Biochar production from sewage sludge and microalgae mixtures: properties, 1 sustainability and possible role in circular economy 2

17 Possible destination for sewage sludge sustainable disposal is its transformation in biochar, 18 achieved by post-processing of the sludge itself through pyrolysis. Biochar from sludge is 19 considered one of the most interesting final products in wastewater-based circular economy, as 20 proven by the multitude of its possible uses tested so far in different areas. Recently, combined 21 activated sludge (AS)-microalgae systems have been proposed to simultaneously remove both 22 carbon and nutrients from wastewaters, as alternative to conventional technologies such as those 23 based on AS. Such innovation could be efficient from the point of view of removal of regulated 24 components from effluents, but it adds potential issues to solid residue disposal practices, as algae 25 normally respond poorly to traditional, mechanical drying processes. In this study, a disposal 26 solution was investigated, consisting of pyrolysation of a mixed sludge/bioalgae matrix under 27 different conditions: in such way, not only landfilled residuals are practically eliminated, but a 28 material with multiple possible beneficial end uses is generated. Initial materials (algae, sludge and 29 combinations thereof) and end-products (biochar and bio-oil) were physically and chemically 30 characterized after pyrolysis under different conditions. Algae alone were also subject to 31 preliminary solvent oil extraction to assess whether increased biochar production would result from 32 this process modification (which did, increasing biochar production by 25-33%). A comprehensive 33 discussion on properties of end products as function of process design, possible applications in a 34 circular economy cycle, and advantages of co-pyrolysis follows. 35 36 inert (N 2 ) gas flow, testing 1 g samples at temperatures between 300 and 700°C, with heating rate of 10°C min -1 The study showed that bio-oil fraction increased with temperature from 30.9% (at 300°C) to a maximum of 60.7% (at 500°C), decreasing afterwards to 48.1% (at 700°C). As for biochar yield, the highest amount was obtained at 300°C (57%), decreasing with temperature to a minimum of 25.5%. HHV of the char also decreased with temperature (from 22.3 to 16.4 MJ kg -1 ),

been proposed to remove simultaneously both carbon and nutrients from liquid streams, as a more 104 energy sustainable and economic alternative to conventional technologies (e.g. AS with nitrification 105 and denitrification). The cultivation of microalgae in wastewater allows direct removal of nitrogen 106 and phosphorus contained within, producing up to 1 kg of dry biomass per m 3 of wastewater [30].

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In this alternative to conventional AS processes, bacteria oxidize the organic substance in 108 wastewater to inorganic compounds consuming oxygen, while microalgae use sunlight to absorb 109 inorganic nutrients released by bacteria, including CO 2 , producing oxygen subsequently used by 110 bacteria for oxidation. Although efficient for liquid streams treatment, such systems generate a 111 residue that is more difficult to handle, as algae normally respond poorly to traditional sludge 112 mechanical separation and drying processes. In fact, algal cells are small (2-20 μm), with density 113 similar to that of water, and rather low (0.5-0.3 g L -1 ) concentration in wastewater [31].

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Purpose of this paper is to evaluate biochar and bio-oil production through thermal pyrolysis 115 processes starting from these initial residues (microalgae and AS waste sludge) and their 116 combination, and to determine which conditions are more favourable to optimal recovery of 117 valuable by-products.

Materials and methods
120 Three different materials were tested, characterized and pyrolyzed at two different 121 temperatures throughout the following experiments. Both initial materials and final products were 122 characterised using thermogravimetric analysis (TGA) and infrared spectroscopy (IR). HHV (higher 123 heating value) in recovered biochar samples was also evaluated.  (Table 1) 127 nitrogen-TGA analysis was carried out on residues of microalgae subject to solvent oil extraction.

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Infrared spectroscopy (IR) was also used to characterize initial materials, liquid and solid residues 166 from pyrolysis, and to detect any presence of water in liquid samples.  on the oven's thermoregulator. After cooling, the process' solid and liquid products were recovered.

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All tests were conducted in triplicate. Table 2    to the solution, that was then filtrated and evaporated.

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Yields of biochar and bio-oil recovered were calculated as follows (Eqs. (1) and (2), respectively): where W biochar is the weight of biochar recovered, Wi is the initial sample weight (20 g) and 2 is 205 the water weight in the initial sample, as determined from TGA analysis, and where W bio-oil is the weight of bio-oil recovered, Wi is the initial sample weight (20 g) and 2 is 208 the water weight in the initial sample, as before.

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TGA was carried out on each initial sample to determine its thermal degradation behaviour.

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Each material was characterized by both air-TGA (oxidative environment, reproducing a 219 combustion process) and nitrogen-TGA (inert environment), between temperatures of 25 -800°C.

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An oxidative environment allows the ash content of the tested material to be evaluated. TGA in 221 inert atmosphere was also needed to determine the temperature range suitable to pyrolyzation of the  Based on ash fractions obtained from TGA analyses, composition of mixed microalgae and 234 sludge from the phytoremediation plant sample was confirmed as 15% and 85% of each, 235 respectively. Ash content in WWTP sludge sample was higher (30.2 ± 1.8%) than in those 236 containing microalgae, meaning that adding even a small amount (15%) of microalgae to the mix 237 positively contributes to the reduction of the ash quantity in residues, improving their energy 238 quality. As for nitrogen-TGA results, it is relevant to see that the quantity of solid residues from the 239 sludge-microalgae mix, is higher than that produced by the single-sludge matrix, leading to 240 increased yield in solid material recovery.         Table 4.

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To assess the effect of solvent oil pre-extraction from microalgae on biochar production 305 yield, as suggested in previous studies [32,33], solid residues after this pretreatment were subject to 14 nitrogen-TGA, comparing the results with those on raw materials. These samples showed 307 significantly better results, than initial ones: biochar production yield after pre-extraction increased 308 from 25% to 33% in microalgae-only samples. However, no benefits were detected from such 309 preliminary oil extraction in the mixed samples (microalgae and sludge), with 38% biochar yield in 310 both cases.

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This work aimed to assess potential advantages in terms of biochar production and

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The most interesting outcomes for these products, regardless of original feedstock, are in 367 fact considered to be those related to the possibilities of their re-use and valorisation, from an urban 368 (wastewater) based circular economy perspective. An appealing use of biochar is that of soil 369 improver in agriculture, that has shown to allow increase in crop productivity, but also to reduce 370 soil pollution by adsorbing metals and other solute contaminants in groundwater [41]. Biochar in 371 fact has excellent adsorbent capacities for both organic and inorganic pollutants, and by virtue of its 372 C content, also acts as a long-term carbon sink. For proper agricultural use, biochar carbon content 373 must be greater than 50% (dry mass), N and P content should be between 1 and 45%, pH should not

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Environmental impact was evaluated in terms of energy demand and GWP (Global Warming 389 Potential) for their production. GWP of biochar generation is usually negative (-0.9 kgCO 2eq kg -1 ),

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against an average 6.6 kgCO 2eq kg -1 of commonly used AC. Energy demand for biochar production 391 is lower than that required for AC by one order of magnitude (1.1 ÷ 16 MJ kg -1 for biochar, 44 ÷ 392 170 MJ kg -1 for activated carbon). However, it has to be considered that spent AC is usually not 393 discarded immediately, but regenerated for reuse, while spent biochar is usually destroyed after its

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This study aimed at assessing the effects of adopting mixtures of sewage sludge and 434 microalgae as feedstock in terms of these residuals' pyrolysis disposal processes, and specifically of 435 possible improvements of biochar production quantity and quality. Final product analysis was not 436 limited at the determination of relative mass produced from each matrix, but also went further to 437 determine the ash fractions, carbon and energy properties of each final product. Results showed that 438 slow pyrolysis of mixed feedstock (85 and 15% sludge and algae respectively) at temperature of 439 350°C, yielded 80% of the initial sample by weight as biochar, of which only 24% as ash.

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Comparing this result to the data from pyrolysis of WWTP sludge at the same temperature, biochar 441 extracted was 74% of the initial sample weight, but with 30% ash content. Therefore, co-pyrolysis