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The treatment and safe disposal of sanitation waste is imperative to human health and the environment. In developed countries, the emphasis is on recovering phosphorus from municipal sewage sludge (SS) and the reduction of landfill. Whilst in developing countries, the focus is on long-term mechanisms to treat fecal sludge (FS) generated from non-sewered sanitation facilities. This chapter summarizes the thermal treatment of FS and SS via slow pyrolysis, and the resultant characterization of FS and SS-derived biochar with the aim of utilization by agriculture. In general, FS and SS biochars have high pH, ash content and macronutrient concentrations, with a low surface area and carbon content. The concentration of potentially toxic elements is a key difference between FS and SS biochars with FS biochars containing lower concentrations of these harmful metals. Assessing the properties of these biochars is challenging because of the different methods involved in the processing of raw sludge. The slow pyrolysis of FS and SS to produce biochar can play a pivotal role in a circular economy through the recovery and re-use of waste. Waste-derived biochar provides an opportunity to utilize an integrated systems-based approach to improve soil health, increase crop yield, and improve water retention.
Faculty of Science and Engineering, Department of Chemistry, Swansea University, Swansea, Wales, UK
Aisling Devine
Faculty of Science and Engineering, Department of Biosciences, Swansea University, Swansea, Wales, UK
Iain Robertson
Faculty of Science and Engineering, Department of Geography, Swansea University, Swansea, Wales, UK
Ian Mabbett
Faculty of Science and Engineering, Department of Chemistry, Swansea University, Swansea, Wales, UK
*Address all correspondence to: larissa.nicholas@swansea.ac.uk
1. Introduction
In developed countries there are sewer systems and wastewater treatment plants that transport and safely treat sewage sludge, however, dramatic population growth, as well as stringent requirements for the treatment of sewage effluent have resulted in a steady increase in the volume of sewage sludge produced [1]. Conventionally the methods for disposing of treated sewage sludge include three main routes: reuse (land application), incineration or landfilling [2]. However, these options are becoming less desirable due to the accumulation of potentially toxic metals and pathogens in sludge which effect its use in agriculture [3]. The impact of EU Directive 2018/851/EC resulted in a ban on landfilling, limited land application of sewage sludge and a focus on sustainable material management and a transition to a circular economy [4].
In developing nations, the emphasis is on sustainable and longer-term solutions to treat fecal sludge from non-sewered onsite sanitation systems. Goal 6 of the UNs 17 Sustainable Development Goals is to “ensure availability and sustainable management of water and sanitation for all” [5].
In low and middle income countries there has been an increase, since 2000, in the proportion of the population reliant on “unimproved” sanitation systems [6]. Globally a total of 3.4 billion are still reliant on onsite sanitation amenities such as composting toilets, septic tanks and pit latrines, [7], with open defecation still practiced by approximately 494 million people globally [8]. Worldwide 2.1–2.6 billion people use onsite sanitation systems that produce large quantities of fecal sludge [9]. Untreated fecal sludge from these facilities is generally discarded straight into the local environment, reused on agricultural land, or disposed of within the household compound [10, 11].
In developing countries, the management, to date, of treatment and disposal of fecal sludge from onsite sanitation facilities has been poor. This had led to pollution of water courses, groundwater and soils [12], negative public and environmental health outcomes and has ultimately resulted in reduced social and economic development [13, 14].
Recently the method of pyrolysis as a solution in treating both sewage and fecal sludge has been a focus of research. Pyrolysis is a thermochemical method where biomass is heated to temperatures of 350–1000°C in an oxygen-free environment [15]. This process effectively and quickly kills pathogens within the sludge [16] and produces a carbon rich product, biochar, which can be used as a soil amendment [17]. The characteristics of biochar are dependent on the composition of the original feedstock, the highest treatment temperature during pyrolysis, the hold time, and the heating rate [11].
The use of biochar as a soil amendment emerged from studies on Amazonian Black Earth (Terra Preta). Terra Preta (Portuguese for ‘black earth’) refers to a specific type of dark, incredibly fertile soil found in the amazon basic that was discovered to contain much higher nutrient levels and organic carbon levels than the neighboring soils [18, 19]. The advantages of treating soil with biochar include an increase in carbon levels, [19], an increase in the cation exchange capacity (CEC) [19], an increase in the water holding capacity of the soil [20, 21], and a decrease of acidity especially in acidic soil [22], as well as the reduction and immobilization of toxic metals [23]. The treatment of soil with biochar also leads to carbon sequestration due to the recalcitrant nature of biochar and therefore is an important tool in achieving net zero targets [24].
The characteristics and effects of sewage sludge derived biochars, on soil fertility, and crop yield has been the major focus of attention [25, 26, 27, 28, 29, 30, 31, 32, 33], with little research in comparison conducted on fecal sludge derived biochars [34, 35, 36].
The end-use of biochar is dependent on biochar properties which in turn depend primarily on two factors: the original feedstock and the high treatment temperature (HTT) used during pyrolysis [37]. Differences in properties of the original feedstock arise from the large variation of processes used to treat the sludges, the sludge holding times and the type of storage facilities used.
There are similarities and differences between the types of human waste discussed in this chapter. The characteristics of each type of waste can vary significantly, depending on several factors outlined below. In general human-waste is a complex heterogeneous mixture which can contain microorganisms, water, oils, nutrients, inorganic material and can be rich in organic matter.
Sewage sludge characteristics can vary with time, type of wastewater treatment facility, the operational method and the sources of the sewage. Wastewater treatment plants receive discharges from industry as well as residential areas. The high concentrations of metals and organic compounds found in sewage sludge can vary greatly depending on nearby industrial activities [38, 39].
Fecal sludge quantities and characteristics can vary greatly depending on several important factors including location, climate, age of the sludge, type of sludge collection and the types of onsite sanitation facilities [40]. These onsite sanitation technologies include septic tanks, aqua privies, pit latrines (including ventilated improved pit latrines VIPs), public ablution blocks and dry toilets. Another difficulty in quantifying FS is that in cities different types of these facilities can be found side-by-side.
Untreated primary sludge can have pH values ranging from 5.0 to 8.0, with digested primary sludge in the range pH 6.5–7.5 [41]. The range of pH of fecal sludge has been reported between 6.55 and 9.34 [42]. A difference in pH of FS between peri-urban areas and rural areas has been reported with a mean pH of 6.7 in rural areas and 7.3 in peri-urban areas in the Ashanti Region of Ghana [43].
The total solids present in FS comprises of organic and inorganic matter. FS total solids concentration has been measured at 12,000–35,000 mg/l [44] and volatile solids measured at between 0.45 and 4.3 g VS/g ash [45]. Total dry solids of untreated primary sludge and digested primary sludge have been reported at 5–9% and 2–5% respectively [41]. Total solids of liquid, dewatered, dried, or compost biosolids have been reported at 2–12%, 12–30%, and 50% TS, respectively [46].
Nitrogen in FS is found in various forms; as nitrate, nitrite, organic forms (amino acids), and ammonium with ammonium in fecal sludge originating from the urine component [47]. Faecal sludge from septic tanks has been found to contain ammoniacal-nitrogen concentrations at 150–1200 mg/l [44], <1000mg/l and 2, −5000 mg/l reported in studies from Ghana, Thailand and Philippines [48]. Typical municipal sewage from tropical countries has been reported to contain ammoniacal-nitrogen levels of 30–70 mg/l [48]. Nitrate concentrations in fecal sludge have been measured at 0.2–21 mg N/L [49]. A considerable proportion of nitrogen (N) in sewage sludge is organically bound and not immediately available for plant uptake [50]. Dewatered anaerobically stabilized primary sewage sludge has a reported nitrate-N content of 0.253 mg/g ± 0.015 dried sludge [51].
Total phosphorus levels found in FS is high, it is usually present in phosphate form (e.g., H3PO4/PO4-P) or in the organic phosphate form found in plant tissue [52]. Phosphorus levels in sludge from VIPs were found to be 3.4 times higher than sludge from septic tanks. The content of phosphorus in SS has been reported at 20.1–28.4 g/kg−3 [53] with phosphorus in sludge mainly present in an inorganic form [54, 55].
Potentially toxic metals that are found in human waste include zinc, cadmium, chromium, nickel, copper, lead and mercury. Potentially toxic metals in sewage sludge originate largely from industrial wastewater entering the sewer system as well as runoff from business effluents and traffic emissions carried via stormwater into the sewer system [56, 57]. Fecal sludge contains lower levels of potentially toxic metals; a recent study reported that FS from pit latrines contained lower concentrations of these metals compared to wastewater sludge [58] and levels in fecal sludge ash have been reported to be below the thresholds for land disposal [59].
Sewage sludge needs to be dewatered and dried before pyrolysis can occur. Sewage sludge is transported through a waterborne sewer system, so it contains a higher liquid content than fecal sludge. Total solids (TS) in biosolids increase from 2 to 12%, in liquid biosolids to 12–30% in dewatered sludge and finally to 50% in dried biosolids [46]. A pelletizing process is sometimes used after the drying step to produce dried pellets of SS which is safer for handling. Dewatering fecal sludge is usually achieved using drying beds [60]. FS total solids concentration have been measured at a range of 12–35 g/l [44], and 20–50 g/l [61, 62].
The source, and drying methods of sewage and fecal sludge can vary considerably. For fecal sludge the sources can range from septic tanks [35], septage drying areas [34], vacuum trucks [63], to latrine waste from ventilated and improved pit latrine (VIP) toilets [64]. The treatment processes at each wastewater treatment plant from where sewage sludge is collected is not always described in the literature but can vary from conventional biological trickling filtration systems [26], to anaerobic digestion and belt-filter-press dewatering systems [1].
3.2 Slow pyrolysis
In this chapter we focus on the most common method of producing biochar: slow pyrolysis. Slow pyrolysis is defined by slow heating rates between 1 and 30°C min°1 [65] with high treatment temperatures of 400–900°C in the absence of oxygen. Slow pyrolysis is often deemed the most practical process for agronomic biochar production [66].
Biochar characteristics and yield are related to the composition of the raw sludge and the pyrolysis process such as the high temperature treatment (HTT) used, and heating rate [67]. HTT is the main factor influencing final biochar characteristics [65, 68].
The pyrolysis of fecal sludge can be conducted using lab-scale technology such as tube furnaces [35], and muffle furnaces [64] as well as large-scale pyrolysis reactors [11, 69]. There are markedly different pyrolysis conditions reported with HTT ranging from 300°C [16] to 750°C [69] as well as a range of holding times from 10 min [63] up to 120 min [64].
Sewage sludge pyrolysis conditions range from self-made stainless-steel reactors heated in a muffle furnace [70], fixed bed laboratory pyrolyzers [31], mechanically fluidized reactors [71], drum pyrolysis reactors fired using coal, biomass gasifiction units [72] to laboratory-scale muffle furnaces [1]. Similar to fecal sludge the pyrolysis conditions of sewage sludge also vary considerably with high treatment temperatures from 200°C [32] to as high as 700°C [32, 53, 73] with the range of residence times from 10 min [63] up to 120 min [64]. Residence times also vary considerably from as short as 15 min [72] to as long as 360 min [26].
4.1 Biochar pH
Both sewage sludge and fecal sludge-biochars are generally alkaline with greater HTTs resulting in biochars with higher (more alkaline) pH values [16, 73]. Examples of pH for SS biochars are presented in Table 1 and FS biochars in Table 2. The alkalinity of biochar results from the increase in alkali salts and salts of alkaline metals such as calcium, and magnesium during pyrolysis [75]. The effect of biochar in altering soil pH is one of several ways in which biochar increases crop yield. Approximately 30% of land cover globally is comprised of acidic soil which leads to diminished crop yield, and also impacts the kind of crops that can be grown. Maize, a cereal corn widely cultivated globally, is negatively affected by acid soil [76], in fact out of all the soils worldwide that are suited to arable agriculture, a large proportion, up to 50%, are acidic [77].
pH, ash, surface area and CEC of fecal sludge biochars—hold times are in brackets.
*Biochar was milled to pass through a 74 μm sieve and demineralized with HCl (2 mol/L).
Biochars typically exhibit neutral to alkaline pH values so can increase the pH of soil by reducing the acidity of acidic soil or increasing the alkalinity of neutral/alkaline soil. This is termed the “liming effect” and contributes to improved plant growth and crop yield especially in acidic soils. It is one of several pivotal mechanisms that contribute to the increased plant growth and yield upon biochar addition [78].
The liming effect improves various soil-plant interactions including:
Increased phosphorus bioavailability and calcium and magnesium bioavailability
The reduction in the available concentration of aluminum, a metal toxic to plant growth [79]
Generally, biochars pyrolyzed at temperatures ≥500° tend to be alkaline whereas sewage sludge derived biochars pyrolyzed at relatively lower temperatures of 300–400°C are more acidic [25, 32, 73]. There are differing results from the effect of FS and SS biochars on the pH of soils [32].
Sewage sludge biochars have increased soil pH, available nutrient concentration and shown a decrease in the bioavailable forms of As, Cr, Co, Ni and Pb [28]. Fecal sludge derived biochar has resulted in an increase of soil pH and cation exchange capacity (CEC) of soil [35]. However, another study showed that sewage sludge biochar treatment resulted in a decrease in soil pH despite the alkalinity of the biochar applied to the soil [32]. Soil pH also affects phosphorus adsorption and bioavailability with this process more evident in acidic soils due to the liming effect of biochar contributing to an increase in phosphorus bioavailability [81]. The availability of nutrients within the biochar itself is also positively affected by soil pH, with an increase in the release of HxPO4 and NH4+ from biochar associated with decreasing pH [82, 83].
4.2 Ash
The concentration of ash in biochar is generally higher than in raw sludge and increasing HTT during pyrolysis results in increased ash content of biochar [84]. The original feedstock has a significant effect on ash content, for example, biochar from hazelnut produced an ash content of only 1.2% compared to poultry litter biochar with an ash content of 51.2% with both pyrolyzed at 350°C [85].
The initial feedstock of human waste tends to be high in ash. Sewage sludges can contain high concentrations of calcium (5.1–7.4%), silica (19–58%), iron (5.2–6.8%) and phosphorus (3.4–4.9%) [53] and ash content of fecal sludge can be as high as 17.0% compared to sawdust measuring only 0.8% [16]. Ash content of SS biochars have been measured at 52.8% at 300°C HTT and 63.3% at 400°C HTT [73]. Biochars produced from mixed urine and feces samples, similar in ratio to sludge from on-site sanitation systems, had ash content of 50.1% at 450°C HTT and 56.3% at a HTT of 650°C [86]. Recently a study comparing mixed urine and feces (MUF) biochar and source-separated feces (SFF) biochar found that MUF biochar exhibited higher ash contents which they associated with greater quantity of inorganic salts in urine [86]. The higher ash content in fecal sludge biochars compared to sewage sludge biochars are due to digestion of the sludge during holding in onsite sanitation amenities [63], the ingress of sand and grit due to poorly lined containment structures [87] and the adhesion of sand to fecal sludge from the surface of drying beds [88]. Biochar may benefit from high ash contents if its end-use is as a soil amendment as the minerals found in ash such as calcium, magnesium, potassium, are essential plant nutrients.
The high ash content of SS and FS biochars is connected to the alkalinity of these biochars. Increasing pyrolysis temperatures leads to an increase in alkalinity due to an increase in ash in biochars derived from sludge feedstocks [16, 73].
Previously the ash content of a FS biochar was reported to play a role in the increase in plant height, below ground biomass and yield of tomatoes grown in acidic soil [36].
4.3 Surface area and porosity
The porous structure of biochar strongly resembles the cellular structure of the original feedstock [84]. In the case of fecal and sewage sludge biochars the cellular porous structures arise from undigested fibrous vegetable matter (Figure 1).
Figure 1.
SEM micrograph of fecal sludge biochar [11].
The addition of biochar to soil can greatly improve soils water retention. A study in 2002 showed that Terra preta exhibited 18% greater water retention compared to neighboring soils that contained very little charcoal [89]. The porous structure of biochar results in greater water holding capacity of soil [21] and increases water availability [90, 91, 92].
The BET surface area of biochars is increased with increasing HTT during pyrolysis, as at higher temperatures there is an increase in volatile matter released.
Municipal sludge pyrolyzed at temperatures between 500 and 900°C produced a greater biochar yield and greater microporous network within the biochar at increasing HTTs [93]. Biochar produced from sewage sludge-derived fertilizer was shown to mainly consist of mesopores with some microporous structure present [94].
Surface area measured by N2 is generally quite low for SS derived biochars, values have been reported ranging from 2.2 m2 g−1 [72] to 54.1 m2 g−1 (Table 1) [53]. Research has shown that sewage sludge biochars have low surface areas due to high ash content [1, 94, 95]. Surface areas are reduced due to the high ash content present which fills or blocks access to the micropores within the biochar [66]. Surface areas of fecal sludge biochars have been measured at 3.7 m2 g−1 and 25.7 m2 g−1 (Table 2) [34, 64]. It has been reported that a greater surface area of fecal sludge biochar (690.8 m2 g−1) can be attained by washing with 2 M HCl acid [35].
Despite comparatively low surface areas, sewage sludge biochar addition has been shown to significantly increase available water of soil [96] and increased sunflower production under the Mediterranean climate without additional water irrigation [97].
4.4 Cation exchange capacity (CEC)
CEC relates to the ability of soil or biochar to adsorb positive ions in exchangeable forms and plays a key role in nutrient leaching and retention in soils [98]. The CEC is largely determined by the oxygen containing functional groups present on the surface of biochar such as C∙O groups [99]. These functional groups allow the adsorption of nutrients in the form of cationic such as K+, Ca2+, and NH4+.
Biochar addition to soil has been shown to increase CEC and pH [19, 35] and limit nutrient leaching, [100] and improve nutrient retention [66].
CEC values of FS sludge biochar has been reported at 23.2 cmol(+) kg−1 for biochar pyrolyzed at 450°C [34]. FS biochar CEC values range from 23.2 cmol(+) kg−1 for biochar pyrolyzed at 450°C [34] to 129 cmol(+) kg−1 for biochar pyrolyzed at between 550 and 750°C [11].
A decrease in CEC values for biochars derived from fecal sludge and sewage sludge with increasing pyrolysis temperature from 350°C to 550°C has been reported [64]. However, both biochars exhibited an increase in CEC values at pyrolysis temperatures of 650°C. Another study conflicts these findings as they recorded increasing CEC values of fecal sludge chars with increasing HTT up to 600°C [63]. Cation exchange capacities of sewage and fecal sludge biochar are sparsely recorded in the literature therefore it is difficult to conclude what impact feedstock and HTT has on sludge-derived biochar CEC values.
The effect of sewage sludge biochars on soil CEC has been reported with application of sewage sludge biochar pyrolyzed at 300°C resulting in increased soil CEC one year after its application and increased maize grain yield [101]. Wastewater sludge biochar has also been found to increase soil CEC by up to 40% [102].
Examples of CEC values for SS and FS biochars are given in Tables 1 and 2.
4.5 Elemental Microanalysis (C, H, N and O)
FS and SS-derived biochars generally have low total C concentrations (11–40%) compared with biochars from lignocellulosic feedstocks [103]. It is the ash content of the original feedstock that influences the biochar ash content and therefore carbon content of the resulting biochar. Fecal and sewage sludges are naturally high in ash therefore produce biochars with high ash and low carbon content. Sewage sludge biochars have reported carbon contents of 21.6–26.2% with a low percentage of H also reported (3.8–5.1%) [53]. The carbon content of fecal sludge biochars have been measured at 21.1–23.8% [11].
Carbon is concentrated within the biochar during the thermochemical process with an increase in carbon content relative to the feedstock commonly reported, however with sewage sludge biochar many studies have reported a decrease in carbon content in biochar relative to the original feedstock [1, 28]. C and N content as well as ash content are reduced with an increase in HTT signifying that as more ash is relatively accumulated, carbon and nitrogen content is reduced. Soils amended with sewage sludge biochar have increased total nitrogen, and organic carbon [28].
Nitrogen in fecal sludge is found in mainly organic form [104] and is volatilized at temperatures of 200°C [80]. Therefore, the nitrogen content of FS and SS biochars can be very low. The total nitrogen content of biochars can vary markedly across a large range [105]. Nitrogen content of wastewater sludge biochar and sewage sludge biochar generally increases with decreasing pyrolysis temperature [73, 106]. Application of SS biochar has increased N uptake and enhanced N use efficiency two years after addition of the biochar, indicating its potential as an alternative to fertilizer [107].
4.6 Potentially toxic metals
The high variability of potentially toxic metal (PTM) levels both in sewage and fecal sludge affects the PTM content in the resultant biochar. The thermochemical process does, however, constrain these metals in immobile and stable forms. The entrapment of potentially toxic metals within the biochar reduces the risk of plant uptake of these metals allowing the potential use of SS biochar as a soil amendment without negatively impacting soil-plant systems.
The levels of potentially toxic metals generally increase as HTT increases [74, 108], however there are conflicting reports on the impact that increasing HTT has on these metals in sludge biochar. The general trend does seem to be an increase in metal concentration with an increase in pyrolysis temperature with some noticeable exceptions at higher temperatures. It has been reported for sludge biochar that toxic metal levels peaked at 450°C and decreased at higher temperatures of 500–550°C [109]. Others have reported a decrease in all PTM concentrations of sludge biochar pyrolyzed at 700°C except for cadmium [73]. Potentially toxic metal levels in FS-biochar conforms to the general trend with an increase in PTM concentrations reported with increasing HTT [63].
Biochar pyrolyzed at higher temperatures can have beneficial qualities for use as a soil amendment including higher pH values and greater surface areas. Consideration needs to be paid to ensure that the higher temperatures do not increase PTM concentration in biochars to greater than the recommended guidelines for PTMs in soils. Potentially toxic metals in most SS and FS derived biochars are below International Biochar Initiative (IBI) accepted upper thresholds [110].
The leaching of potentially toxic metals from SS and FS biochar has also been investigated and it was found that in contrast to the original sludge, biochar contains significantly less total concentrations of these metal as well as less soluble and extractable fractions [111]. Toxic metal mobility of biochar from fecal sludge co-treated with agricultural waste has been shown to be markedly reduced compared with metal mobility of original feedstock [69]. There are conflicting reports on the effect of HTT on extractable fractions of potentially toxic metals. DTPA-extractable concentrations of these metals have been reported to decrease with increasing HTT (from 300 to 700°C) in wastewater sludge biochar [73] and in a separate study extractable potentially toxic metal concentrations in SS biochar increased with increased HTT (from 300 to 500°C) [74].
4.7 Phosphorus
There is an abundance of mineral nutrients found in sewage and fecal sludges such as potassium, ammonium, nitrate, and trace elements. SS and FS are rich in phosphate which is of particular importance as phosphorus is a finite resource and an essential plant limiting nutrient [112]. Concentrations of phosphorus on a dry weight basis range from <0.1 to 14% in sewage sludge [113]. The resultant concentration of phosphorus in biochar is greater compared to the original feedstock as other elements such as carbon, hydrogen, and oxygen are volatilized at the high pyrolysis temperatures [25]. In general, an increase in HTT results in increased phosphorus concentration within the biochar. Confirming the general trend, phosphorus concentrations in sewage sludge biochar increased from 5.6% at a HTT of 250°C to 12.8% at a HTT of 800°C [114].
Phosphorus within sewage sludge is found mainly in the inorganic fraction and is significantly affected by volatilization losses at HTT greater than 700°C [115]. This trend has been observed in SS biochar [53, 74] and FS biochar [16, 63], however decreases in phosphorus concentrations in FS biochar have been reported at HTTs of 700°C [16]. The reduction in phosphorus content at higher pyrolysis temperatures may be due to different sludge types containing different forms of phosphorus. Chemical and biological treatment process of sewage and fecal sludges can also affect the forms of phosphorus present and the degree to which phosphorus is volatilized at high HTTs >700°C [116]. There are recorded increases in total phosphorus levels in FS biochar with increasing HTT from 3.2% at 350°C to 3.9% at 600°C [63] and 5.4% at 300°C to 8.1 wt.% at 600°C. The latter study did however, report a slight decrease in P concentration at 700°C [16].
The bioavailable phosphorus within biochars is generally less than the total P content with an increase in HTT resulting in increased available phosphorus compared to the raw sludge feedstock [31]. One of the most important effects of SS biochar application to soil is the increase in bioavailable P content [71].
Biochar-treated soils have higher organic bioavailable phosphorus concentrations relative to soil without biochar treatment but these mechanisms concerning the release of nutrients from biochar are not fully understood. The potential mechanisms include the supply of nutrients from the biochar itself, the liming effect of biochar (especially alkaline biochar) which increases the plant-available phosphorus levels [117, 118] and enhanced nutrient retention capacity [119].
Bioavailable P within sewage sludge biochars has been shown to decrease with increasing HTT [32, 73], however analysis of FS biochar has revealed the opposite trend with an increase in available phosphorus with increasing HTTs from 350°C [63].
4.8 Macronutrient concentrations (Ca, Mg and K)
Sewage and fecal sludge biochars contain large amounts of macro-nutrients such as calcium, potassium, and magnesium. Thermochemical treatment of sludge increases these elements concentrations in biochar compared to raw sludge. Increases in Ca, K, and Mg have also been reported with increasing HTT. These metallic elements cannot be volatilized at the pyrolysis temperatures, so these are concentrated within the biochar as C, H, and O are gradually lost at higher HTTs [120]. Large concentrations of Ca, Mg and K in SS and FS biochar has been reported (Figure 2) [31, 34, 69, 72, 73]. Both SS and FS biochars have been reported to contain increasing Ca, Mg, and K concentrations with increasing HTT [53, 74] [16]. The treatment process of sewage sludge can impact the concentration of certain elements; it was noted that a relatively high proportion of calcium present in sludge biochar was caused by to the addition of CaO during the sludge conditioning process [74]. Soil amended with SS biochar has recorded greater levels of potassium [121] and calcium and magnesium [25]. In the latter study an increase in Mg concentration was recorded in leaves of radish plants grown in the soil, however there was no effect of the biochar on calcium concentration in radish leaves despite the increased calcium levels recorded in the soil [25]. Other studies have found SS biochar application to soil, has not affected the levels of exchangeable calcium and magnesium ions in the soil over an average of 5 years [122]. The conflicted results of these studies highlight the many factors that influence the concentration levels of macronutrients in soils. These factors include the treatment of the raw feedstock, the HTT used during pyrolysis and the rate of biochar application (Tables 3–5).
Figure 2.
SEM-EDX map for all elements distribution across the area highlighted in image and associated energy dispersive X-ray (EDX) quantification of fecal sludge biochar with calcium the most abundant metal [11].
In this chapter, the focus is on assessing the similarities between fecal sludge and sewage sludge biochars, with the observed distinctions arising from variances in the transportation and treatment of the raw material. Sewage having traveled through a sewered sanitation system, receives discharges from industry as well as residential areas and thus, generally exhibits higher levels of potentially toxic metals compared to fecal sludge which is contained in onsite sanitation systems. The variances in potentially toxic metal concentrations between fecal sludge and sewage sludge may not be significant since these metals are confined within the biochar in inert and stable forms. However, higher temperatures during pyrolysis can lead to elevated concentrations of potentially toxic metals. To prevent exceeding recommended guidelines for these metals in soils, it is advisable to employ lower pyrolysis temperatures when producing sewage sludge biochar. The similarities between fecal sludge (FS) and sewage sludge (SS) biochars include high pH, ash content, and nutrient composition. These properties suggest their potential to enhance soil fertility and improve crop yields, particularly in acidic and nutrient-deficient soil conditions.
Assessing the properties of these biochars is challenging because of the different processes used in collection, storage, and transportation of the raw sludge. Properties of fecal sludge vary depending on location, season, climate, sanitation technology, and sludge age. These aspects combined contribute to the difficulty in being able to characterize, in general terms, fecal sludge and therefore the properties of fecal sludge biochar. It is suggested that characteristics of large-scale produced FS biochar should be examined on a case-by-case basis considering the factors described.
This chapter emphasizes the importance of the physical and chemical properties.
of sludge biochars and also the physical and chemical properties of the soil to which the biochar is added.
Future research should concentrate on short-term and long-term field studies of sludge biochar application to acidic, low nutrient soils. Long-term field trials are needed to determine the duration of the reported positive liming effects of sludge biochars and the long-term effects of repeated biochar applications on potentially toxic metal content in soils.
This work was supported, in whole or in part, by the Bill & Melinda Gates Foundation [OPP1149054], and under the grant conditions of the Foundation, a Creative Commons Attribution 4.0 Generic License has already been assigned to the Author Accepted Manuscript version that might arise from this submission. The work was also supported by Swansea University’s ‘SUNRISE’ project funded through GCRF via EPSRC [EP/P032591/1].
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Written By
Hannah Larissa Nicholas, Aisling Devine, Iain Robertson and Ian Mabbett
Submitted: 03 June 2023Reviewed: 13 June 2023Published: 13 November 2023
Open access peer-reviewed chapter
