste. It is recommended that site-specific decisions are made for torrefaction of SS with respect to its HM content, as the resulting biochar could be considered as hazardous waste, depending on the feedstock. Moreover, the biochar produced under the whole range of temperatures during 20 min retention time significantly (p < 0.05) increased the Daphnia magna Straus mobility inhibition by up to 100% in comparison to the biochar obtained during 40 and 60 min torrefaction. Taking into account the increased leachability of specific HMs and D. magna Straus mobility inhibition, biochar should be considered a potentially hazardous material. Future research should focus on biochar dosage as a fertilizer in relation to its toxicity. Additional research is warranted to focus on the optimization of SS torrefaction process parameters affecting the toxicity.

Biochar can mitigate co-selection and control antibiotic resistant genes (ARGs) in compost and soil

“…Freshly produced PAHs-contaminated biochar released PAHs in soils and the level of PAHs released decreased with aging ( Sigmund et al., 2017 ). The pattern of PTEs leaching observed in sewage sludge biochar is similar to what is obtainable with the non-pyrolyzed biosolids ( Bialowiec et al., 2020 ). This implies that conversion of biosolids to biochar has no significant impact on the reduction of PTEs.…”

Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery

“…Moreover, Białowiec et al (2020) [ 135 ] conducted research on toxicity of the bio-char from torrefaction of SS, the results showed that increasing temperature and resident time leads to the toxicity reduction. However, toxic HMs was independent on time and temperature.…”

“… Generation of leachate, waste of valuable organic/inorganic materials, emission of greenhouse gases if not engineered Cheap process with no energy consumption, reduces SS volume [ 85 , 87 ] Farm land – Direct use of SS without any treatment Contamination of heavy metals Great source of inorganic/organic substances as fertilizer or to improve soil properties [ 81 , 149 ] Drying 60–93 °C. Spread of waste/SS on open bed of sand and allowed to remain until dry Climate dependent, unstable temperature conditions, cover large land surface, emit odor, heavy metals pollution Performed in natural way, affordable, destroy pathogen, reduce sludge volume [ 136 , 138 ] Dewatering 30–70 °C and 1–300 bar Minimization of water content from SS for its effective disposal This treatment process can't mitigate HMs pollution in SS Improve bio-energy utilization, reduce volume, easy transportation, lessen leachate in landfills, does not contribute to carbon emission [ 61 ] Torrefaction >300 °C Low/slow thermal process in oxygen starving environment Pollution of HMs through different application of torrefied product Produce high amount of biochar, improves solid fuel properties, low energy consumption, short torrefaction time, enhance carbon fixation and hydrophobicity of torrefied material [ 134 , 135 ] Incineration Temperatures of 750–925 °C (1400–1700 °F). Residence times are typically 2–5 s. Straight combustion in excess oxygen supply Expensive, emit some pollutant gases such us furans and nitrous, ammonia emission, leads to the second pollution via residue (ash) Odor minimization, reduction of raw waste mass (SS), production of energy.…”

Improving Lignocellulosic and Non-Lignocellulosic Biomass Characteristics through Torrefaction Process

“…Even though forests cover approximately 58% of the country's area, municipal solid waste, industrial waste, or sewage sludge could replace lignocellulosic biomass in thermochemical processes [30,31] due to environmental friendliness and costeffectiveness [32,33]. Sewage sludge (SS) co-combustion has been studied recently for energy utilization while improving organic compounds share [34,35]. Sewage sludge is an incidental product generated in biological wastewater treatment plants [36].…”