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

In addition to the various options available for sewage sludge disposal, a possible process for sustainable resource recovery from this residue is its transformation into biochar, achieved by post-processing through pyrolysis. Biochar obtained from sewage sludge is considered one of the most interesting final products in a wastewater-based circular economy, as proven by the multitude of its possible uses tested so far in various applications. Recently, combined activated sludge (AS)-microalgae systems have been proposed to simultaneously remove both carbon and nutrients from wastewaters, as alternative to conventional technologies such as those based on denitrification and chemical phosphorus precipitation. Although this combined process could be efficient from the point of view of component removal from effluents, it generates potential issues to solid residue disposal practices, as algae normally respond poorly to traditional, mechanical drying processes. In this study, a disposal solution was investigated, consisting of pyrolysation of a mixed sludge/bioalgae matrix under different conditions: in such way, not only landfilled residuals are practically eliminated, but a material with multiple potential end uses is recovered. Process feedstock (algae, sludge and combinations thereof) and end-products (biochar and bio-oil) were characterised after pyrolysis under different conditions. Algae alone were also subject to preliminary solvent oil extraction to assess whether increased biochar production would result from such process variation (which it did, increasing biochar production by 25–33%). A comprehensive discussion on properties of end products as function of process design, possible applications in a circular economy cycle and advantages of co-pyrolysis follows.


Introduction
Increasing industrialisation, demographic expansion and expansion of the transportation and mobility sector worldwide, and especially in developing countries, are the cause of excessive conventional fossil fuel exploitation, leading not only to repeated energy shortages worldwide, but also to increasing global levels of greenhouse gas emissions [1]. Renewable feedstocks and energy sources are thus being investigated to face the demand for cleaner energy alternatives, in order to fulfil growing energy demands. Moreover, increasing carbon dioxide and greenhouse gas emissions into the atmosphere have prompted the ethical obligation to investigate more sustainable and environmentally neutral energy sources [2,3]. A current area of intense investigation is the exploitation of biomasses for energy production [4,5]. Among them, sewage sludge, the final residue of wastewater treatment in the integrated water cycle, is getting increasing attention as not only it normally requires additional expensive treatment and disposal costs by generating utilities, but it is also targeted for sustainable recovery of materials and energy, in compliance with increasingly ambitious EU objectives of generating circular economy cycles from waste streams [6] in accordance with new paradigms in urban water management [7]. Cost of sludge disposal has been estimated at around 50% of the total cost of wastewater treatment [8] while, at the same time, disposal alternatives under current strategies are getting increasingly limited, since accumulation of heavy metals, organic pollutants and pathogenic organisms in the sludge narrow its continued use in commonly adopted practices, such as direct land disposal and composting [9]. Among possible alternatives, incineration would significantly reduce the quantity of waste to be disposed of, allowing energy cogeneration at the same time [10]. However, this involves high costs for effluent gas treatment, which may contain metals, acidic components and dioxins. In addition, this process generates residual ashes considered hazardous waste and may be poorly accepted, or outright opposed, by public opinion. Therefore, researchers' interest has switched to non-combustion, more environmentally sustainable technologies, such as gasification and pyrolysis. Pyrolysis is the thermal degradation of biomass in the absence of oxygen, resulting in the production of liquid (biooil) and solid (biochar) residues, and gaseous products (pygas), effectively transforming wastes into valuable products [11][12][13][14]. These show different possible applications; in particular, biochar has proven multiple uses as solid fuel, soil conditioner for agricultural land and industrial applications in flue gas cleaning, as building material, or aid in contaminated site remediation [15]. Also, high process temperatures favour fixation of metals concentrated in sludge into the carbonaceous char matrix, considerably reducing the possibility of their release into soil, and ultimately into the food chain [12,16]. Depending on heating velocity and residence time of the process, pyrolysis can be broadly classified as slow (conventional) or fast. Slow pyrolysis maximises solid fraction (biochar) production and occurs at long residence times and slow heating rates, while liquid and gaseous energy-rich products (bio-oil or py-gas) fractions are increased during fast pyrolysis [17]. An increase of pyrolysis temperature generally maximises the gaseous fraction, minimizing the solid yield [18]. Properties of the solid residue (biochar) also vary in terms of carbon content and composition. Concerning energetic aspects, bio-oil and biochar could be used as fuels, meeting increasing needs for energy from non-fossil fuel sources [19,20]. However, biochar derived from sewage sludge generally presents high ash content and lower heating value, diminishing its energetic worth [11].
For this reason, an interesting opportunity could consist in the application of co-pyrolysis of sludge with microalgae, which have been recently investigated both as a wastewater treatment process and potential energy feedstock [21]. Microalgae are unicellular photosynthetic microorganisms capable of fixing carbon dioxide by photosynthesis, with several characteristics that make them suitable for energy recovery [22]. These include (i) absence of competition with food supply, (ii) high productivity with reduced cultivation areas (dried biomass' oil yield of about 70% by weight, with area requirement of just 0.1 m 2 /year per kg extracted), (iii) growth possibility on areas not suitable for other crops, (iv) production in most types of water (fresh, brackish and waste water), with minimal or positive impact on water resources use [23].
Microalgae present positive impact also on carbon dioxide emissions, in fact they contain about 50% C over dry weight derived mainly from atmospheric CO 2 ; therefore, production of 100 tons of microalgae allows fixation of about 183 tons of carbon dioxide [24]. High growth rate, cultivation ease, high lipid and low ash contents make microalgae highly appealing, compared to other biomasses, with high yields in terms of both bio-oil and biochar [25], as determined with satisfactory results by numerous studies [26][27][28]. Growth and productivity of microalgae are strongly influenced by environmental and physiological factors, such as temperature, pH, light intensity and nutrient availability [29]. Microalgal biochar has lower carbon content than biochar from other feedstocks, lower surface area and lower cation exchange capacity, while pH, ash and nitrogen contents and extractable inorganic nutrients are high. These properties make it a useful additive to enhance soil characteristics and improve crop productivity, particularly for acidic soils [12].
Recently, combined activated sludge (AS)-microalgae wastewater treatment systems have been proposed to remove simultaneously both carbon and nutrients from liquid streams, as a more energy and economically sustainable alternative to conventional technologies (e.g. AS with nitrification and denitrification). The cultivation of microalgae in wastewater allows direct removal of nitrogen and phosphorus contained within, producing up to 1 kg of dry biomass per m 3 of wastewater [30]. In this alternative to conventional AS processes, bacteria oxidise the organic substance in wastewater to inorganic compounds consuming oxygen, while microalgae use sunlight to absorb inorganic nutrients and CO 2 released by bacteria-producing oxygen, used immediately for oxidation. Although efficient for liquid stream treatment, such systems generate a residue that is more difficult to handle, as algae normally respond poorly to traditional sludge mechanical separation and drying processes. In fact, algal cells are small (2-20 μm), with density similar to that of water, and rather low (0.5-0.3 g L −1 ) concentration in wastewater [31].
The purposes of this paper are to evaluate biochar and biooil production through thermal pyrolysis processes starting from these feedstocks (microalgae and AS waste sludge) and their combination and to determine which conditions are more favourable to an optimal recovery of valuable by-products.

Materials and methods
Three different materials were tested, characterised and pyrolyzed at two different temperatures throughout the following experiments. Both initial materials and final products were characterised using thermogravimetric analysis (TGA) and infrared spectroscopy (IR). The higher heating value (HHV) in recovered biochar samples was also evaluated.

Sample preparation
A mixed culture of microalgae Chlorella was cultivated in four lab-scale open-air reactors (dimensions 0.35 × 0.20 × 0.10 m) filled with water to a depth of 3 cm, in BG-11 medium (Table 1).
A domestic aquarium air pump provided air bubble agitation to keep microalgae in suspension, simulated solar irradiation was provided by a full-spectrum LED lights (40 W) under a 16:8 light/dark sequence. Once the culture reached stable growth, microalgae were harvested, dried on nylon filters (ø = 0.25 μm) for 12 h and pulverised to uniform size in a mortar (feedstock samples 1 and 2).
Feedstock samples 2 and 3 consisted of sewage sludge (mixture of primary and secondary sludge) collected from a nearby wastewater treatment plant and dried at 100°C for 12 h (reaching humidity content below 10%).
The third feedstock was a mixture of sludge and microalgae with high humidity content, collected from a full-scale phytoremediation plant in Spain (kindly supplied by FCC Aqualia S.A.). Fresh material was distributed in 2-cm layers in a crystalliser, and then dried at 100°C for 12 h to reduce humidity below 10%. Subsequently, dried material was shredded, to obtain a resulting grain size as uniform as possible.

Oil extraction from microalgae
Previous studies assessed that preliminary oil extraction from dried microalgae samples could lead to enhanced bio-oil and biochar recovery yields from a subsequent thermal processing. Combination of a two-step lipid extraction and slow pyrolysis processing regime may in fact yield an oil product high in valuable fatty acids, with no variation on its quality, compared to the one-step process, with overall increased yields of liquid and solid fractions over the gaseous one [32,33]. Therefore, preliminary microalgae solvent oil extraction was performed using a chloroform-methanol ratio 2:1, as described in [29]. From a fraction of the two algae-containing feedstocks described in the previous section, 1 g of dried sample was immersed in 20 mL of solvent solution in a flat-bottomed pyrex glass flask, stirred for 25 min, then centrifuged for 20 min at 4000 rpm. The liquid fractions were then filtered and evaporated in a rotary evaporator (Rotovapor, Buchi) to remove solvent and determine the weight of the extracted oil.

Thermogravimetric analysis and infrared spectroscopy
Aliquots (20 g each) of the raw and processed feedstocks (sludge, algae and sludge mix, powdered algae) were subject to thermogravimetric analysis (TGA, 25 ÷ 800°C, heating speed 20°C min −1 , with TGA1 Star System, Mettler Toledo). TGA analysis weights any changes in samples as a function of increasing temperature, as their thermal degradation occurs in multiple stages within the temperature range. TGA was first conducted under nitrogen (nitrogen-TGA) atmosphere (0.4 L min −1 ) to identify the temperature at which pyrolysis process began, later under air (air-TGA), to determine samples' ash and inorganic material content. Both nitrogen-and air-TGAs were subsequently carried out also on solid residue samples deriving from pyrolysis, to assess the characteristics of processed feestocks and compare their ash content. Subsequently, a nitrogen-TGA analysis was carried out on residues of microalgae subject to solvent oil extraction. Infrared spectroscopy (IR) was also used to characterise initial feedstocks, liquid and solid residues from pyrolysis, and to detect any presence of water in liquid samples.

Pyrolysis process and product recovery
Initial substrates were pyrolyzed in a thermostatic sand bath S-70 (FALC instruments) during the experiments. Process equipment is schematised in Fig. 1. A flat-bottomed pyrex glass flask containing 20 g of sample was immersed within the heating sand medium in contact with its bottom. The absence of oxygen was ensured by continuous flow of nitrogen blown directly inside the reactor. A three-way glass fitting was connected by silicone tubing to a solvent trap containing acetone and immersed in crushed ice, for recovery of the oily fraction. Py-gas thus flowed through the tubing, entering the trap where it condensed. The non-condensable py-gas was not further characterised and eliminated from the system. Experiments were conducted at 500°C and 350°C temperatures for each sample. In tests at 500°C, the oven was kept operating at maximum temperature, monitoring the temperature curve with a thermocouple inserted in the sand bath. Once the desired set-point was reached, temperature was kept As for the remaining tests, temperature was monitored with the thermocouple until reaching 350°C, manually maintaining this value for about 30 min by acting on the oven's thermoregulator. After cooling, the process' solid and liquid products were recovered. All tests were conducted in triplicate. Table 2 summarises sample analyses throughout the experiment. Solid (biochar) and liquid (bio-oil) product fractions were recovered from each test, while the uncondensed gas fraction was considered irrelevant for the purpose of this work, and only estimated through mass balance. After completion of each pyrolysis test, all glassware and tubing were washed with acetone to remove all residual solid and oil particles still contained therein. This resulted in a mixture of biochar, biooil, acetone and water, subjected to further treatment for component separation. For the solid fraction, filtration with Buchner funnel, with weight determination before and after filtration to quantify the separated fraction was performed. The liquid fraction (a mixture of acetone and oil) was transferred into a balloon flask, and vacuum evaporated using Rotavapor R-100 (BUCHI) to remove the solvent, weighting the flask before and after the process. In case water was detected in the sample during IR analysis, anhydrous Na 2 SO 4 was added to the solution, which was then filtrated and evaporated.
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, W i is the initial sample weight (20 g) and W H 2 O is the water weight in the initial sample, as determined from TGA analysis, and where W bio-oil is the weight of bio-oil recovered, W i is the initial sample weight (20 g) and W H 2 O is the water weight in the initial sample, as before.

Biochar thermal properties
The calorific value (HHV-higher heating value) of recovered biochar samples was measured with adiabatic calorimeter IKA C6000 Global Standard, in accordance with UNI EN 14918:2010.

Initial material characterization
TGA was carried out on each sample to determine its thermal degradation behaviour. Each material was characterised by both air-TGA (oxidative environment, reproducing a combustion process) and nitrogen-TGA (inert environment), between temperatures of 25-800°C. An oxidative environment allows the ash content of the tested material to be evaluated. TGA in inert atmosphere was also needed to determine the temperature range suitable to pyrolyzation of the samples tested. The thermochemical process in absence of oxygen leads to degradation of volatile substances, leaving char as residue. Results of the TGA in both air and nitrogen are summarised in Table 3. According to derivative thermogravimetry (DGT) analyses, thermal degradation of microalgae takes place in one single stage, as reported in previous studies [34], while that of mixed sludge and algal samples occurs in two distinct phases. It should be highlighted that the temperature range 200-500°C includes the highest degradation peaks for all samples (Fig. 2). These are generally associated with carbohydrate and protein de-volatilisation [35]. In mixed feedstock a second peak between 600 and 700°C also appears, corresponding to degradation of lipids and solid residues [36]. Based on ash fractions obtained from TGA analyses, composition of mixed microalgae and sludge from the phytoremediation plant sample was confirmed as 15% and 85% of each, respectively. Ash content in WWTP sludge sample was higher (30.2 ± 1.8%) than in those containing microalgae, meaning that adding even a small amount (15%) of microalgae to the mix positively contributes to the reduction of the ash quantity in residues, improving their energy quality. As for nitrogen-TGA results, it is relevant to see that the quantity of solid residues from the sludge-microalgae mix is higher than that produced by the single-sludge matrix, leading to increased yield in solid material recovery.

Biochar production and characterization
Resulting pyrolysis products from tests at 350°C and 500°C were solid (biochar) and liquid (bio-oil) residues. After recovering and separating solid and liquid particles remained in the testing equipment, biochar was directly weighed. Figure 3 represents the product fractions obtained from tests. For all feedstocks examined, pyrolysis at 350°C produced the greatest amount of solid residue (biochar), while higher temperatures (500°C) generally yielded higher production of bio-oil. Considering only the production yield of biochar, WWTP sludge processed at 350°C gave higher values (82.0 ± 4.4%) along with mixed feedstock at the same temperature (82.7 ± 2.1%). As for liquid residues (bio-oil) yields, higher temperatures usually originate higher fractions than those obtained in the present work [37]; nevertheless, all feedstocks processed at 500°C produced 13 ± 3% of bio-oil, a fraction higher than at lower temperature.
Due to the focus of the present work, only the process' solid residues were fully characterised. Biochar samples from pyrolysis tests were subject to TGA, IR analysis and HHV (high heating value, UNI EN 14918:2010). Under visual analysis, all samples appeared different from each other, with appearance changing according to process temperature and feedstock (Fig. 4). Samples 2 and 4 from tests at 350°C (Fig. 5e, f) presented fairer colour (brownish), compared to all others (black or blackish). Among microalgae-derived biochar   (Fig. 5a, d, respectively), no colour differences were detected, but they significantly differed in consistence: sample 2 (Fig. 5d) had a dusty structure, while sample 1 was mostly solid (Fig. 5a). Air-TGA analyses were performed to evaluate ash content of the biochar samples, while nitrogen-TGA was used to evaluate the efficiency of the pyrolysis process (Fig. 4), by assessing their supplemental weight loss. IR analysis was performed before and after pyrolysis to evaluate variation of internal material bonds induced by the process (Fig. 6), by determining functional groups and bonds within samples. The most significant information in the graphs is deduced by the wavelengths representing water and carboxyl groups (3600-2500 cm −1 ), C-C and C-H bonds (3300 cm −1 ), esters and fatty acids (1700 cm −1 ) and Si-O bonds in inorganic material (1100 cm −1 ). By comparing the different spectra, all samples appear very similar to each other prior to pyrolysis, although some relationships between components may vary. Pyrolyzed samples (the pattern for only one such sample is shown) indicate removal of water and organic acids during the process and reduction of many of the functional groups present. This corresponds to formation of compounds with high carbon content, even if some C-C and C-H bonds are still present. Obviously, Si-O bonds are preserved, as not involved in pyrolysis reactions. Further increasing duration and temperature of pyrolysis would lead to formation of a graphitic carbon, with absence of IR bands detected.
HHV analysis shows that microalgae-derived biochar has higher heating value (samples 1 and 2) than others, decreasing with decreasing process temperature. HHVs of remaining samples are lower, suggesting that thermal uses might not be indicated as the main final application of these biochars. Significant results are summarised in Table 4.
To assess the effect on biochar production yield of solvent oil pre-extraction from the microalgae, as suggested in previous studies [32,33], solid residues after this pre-treatment were subject to nitrogen-TGA, comparing the results with those on raw feedstock. These samples showed significantly better results than initial ones: biochar production yield after pre-extraction increased from 25 to 33% in microalgae-only samples. However, no benefits were detected from such preliminary oil extraction in mixed feedstock (samples 5 and 6), with 38% biochar yield in both cases.

Discussion
This work aimed to assess potential advantages in terms of biochar production and characteristics of the combination of sewage sludge and microalgae as feedstock in a pyrolysis processes, with a view to improve the final use value of the recovered resources. Product analysis was not limited at observing mass weight obtainable from each matrix, but was extended to determine the ash fractions and their quality in final products. In order to couple these two feedstocks in a single matrix, separate microalgae production with direct addition to sludge at the time of pyrolysis could also be feasible. However, this strategy would be of small benefit compared to the direct use of an original microalgae-sludge mix from a combined AS/microalgae wastewater treatment facility of new conception. In this novel type of process, in fact, simultaneous nutrient removal from wastewater by microalgae occurs without costly bio-denitrification processes, producing a mixed biomass (sludge and microalgae) that could originate after thermal treatment a solid residue with excellent properties for reuse.

Comparative analysis
Observed product yields were compared with those obtained by other authors, to validate present results (Table 5). Sewage sludge biochar was obtained by slow pyrolysis in helium atmosphere using a quartz tubular reactor containing 30 g sludge samples by Sanchez et al. [38]. In this study, the original matrix had ash content of 3.4% by weight. Tests were conducted at 350, 450, 550 and 950°C, with the largest amount of biochar (52%) produced at the lower temperature of 350°C, in accordance with the present study.
Microalgae-derived biochar was obtained by Gong et al. [35] using a quartz, fixed-bed reactor under 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 ), while gas yield increased along with temperature (from 0.4 to 15.5%). Compared to the results of this study, char production yield from the Chlorella culture was higher, as well as the product HHV.
Results from microalgae and sewage sludge mixtures in this study could not be compared due to the lack of similar literature data concerning these feedstock co-pyrolysis. The highest productions of biochar were detected in pyrolysis of microalgae alone, while intermediate results were obtained from co-pyrolysis of sewage sludge and microalgae, as expected by preliminary information reported in other studies on co-pyrolysis of microalgae with other, non-sludge matrixes [12].

Possible beneficial applications of biochar
Pyrolysis operating conditions are paramount to determine optimal final uses of derived biochar, since these factors directly contribute to the development of different intrinsic characteristics of the product [40]. It is therefore important to analyse feedstock materials before thermal processing, in order to establish a priori the best application for the biochar that will be obtained under given operating conditions. Results obtained in this study from HHV analysis on obtained biochars, compared with HHV of hard coal (around 30 MJ/kg), prove that biochar from microalgae could in fact be used as fuel (HHVs of 29.1 and 26.9 MJ/kg, not dissimilar from coal's value). As biochar is the product of renewable feedstock, this would substitute the caloric equivalent amount of fossil fuels, offsetting related GHG emissions. However, other alternative uses of this product, regardless of its origin, are those related to the possibilities of its re-use and valorisation from an urban (wastewater) based circular economy perspective.
Interesting uses in agriculture as soil enhancement, or in wastewater or contaminated site remediation as pollutants adsorbent [41], are both applications with greater added value compared to outright combustion. In addition to allowing increase in crop productivity, agricultural soil use will effectively work as long-term carbon sequestration (also valuable under current environmental policies), while uses as adsorbent (biochar in fact has excellent adsorbent capacities for both organic and inorganic pollutants by virtue of its C content) could substitute other energy-embedded commercial products, after which the spent biochar could be sent to controlled combustion, which would simultaneously serve as final contaminants destruction and exploitation of the char's energy content. For proper agricultural use, biochar carbon content must be greater than 50% (dry mass), N and P content should be between 1 and 45%, pH should not exceed 10 and particles' specific surface should be greater than 150 m 2 g −1 [42,43]. Biochars derived from bagasse and vegetal biomass feedstocks generally fit these specifications, and some studies confirmed that also microalgal biochar presents compatible characteristics [44,45]. Effects of biochar on physical-chemical improvement of soils also depend strongly on the original soil characteristics and on feedstock used for its production [46].
A recent study from Oliveira and co-workers [47] showed that low pyrolysis temperatures (< 500°C) favour partial carbonisation, producing biochar with smaller pores and reduced surface area, while increasing the presence of oxygen-containing functional groups, making it ideally suitable for removal of inorganic pollutants. On the contrary, biochar produced at high temperatures (> 500°C) could be applied for adsorption of organics, due to higher specific surface area, making it highly suitable for environmental bioremediation of organic pollution and for wastewater treatment applications, specifically for removal of toxic compounds, instead of activated carbon (AC) [48]. In that respect, Alhashimi and Aktas [49] performed a LCA (life cycle assessment) analysis evaluating the relative economic and environmental performance of biochar as adsorbent, compared to AC. Environmental impact was evaluated in terms of energy demand and GWP (global warming potential) for their production. GWP of biochar generation is usually negative (− 0.9 kgCO 2eq kg −1 ), against an average 6.6 kgCO 2eq kg −1 of commonly used AC. Energy demand for biochar production is lower than that required for AC by one order of magnitude (1.1 ÷ 16 MJ kg −1 for biochar, 44 ÷ 170 MJ kg −1 for activated carbon). However, it has to be considered that spent AC is usually not discarded immediately, but regenerated for reuse, while spent biochar is usually destroyed after its first use. As for economic aspects, biochar and AC industrial production costs are comparable, estimated as $5 kg −1 and $5.6 kg −1 , but this does not factor in the missed high costs for the original sewage sludge disposal that would otherwise be needed.
Some drawbacks of biochar as adsorbent must also be considered, such as less controllable quality and fluctuating efficiency, longer time needed for absorption of certain contaminants, differences in performance of products from different feedstocks. However, environmental advantages are obvious and with adequate optimization biochar may be considered suitable for most adsorption applications. Finally, due to its high carbon content, biochar has found other applications, for example for use as electrode material in bioelectrochemical systems (BES) in lieu of granular graphite or AC, and many others [42]

Implications for a biochar-based circular economy
All the above-mentioned and other additional applications of biochar will gradually be investigated and validated as circular  [50] and the European Biochar Foundation [51]. The former concerns the use of biochar in soils, the latter consists of guidelines for its sustainable production. Both these guidelines define biochar as material produced by pyrolysis of biomass under low oxygen conditions, without limitations to the feedstock origin; therefore, products from both sewage sludge and algae fall under this definition. Both guidelines include specifications about maximum toxicant assessment, and their maximum allowed thresholds. Product certification, although at present existing only for a restricted range of applications, is an important step for setting up reliable and lasting circular economy circuits. While circular economy strategies centred on wastewater treatment by-products in the EU generally postulate their direct re-use in energy production, and alternative could be based on their transformation into new products, not necessarily limited to agricultural use. In order to fulfil future certification and regulation requirements for these products, the next challenge for research and development of biochar-based products lies in achieving a greater understanding and control of pyrolysis processes staring with feedstock pre-treatment, additive addition, effect of process operating parameters, process yield in terms of specific product properties, such as heavy metal immobilization, specific surface area, elemental analysis, phosphorus and micropollutants contents.
Decentralised biochar production units would constitute the most efficient way to meet local by-product demand with specific characteristics by using site-produced, homogeneous feedstock under purpose-designed process conditions, avoiding the economic and environmental impacts of longrange transportation, promoting local business and employment and locally improving resource efficiency and synergistic opportunities for various local actors in the transition to circular economy paradigms.

Conclusions
This study aimed at assessing the effects of adopting mixtures of sewage sludge and microalgae as feedstock to pyrolysis processes in terms of these residuals' disposal/reuse, and specifically of possible improvements of biochar production quantity and quality. Final product analysis was not limited at the determination of relative mass produced from each feedstock matrix, but also went further to determine the ash fractions, carbon and energy properties of each product. Results showed that slow pyrolysis of mixed feedstock (85 and 15% sludge and algae, respectively) at temperature of 350°C yielded 80% of the initial sample by weight as biochar, of which only 24% as ash. Comparing this result to the data from pyrolysis of WWTP sludge at the same temperature, biochar extracted was 74% of the initial sample weight, but with 30% ash content. Therefore, co-pyrolysis of sewage sludge and microalgae yielded a more valuable product, with multiple possible applications. This solution could contribute to the reduction of problems deriving from expensive and/or inappropriate disposal of wastewater treatment residuals.
Various possibilities in terms of implementation of productive biochar reuse have been described. Within a wastewaterbased circular economy cycle, biochar is a very valuable material, with multiple possible interesting outlets that need further careful evaluation beyond currently known applications. Standardization and certification of final products characteristics are the keys to a successful circular economy implementation. Some attempts in this sense have been already developed for specific biochar applications.
Decentralization of biochar production from local feedstock sources would be the most logical and effective way to implement efficient biochar-based circular economy. Such systems would benefit from more homogeneous feedstock characteristics and the possibility to custom-define and design the required final products characteristics according to local applications, and minimise additional environmental impact.
Acknowledgements The authors thank Aqualia SA (Spain) for providing residual material from their phytoremediation plant.