Co-Pyrolysis of Sewage Sludge and Wetland Biomass Waste for Biochar Production: Behaviors of Phosphorus and Heavy Metals

Abstract

Large amounts of sewage sludge (SS) and wetland plant wastes are generated in the wastewater treatment system worldwide. The conversion of these solid wastes into biochar through co-pyrolysis could be a promising resource utilization scheme. In this study, biochar was prepared by co-pyrolysis of SS and reed (Phragmites australis, RD) using a modified muffle furnace device under different temperatures (300, 500, and 700 °C) and with different mixing ratios (25, 50, and 75 wt.% RD). The physicochemical properties of biochar and the transformation behaviors of phosphorus (P) and heavy metals during the co-pyrolysis process were studied. Compared with single SS pyrolysis, the biochar derived from SS-RD co-pyrolysis had lower yield and ash content, higher pH, C content, and aromatic structure. The addition of RD could reduce the total P content of biochar and promote the transformation from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). In addition, co-pyrolysis also reduced the content and toxicity of heavy metals in biochar. Therefore, co-pyrolysis could be a promising strategy to achieve the simultaneous treatment of SS and RD, as well as the production of value-added biochar.

Biochar synthesis from mineral and ash-rich waste biomass, part 2: characterization of biochar and co-pyrolysis mechanism for carbon sequestration

“…Here, even the cellulose-rich M010 (H/C org = 1.09) has not been sufficiently transformed, implying that the residence time is the limiting factor. Furthermore, except for B541 and B505, co-pyrolysis has generally increased the carbonization (H/C org ) relative to B100 as seen in the case of sludge and wetland biomass waste [30].…”

Review of Phosphorus Chemistry in the Thermal Conversion of Biomass: Progress and Perspectives

“…Humans lack the ability to digest phytic acid, and a smaller fraction of the phosphorus content is therefore present in this form . However, most of the phosphorus is present as inorganic phosphorus, often aluminum- and/or iron-phosphates because iron- and aluminum-based additives are used as coagulation or precipitation agents in wastewater treatment processes. , Using chemical fractionation methods (the SMT protocol), it was shown that sewage sludge normally contain more than 70% inorganic phosphorus, out of which 60–95% was reported to be present as nonapatite phosphorus. ,− This includes, for example, Al- and Fe-phosphates. Solution state 31 P NMR analysis has shown similar results, and the phosphorus extracted for analysis was confirmed to be mainly orthophosphate. ,, However, some chemical fractionation studies (using the Hedley protocol) showed that more than 50% of the phosphorus is found in the acid-leachable (HCl) fraction, indicating phosphorus is associated as apatite in these sewage sludge samples. − …”

“…The decomposition of organic phosphorus species in biomass has been studied primarily using solution state 31 P NMR − ,,,,,, and chemical fractionation following the SMT protocol. ,− ,− …”

“…The SMT protocol is a chemical fractionation method (introduced in section ) that divides the phosphorus into inorganic and organic phosphorus. A compilation of results ,− ,− based on chemical fractionation using the SMT protocol of thermally treated sewage sludge, is shown in Figure . The fraction of phosphorus present as inorganic phosphates (according to the SMT protocol) appear to increase (meaning that organic phosphorus is decreasing) when the biomass is exposed to rather low temperatures, which is qualitatively in line with the results based on 31 P NMR shown in Figure .…”

Investigation of Pyrolysis Behavior of Sewage Sludge by Thermogravimetric Analysis Coupled with Fourier Transform Infrared Spectrometry Using Different Heating Rates

“…As a result of our improving understanding of pyrolysis processes and the influences of scaling them up, investigating their associated reaction kinetics is necessary. However, it is known that due to the large number of complex and consecutive reactions involved, the thermal decomposition process is very complex [12][13][14][15]. Thus, it is not possible to describe all reactions individually.…”