Advantages of Co-Pyrolysis of Sewage Sludge with Agricultural and Forestry Waste

Abstract

This paper explores the advantages of the co-pyrolysis of municipal sewage sludge with agricultural and forestry biomass, emphasizing its potential for environmental and economic benefits. Co-pyrolysis with lignocellulosic biomass significantly enhances biochar quality, reduces the heavy metal content, increases porosity, and improves nutrient retention, which are essential for soil applications. The biochar produced through co-pyrolysis demonstrates enhanced stability and a lower oxygen-to-carbon (O/C) ratio, making it more suitable for long-term carbon (C) sequestration and pollutant adsorption. Additionally, co-pyrolysis generates bio-oil and syngas with improved calorific value, contributing to renewable energy recovery from sewage sludge. This synergistic process also addresses waste management challenges by reducing harmful emissions and immobilizing heavy metals, thus mitigating the environmental risks associated with sewage sludge disposal. This paper covers key sections on the properties of waste materials, improvements in biochar quality and energy products, and the environmental benefits of co-pyrolysis, such as emissions reduction and heavy metal immobilization. The paper highlights trends and challenges in co-pyrolysis technology, aiming to optimize parameters for maximizing biochar yield and energy recovery while aligning with sustainability and circular economy goals. The paper concludes with recommendations for optimizing co-pyrolysis processes and scaling applications to support sustainable waste management. Overall, co-pyrolysis represents a sustainable approach to valorizing sewage sludge, transforming it into valuable resources while supporting environmental conservation.

1. Introduction

With the global increase in urbanization, waste management has become a pressing need, and sewage sludge, a byproduct in the process of municipal wastewater treatment, also increases. In 2020, the total production of sewage sludge in European Union (EU) countries was above 3.6 Mt [1]. Sewage sludge recovery methods can typically be divided into an organic recycling pathway, which encompasses options like agricultural application and composting [2,3]. The recycling of sewage sludge can be limited due to its composition (i.e., variability in pollutant contents—heavy metals and organic pollutants—or nutrient availability) [4]. On the other hand, sewage sludge has potential as recoverable energy, as it can provide approximately 10% of the global energy supply [5,6]. Thus, the second pathway includes materials and energy recovery, which includes methods such as thermochemical conversion (e.g., pyrolysis, gasification) and anaerobic digestion, among others [2,3].

The pyrolysis of sewage sludge is currently under consideration and investigated as an alternative to other sewage sludge disposal methods [7]. Pyrolysis technology for the thermal utilization of sewage sludge has attracted extensive attention both on the laboratory scale [8], pilot pyrolysis scale [7,9], or full pyrolysis scale [10,11]. Pyrolysis of sewage sludge can result in the recovery of precious resources, such as nutrients and organic matter, and their conversion into valuable fractions, i.e., biochar, bio-oil, and pyrolysis gas. These products are usable in innovative biorefinery pathways toward a wide range of value-added materials [3]. However, due to the high ash content, low volatile matter content, high moisture content, and low calorific value, individual pyrolysis of sewage sludge may not be efficient. If referring to dry mass, preliminary drying may be necessary to improve process efficiency [8].

Co-pyrolysis is the thermal decomposition of two or more types of organic materials in an oxygen-limited environment to produce biochar, bio-oil, and syngas [12]. Co-pyrolysis is considered a simple and effective way to improve the quality of pyrolysis products [13]. In the context of sewage sludge co-pyrolysis, this process involves combining sewage sludge with other organic materials—commonly agricultural and forestry residues—for simultaneous pyrolysis [14]. The residues from agricultural and forestry practices are generated in large quantities. If they are not properly managed, they contribute to environmental pollution through open burning or decomposition. Co-pyrolysis often uses ratios with ranges based on the specific properties and goals of the process, but the common ratio between sewage sludge and forestry and agricultural waste is 1:1 [13]. Co-pyrolysis has been proposed as an alternative to pyrolysis primarily to overcome the limitations associated with pyrolyzing sewage sludge alone. The idea behind co-pyrolysis is that combining different feedstocks results in synergistic effects due to the different properties between sewage sludge and agricultural/forestry waste that improve the quality and quantity of the products compared to pyrolyzing sewage sludge alone [15].

The main reasons why co-pyrolysis is proposed instead of pyrolysis alone for sewage sludge are as follows

(a)

Heavy metal reduction—The addition of biomass in sewage sludge co-pyrolysis leads to a reduction in the heavy metal content due to two main mechanisms: the dilution effect and volatilization of metals. Biomass, which typically contains minimal heavy metals, dilutes the concentration when mixed with sludge during co-pyrolysis. Certain metals, like cadmium (Cd) and lead (Pb), volatilize under high-temperature conditions, with biomass facilitating their escape in gaseous form, further reducing their presence in the final biochar. Additionally, the interaction between the sewage sludge and co-substrates during co-pyrolysis stabilizes heavy metals, making them less bioavailable and reducing their environmental risks [16,17].

(b)

Improved biochar quality—When sewage sludge is co-pyrolyzed with lignocellulosic biomass, the resulting biochar has enhanced physical properties, such as increased porosity, a larger surface area, and improved structural integrity. The organic material from forestry or agricultural waste contributes to the development of a more stable C structure, which enhances the biochar’s capacity to retain water and nutrients, and adsorb pollutants [18].

(c)

Balancing the nutrient content—Sewage sludge is rich in nutrients like nitrogen (N), phosphorus (P), and potassium (K), but the high nutrient content can lead to imbalances or excessive nutrient release when biochar is applied to soil. The addition of biomass during co-pyrolysis can balance the nutrient profile, making it more suitable for long-term soil applications [19].

(d)

Enhanced energy recovery—The energy content of the syngas produced from the pyrolysis of sewage sludge alone may not be as high as desired due to the lower hydrogen content of sewage sludge-derived syngas. Co-pyrolysis produces syngas that is richer in hydrogen, which can be utilized as a renewable energy source [20].

(e)

Reduction of greenhouse gas emissions—Co-pyrolysis results in lower overall greenhouse gas emissions compared to pyrolysis alone. This is because the process captures more C in the stable biochar form, reducing the amount of carbon dioxide (CO₂) released into the atmosphere [21].

(f)

Waste diversion—Co-pyrolysis provides an opportunity to simultaneously manage multiple waste streams. This maximizes resource recovery and reduces the overall waste burden on the environment [22].

As research interest in sewage sludge co-pyrolysis is still growing, the aim of this paper is to summarize the potential benefits of the co-pyrolysis of sewage sludge with agricultural and forestry waste biomass. The scope of the paper includes a comparative analysis of the properties of sewage sludge, agricultural waste, and forestry waste, highlighting the synergistic effects of co-pyrolysis on enhancing biochar quality. This includes improvements in heavy metal immobilization and the nutrient profile of biochar. Additionally, the paper examines how the co-pyrolysis process enhances the energy potential of sewage sludge and reduces greenhouse gas emissions by capturing more C within the biochar matrix, thereby contributing to sustainable C sequestration. Overall, co-pyrolysis represents a sustainable approach to converting waste into valuable resources while contributing to environmental protection and C sequestration.

2. Characterization of Agricultural and Forestry Waste as the Primary Co-Substrates in Sewage Sludge Co-Pyrolysis

Global sewage sludge production is rising due to rapid population growth, urbanization, and industrialization. In 2017, global sewage sludge production was 45 Mt and continues to grow, with projections suggesting a 24% increase by 2030. Among EU countries, the highest average sewage sludge generation in 2012–2020 was in Germany (1.8 Mt), Spain (1.2 Mt), France (1.1 Mt), and Poland (0.6 Mt) [1]. Managing sewage sludge is challenging due to its substantial volume, high moisture content, complex composition (inorganic and organic compounds, nutrients, and microorganisms), and the presence of numerous contaminants [23].

Agricultural and forestry waste are commonly used as co-substrates in sewage sludge co-pyrolysis due to their abundance, renewable nature, and beneficial properties (Figure 1). In the EU, agricultural and forestry waste are key resources for enhancing the bioeconomy, contributing to energy production, material manufacturing, and sustainable agricultural practices. In 2017, the total agricultural biomass supply amounted to approximately 927 Mt of dry vegetal biomass equivalents [24].

Agricultural waste in the EU includes a diverse array of byproducts, mainly crop residues and grazed biomass that are either left in the field or collected for secondary uses. Agricultural waste, byproducts of crop cultivation and processing, is generated in large quantities worldwide, especially in regions with intensive agricultural production [25]. In 2017, approximately 108 Mt of dry crop residues were generated in the EU [24]. While a portion of these residues is left on fields to improve soil fertility and reduce erosion, a significant amount is collected and repurposed. Of the collected residues, about 33% is used for animal feed, while the remainder is often used for bioenergy and biomaterial production [24]. Currently, agricultural biomass consists primarily of byproducts from traditional food and forage crops like rapeseed, sugar crops, and starch crops. These residues, including cereal straw and other organic materials, have the potential to significantly contribute to the EU’s renewable energy goals as the focus shifts toward energy independence. Researchers estimate that by 2030, the potential of traditional agricultural byproducts could reach 7.3 exajoules (EJ) per annum [26].

Forestry waste, often referred to as woody biomass residues, is a crucial source for both energy and material uses. Forestry biomass waste in the EU primarily comes from two sources: primary residues, such as branches and treetops left over after logging, and secondary residues, like sawdust, bark, and wood chips from processing operations. In 2015, the total supply of woody biomass, including waste, amounted to approximately 370 Mt of dry matter [24]. While the current supply of forestry biomass is primarily dependent on planned forestry activities, estimates suggest that the contribution of wood industry residues to biomass energy could increase by around 30% in the coming years [26]. The real potential of forestry biomass depends on technical feasibility, economic conditions, and environmental considerations. Estimates of the forestry biomass potential in the EU vary, ranging from 1.4 EJ to as high as 18 EJ per annum [26].

The properties of sewage sludge, agricultural waste, and forestry waste differ significantly, influencing their potential uses, especially in energy production. Sewage sludge is characterized by a high moisture content, typically over 80%, which makes drying an energy-intensive step. Additionally, sewage sludge has a high ash content, a high N content, and a relatively high sulfur (S) presence. These components present environmental challenges during pyrolysis, as they lead to the emission of some gases, such as NH₃ and H₂S. The volatile matter is moderate, which implies limited gaseous fuel production potential compared to other biomasses. The lignocellulosic content of sewage sludge is low, consisting mainly of moderate amounts of lignin and low cellulose, giving it a lower energy yield. Sewage sludge also contains various contaminants like heavy metals (e.g., zinc (Zn), copper (Cu), Cd, and Pb) and other organic pollutants [27] (

Table 1. Characterization of sewage sludge according to different authors.

Properties	Composition	[28]	[29]	[30]	[31]
Proximate analysis (wt.% dry basis)	Ash	29.5	48.7	38.0	43.0
	Volatile matter	59.5	46.6	55.3	53.8
	Fixed carbon	11.0	4.7	6.7	3.2
Ultimate analysis (wt.% dry basis)	C	34.4	26.2	33.3	30.0
	N	5.7	4.6	5.1	5.1
	S	0.9	-	1.6	1.5
Biochemical analysis (wt.% dry basis)	Cellulose	-	-	3.3	4.5
	Hemicellulose	-	-	8.3	24.2
	Lignin	-	-	21.1	0.03
Major heavy metals (mg/kg)	Cu	700.0	334.0	-	410.0
	Zn	870.0	587.0	-	<26.0
	Pb	-	38.0	7.5	183.0
	Cd	-	1.7	25.2	<4.0
HHV (MJ/kg)	-	14.7	-	11.9	12.5

).

Agricultural waste varies in composition depending on the type of crop (

Table 2. Characterization of agricultural waste.

Properties	Composition	Wheat Straw	Cotton Stalks	Corn Stalks	Brewers’ Spent Grain	Digested Manure
		[28]	[29]	[32]	[33]	[31]
Proximate analysis (wt.% dry basis)	Ash	6.8	8.5	7.3	-	23.0
	Volatile matter	78.5	78.3	75.6	83.4	62.1
	Fixed carbon (%)	14.6	13.1	17.2	-	14.9
Ultimate analysis (wt.% dry basis)	C	42.8	47.4	39.6	42.5	36.4
	N	1.1	1.3	1.8	3.7	2.2
	S	-	-	0.2	-	0.6
Biochemical analysis (wt.% dry basis)	Cellulose	-	-	31.8	20.2	34.9
	Hemicellulose	-	-	28.2	27.2	3.2
	Lignin	-	-	5.5	13.5	18.7
Major heavy metals (mg/kg)	Cu	<5.0	3.7	-	-	<34.0
	Zn	<5.0	3.3	-	-	330.0
	Pb	-	8.7	-	-	<133.0
	Cd	-	0.3	-	-	<4.0
HHV (MJ/kg)	-	17.0	-	-	-	13.9

). The ash content is moderate, while the volatile matter content is generally high, providing significant potential for gaseous energy production. The N content in agricultural waste is moderate, which can lead to some nitrogenous emissions during pyrolysis, although lower than sewage sludge. The S content is generally low, reducing concerns about sulfur emissions. Agricultural waste is rich in lignocellulosic materials such as cellulose, hemicellulose, and lignin, making it a good candidate for energy recovery with a higher heating value (HHV) of around 17 MJ/kg, depending on the biomass type. Contaminant levels are moderate, typically including some heavy metals and pesticide residues.

Forestry waste is considered the most favorable for energy production due to its overall properties. The ash content is much lower compared to sewage sludge, generally below 2%. Forestry waste has a high volatile matter content (above 80%) and low N and S levels, which reduce the complexity of the emissions treatment process during pyrolysis. The lignocellulosic composition of forestry residues includes high levels of lignin, cellulose, and hemicellulose, providing a higher heating value (>20 MJ/kg) and making forestry waste an efficient source of biomass for energy recovery [27] (

Table 3. Characterization of forestry waste.

Properties	Composition	Rubberwood Sawdust	Salix Residues	Poplar Sawdust	Pine Sawdust
		[30]	[34]	[35]	[36]
Proximate analysis (wt.% dry basis)	Ash	2.1	0.8	1.0	1.5
	Volatile matter	80.4	85.5	84.6	79.7
	Fixed carbon	17.4	13.7	14.4	18.8
Ultimate analysis (wt.% dry basis)	C	47.6	47.4	48.1	48.2
	N	0.2	0.16	0.02	0.8
	S	0.04	0.1	1.3	0.1
Biochemical analysis (wt.% dry basis)	Cellulose	56.9	-	-	-
	Hemicellulose	15.2	-	-	-
	Lignin	17.4	-	-	-
Major heavy metals (mg/kg)	Cu	-	-	-	-
	Zn	-	-	-	-
	Pb	not detected	-	-	-
	Cd	not detected	-	-	-
HHV (MJ/kg)	-	20.1	18.5	18.5	17.8

).

Based on the data in

Table 4. General differences between sewage sludge, agricultural waste, and forestry waste properties.

Property	Sewage Sludge	Agricultural Waste	Forestry Waste
Moisture	high	variable	moderate
Ash content	high	moderate	low
Volatile matter	moderate	high	high
N content	high	moderate	low
S content	moderate	low to moderate	low
Lignocellulosic composition	low cellulose, moderate lignin	moderate cellulose, hemicellulose, and lignin	high lignin, cellulose, and hemicellulose
Contaminants	high (heavy metals and organics)	moderate (some heavy metals and pesticides)	low
Energy yield	moderate	moderate	high

, the properties of agricultural and forestry waste complement the properties of sewage sludge. Co-pyrolysis of sewage sludge with biomass provides multiple documented advantages that enhance both the process efficiency and sustainability. Specifically, this combination increases the calorific value of bio-oil and syngas while improving biochar quality through greater C stability and effective heavy metal immobilization. These synergistic benefits support the transition to a circular economy by transforming waste into valuable resources, thereby simultaneously addressing waste management challenges and renewable energy generation.

3. The Effects of Sewage Sludge Co-Pyrolysis on Nutrient Recovery and Bioavailability

As global P resources face depletion, finding sustainable alternatives for P recovery is essential for ensuring long-term agricultural productivity. Sewage sludge, an abundant secondary P resource, offers an opportunity to recycle P, though direct pyrolysis often results in low bioavailability due to the formation of poorly soluble metal phosphates. To overcome this, biomass ash rich in K, calcium (Ca), and magnesium (Mg) is used as an additive during the pyrolysis process to transform P into plant-available forms.

The co-pyrolysis of sewage sludge with sunflower straw ash markedly enhanced the bioavailability of P, as evidenced by Diffusive Gradients in Thin Films (DGT) technology, which exhibited 94% accuracy in predicting plant P uptake. Moreover, pot experiments revealed elevated P uptake by ryegrass, with enhancements ranging from 11.9% to 114.9% in shoots and 28.9% to 69.7% in roots relative to control soils. The presence of the K, Ca, and Mg elements in the straw ash facilitated the formation of bioavailable P compounds, including monetite (CaHPO₄), hydroxyapatite (Ca₅(PO₄)₃OH), and pyrocoproite (K₂MgP₂O₇). These compounds are more readily absorbed by plants, thereby enhancing the efficiency of P recovery in agricultural soils [37]. The use of agricultural biomass ash as an additive in the pyrolysis process represents a cost-effective and environmentally sustainable alternative to chemical reagents, further promoting the reutilization of sewage sludge as a valuable resource for P recovery.

The co-pyrolysis of C-rich and nutrient-rich biomass feedstocks represents an effective method for enhancing the retention of P and K in biochar. Co-pyrolyzing sewage sludge with nutrient-rich materials, such as chicken bones, and C-rich agricultural residues, including banana stem and sunflower stalk, has been demonstrated to enhance the recovery of P and K. In particular, the recovery of P increased from 1.8% to 25.5%, while the recovery of K improved from 5.3% to 19.5%. This process not only optimized the yield of biochar but also enriched its nutrient profile, making it a valuable source of essential nutrients for agricultural soils [38].

The co-pyrolysis of sewage sludge with wetland biomass (Phragmites australis) resulted in the transformation of a significant portion of the P in the sewage sludge, which was initially present in the form of non-apatite inorganic P, into apatite P. The recovery rate of P in biochar remained high, generally between 81.8% and 99.2%, depending on the mixing ratio and pyrolysis temperature. A higher biomass mixing ratio and a lower pyrolysis temperature were found to promote synergistic effects. The mixture with 25% biomass (25 wt.% Phragmites australis) appeared to be the optimal ratio for producing biochar with a higher bioavailable P content. This ratio achieved a balance between enhancing P stability and maintaining a reasonably high P concentration [39].

Co-pyrolysis of sewage sludge with rice husk significantly increased the bioavailability of P in the resulting biochar. The transformation of organic P (OP) to inorganic P (IP) as orthophosphate was enhanced due to the addition of rice husk. For example, when co-pyrolyzed with 50 wt.% rice husk, the inorganic P content increased from 71.5 wt.% in sludge-derived biochar to 92 wt.% in co-pyrolyzed biochar, indicating that rice husk addition promoted the conversion of OP to IP, making P more available for plant uptake. Rice husk is rich in minerals, such as Ca, Mg, and silica (Si), which participate in the pyrolysis reactions. These minerals can react with the P released during the thermal degradation of sewage sludge to form stable compounds like Ca phosphate [40]. The increase in the orthophosphate content also corresponded to the conversion of non-apatite inorganic P (NAIP) into apatite P (AP). Apatite P is a stable mineral form of P that provides a slow-release nutrient source for plants.

The incorporation of biomass and mineral materials during co-pyrolysis has a synergistic impact on enhancing the nutrient profile and overall nutrient stability of sewage sludge biochar, as well as improving its adsorption potential for nutrient removal from aqueous solutions. The incorporation of biomass, including maize, tobacco, and chili stalks, resulted in an enhancement of the nutrient profile of the sewage sludge biochar. In particular, co-pyrolysis resulted in a discernible increase in the concentrations of essential elements, including C, K, and N, which are vital for improving soil fertility. The C content increased by approximately 20.3% to 28.3%, the K content by 24.4% to 32.6%, and the N content by 2.5% to 21.0%, contingent on the specific type of biomass employed [41]. The incorporation of mineral materials, including manganese ore powder, graphite powder, and clay, resulted in an increase in the metal element content, specifically manganese (Mn), iron (Fe), and aluminum (Al), within the biochar. To illustrate, the Fe content increased by between 6.2% and 17.2%, while the Al content increased by between 2.0% and 13.7%. The presence of these metal elements enhances the adsorption properties of biochar by providing additional surface adsorption sites, thereby improving the biochar’s ability to bind anionic nutrients such as nitrate and phosphate, and reducing nutrient leaching. The results of the adsorption capacity for nutrients in aqueous solutions demonstrated a nitrate removal rate as high as 94%, specifically with biochar produced from the co-pyrolysis of sewage sludge and 6.67% clay at 450 °C (a biochar to solution ratio of 1:40 and adsorption time of 24 h). This high nitrate removal was attributed to electrostatic interactions between positively charged metal ions and negatively charged nitrate ions. The adsorption capacity for ammonium and phosphate was also enhanced, albeit to a lesser extent than for nitrate [41].

The availability of nutrients in co-pyrolyzed biochar is influenced by the pyrolysis temperature, type of organic additive, and addition rates, as shown by Yin et al. [19]. The data indicated that lower temperatures resulted in higher nutrient availability. At 350 °C, greater retention of plant-available P (Olsen P) was observed due to the reduced likelihood of P crystallization into less soluble forms. Conversely, at higher temperatures (700 °C), P was more likely to undergo conversion into stable mineral forms. With regard to N, the retention of ammonium was found to decrease with increasing temperature due to the volatilization of nitrogenous compounds. However, higher temperatures also led to improved adsorption when combined with additives such as brewers’ spent grain. The type of organic additive employed had a significant impact on the availability of nutrients. The use of brewers’ spent grain was found to be particularly effective for P at 350 °C, resulting in an increase in the Olsen P content by 52.0% to 81.7%. Reed straw also enhanced the availability of P, with increases ranging from 19.2% to 38.1% at 350 °C. It is noteworthy that this increase was sustained even at 700 °C. This was attributed to the mineral content of reed straw, which facilitates the retention of P in a more accessible form for plants. In contrast, sawdust proved less effective. At both low and high temperatures, higher addition rates of sawdust led to a reduction in the Olsen P content, which was attributed to the dilution effect of its lower inherent P content.

Furthermore, addition rates were found to be a significant factor determining the outcomes with regard to nutrients. A 50% addition rate of reed straw was found to be particularly effective at high temperatures (700 °C) for maintaining a high Olsen P content. In contrast, higher addition rates of sawdust led to no significant improvement in the available P content, and in some cases, even resulted in a reduction due to its lower P content. With regard to ammonium, higher addition rates of brewers’ spent grain and sawdust at 700 °C demonstrated a notable increase in ammonium retention, reaching up to 552%. This suggests an enhancement of the N adsorption capacity at elevated temperatures when biochar with a developed porous structure was produced [19].

Recent advancements in sustainable waste management have highlighted a promising solution for the effective treatment of sewage sludge. This combined approach of biomass co-pyrolysis with KH₂PO₄-modified biochar addition offers an effective solution for improving N retention during pyrolysis of sewage sludge. By incorporating biomass, such as sugarcane bagasse, along with KH₂PO₄-modified biochar, the method not only inhibits the release of nitrogenous gases but also enhances the N fixation rate significantly [42]. The co-pyrolysis of sewage sludge with 30% sugarcane bagasse yielded the optimal N fixation rate of 39%. This indicates that the N initially present in the sewage sludge was effectively retained in the biochar, thereby reducing its susceptibility to emission as harmful nitrogenous gases. Furthermore, the incorporation of 5% KH₂PO₄-modified biochar facilitated a supplementary adsorption phase, resulting in an augmented total N fixation rate of 47.6%. This was achieved by capturing nitrogenous volatiles and transforming them into stable N oxides and pyrrole/pyridine N. The KH₂PO₄ modification augmented the adsorption capacity of biochar by introducing functional groups that interact with nitrogenous volatiles. The modification also resulted in increased acidity and availability of adsorption sites, which contributed to an effective N fixation mechanism. The process facilitated the adsorption of NH₃, HCN, and other intermediates formed during pyrolysis, thereby reducing NOx emissions and increasing the N content of the biochar.

Co-pyrolysis of sewage sludge with various biomass additives increases the availability of essential nutrients in biochar. By optimizing pyrolysis conditions, biomass type, and addition rates, this approach not only aids in nutrient recovery (specifically N, P, and K) but also creates biochar that serves as an effective, sustainable soil amendment capable of improving soil fertility and agricultural productivity. This strategy contributes to sustainable nutrient cycling and waste management, offering a valuable alternative for addressing both environmental and agricultural challenges.

The results show significant potential for the recovery of nutrients from co-pyrolyzed biochar, especially P and K. However, achieving the optimal bioavailability of nutrients for agricultural use remains a challenge that requires further research to adjust the process parameters and co-substrate selection.

4. The Effect of Sewage Sludge Co-Pyrolysis on Selected Biochar Properties

The co-pyrolysis of sewage sludge with agricultural and forestry biomass generally enhances biochar properties, including the organic content, specific surface area, and functional group diversity, though the effects vary with the biomass type, ash content, and particle size. These improvements, especially in adsorption capability, make co-pyrolyzed biochar a valuable material in environmental applications, though careful selection of biomass and process conditions is required to optimize biochar performance [43]. The general effects of co-substrate addition on selected biochar properties are summarized in

Table 5. The effects of co-substrates on sewage sludge biochar properties.

Biochar Property	Effect of Co-Substrate Addition	Example of a Co-Substrate Type	Explanation
Yield and organic C content	Co-substrates with a low ash content decrease the yield, but increase the organic C content in biochar; co-substrates with a higher ash content increase the yield	rice straw, sawdust, peanut shells	A lower ash content in the co-substrate leads to volatile loss as gases, while a high ash content in the co-substrate retains more material, influencing yield.
Specific surface area and total pore volume	Improved surface area and porosity, but it depends on the co-substrate composition, its particle size, and amount	rice straw, willow, bamboo, cotton stalks	Biomass rich in volatile organic matter, like cellulose and lignin, decomposes during pyrolysis, increasing pore formation and the surface area, especially at higher temperature.An optimal particle size enables better mixing with the sludge and an expanded pore structure. Large particles float and reduce mixing, while small particles can block pores, both leading to a reduced surface area.Too much biomass produces excess volatiles, which can clog pores and reduce the surface area.
Functional groups	Increased diversity of functional groups depending on the co-substrate type and its amount	bamboo, rice husk	Breaking and rearrangement of the chemical bonds occur during co-pyrolysis. With increased co-substrate proportions, the number of these functional groups generally increases. Some groups (e.g., O-H and C-O) can decrease due to dehydration and decomposition reactions. Biomass can enrich biochar in specific compounds, e.g., Si compounds

, while a comparison of biochar properties produced from the co-pyrolysis of sewage sludge with agricultural and forestry waste is presented in

Table 6. Example of the effect of the co-pyrolysis of sewage sludge with corn stalks and rubberwood sawdust on selected biochar properties.

Property	Agricultural—Corn Stalks at 650 °C		Forestry—Rubberwood Sawdust at 550 °C
	[44]		[30]
	SS	SS50:CS50	SS	SS50:RWSD50
Yield (%)	47.0	42.0	49.7	33.0
C content (%)	62.3	67.9	24.3	42.9
Surface area (m2/g)	78.8 *	109.9 *	17.6	10.3
Pore volume (cm3/g)	0.18	0.13	lower	higher
Bulk density (kg/m3)	-	-	568.33	315.9
Surface morphology	tiny pores merge into larger ones, reducing the surface area	promotes pore formation by reducing oily blockages and producing smaller C particles	lack of well-defined pores; compact and dense surface; covered with small particles, indicating insufficient pore development	much rougher surface covered in non-uniformly shaped pores, development of a porous structure
Ash content (%)	-	-	67.6	46.8
Pore size distribution	mainly particles of 5–15 nm	increase in the particle size of 2–3.5 nm	0.1–1.15 mm; 44% are 0.5–0.6 mm	0.1–1.15 mm; 50% are 0.5–0.6 mm
Average pore diameter (nm)	9.1	4.8	143.4	136.8

SS—sewage sludge, CS—corn stalks, RWS—rubberwood sawdust, * in m³/g.

.

Zou et al. [44] found that the characteristics of biochar produced from the co-pyrolysis of sewage sludge with corn stalks varied depending on the pyrolysis conditions. The addition of corn stalks increased the C content in the biochar and improved its surface properties (Table 6). Consequently, the co-pyrolyzed biochar offers significant agricultural benefits by enhancing corn growth and facilitating C sequestration in degraded soils typical of coal mining areas. Similarly, co-pyrolysis of sewage sludge with rubberwood sawdust improved the biochar by increasing its C content and reducing the ash content and changes in surface morphology by improving the porous structure [30].

Co-pyrolysis improves the physical and chemical properties of biochar, making it an attractive amendment for soil health and C sequestration applications. Nevertheless, scalability remains a limitation, as variations in co-substrate types and pyrolysis conditions could affect the uniformity of biochar on an industrial scale.

5. The Effect of Sewage Sludge Co-Pyrolysis on Heavy Metal Immobilization in Biochar

Co-pyrolysis of sewage sludge with biomass effectively reduces the mobility and bioavailability of heavy metals in the produced biochar. During co-pyrolysis, heavy metals present in the sludge interact with the biochar matrix and undergo transformations that immobilize them. Pyrolysis conditions such as the temperature, mixing ratio, and the type of biomass used significantly influence the extent of heavy metal immobilization [45,46].

Increasing the pyrolysis temperature generally increases the immobilization of heavy metals in biochar. At temperatures above 350 °C, metals become more stable, resulting in reduced environmental toxicity and extractability. Higher temperatures (up to 600 °C and beyond) promote the conversion of metals from mobile and exchangeable forms to more stable forms, such as oxidizable and residual fractions, which are less bioavailable. For instance, pyrolysis at 600 °C has been found to reduce the bioavailability of metals significantly and enable sewage sludge nutrient recycling [47]. However, at very high temperatures, heavy metals can become more concentrated due to increased mass loss during the co-pyrolysis process, although their leachability is reduced [48].

Several studies highlight the significant role of biomass mixing ratios in co-pyrolysis for reducing heavy metal concentrations and toxicity in biochar compared to biochar from singular sewage sludge pyrolysis. The incorporation of adequate biomass reduces the heavy metal content through the dilution effect [49]. Increasing the biomass content also promotes metal transformation from mobile forms to more stable fractions, decreasing environmental risks. The use of different biomass types increases the immobilization of heavy metals, transforming them into less bioavailable and stable forms, like oxidizable and residual fractions. This synergy between sewage sludge and biomass during co-pyrolysis effectively mitigates the potential environmental toxicity of metals by promoting the formation of stable compounds [49].

The choice of the pyrolysis atmosphere (N₂ or CO₂) can influence the physicochemical properties of biochar and its capacity to immobilize heavy metals. Studies show that using CO₂ instead of N₂ as the carrier gas can enhance the surface properties of biochar, such as increasing its oxygen content and specific surface area. This improves the metal adsorption capacity of biochar [49,50]. Additionally, CO₂ has been found to reduce the leachable metal content and lower the overall toxicity of biochar produced from sewage sludge co-pyrolysis with biomass [51].

Recent studies have largely focused on analyzing metal stability in biochar produced from co-pyrolysis with biomass. The type of biomass significantly influences heavy metal immobilization in biochar during co-pyrolysis. Forestry and agricultural biomass have distinct characteristics that affect the ability of biochar to stabilize heavy metals, including differences in the ash content and lignin, cellulose, and mineral compositions (

Table 7. The main biomass properties and their effects on metal immobilization within the co-pyrolyzed biochar.

Biomass Type	Key Properties	Main Effect on Metal Immobilization
Agricultural	High ash content (Ca, Mg, and P)	It promotes the formation of stable metal compounds (carbonates, phosphates, and oxides).
	High Si content	The content of Si helps trap metals like arsenic (As) and Cd.
	Alkaline pH	High alkalinity enhances metal immobilization through chemical reactions (e.g., hydroxide formation).
Forestry	High lignin and cellulose contents	Produces a stable, porous biochar that physically traps heavy metals.
	Low ash content	It is less reliant on chemical precipitation but good for physical adsorption.

).

Forestry biomass contributes to biochar with a more stable porous structure, which enhances the physical entrapment of metals, while agricultural biomass, due to its higher ash and mineral contents, promotes the chemical immobilization of metals through the formation of stable metal compounds like carbonates, phosphates, and silicates. Agricultural waste like rice husk and forestry residues like willow offer significant benefits in reducing the ecological risks associated with heavy metals in biochar [45,46,49].

The addition of both forestry and agricultural biomass in the co-pyrolysis process with sewage sludge improves heavy metal stabilization by enhancing biochar’s structural and chemical properties. The porous structure, increased surface area, and the formation of stable metal complexes all contribute to the immobilization of metals.

The co-pyrolysis of sewage sludge and poplar sawdust under low-oxygen conditions significantly enhances the immobilization of heavy metals in biochar. The presence of 10% oxygen during pyrolysis, particularly at higher temperatures (e.g., 650 °C), promotes the transformation of heavy metals like Zn, chromium (Cr) and Cu from exchangeable and reducible states to more stable oxidizable and residual states [52].

The co-pyrolysis of sewage sludge with biomass, such as reed (Phragmites australis), significantly influences the transformation of heavy metals, enhancing their immobilization in biochar. The proportion of Zn in the stable residual fraction increased significantly with the addition of biomass and rising temperatures. At 700 °C, the stable residual fraction of Zn increased to 81.3% with 75% biomass addition, compared to 33.6% in biochar derived solely from sewage sludge [39].

The immobilization of metals in co-pyrolyzed biochar is a combination of physical encapsulation, chemical reactions that form stable mineral phases, and surface interactions involving aromatic compounds that bind heavy metal cations. Yang et al. [53] distinguished the main pathways for the immobilization of Pb and Cd during the co-pyrolysis of sawdust and sewage sludge. Firstly, stable mineral phases, such as PbAl₂O₄ or CdAl₂O₄, form due to interactions between metals and the Si-Al compounds present in the biomass, which are rich in SiO₄ and AlO₄ tetrahedral structures. These newly formed minerals had low solubility, contributing to metal immobilization. Secondly, metal hydroxides and carbonates form due to the alkaline pH of biochar and the presence oxides of Si, Al, Ca, and P in the ash. Thirdly, metals are immobilized via aromatic structures that can supply π-electrons and have a strong ability to bond heavy metal cations.

Similarly, co-pyrolysis of sewage sludge with biomass (either rice husk or bamboo sawdust) at higher temperatures (400 °C and 700 °C) resulted in increased aromaticity of the biochar. The aromatic clusters formed during co-pyrolysis were larger and more condensed compared to those formed by sewage sludge alone. Higher aromaticity decreases the hydrophilicity of biochar, leading to less mobility of metals. Additionally, larger aromatic clusters in biochar supply more π-electrons to bind metal cations [54]. These mechanisms effectively stabilize metals like Pb, Cu, Zn, Cr, Mn, and nickel (Ni) in the biochar, reducing their bioavailability and environmental risks.

Increasing the residence time during co-pyrolysis has a significant effect on metal immobilization in biochar by promoting the transformation of metals into stable forms and reducing their environmental mobility and risk. Wang et al. [29] found a clear relationship between the residence time and mobility of different metals (Pb, Cu, Zn, Cr, Ni, and Cd) in biochar from the co-pyrolysis of sewage sludge and cotton stalks at 600 °C. At a short residence time (30 min), a high proportion of heavy metals in bioavailable fractions and relatively low immobilization in the residual fraction were observed. Significant transformation of metals from mobile fractions to more stable forms with an increased specific surface area and improved binding capacity was observed at a moderate retention time (60–90 min). An extended residence time (120–150 min) enhanced the stabilization of heavy metals, with the vast majority found in the residual fraction, and reduced the environmental risk.

To address the concerns of elevated heavy metal concentration in sewage sludge biochar, researchers have explored various methods to modify biochar and reduce its ecological risks. One promising approach is the combination of chemical impregnation using agents like zinc chloride (ZnCl₂) and co-pyrolysis with biomass [55]. The method enhances the physical and chemical properties of biochar and immobilizes heavy metals, making the biochar safer for environmental applications. The impregnation reduced the total content of heavy metals via chlorination. Nonetheless, the oxidizable fraction of heavy metals showed an increase, implying more complex interactions with biochar components.

Hakeem et al. [28] utilized a mild acid pre-treatment (3% H₂SO₄) to effectively reduce the ash content and remove inorganic elements, including heavy metals and alkaline earth metals. This pre-treatment step was particularly effective in sewage sludge, leading to a notable decrease in the concentrations of metals like Zn, Cu, Ca, and Fe by as much as 75%. Combining the acid pre-treatment of sewage sludge with co-pyrolysis of wheat straw resulted in the highest degree of metal immobilization compared to other combinations (e.g., non-pretreated sewage sludge with wheat straw). Together, these methods produce a biochar that has lower concentrations of heavy metals and is safer for environmental applications, particularly in agriculture.

In summary, the main factors affecting metal immobilization in biochar produced from co-pyrolysis are listed in

Table 8. Factors affecting heavy metal immobilization in biochar produced from co-pyrolysis.

Factor	Description	Effect on Heavy Metal Immobilization
Pyrolysis temperature	Temperatures above 600 °C promote the formation of stable metal oxides, carbonates, and other mineral forms.	Reduces heavy metal leachability and bioavailability
Biomass mixing ratio	Higher ratios of biomass like rice straw or sawdust help dilute the concentrations of heavy metals in the biochar, reducing their mobility and increasing the immobilization of heavy metals.	Lowers the heavy metal content and increases metal immobilization efficiency
Biomass type	Different biomass types (e.g., cotton stalk, rice husk, and hazelnut shell) enhance the biochar pore structure and chemical interactions with heavy metals.	Improves the physical entrapment and chemical stabilization of metals
Pyrolysis atmosphere	The use of inert gases like CO2 instead of N2 can affect the physicochemical properties of the biochar and reduce leachable heavy metals.	Reduces toxicity by increasing the formation of stable, non-leachable metal fractions.
Catalyst use	Catalysts like CaSO4 and ZnCl2 can promote the formation of stable metal compounds (e.g., metal phosphates and oxides).	Further immobilizes metals through chemical reactions with pyrolysis intermediates

.

The ability of co-pyrolysis to stabilize heavy metals in biochar suggests valuable environmental benefits. However, understanding the long-term stability of these immobilized metals under different environmental conditions is critical to determine the safety and efficacy of biochar as a soil amendment.

6. The Effect of Sewage Sludge Co-Pyrolysis on C Sequestration

A major product of co-pyrolysis is biochar, a C-rich solid. This biochar can be used for C sequestration, effectively capturing C that would otherwise be released into the atmosphere as CO₂. Moreover, biochar can also be utilized as a soil amendment, further enhancing its environmental benefits by improving soil quality and helping to capture and store C.

The co-pyrolysis of sewage sludge with mineral- and ash-rich biomass such as banana peduncles and anaerobic digestate improves the quality of biochar, making it more suitable for C sequestration. The biochar derived from co-pyrolysis can sequester up to 0.22 kg of CO₂ per kg of biomass. The biochar also contributes to C sequestration in nutrient-deficient soils by enhancing the stability and carbonization of biomass, reducing the emission of greenhouse gases from soil [56].

Ali et al. [57] assessed the C sequestration potential (CS) and recalcitrance index (R50) of various biochar types, including rubberwood sawdust biochar, sewage sludge biochar, and their blends. The R50 index is used to measure biochar’s resistance to biotic and abiotic degradation, which is crucial for ensuring long-term C retention in soils. The calculated R50 values for the biochar samples ranged from 0.28 (sewage sludge biochar) to 0.48 (25% sewage sludge and 75% rubberwood sawdust biochar). Thus, co-pyrolysis can offer a greater resistance to thermal degradation and higher stability of the biochar in the soil environment. The CS, which incorporates the C content, and the R50 index were also greater for biochar derived from co-pyrolysis (16.9–19.9%) than sewage sludge biochar (11.0%).

High pyrolysis temperatures, appropriate biomass selection, and an increased fixed carbon content all contribute to producing biochars that are highly effective at capturing and sequestering C in a stable form, thus offering a promising strategy for mitigating climate change through long-term C storage. Higher temperatures facilitate the formation of aromatic C structures, which are more resistant to decomposition. Conversely, low pyrolysis temperatures result in biochar with a high C yield but limited stability due to smaller pores and lower resistance to microbial mineralization [57].

Co-pyrolysis of sewage sludge and rice straw at optimal mass ratios and temperatures significantly enhances the fixation of both C and N in biochar [58]. The synergistic effects are driven by specific reactions, such as deoxygenation and dehydrogenation, which enhance the C structure of the biochar, making it more suitable for long-term C storage. These reactions vary by the feedstock ratio. Deoxygenation is enhanced at 1:3 sewage sludge/rice straw ratio, dehydrogenation and deoxygenation at a 1:1 ratio, and dehydrogenation at a 3:1 ratio.

The co-pyrolysis of sewage sludge with rice husk or bamboo sawdust resulted in biochar with larger aromatic clusters compared to sewage sludge alone. This higher degree of aromaticity indicates an enhanced C structure that is less prone to degradation. Specifically, the sewage sludge–bamboo sawdust biochar produced at 700 °C had an O/C ratio of 0.02, which was significantly lower than that of sewage sludge–rice husk biochar (0.27), suggesting a greater degree of C recalcitrance [54].

Another key indicator that helps determine the recalcitrance of biochar is fixed carbon and elemental ratios such as H/C and O/C. Fixed carbon in biochar refers to the C content that remains after volatile compounds are released during pyrolysis. The fixed carbon content of biochar increases significantly with rising pyrolysis temperatures, though the extent of this increase varies depending on the co-substrate type and its ratio in the mixture with sewage sludge [59] (Figure 2).

Producing biochar from lignocellulose-rich materials, like wood chips and crop residues (straw, seed husks, and cobs), increases the stability of the biochar product. Conversely, biomass with higher levels of simple sugars and proteins is transformed thermally into C forms with predominantly aliphatic structures, making them more vulnerable to abiotic and biotic factors that promote biochar decomposition [63]. A lower O/C ratio typically suggests a higher degree of carbonization and stability in biochar, making it more resistant to degradation and therefore more effective for long-term C sequestration in soil [63]. Conversely, a higher O/C ratio implies a greater presence of oxygenated functional groups, which, while beneficial for nutrient retention, may reduce the biochar’s stability and permanence as a C sink. Co-pyrolysis of sewage sludge with agricultural and forestry waste can effectively influence the molar O/C ratio of the resulting biochar, impacting its stability and potential for C sequestration (Figure 2).

The results support the role of biochar in C sequestration and thus contribute to climate mitigation efforts. Future research should investigate the durability of C in different soil types and climates.

7. The Effect of Sewage Sludge Co-Pyrolysis on the Reduction of Harmful Gas Emissions

Co-pyrolysis often involves combining sewage sludge with materials such as agricultural biomass or non-lignocellulosic waste. The inherent properties of different feedstocks can complement each other. For example, sewage sludge is typically rich in N and other nutrients, while biomass and plastic waste can offer higher C contents. During co-pyrolysis, these materials interact to reduce the release of harmful gases.

One significant advantage of co-pyrolysis is the reduction of nitrogen oxide (NO_x) emissions. Chen et al. [64] observed a significant reduction in NO emissions during the co-pyrolysis of sewage sludge and corn straw. Specifically, the combination of 75 wt.% sewage sludge with corn straw resulted in a 25.3% reduction in NO emissions compared to the expected emissions from independent pyrolysis of the individual feedstocks. This synergistic reduction was attributed to interactions between S compounds from sewage sludge and oxygen-containing radicals from corn straw, which suppress the formation of NO. The interaction between the components from sewage sludge and corn straw became more effective at higher sludge ratios, likely due to enhanced S-related interactions that mitigate NO formation. The heating rate also had a notable effect on NO emissions. Lower heating rates were found to increase the reaction time, favoring NO formation, while higher heating rates promoted the production of carbon monoxide (CO), which then reacted with NO, reducing its overall emissions [64]. Controlling the gasifying environment can be an effective way to manage emissions during co-pyrolysis. In a 5% O₂/95% Ar atmosphere, more N-containing compounds remained in the biochar, indicating less NO formation. In contrast, a CO₂-rich atmosphere promoted the release of N from the fuel, resulting in increased NO emissions [64].

Similarly, the co-pyrolysis of sewage sludge with coffee grounds resulted in a significant reduction in acid and nitrogenous compounds in the gas emissions [65]. Specifically, the relative content of acid compounds decreased from 19.78% to 7.85%, and nitrogenous compounds were reduced from 13.42% to 8.47%. This indicates a cleaner gas profile when sewage sludge and coffee grounds are co-pyrolyzed compared to their individual pyrolysis. The types of evolved gases differed significantly between the N₂ and CO₂ atmospheres. In an N₂ atmosphere, the temperature-dependent evolution of gases followed the sequence ethers/esters/acids/ketones/aldehydes/CO₂ hydrocarbons. However, in a CO₂ atmosphere, the order shifted to acids/ketones/aldehydes/esters/ethers/hydrocarbons. This suggests that using CO₂ as the pyrolysis medium changes the dynamics of volatile compound release, reducing the presence of acids and nitrogenous compounds, which are typically harmful emissions.

Sewage sludge often contains S compounds that can result in sulfur dioxide emissions during thermal processing. Co-pyrolysis with biomass or other carbonaceous materials helps bind S in the solid phase [66], thereby reducing the release of S gases. This can help mitigate acid rain and other environmental problems related to S emissions.

Zou et al. [67] found that the co-pyrolysis of textile dyeing sludge and waste biochar led to significant changes in gas emissions depending on the atmosphere. In an N₂ atmosphere, the emission of NH₃ and SO₂ was notably reduced. In a CO₂ atmosphere, there was a delay in N decomposition and a transformation of S compounds into more stable forms, leading to lower emissions of harmful sulfur gases. The reduction in NH₃ emissions was achieved by converting volatile N into more stable forms within the biochar, while SO₂ emissions were minimized by transforming S into stable sulfones, particularly in a CO₂ atmosphere.

The co-pyrolysis of sewage sludge and rice husk in a cyclic catalytic integrated process system not only increased the syngas yield and quality but also significantly reduced the emission of harmful gaseous byproducts [68]. Utilizing a cyclic catalytic integrated pyrolysis (CCIP) system, the co-pyrolysis process significantly improved gas emissions. The innovative CCIP system increased the residence time of gaseous tar in the reactor through steam circulation, leading to the secondary cracking of heavy tars into more valuable gaseous products such as H₂, CH₄, and CO, and thereby reducing the overall yield of tar and increasing the lower heating value (LHV) of the syngas by 41.5% compared to traditional pyrolysis.

While co-pyrolysis reduces harmful emissions compared to sewage sludge pyrolysis alone, the management of residual emissions and the optimization of process conditions remain crucial. Further studies should focus on emission control technologies and the use of catalysts to further improve environmental outcomes.

8. The Effect of Sewage Sludge Co-Pyrolysis on Enhanced Energy Recovery

The co-pyrolysis of sewage sludge with forestry and agricultural waste significantly enhances energy recovery, creating a more efficient and sustainable process for converting waste into valuable energy products. Sewage sludge, when pyrolyzed alone, typically has a lower energy output due to its high moisture content and low C content. However, co-pyrolysis with forestry residues (e.g., wood chips and bark) and agricultural waste (e.g., straw and husks) enhances the overall energy efficiency by increasing the thermal stability and calorific value of the feedstock mixture (

Table 9. Calorific values of biochar and bio-oil from the pyrolysis and co-pyrolysis of sewage sludge.

Type of Feedstock	Biochar HHV (MJ/kg)	Bio-Oil HHV (MJ/kg)	Ref.
Sewage sludge	11.3	12.9	[28]
Sewage sludge + wheat straw	15	10.3
Sewage sludge pretreated with acid	15.7	13.0
Sewage sludge + wheat straw pretreated with acid	17.3	10.4
Sewage sludge	9.9	14.7	[30]
Sewage sludge 75% + rubberwood sawdust 25%	14.8	16.9
Sewage sludge 50% + rubberwood sawdust 50%	21.8	18.6
Sewage sludge 25% + rubberwood sawdust 75%	27.7	20.5
Sewage sludge	8.8	-	[60]
Sewage sludge 70% + rice husk 30%	9.6	-
Sewage sludge 50% + rice husk 50%	10.1	-
Sewage sludge 30% + rice husk 70%	10.7	-
Sewage sludge 70% + hemp straw 30%	10.4	-
Sewage sludge 50% + hemp straw 50%	11.4	-
Sewage sludge 30% + hemp straw 70%	12.4	-
Sewage sludge + rice husk without a catalyst	-	25.7	[69]
Sewage sludge + rice husk with a sewage sludge biochar catalyst	-	32.3
Sewage sludge + rice husk with a rice husk biochar catalyst	-	30.5
Sewage sludge + rice husk with a mixed biochar catalyst	-	34.7
Sewage sludge + rice husk with a rice husk ash catalyst	-	30.7
Sewage sludge	8.0	34.0	[31]
Sewage sludge 50% + cattle manure 50%	11.0	29.0

and

Table 10. Calorific values of syngas from the pyrolysis and co-pyrolysis of sewage sludge.

Type of Feedstock	LHV (MJ/Nm3)	Ref.
Sewage sludge, no rotor revolutions	7.9	[68]
Sewage sludge 75% + rice husk 25%, no rotor revolutions	8.7
Sewage sludge 50% + rice husk 50%, no rotor revolutions	8.5
Sewage sludge 50% + rice husk 50%, no rotor revolutions	8.9
Sewage sludge 25% + rice husk 75%, 1050 rpm, without a catalyst (CaO)	11.1
Sewage sludge 25% + rice husk 75%, 1050 rpm, with 2.5 g of CaO	11.3
Sewage sludge 25% + rice husk 75%, 1050 rpm, with 5 g of CaO	10.9
Sewage sludge 25% + rice husk 75%, 1050 rpm, with 7.5 g of CaO	10.3
Sewage sludge	3.3	[70]
Sewage sludge 50% + rice husk 50%	9.2
Sewage sludge	3.4	[20]
Sewage sludge + sawdust 20%	4.8
Sewage sludge + sawdust 40%	7.4
Sewage sludge + sawdust 60%	8.8
Sewage sludge + sawdust 80%	9.6
Sewage sludge + sawdust 40%, 600 °C	15.6	[71]
Sewage sludge + sawdust 40%, 700 °C	15.2
Sewage sludge + sawdust 40%, 800 °C	14.4
Sewage sludge + sawdust 40%, 900 °C	13.6
Sewage sludge + sawdust 40%, 1000 °C	13.2

).

Co-pyrolysis improves the production of the energy-rich byproducts biochar, bio-oil, and syngas. Biochar retains a high level of fixed carbon, making it valuable as a fuel. Additionally, the bio-oil yield increases with a richer composition of volatile compounds, while syngas—composed of hydrogen (H₂), CO, methane (CH₄)—offers an enhanced energy potential due to the improved gasification of organic materials.

In the Ruiz-Gómez et al. study [31], energy recovery from the co-pyrolysis of sewage sludge and digested manure followed distinct patterns across biochar, bio-oil, and syngas, with biochar and bio-oil showing more favorable energy characteristics compared to syngas, which had a lower-than-expected calorific value. The biochar produced from co-pyrolysis had a higher HHV than the HHV of biochar produced from sewage sludge and manure alone. Despite manure biochar having a higher calorific value, the co-pyrolysis biochar still retained a significant portion of the feedstock’s energy, contributing 40% of the total energy recovered during the process and making it a valuable energy source, even in co-pyrolysis. The bio-oil produced from co-pyrolysis had an HHV that was slightly lower than bio-oil produced from sewage sludge alone, but comparable to that from manure. Bio-oil contributed 24% of the total energy recovered, similar to individual pyrolysis. However, the N and sulfur contents of the bio-oil would require additional refining to avoid harmful emissions during combustion. Syngas produced from co-pyrolysis had a lower LHV compared to sewage sludge alone. This was primarily due to lower-than-expected hydrogen yields during the process. The syngas contributed only 4% of the total energy recovered, indicating that it was the least energy-dense product of co-pyrolysis. Although its lower calorific value limits its overall contribution, syngas can still be used for power generation or heating after gas cleaning.

The introduction of biochar catalysts during the co-pyrolysis of sewage sludge and rice husk significantly improves the HHV of the resulting bio-oil. Among the tested catalysts in form of biochars, i.e., sewage sludge biochar, rice husk biochar, mixed sewage sludge and rice husk biochar, and risk husk ash, the rice husk biochar showed the best catalytic performance, leading to a marked increase in the HHV from 25.7 MJ/kg (without the catalyst) to 34.7 MJ/kg (with the catalyst). This improvement is mainly due to enhanced hydrocarbon formation, effective deoxygenation, N removal, and improved stability of the bio-oil [68].

Co-pyrolysis of sewage sludge and sawdust improves the calorific value of syngas through a synergistic mechanism, where the addition of sawdust enhances the thermal breakdown of sludge, leading to more efficient gas production. The optimal conditions for maximizing syngas calorific value were identified as temperatures of 700–750 °C and 10 wt.% sawdust addition, which achieved the best balance between yield and the energy content of the syngas [72].

Co-pyrolysis using the cyclic catalytic integrated process system significantly improved the calorific value of syngas. The integration of tar cracking, steam circulation, and optimized catalyst use increased the proportion of energy-dense gases, leading to a substantial rise in the syngas’s LHV from 7.9 MJ/Nm³ to 11.3 MJ/Nm³. This makes the co-pyrolysis of sewage sludge and rice husk a highly effective method for producing high-quality syngas [68].

The synergy during co-pyrolysis of sewage sludge and rice husk increases the LHV of syngas by promoting gasification reactions that increase the production of hydrogen and CO and reduce the CO₂ content, benefiting from the catalytic effects of metals found in the feedstocks. This leads to a higher calorific value of the syngas than what would be expected from the individual pyrolysis of sewage sludge or rice husk alone [70].

Based on the paper by Zhu et al. [20], the calorific value of syngas produced from the co-pyrolysis of wet sewage sludge and sawdust was influenced by the ratio of sawdust added and the pyrolysis conditions. The LHV of the syngas produced varied with the sawdust content. When sawdust content increased from 0% to 100%, the LHV of syngas was slightly decreased. The total syngas heat produced from 1 kg of raw material increased significantly. It increased from 3.4 MJ/kg (with 0% sawdust) to 9.9 MJ/kg (with 100% sawdust), indicating a more efficient energy recovery as the sawdust ratio increased.

In the study by Yang et al. [71], the co-pyrolysis of wet sewage sludge and sawdust at high temperatures (600 °C to 1000 °C) resulted in significant syngas production, with an increase in gas yield from 40.8% to 57.8% as the temperature rose. The syngas primarily consisted of H₂, CO, CH₄, and CO₂, with the H₂ and CO proportions increasing at higher temperatures. The LHV of the syngas decreased slightly from 15.6 MJ/Nm³ at 600 °C to 13.2 MJ/Nm³ at 1000 °C due to a reduction in energy-rich hydrocarbons like CH₄. Despite this, the syngas remained suitable for industrial applications such as power generation, highlighting the process’s potential for producing valuable fuel gas from sewage sludge and biomass.

By combining sewage sludge with C-rich materials such as wood chips, rice husks and sawdust, this process produces energy-rich byproducts (biochar, bio-oil, and syngas), each of which has different energy applications. The synergy between sewage sludge and these biomass materials optimizes the calorific value, stability, and overall quality of the fuel. With innovations such as catalytic processes and optimal feedstock ratios, co-pyrolysis becomes a viable, sustainable option for converting waste into renewable energy sources.

9. Conclusions and Perspectives

This paper highlights the advantages of the co-pyrolysis of sewage sludge with agricultural and forestry biomass residues, offering substantial environmental and economic benefits. Co-pyrolysis significantly enhances the quality of the biochar produced, achieving a reduced heavy metal content, increased porosity, and enhanced nutrient retention, making it more suitable for agricultural and environmental applications. The synergistic effect of co-pyrolysis also improves the calorific value of bio-oil and syngas, boosting their viability as renewable energy sources. This process mitigates the challenges of sewage sludge management by reducing harmful emissions, immobilizing heavy metals, and optimizing resource recovery. Co-pyrolysis thus represents a sustainable, circular economy approach to managing sewage sludge by converting it into valuable bio-products, supporting both waste reduction and energy production.

Future research should prioritize optimizing co-pyrolysis parameters to enhance biochar quality and energy yields. Testing a broader range of biomass sources and ratios could provide valuable insights into nutrient recovery and heavy metal immobilization. Additionally, efforts should aim to scale the co-pyrolysis process effectively, ensuring both economic feasibility and environmental compliance, while integration with other waste management and biorefinery systems could maximize resource efficiency.

Investigating catalytic co-pyrolysis also holds significant potential for improving the bio-oil yield and quality. Catalysts can enhance the deoxygenation and denitrogenation of bio-oils, resulting in higher heating values and cleaner emissions. The application of activated carbon, which could function without additional energy input, is a promising method for removing contaminants from flue gases, and its effectiveness should be explored in greater depth.

Since many studies on sewage sludge co-pyrolysis are confined to laboratory-scale reactors, the industrial feasibility of co-pyrolysis technologies must be tested through scaled-up processes. This is essential to assess engineering challenges, economic viability, and operational stability at larger scales.

Finally, understanding the long-term impacts of biochar derived from sewage sludge co-pyrolysis on soil health and C sequestration is the key to unlocking applications in sustainable agricultural and climate mitigation, highlighting the need for in-depth assessments of biochar’s performance as a soil amendment under field conditions.