With the rapid development of the energy industry, great challenges in treating all types of sludge (oil sludge, paper sludge, municipal sludge, etc.) have emerged [1]. It has been predicted that the total growth rate of various types of sludge in China will increase by approximately 10% per year [2]. Thus, the development of sludge treatment technology worldwide has drastically increased. Currently, the main methods used in sludge treatment include incineration, land cultivation, and thermochemical conversion. Although they have certain applications, they still have their own limitations, and it is difficult to meet the requirements of various countries for sludge treatment and disposal. Among these methods, pyrolysis is considered a promising method for sludge valorization owing to the associated low heavy metal content, volume minimization, zero-waste conversion, and high-value product recovery [3]. In addition, compared with incineration, fewer sulfur oxides and a lower heavy metal content are produced in residues during sludge pyrolysis, and the by-products, such as pyrolysis oil, pyrolysis residues, and combustible gas, have reutilization value [4].

2. Types and Characteristics of Sludge

There are many ways to classify sludge. It can be classified as organic, inorganic, or hydrophobic sludge according to its composition and the characteristics produced during sewage treatment. Based on the different stages of treatment, sludges are categorized as raw, thickened, and dried sludge, among other types. Further, according to one source, it can be divided into municipal, oily, papermaking, printing, and dyeing sludge, among others. However, they are usually treated according to the source of the sludge. In sludge treatment, the different sources of sludge and their basic characteristics are shown in Table 1. It can be seen from Table 1 that the main composition and characteristics of sludge from different sources are relatively obvious. Therefore, the classification of sludge by source is helpful for researchers to further clarify the reasons for potential synergy between sludge and different additive co-pyrolysis.

3. Sludge Co-Pyrolysis Technology

Pyrolysis, one of the main technologies for sludge treatment, has the advantages of thorough treatment, volume minimization, and the recovery of high-value products ^[26][31]. However, sludge pyrolysis is often associated with problems such as high volatility and low ash content, as well as poor application performance of pyrolysis residues. Therefore, researchers have carried out investigations into sludge co-pyrolysis technologies with biomass and other additives in recent years. Generally, sludge, biomass, and solid waste are different in chemical and physical properties, such as ash content, volatile matter, and oxygen content, which can result in synergetic interactions during co-pyrolysis ^[27][32]. The use of the synergistic effect with the co-pyrolysis of sludge and additives can solve the shortcomings of sludge pyrolysis and realize associated resource utilization.

3.1. Co-Pyrolysis of Sludge and Biomass

Biomass comprises an organism formed through photosynthesis. The use of clean, renewable biomass energy has always been a hot topic for researchers ^[28][29][33,34]. Therefore, in recent years, researchers have studied the multi-phase products (gas, pyrolysis oil, biochar) of the co-pyrolysis of sludge and biomass and found that it not only solves the problem of sludge and biomass resource utilization, but can also improve the quality of multi-phase products and the prospects of product reuse ^[30][31][32][35,36,37].

3.1.1. Effect of Sludge and Biomass Co-Pyrolysis on the Performance of Biochar

Hua has obtained a heavy-metal adsorbent with a rich pore structure and more mineral particles from the co-pyrolysis of municipal sludge and banana peel ^[33][38]. Hong found that an increase in the pyrolysis temperature results in an increase in ash content and the specific surface area of the residue in the co-pyrolysis of dehydrated sludge and water hyacinth, and the adsorption capacity of Cr³⁺ reached 44.96 mg g^{−1 [34]}[39]. Wang obtained potential soil amendment products with higher carbon storage capacity in the soil from the co-pyrolysis of sewage sludge and cotton straw in terms of higher carbon content and lower H/C and N/C values ^[35][40]. In addition, because the residue has a high cation exchange capacity, it can enhance the nutrient supply and nutrient retention capacity in the degraded soil, and thus, it has high economic and environmental value. Xu et al. ^[36][41] used bamboo scraps as an additive to pyrolyze sewage sludge at 700 °C and found that the residue yield, pH value, ash content, specific surface area, and residue aromatization degree increased significantly, whereas H/C remained relatively low. At the same time, with the increases in bamboo chip proportions, the potential ecological risk factor of heavy metals in the residue drops below 40. Heavy metals are also reduced to a low risk level, indicating that the addition of bamboo chips is beneficial to improve the quality of the residue.

3.1.2. Effect of Co-Pyrolysis of Sludge and Biomass on Gas and Liquid Phases

The co-pyrolysis of sludge and biomass will not only change the properties of the biochar but also improve the quality of gas–liquid products ^[37][38][43,44]. Many researchers have evaluated the co-pyrolysis of sludge and biomass to enhance the properties of the gas and liquid phases and reduce activation energy during pyrolysis ^[39][40][45,46]. For example, Li found that the addition of peanut shells in the pyrolysis of municipal sludge can decrease the ammonia nitrogen content of the liquid phase product to 1369.00 mg/L, and the water phase product is one of wood vinegar ^[41][47]. Wan found an increase in the H₂ yield of combustible gas with an increase in the pyrolysis temperature during the co-pyrolysis of domestic sludge and pine wood chips ^[42][48]. Wang also observed that the addition of pine wood chips to the pyrolysis of domestic sludge can decrease the pyrolysis activation energy by approximately 117 kJ/mol ^[43][49]. Li studied the reaction kinetics and product distribution characteristics of the co-pyrolysis process from the co-pyrolysis of municipal domestic sludge and vinegar grains in a fixed-bed reactor and revealed that the presence of grains not only increases the H₂ and CO yield and the distribution of phenols and esters in the bio-oil, but also reduces the final temperature of the pyrolysis reaction ^[44][50].

3.2. Co-Pyrolysis of Sludge and Coal

China has abundant coal reserves, and coal is an important primary energy source. Some high-rank coals, such as anthracite, have been exploited in large quantities, but low-rank coals, such as lignite, long-flame coal, and bituminous coal, have the disadvantages of low calorific value and high moisture content ^[45][52]. Therefore, if improperly handled, they are likely to cause secondary pollution in the environment. Studies have found that coal and sludge have a synergistic catalytic effect on the co-pyrolysis process. On one hand, the synergetic effect can improve sludge pyrolysis characteristics during the co-pyrolysis of sludge and coal, and achieve the effective utilization of solid waste. On the other hand, sulfur contaminant emissions can also be controlled. For example, Li found that the addition of bituminous coal to the pyrolysis of dried sludge can reduce the pyrolysis temperature at the peak of gas production by 100 °C compared to that with the pyrolysis of sludge alone, effectively decrease the activation energy, and increase the yield of H₂ by 50% ^[46][53]. Chang obtained two independent pyrolysis zones of the co-pyrolysis process from the co-pyrolysis of low metamorphic coal and municipal sludge, and the results showed that the pyrolysis of sludge occurred mainly below 450 °C, whereas higher temperatures were mainly associated with the pyrolysis of coal ^[47][54]. However, owing to the synergetic effect, the activation energy for the co-pyrolysis of sludge and coal is less than that of the pyrolysis of sludge and coal separately. The concentration of small-molecule combustible gases, such as H₂ and CO, in mixed pyrolysis accounts for more than 85% of the total gas phase yield, and the calorific value can reach 32.05 MJ/Nm³. Moreover, the iodine value can reach 277 mg/g. These properties make pyrolysis residues promising as fuels and for adsorbent applications. Chen found that the synergetic effect between domestic sludge and Shenmu coal results in the comprehensive release characteristic index of volatile matter being increased by 1.86 times, but the activation energy was determined to be only 75% of the pyrolysis of sludge ^[48][55]. In addition, he revealed that co-pyrolysis has stable devolatilization and low reaction activation energy and also observed an increase in the yield of CH₄ and H₂ but a decrease in the emission of CO₂ and nitrogen-containing gas yield ^[49][56]. Zhao et al. ^[50][57] also observed a significant inhibitory effect on the release of H₂S and SO₂ yield during the co-pyrolysis of Zhundong coal and domestic sludge, and when the mass ratio of Zhundong coal to sludge was 1:1, the inhibitory effect of sulfur pollutants was best. The co-pyrolysis of coal and sludge can reduce the activation energy of the pyrolysis reaction, improving the utilization rate of coal and sludge and reducing the emission of sulfur pollutants ^[51][52][58,59]. In this context, the co-pyrolysis of sludge and coal could be a feasible approach to achieve the sustainable development and resource utilization of sludge.

3.3. Co-Pyrolysis of Sludge and Domestic Waste

With the growth of social and urban living standards, a large amount of domestic waste is produced. Domestic waste refers to the solid waste generated in daily life, and its presence is a huge threat to the living environment. The harmless treatment of domestic waste is an important topic for ecological development ^[53][60]. As sludge and domestic waste share many usable resources, co-pyrolysis technology can be used to reuse sludge and domestic waste resources. Fang studied the co-pyrolysis of combustible solid waste and paper mill sludge and found that when the blending mass ratio is 10%, oxygen-containing substances increase by 20.11% ^[54][61]; further, when the blending mass ratio is 50%, it can promote sulfur and nitrogen fixation and minimize pollutant emissions. In addition, upon assessing the thermal characteristics and kinetics of the co-pyrolysis of municipal solid waste and paper mill sludge by TG-GC/MS, the activation energy of the solution was found to be only 95.70 kJ/mol, with further increases in the yields of gas–liquid products ^[55][62]. Therefore, using domestic waste as an additive for sludge pyrolysis not only promotes the pyrolysis of sludge but also helps to deal with the pollution problems caused by domestic waste ^[56][57][64,65]. This research deserves to be extended in the future.

3.4. Research on the Co-Pyrolysis of Sludge and other Additives

Beyond the additives discussed previously herein, some special substances are also used for co-pyrolysis with sludge. For example, Liu prepared magnetic biochar based on the co-pyrolysis of sewage sludge with nano-zero-valent iron and used it to remove Cr⁶⁺ in wastewater ^[58][66]; they found that the removal rate of Cr⁶⁺ was good, and the adsorption amount could reach 11.56 mg g⁻¹. Milato used polyolefins as additives to co-pyrolyze oily sludge in a fixed-bed reactor at 450 °C ^[59][67]. They found that different products were produced due to the presence of polyolefins in the pyrolysis of oily sludge. At the same time, due to the synergistic effect of tertiary carbon in polyolefin and oily sludge, the pyrolysis process was optimized, and the content of heavy hydrocarbons (≥C₂₅) was found to increase. Overall, based on the characteristics of sludge and the additives, sludge co-pyrolysis technology could increase the application of co-pyrolysis residues (such as adsorbents, catalysts, soil nutrients, pesticide additives, antibacterial agents, etc.). Therefore, it is suggested future research focus on the co-pyrolysis of sludge with more large-scale additives, which have a greater synergistic effect with sludge, to further improve the potential value of sludge co-pyrolysis products.