Physico-chemical properties of waste derived biochar from community scale faecal sludge treatment plants

Background: The dumping of untreated faecal sludge from non-sewered onsite sanitation facilities causes environmental pollution and exacerbates poor public health outcomes across developing nations. Long-term mechanisms to treat faecal sludge generated from these facilities are needed to resolve the global sanitation crisis and realize the Sustainable Development Goal (SDG) 6 “ensure availability and sustainable management of water and sanitation for all” by 2030. Pyrolysis of faecal sludge removes pathogens and generates biochar, which can be used as a soil enhancer. Methods: The properties of faecal sludge biochars from three full-scale treatment plants in India were determined via Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX) spectroscopy, crystal x-ray diffraction (XRD), proximate analyses, and BET surface area porosimetry. Results: Results showed that all three biochars had low specific surface area, high alkaline pH values, high ash content, and negative surface charge. Fourier transform infrared spectra showed the same surface functional groups present in each biochar. X-ray diffraction analysis showed the mineral composition of each biochar differed slightly. Scanning electron microscopy analysis indicated a porous structure of each biochar with ash particles evident. Conclusions: Slight differences in the ash content, surface area, pH and mineral content was observed between the three biochars.


Introduction
Improving sanitation along with hygiene practices and access to safe water are essential for improving socioeconomic development and health globally. Inadequate sanitation facilities and lack of clean water are key factors in the contraction of diarrhoeal diseases world-wide leading to 1.6-2.5 million deaths every year (Kosek et al., 2003).
In 2000 the international community set out eight Millennium Development Goals, a template aimed at tackling the needs of the world's most poverty stricken and underprivileged (Hutton & Bartram, 2008). Target 7C of the Environmental Sustainability Goal was to reduce by half the proportion of citizens "without sustainable access to safe drinking water and basic sanitation". Since 2000, however, the proportion of the population in low and middle-income nations that use "unimproved" sanitation facilities has increased (WHO & UNICEF, 2017).
In 2015 the international community set out 17 new Sustainable Development Goals including Goal 6, to "ensure availability and sustainable management of water and sanitation for all" (UN, 2015). It is estimated that 3.6 billion people in the world still do not have access to safely managed sanitation facilities (UNICEF/WHO, 2021). Approximately 2.1 -2.6 billion of these depend on onsite sanitation facilities (UNICEF & WHO, 2017) that generate vast quantities of untreated faecal sludge each day. In developing countries, faecal sludge (FS) collected from onsite sanitation facilities has been poorly managed, which has led to negative public and environmental health outcomes from eutrophication of surface water bodies, and contamination of groundwater and soils (Gwenzi & Munondo, 2008), and poor social and economic development (Haller et al., 2007;Mara et al., 2010). Long term and more sustainable solutions to deal with faecal sludge that don't involve expensive, water intensive and energy intensive sewer systems are needed.
Recent research has focused upon thermochemical treatment, with an emphasis on pyrolysis as a safe method of disposing of faecal sludge (Krueger et al., 2020). Pyrolysis is the heating of biomass to temperatures of 350°C -1000°C in an oxygen-free environment (European Biochar Foundation, 2016) which eliminates harmful pathogenic organisms within the sludge. Carbon-rich biochar produced from pyrolysis does not readily burn like charcoal (Crombie et al., 2013), is safe to handle, and has been demonstrated to be an important soil amendment (Chan et al., 2007). The original feedstock source, pyrolysis temperature, hold time, and heating rate are the main factors determining the characteristics of biochars (Chen et al., 2008;Crombie et al., 2015;Lehmann & Joseph, 2012;Tomczyk et al., 2020;Weber & Quicker, 2018).
The theory behind the utilization of biochar to improve soil fertility and increase crop yield originated from observations made on the Amazonian Black Earth (Terra Preta). Terra Preta is a specific type of very dark, fertile soil discovered in the Amazon basin, containing higher nutrient levels and higher organic carbon content than the surrounding soils which are generally low in fertility (Glaser et al., 2001).
There are multiple benefits to adding biochar to soil aside from improving carbon content and nutrient levels. Surface functional groups on the surface of biochar can lead to an increase in the cation exchange capacity of the soil (CEC) (Glaser et al., 2001); the microporous structure of biochar can increase the water holding capacity of the soil (Gaskin et al., 2007), and alkaline biochars can increase pH levels in acidic soil (Novak et al., 2009).
Biochar addition to soil has also been shown to reduce the bioavailability of heavy metals in soils (Park et al., 2011;Uchimiya et al., 2011;Zhang et al., 2013) including that of wastewater sludge biochar (Hossain et al., 2010) and sewage sludge biochar (Méndez et al., 2012). Biochar addition also reduces plant uptake of pesticides (Yu et al., 2009) and reduces the leaching of applied pesticides which can impact underground water contamination (Ahmad et al., 2014). Changes in soil microbial properties upon biochar addition have also been reported (Lehmann et al., 2011), including alteration of soil microbial community structure (Farrell et al., 2013), and an increase in microbial abundance (Awad et al., 2018).
There is a considerable amount of research investigating characteristics of sewage sludge-derived biochar but less on faecal sludge biochar (Gold et al., 2018). Many studies exist on the properties of faecal sludge itself (Awere et al., 2020;Bassan et al., 2013;Fanyin-Martin et al., 2017;Lama et al., 2022;Schoebitz et al., 2014;Septien et al., 2020). Only a handful of articles examine properties of faecal sludge biochar and these studies have a diverse range of objectives including biochar as a soil amendment to increase lettuce yields (Woldetsadik et al., 2018), biochars solid fuel characteristics (Krueger et al., 2020), cadmium adsorption by biochar (Koetlisi & Muchaonyerwa, 2017), recovery of ammonium in urine by biochar (Bai et al., 2018) and energy balance analysis of slow pyrolysis of human manure (Liu et al., 2014).

Amendments from Version 1
The article was accepted by the reviewer, only minor corrections have been made, some grammatical errors have been corrected, additional information in the introduction and conclusion has been supplied and methods section is clearer.
Any further responses from the reviewers can be found at the end of the article

REVISED
Most of the research into faecal sludge-derived biochar has focused on characterization of small-scale laboratoryproduced biochar (Bleuler et al., 2021;Gold et al., 2018;Liu et al., 2014;Woldetsadik et al., 2018) while data from full-scale operations are very limited (Krueger et al., 2020). Investigating the feasibility of resource recovery of operational up-scaled sludge treatment technologies and production of FS biochar with consistent properties is imperative to alleviate the sanitation crisis (Andriessen et al., 2019;Strande et al., 2014). Krueger et al. (2020) investigated the physico-chemical properties of full-scale faecal sludge biochars from treatment plants in Warangal and Narsapur, India. They focused on solid fuel properties of biochar, particle size distribution and heavy metal concentration. Heavy metal concentrations were found to be within the limits for land application set out by the EU (EEC, 1986) and the International Biochar Initiative (IBI, 2015) apart from the Narsapur biochar which contained concentrations of lead over the IBI stated threshold.
The objective of this investigation was to assess the uniformity of biochar characteristics produced from three full-scale faecal sludge treatment plants in Wai, Warangal and Narsapur, India. This study focused more on physico-chemical properties that would contribute to biochars end-use as a soil amendment. The biochar properties determined were ash content, pH, carbon content, organic surface groups, surface charge, mineral content, pore volume, and specific surface area.

Biochar preparation
The faecal feedstocks for the preparation of the biochars used in this study were sourced from three different faecal sludge and septage processors in India: Narsapur in Andhra Pradesh, Warangal in Telangana and Wai, Maharashtra ( Figure 2). Warangal and Narsapur treatment plants currently have a capacity of 15m 3 per day, whereas the Wai treatment plant has a capacity of 70 m 3 per day. FS collected from septic tanks is delivered to each processing plant where it is stored in holding tanks for the homogenization of the sludge. Tide Technocrats Private Limited have several community scale faecal sludges and septage processors which sanitize faecal waste and dewaters the sludge (5-10% moisture content) using solar energy in drying beds. Solar drying was managed on-site and expedited by spreading thesludge in a 10 mm layer. The sludge was pyrolyzed into biochar using a flame temperature operating range of 550-750°C. The process relies on autothermal operation, thus a limited supply of oxygen flows through an air fan into the main reaction chamber to allow for partial oxidation. process at the community scale faecal sludge and septage processors are outlined in Figure 1. Three 5kg biochar samples were collected fromeach processor in September 2018.

Characterization of biochars
The biochars characterized were collected from the pyrolyzer.
Chemical analysis. Elemental C, N, S and H abundances were determined at Environmental Geosciences, University of Vienna, Austria using an elemental analyzer (Vario MACRO, Elementar).
The elemental composition (C, H, N, S and O) and ash content of the biochars were used to calculate the molar element ratios H/C, C/N, and O/C. The amount of oxygen in the samples was calculated from the subtraction of total percentage carbon, hydrogen, nitrogen, and sulphur and ash content from 100 (Castan et al., 2019).

Proximate analyses.
Moisture and ash content of the three biochars were determined in triplicate by methods adapted from the literature (ASTM D 1762(ASTM D -84, 2011Enders et al., 2012).
Crucibles and covers were cleaned by heating at 750°C for 6 hours and then cooling to 105°C. This procedure volatilized residual material on the crucibles. The crucibles were transferred to desiccators and cooled to ambient temperature. The mass of crucibles and crucible covers were recorded to 0.1mg and masses determined for all samples. Approximately 1.0g of biochar was added into each crucible. For the moisture determination the crucibles and covers were heated at 105°C for 18 hours and then transferred to desiccators whilst hot. The covers were removed briefly in order to safely remove the crucibles and covers from the oven. After cooling to ambient temperature, the mass of crucibles, covers and sample were recorded to 0.1mg for all samples.
For ash determination the covered crucibles with 105°C dry biochar was placed in the furnace. The covers were adjusted so that they were askew to allow air flow into the crucibles, while reducing the possibility of physical losses. The samples were heated from ambient to 750°C at a rate of 2°C per minute. The furnace was programmed to hold the temperature at 750°C for 6 hours then allowed to cool down to ~130°C. Crucible lids were adjusted to sit flush when the temperature of 105°C was reached. The crucibles were then removed from the furnace before placing in desiccators and left to cool to ambient temperature. The mass of each crucible and crucible cover with sample was recorded to 0.1mg.
Chars were ground to <850µm in a pestle and mortar to enhance representativeness of the sample and sieved to >149µm as this lessens physical losses upon rapid heating (Enders & Lehmann, 2015).
pH and electrical conductivity. The pH of biochar samples was measured by suspending 5.0g (ground to <2mm) biochar in deionised water in a 1:10 ratio (Singh et al., 2017). After 1 hour of shaking, suspensions were allowed to stand for 30 minutes before pH measurements were taken using a Voltcraft soil pH meter calibrated using pH 7 and pH 10 buffers. Electrical conductivity (EC) was measured on the same samples using a calibrated Whatman CDM 400 EC meter. The analyses of pH and EC were performed in triplicate.

FT-IR analysis.
Fourier transform infrared (FTIR) spectra were used to identify the surface organic functional groups present in the biochar. High ash content in sludge-derived biochars leads to a high mineral content with bands in the infrared spectrum arising at similar wavenumbers to organic functional groups. To elucidate the different groups present, FTIR spectra of ashed biochars and de-ashed (acid washed) biochars were also generated.
Acid washing biochar to remove ash content (Klasson et al., 2009;Lima et al., 2016;Thomas Klasson et al., 2014) was achieved with 0.1 M HCl at a ratio of approximately 50:1 (v/w). Samples were shaken in a Uniwist 400 at 180 rpm for 2 hours before being filtered and washed with deionised water until a pH of 7 was reached. Samples were oven-dried at 80°C overnight.
The samples were gently ground using a pestle and mortar and analyzed using a Perkin Elmer Spectrum 2 FTIR spectrophotometer applying the Attenuated Total Reflectance (ATR) method with a diamond crystal. The resulting spectra were an average of 16 scans obtained in the range from 400 to 4000 cm −1 with a spectral resolution of 2 cm −1 for biochars and 4 cm −1 for acid washed and ashed biochars. Surface area. The BET (Brunauer, Emmett, and Teller) method is frequently used to determine the total surface area and pore size of materials. The BET analysis was conducted using the NOVA 2200e surface area and pore size analyzer (Quantachrome Instruments). The BET specific surface area of the three biochar samples were determined using two methods: N 2 as adsorptive gas at 77 K and CO 2 at 273 K.
Prior to these measurements, 200mg -300mg of biochar (<2mm) were heated to 130°C under vacuum for a minimum of 4 hours. Then the samples were transferred to the instrument and outgassed at 105°C for a minimum of 4 hours following standard protocols. Samples were analyzed in triplicates.
For N 2 isotherms and CO 2 isotherms the BET equation was used to determine the specific surface areas from six points in the pressure region P/P 0 = 0.01-0.30 (Brunauer et al., 1938). For N 2 the pore size-distributions in the pressure region P/P 0 = 0.01-0.98 were ascertained using the built-in Density Functional Theory (DFT) model assuming slit-like pores. DFT considers micropore filling process, the development of the adsorbed film thickness, and importantly capillary condensation and evaporation, thus it can model hysteresis in the adsorption/desorption mesopore region of the isotherm.
For CO 2 isotherms the pore size-distribution, the cumulative pore volume (µPV) and the cumulative surface area (µSSA) in the pressure region P/P 0 = 0.001-0.030 were determined using the built-in Grand Canonical Monte Carlo (GCMC) simulation, again, assuming pores were slit-shaped.
The benefit of using of DFT and Monte Carlo simulation methods is that they provide a combined micro-mesopore analysis.
Measurement of zeta potential. Zeta potential measurements were undertaken according to methods reported in the literature (Samsuri et al., 2013;Yuan et al., 2011). The zeta potential values were determined by weighing 0.045g of 63µm sieved biochar into a 250ml conical flask and adding 180 mL of 0.1M NaCl solution to each flask. Five suspensions were prepared for each biochar at pH values between 5.0 -9.0 with the pH of each suspension adjusted using HCl . Suspensions were then dispersed ultrasonically for 30 minutes at 30 ± 1 °C in a bath-type sonicator at a frequency of 40 kHz and a power of 300 W. The samples were then left to stand for 72 hours before being measured with a Malvern Zeta Sizer Nano. In total 15 suspensions were prepared and each suspension was measured a minimum of three times.

X-ray diffraction (XRD)
. The X-ray diffraction (XRD) analysis of the chars was conducted on a Bruker D8 Discover XRD. This was operated at 40 kV and 40 mA and the data collected over a 2θ range of 20-70° using the Cu-Kα radiation at a scan rate of 2° min −1 . The main phase peaks were identified by comparing the observed XRD patterns to the standards compiled by the Crystallography Open Database (COD) (Downs & Hall-Wallace, 2003;Gražulis et al., 2009;Gražulis et al., 2015, Gražulis et al., 2012Merkys et al., 2016;Quirós et al., 2018).

SEM/EDX. SEM-EDX analysis offers detailed imaging data
about the morphology and surface texture of individual particles, with characterization of the elemental composition of the analyzed volume. Scanning electron microscope (SEM) analysis was performed using a Hitachi TM3000 SEM fitted with a Bruker X-ray energy dispersive spectrometry (EDS). The two modes of operation in SEM analysis utilized here were backscattered electron imaging (BSE) and energy dispersive x-ray EDX. Biochar particles used were in the size range 150 µm-850 µm. Prior to analysis samples were spread onto double-sided carbon tape and mounted on a SEM stub.

Cation exchange capacity.
Cation exchange capacity measurements were performed in University of Santiago de Compostela, Spain, by the summation method of the exchangeable base cations of Ca, Mg, Na, K, and Al. NH 4 Cl 1M (25ml) was added to the biochar sample (5g) and shaken manually before being left to stand overnight (16 hours). The following day, 75 ml of NH 4 Cl 1M was added and then filtered using quantitative, low ash filter paper. (Peech et al., 1947). Ca, Mg and Al were measured by PerkinElmer PinAAcle 500 Atomic Absorption Spectrometer and Na, K, were measured by Atomic Absorption Spectophotometer with an Emission Flame.

Statistical analysis
Electrical conductivity, pH, moisture content, ash content, CEC, surface area and pore volume measurements were all performed in triplicate and experimental uncertainty given by standard deviation.

Proximate analyses EC, pH and elemental analyses
All biochars collected from the pyrolyzer had high ash contents (Nicholas, 2022). Warangal biochar (WGL_BC) recorded the highest ash content at 88.3% and Narsapur biochar (NSP_BC) and Wai biochar (WAI_BC) had lower ash contents at 67.0% and 62.3% respectively (Table 1.). Warangal biochar (WGL_BC) also had the lowest moisture content at 0.98% in comparison with 2.15% and 3.08% for Narsapur biochar (NSP_BC) and Wai biochar (WAI_BC) respectively. Measured pH values were high for all three biochars (11.86 -12.45). The measured high ash content is consistent with the literature (Gold et al., 2018;Koetlisi & Muchaonyerwa, 2017;Liu et al., 2014). The initial feedstock of sewage sludge is high in ash and sewage sludges have been found to contain very high concentrations of Si (19-58%), Ca (5.1-7.4%), and P (3.4-4.9%) (Zielińska et al., 2015). Ash content of faecal sludge is also high and has been measured at 17.0 wt.%, significantly higher than measured ash content of sawdust at 0.8% (Liu et al., 2014). Ash content of faecal sludge has been found to be higher than that of sewage sludge (Koetlisi & Muchaonyerwa, 2017). Several reasons for this high ash content have been suggested including loss of volatile solids from latrine waste due to long storage times in the latrine (Zuma et al., 2015), ingress of grit and sand from poorly lined containment structures (Niwagaba et al., 2014) and in community toilets a higher rate of disposal of polluting waste can lead to higher ash contents (Barani et al., 2018). Increases in pH due to increases in ash content in biochars derived from sewage sludge feedstocks have been previously reported (Hossain et al., 2011;Liu et al., 2014). The general alkaline character of biochar likely results from the increase in quantities of alkali salts (Na, K) and salts of alkaline elements (Ca, Mg) during the pyrolysis process (Singh et al., 2010).
WGL_BC recorded the most alkaline pH value (12.45) and the largest EC value (Table 1.) which is due to the higher ash content recorded for WGL_BC (Rehrah et al., 2014). There could be several reasons why the WGL biochar had a significantly higher ash content. Digestion during storage in onsite sanitation technologies can play a part in the high ash content of FS biochar (Gold et al., 2018) as well as contamination of FS by sand and grit caused by poorly lined containment structures (Niwagaba et al., 2014). A recent study investigating biochar from the same treatment facilities in India observed that sintered mineral depositions had to be removed from the reactor on a weekly basis (Krueger et al., 2020). Therefore, it is likely that ash concentrations from biochars from these types of treatment plants will fluctuate over time.
A high ash content of biochar could be useful with regards to its end-use a soil amendment. Increased crop growth with a highly alkaline (12.1), high ash biochar treatment of acidic soil has been previously reported (Smider & Singh, 2014) The authors deemed this was a result of the release of nutrients from the ash in the biochar itself and the biochar's liming effect. It has been proposed that this liming effect is one of the main processes influencing the enhanced plant growth seen on biochar addition to soils (Jeffery et al., 2011). Altering soil pH is one of several mechanisms by which biochar can improve soils and increase agricultural productivity. Therefore, highly alkaline biochars could be of benefit to acidic soils are responsible for the severe limitation of crop agriculture worldwide.
Currently only a small fraction of acidic soil is used for arable crops globally but approximately 50% of the earth's potential arable lands are acidic (von Uexküll & Mutert, 1995).
The elemental composition (Table 1.) shows a relatively low percentage of carbon within the samples, 21-23% for NSP_BC and WAI_BC and a very low 8% for WGL_BC which is consistent with the measured ash content. Pyrolysis generally concentrates carbon in the biochar with an increase in C content relative to the feedstock frequently reported. However, most studies on sewage sludge (SS) -derived biochar show a decrease in the percentage of C in the final product relative to the feedstock (Agrafioti et al., 2013;Khan et al., 2013). FS-and SS-derived biochars generally have low total C concentrations in comparison with cellulose derived biochars (Tomczyk et al., 2020). This is due to the high ash content in the original feedstock of faecal and sewage sludge. Fourier transform infrared (FTIR) spectroscopy FTIR spectra indicated that all three sludge biochars have a complex chemical bond structure with both organic matter and mineral compounds evident within the biochar. The FTIR spectra of all three biochars were similar with the same functional groups present on the surface; an indication of the homogenous nature of faecal sludge (Figure 3).
High ash content in sludge-derived biochars leads to a high mineral content with bands arising on the spectrum at similar wavenumbers to organic functional groups. For example, a broad peak in the 1000-1200cm -1 region can arise due to several functional groups such as inorganic and organic silicon, phosphorus compounds, as well as C-O stretching and sulphate groups (Coates, 2004) Low intensity peaks evident in the 3800cm −1 -3600cm −1 region relate to OH group vibrations within mineral matter (Hossain et al., 2011) which indicates the presence of clay type compounds within the biochar (Table 2). Two peaks at 2980cm -1 and 2890cm -1 indicate asymmetric and symmetric aliphatic ν(CH) from terminal -CH 3 groups respectively (Socrates, 2001).  However, these CH bands disappear at high temperatures due to demethylation and dehydration (Zhang et al., 2015) therefore in biochar pyrolyzed at 550 -750°C the peaks are negligible.
Small peaks in the 2700-2100 region could be due to P-OH groups in phosphorus acids and esters which produce one or two broad bands (Stuart, 2004).
A peak at 1424cm −1 in the biochar spectra (Figure 3.) corresponds to asymmetric stretches of carbonate groups, which correlates with the small peak at 874 cm −1 due to the out-of-plane bending for CO 3 2− (Zhao et al., 2013). This could indicate the presence of calcite (calcium carbonate) in the sample. The presence of carbonate was verified as the FTIR spectrum of the acid washed biochar showed no clear peaks at 1424 cm −1 or 874 cm −1 confirming that acid washing removed carbonates from the sample (Figure 5.).
Another interesting difference between the ashed (Figure 4.) biochars and deashed biochars is a broad trough between 3400cm -1 and 2500 cm -1 . This implies the presence of O-H in carboxylic acids however there is only a very weak intensity peak at ~1700 cm -1 which could correspond to C=O in carboxylic acids. Other possible groups responsible for peaks within the 3400cm -1 and 2500 cm -1 region include ν(OH) from sorbed water and hydrogen bonded OH (Keiluweit et al., 2010). The low intensity peak in biochar between 1540 and 1650 could be indicative of C=O stretching vibrations for amides (Calderón et al., 2006), aromatic C=C stretching and carboxylate anion vibrations (Deacon & Phillips, 1980). The peak in the deashed biochar at 1580 cm −1 to 1600 cm −1 is indicative of a carboxylate ion, the conjugate base of a carboxylic acid (Deacon & Phillips, 1980;Ellerbrock & Gerke, 2021).
This peak was not evident in the ashed biochar (Figure 4.). It's been suggested that a reduction in inorganics by acid demineralization allows previously hidden carbon to emerge so increasing the amount of acidic functional groups (Lou et al., 2011). In the ashed biochar there are very visible peaks ~1450cm -1 indicative of a carbonate stretch (CO 3 2-) whereas as the peaks in the acid washed samples are much less visible indicating some carbonate salts within the ash content have been successfully removed by acid demineralization. The very broad band in the range 1200-970cm -1 is indicative of several functional groups. Inorganic and organic silicon and phosphorus compounds, as well as carbohydrates and sulphates can contribute to this broad peak (Wen et al., 2007). Sewage chars are known to contain high phosphorus levels suggesting that the peaks observed in 1200-950cm -1 band arise from P containing functional groups such as asymmetric and symmetric stretching of PO 2 and P(OH) 2 in phosphate (Jiang et al., 2004). Si-O asymmetric stretching could also be present between 1000-1100cm -1 (Falaras, 1999) as well as symmetric C-O stretching of ethers. There are more signals recorded in the 900-400 cm -1 region for the ashed biochar than the deashed biochar which relate to clay minerals associated with biochar. Bands below 600 cm -1 can be caused by stretching inorganic compounds such as KCl and CaCl 2 (Hossain et al., 2011).
The oxygen containing functional groups (OCFGs) present on biochars surface such as C=O groups determine its cation exchange capacity (CEC) (Banik et al., 2018). It is this property that enables biochars to adsorb cationic nutrients such as NH 4 + , Ca 2+ , K + within the soil and increases soils nutrient retention capability. The lack of C=O groups present in WGL_BC could affect its ability to retain nutrients and therefore its suitability as a soil amendment.

Surface area
The shape of the isotherms indicate a Type II isotherm, however, Type II isotherms are generally typified by a lack of hysteresis and no saturation at P/P 0 near to 1; typical of nonporous and macroporous adsorbents (Thommes et al., 2015). A deviation from a true Type II isotherm can be described as a pseudo-type II isotherm. These isotherms are associated with delayed capillary condensation due to the small degree of pore curvature and non-rigidity of the aggregate structure of the adsorbent. (Sing & Williams, 2004).
Hysteresis is present in all isotherms and can be classified as as either type H3 hysteresis loop or type H4 hysteresis loop according to International Union of Pure and Applied Chemistry (Thommes et al., 2015). Hysteresis is caused by capillary condensation and is typical of mesoporous materials. H3 and H4 loops do not tend to close until equilibrium pressure is at or close to saturation pressure. H3 type is typical for loose aggregates of plate-like particles and in porous materials typical of pore networks containing macropores not entirely filled with condensate. H4 type loops suggest presence of slit-shaped pores including pores in the micropore region and plate-like particles with spaces between the parallel plates (Mokaya & Jones, 1995) and are common with activated carbons. H4 hysteresis loops are commonly observed with more complex materials consisting of both micropores and mesopores.
Adsorption and desorption N 2 isotherms for all biochars (Figure 6.) showed low surface areas of between 3.52 -12.07m 2 g -1 (Table 1)  The low nitrogen uptake of all three biochars can be characteristic of materials with small ultra-micropores that are close to the kinetic diameters of nitrogen, since molecules cannot overcome the activation energy for passing through the pores at cryogenic temperatures (Kim et al., 2011). To investigate this potential microporosity further CO 2 adsorption isotherms at 273K were recorded for the three biochar samples (Figure 7).
The CO 2 -based BET specific surface areas (S BET , µSSA) and pore volume (µPV) values were significantly larger than the N 2 -derived BET specific surface area (S BET ) and pore volume (TPV) values signifying that kinetic limitations with N 2  physisorption were present for all biochars and there is some degree of microporosity present.
NSP_BC showed the largest surface area measured with CO 2 and the lowest with N 2 indicating a more microporous structure whereas WGL_BC had the highest N 2 SSA and lowest CO 2 µSSA signifying a slightly less microporous and more mesoporous structure. The greatest pore size distribution at pore diameters 4-15Å was recorded for NSP_BC also indicating it had more of a microporous structure than WGL_BC which recorded relatively sparse pore size distributions in this region (Figure 8b). In the mesoporous region, (16-150 Å), WGL_BC pore size distributions were much greater than both NSP_BC and WAI_BC confirming WGL_BC has a more mesoporous structure (Figure 8a).
The values obtained demonstrate the complex pore network within the biochar, even though the surface area values are generally low compared to other biochars there is still a degree of both microporosity and mesoporosity within the biochars. Low surface area biochars may be unsuitable for use as soil amendments as the water holding capacity is relatively low and the low porosities are not conducive to promoting soil microbial growth (Ishii & Kadoya, 1994;Thies & Rillig, 2009), which play an important role in nutrient cycling (Lambers et al., 2008). The surface area could be increased by increasing the pyrolysis temperature (Song & Guo, 2012;Tomczyk et al., 2020). The fast pyrolysis of municipal sludge biochar at temperatures 500 -900 °C showed that increasing temperatures resulted in a greater microporous network within the biochar (Chen et al., 2014). Previous work has shown that the greatest enhancement of sewage sludge biochar porosity occurred between 400 -600°C (Bagreev et al., 2001). However heavy metal concentration in biochars generally increase with pyrolytic temperature (Lu et al., 2013). This is because heavy metals do not volatilize, so their concentration within the biochar increases with pyrolysis temperature (Chanaka Udayanga et al., 2019;Hossain et al., 2011;Wang et al., 2021).
The larger surface areas (with CO 2 ) and more microporous structure of NSP_BC and WAI_BC relates to their lower ash content of 67.0% and 62.3% respectively. The lowest surface area measured (with CO 2 ) was that of WGL_BC which recorded the highest ash content of 88.3%. These findings support the notion that lower surface areas relate to ash filling or blocking access to the biochar micropores (Song & Guo, 2012).
Zeta potential Zeta (electrokinetic) potential signifies the net charge between the surface plane and slip plane of a colloidal particle (Hiemenz & Rajagopalan, 1997). Zeta potential values yield information about the external surface charges of biochar particles in solution and indicates the sorption and nutrient holding characteristics of the biochar in soil. Negatively charged surfaces are unlikely to sorb negatively charged ions such as phosphate but are more likely to sorb positive cations such as heavy metal ions and ammonium ions.
The zeta potential values for all three biochar samples were negative in the pH range 5.0-9.5, revealing that negative charges are carried on the surface of the biochar particles ( Figure 9). FTIR spectra revealed the existence of oxygen containing functional groups (-COO − and-OH) on the biochars surface which can contribute considerably to surface charge of the biochars. The negative zeta potentials of all three biochars in the pH range 5.0 -9.5 support this interpretation.
At acidic pH, the zeta potentials of the biochar samples became less negative, indicating that the association of -COO − and -O − with H + reduced the negative charge of the biochars. With increasing pH, the zeta potential of WAI_BC and NSP_BC biochars become more negative due to increasing deprotonation of the biochar surface functional groups (Yuan et al., 2011). However, at pH above 7 there was an increase in zeta potential for WGL_BC from -23.4mV to -7.7mV indicating a decrease in negative surface charge. WGL_BC contains the highest ash content of all the biochars (Table 1), and it is likely this that contributes to the increase in zeta potential values at pH>7. The mechanism by which surface charge increases at high pH values cannot be explained by deprotonation of the surface functional groups in the case of WGL_BC. The higher ash content indicates some other mechanism occurring. Zeta potential of fine coal tailings containing several ash-forming minerals showed a similar trend which the authors attributed to the presence of alumina and silicate particles, which result in lower negative zeta potential values. They also noted that varying zeta potential values at high pH could be attributed to the binding of more cations such as Ca 2+ (Kumar et al., 2014). Positively charged calcium monohydroxide ions on the biochar surface would to some degree neutralize the negative surface charges resulting in less negative zeta potential values. (Liu et al., 2002). It is possible that at pH values >9 WGL_BC would have positive zeta potential values and thus more likely to sorb negatively charged ions such as nitrate or phosphate.
X-ray diffraction (XRD) X-ray diffraction (XRD) analysis of the biochars revealed that mineral components in the crystal form were present in all Figure 8. Pore volume weighted pore size-distribution derived from a) N 2 (mesopore region 20-500Å) and b) CO 2 (micropore region<20Å) for NSP, WAI and WGL biochars. three biochars (Figure 10). Quartz was identified as the predominant crystalline phase with the highest peak at 2θ around 26.6° (d = 3.33 Å) in NSP and WGL biochars. WAI biochar exhibited a more intense peak relating to CaSO 4 (anhydrite). Quartz, sylvite, calcite, calcium sulphate, albite were the most common phases identified. These minerals are formed during pyrolysis due to a reaction between CO 2 and alkaline-earth metals and alkaline oxyhydroxides.
Previous research has shown sewage sludge biochar to have a turbostratic structure where the carbon fraction is dominated by disordered graphitic crystallites (Srinivasan et al., 2015;Uchimiya et al., 2011). This is in discordance with the XRD results which show a lack of C (002) diffraction peaks (2θ = 15-30°) and C (101) diffraction peaks (2θ = 40-50°) due to amorphous carbon structures and graphite structures respectively. The biochars studied here do have a very high ash content and the lack of these peaks could be as a result of interference of high-intensity quartz peaks. Studies have shown that the high content of minerals, specifically quartz can affect the structural characterization of biochar carbon fraction (Feng et al., 2015).
The difference in mineral composition between the three biochars could be due to possible contamination of FS by sand and grit caused by poorly lined containment structures (Niwagaba et al., 2014). The containment structures at each location would have to be investigated to reach a definitive conclusion. Overall, the mineralogical composition of the biochars is in agreement with their high ash contents.
Scanning electron microscopy (SEM) with energy dispersive x-ray analysis (EDX) SEM analysis reveals a complex porous structure evident in all biochars ( Figure 11). The porous structure of biochars strongly resembles the cellular structure of the original feedstock (Fuertes et al., 2010;Yao et al., 2011). In the case of faecal sludge, cellular macroporous structures arise from undigested fibrous vegetable matter. The morphology of the biochar is honeycomb-like with cylindrical and slit like holes clearly observable. This porous structure can provide a specialized environment for the colonization of microbes (Thies & Rillig, 2012). This increase in mycorrhizal fungi contributes to increased mineralization of recalcitrant soil organic matter, ultimately improving soil and plant health (Anderson et al., 2011;Zimmerman et al., 2011). SEM images also show all three biochars have high ash content with EDX confirming the presence of mineral elements (Figure 12). The SEM images clearly showed a high presence of clay mineral particles/ash (white/grey) with a smaller amount of biochar particles present (black).
Visually WGL biochar had a higher ash content which is concurrent with the ash percentage from proximate analyses. EDX results on the biochar particles themselves revealed high volumes of carbon and oxygen (Figure 12). Also present were  silicon, calcium aluminium, potassium, magnesium, phosphorus, and sodium all of which are beneficial to plant health.

Cation exchange capacity
Cation exchange capacity (CEC) enables biochars to adsorb cationic nutrients such as NH 4 + , Ca 2+ , and K + . It is thought this characteristic of biochar results predominantly from formation of carboxylic functional groups during oxidation (Cheng et al., 2006).
There was a large variation in CEC values with WGL biochar (WGL BC) the highest CEC at 129.3 cmolKg -1 and NSP biochar the lowest CEC at 41.9 cmolKg -1 (Table 1). Fresh biochars from lignocellulosic biomass generally have lower CEC, with manure-based biochars exhibiting higher CEC values (Tag et al., 2016). In the literature CEC values for biochar are highly variable, commonly ranging from 6 cmol (+) Kg −1 (Munera-Echeverri et al., 2018) to 36.3 cmol (+) Kg −1 (Song & Guo, 2012) to as high as 304 cmol (+) Kg −1 . Yuan et al. (2011) proposed that high ash content biomass creates high CEC biochars and that K, Na, Ca, Mg, and P in the feedstock would promote formation of O-containing acidic functional groups such as carboxylic, and phenolic groups on biochar surface during pyrolysis and thus, result in higher CEC (Gaskin et al., 2008). However, FTIR analysis showed a lack of acidic functional groups such as phenolic groups in these biochars. It is possible that the high ash content of these biochars could contribute to methodological problems in determining CEC (Graber et al., 2017). There is a large range of CEC values reported in the literature and measurements are often poorly reproducible (Munera-Echeverri et al., 2018). FTIR shows that there are carbonates and silicates present in these biochars which would result in the release of base cations and interference with the sum of measured exchangeable base cations (Munera-Echeverri et al., 2018). WGL biochar records the highest CEC value (129.3 ± 2.3 cmol.kg -1 ) and the highest ash content of all three biochars implying that it is the high ash content that is responsible for the high CEC value.

Conclusion
Overall, all three faecal sludge biochars had a high ash content, high pH, low carbon content, negative surface charge and low specific surface areas and pore volumes. The similarity of FTIR spectra between biochars signifies a uniformity of the organic component of all three biochars. Warangal biochar had a significantly higher ash content and pH compared to the Narsapur and Wai biochar. There were also differences in XRD spectra between biochars. These differences are likely related to the contamination of faecal sludge in the containment structure by sand or grit, or the sintered mineral deposits in the reactor itself. The variability of these faecal sludge biochar properties highlights the differences between small-scall laboratory and full-scale "real world" biochar production. Control over every single variable in large-scale faecal sludge biochar production is difficult and routine inspections of every containment structure at every location would be time-consuming. However, the pH and ash content of the biochars could be monitored periodically at the treatment plant. The low surface areas and porosity of these biochars could prove detrimental in its end use a soil amendment as these properties relate to water holding capacity and microbial activity. However, increasing the porosity of faecal sludge biochar is possible through techniques such as chemical and physical activation. Overall, the properties of these biochars, in particular the high alkalinity, shows their potential use as soil amendment particularly with acidic soil. The liming effect from these biochars and release of nutrients from the ash in the biochar itself could contribute to increased agricultural productivity especially in developing nations where the use of inorganic fertilizer on smallholder farmers is much lower. Future work should determine the biochars total and plant available macro-and micronutrient concentrations. Further investigation into the evaluation of these biochars as soil amendments with a focus on application to acidic soils is also recommended.

Sadib Bin Kabir
Monash University, Clayton, Victoria, Australia This study focuses on the biochar assessment derived from faecal sludge pyrolysis. Even though numerous research on small-scale faecal sludge-derived biochar characterization has been studied before, this study conducted the characterization of biochar in a full-scale operation. The work is clearly and accurately presented and it cites the current literature.
The study design is appropriate and the work is technically sound. This study's characterization methods include Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX) spectroscopy, crystal x-ray diffraction (XRD), proximate analyses, and BET surface area porosimetry. The methods are explained in detail and are sufficient to allow replication by others.
Results showed that biochar samples had low specific surface area, high alkaline pH values, high ash content, and negative surface charge. In the Results section, the Fourier transform infrared section requires more improvement and critical discussion. The authors listed all the compounds present without proper explanation. X-ray diffraction analysis showed the mineral composition of each biochar differed slightly. Scanning electron microscopy analysis indicated a porous structure of each biochar with ash particles evident.
All the source data underlying the results ensure full reproducibility. The conclusions are drawn adequately supported by the results.

Is the study design appropriate and is the work technically sound? Yes
Are sufficient details of methods and analysis provided to allow replication by others? © 2023 Manga M. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Musa Manga
The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA

Broad overview
The authors discuss and compare the properties of biochar obtained from the pyrolysis of faecal sludge at three treatment plants in India. They examined properties that influence its use as soil amendment/conditioner such as the ash content, pH, carbon content, organic surface groups, surface charge, mineral content, pore volume. This is a very insightful paper as the authors investigate an understudied area; they studied the characteristics of biochar from full scale treatment plants as opposed to former studies which have examined biochar from laboratory experiments. The manuscript is well written, and the authors provided broad discussion on their findings. However, there are still some shortfalls in the manuscript (as outlined below) that the authors need to address.

General comments
The authors did not state anywhere in the manuscript the reason for choosing the three different faecal sludge treatment plants (FSTPs). It will be good if the authors can clearly state why they chose these plants: are they the only three FSTPs in India? are they representative of FSTPs in India?

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Since the authors studied properties of biochar from large scale treatment plants, more value will be given to the paper if the authors can include in their discussion, comparisons between properties of biochar obtained from treatment plants and those obtained from laboratory experiments.

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In addition, since the authors decided to study the properties of biochar for use as a soil amendment, the authors should provide a separate section where they elaborately discuss the implications of these properties on biochar's use as soil amendment. In this section, they should also discuss what can be done to improve some of the properties that limit its use as soil amendment.

Specific comments Methods section:
Under biochar preparation: Figure 2 was mentioned before Figure 1, this should be corrected.

Under Characterization of biochar:
Rather than saying "The biochar characterized were collected from the pyrolizer", it will be better to say "The following analysis were carried out on the collected biochar"

Under chemical analysis:
The full meaning of C, H, N, S should be provided at the point of first mention.
Paragraph 6: there are more advantages of the use of biochar as soil conditioner that you can mention (e.g., removal of pollutants, reduction of pollution of underground water, support the growth of microbial communities beneficial for plant growth, retention of nutrients). biochar). It seems to me that biochar and ashed-biochar are the same. Please clarify. Paragraph 2: in Figure 2, 3 and 4, we cannot see the spectrum in the 3800 cm-1 to 3600 cm-1 region (related to OH group variations within mineral matter).

If applicable, is the statistical analysis and its interpretation appropriate? Partly
Are all the source data underlying the results available to ensure full reproducibility? Yes

Results and Discussions:
"Proximate analyses, EC, pH and elemental analyses" section: ○ -Comment 13: You should relate the higher ash content of the biochar to the initial ash content of the sludge.-Authors response: Agreed and amended. Comment 14: As an explanation of the high ash content in faecal sludge, the disposal of liquid and solid waste in the toilets could also contribute to the high amount of inorganic material in the sludge. It would have been interesting to measure the carbon content in the sludge to verify if this parameter increases or decreases after pyrolysis in your case. "Fourier transform infrared (FTIR) spectroscopy" section: Comment 15: You could conduct an element analysis of the ash from the three biochar samples and correlate the FTIR results to these results.

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Authors response: This would have been a really good idea. Unfortunately, as I have finished my PhD, I cannot do any further experimental work. Also, I didn't have access to an elemental analyser so samples had to be sent away for analysis which makes it a bit trickier. Comment 16: What could be the reasons that the sludge biochar have the same composition according to the FTIR spectra? What are the implications of this in the application? ○ Authors response: I think the homogeneous nature of faecal sludge leads to the same composition and the similar temperatures that the sludge was pyrolzed at. Comment 17: There is not a clear difference between Figure 2 (biochar) and 3 (ashedbiochar). It seems to me that biochar and ashed-biochar are the same. Please clarify.-○ Authors response: They are different, although not massively due to the very high ash content of the biochars. There is a low intensity peak in biochar between 1540 and 1650 whereas this peak is not evident in the ashed biochar (Figure 3.). Comment 18: Paragraph 2: in Figure 2, 3 and 4, we cannot see the spectrum in the 3800 cm-1 to 3600 cm-1 region (related to OH group variations within mineral matter). -○ Authors response: I did cut that out to make the graph clearer but there were no peaks in that region. I would like to keep the graph as it is. "Surface area" section: Comment 21: Paragraph 2: it contradicts what has been stated in the previous paragraph. If the sorption isotherms are of Type II, it could be expected a nonporous and microporous adsorbent, but it was found that the biochar samples have a degree of microporosity and mesoporosity. Please clarify. -○ Authors response: So later on we do clarify that the isotherms are pseudo type II. This is actually quite tricky. The shape of the adsorption isotherms is best suited to type II, but type II isotherms are characterized by a lack of hysteresis and there is hysteresis to some degree here, especially when looking at NSP and WAI biochars. Type IIs are characteristic for nonporous and macroporous adsorbents. As there are indications of hysteresis loops especially for WAI and NSP biochar, this would indicate a type IV isotherm according to the IUPAC classification but there is no plateau characteristic for this type. So really, I can't say its type II or type IV. According to (Sing and Williams, 2004) the isotherms can be classified as pseudo-type II, describing delayed capillary condensation due to the small rigidity of the aggregate structure of the adsorbent. So, I have essentially concluded it is a pseudo type II isotherm. I hope this makes sense! Comment 31: Paragraph 3: How were the high content of the biochar samples in Ca, Si and K, as well as its high alkalizing capacity, determined? ○ Authors response: Agreed that this wasn't determined clearly enough, so this section has been removed Comment 32: Paragraph 3: "Thermonatrite" must be written without capital letter. ○ Authors response: Amended.

Conclusion:
Comment 33: Indicate the weak points of using biochar derived from faecal sludge as soil condition and suggest methods to overcome these weaknesses.

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Authors response: Agreed this has been added to conclusion. Comment 34: Precis in which context the high alkalinity of the biochar would be beneficial (i.e., acidic soils).