1. Biosolids/Biosludge

Biosolids or biosludges will refer to any form of sludge that has undergone some form of treatment (e.g., chemical, biological, heat treatment, etc.) in order to transform it into a less hazardous and organic form. Additional forms of SS resulting from industrial activities involving the above are also considered (e.g., due to the use of a bioreactor or from the pharmaceuticals industry). Figure 1 is an illustration of the different classes of biosolids.

Figure 1. Different classifications of what constitutes biosolids

The pyrolysis process holds great potential as a thermal treatment method [35,36], that is capable of transforming biomass into value-added products. Typically, a biomass feedstock will undergo thermal treatment to an elevated temperature that goes beyond 400 °C in the pyrolysis process in an inert environment, that involves partial or total removal of oxygen. In addition to containing the gas produced resulting from the thermal degradation of the organic components [37,38], the pyrolysis process results in the production of three major products, namely, biochar, bio-oil, and biogas (Figure 1).

Figure 1. Different classifications of what constitutes biosolids

2. Management of Biosolids and the Use of the Pyrolysis Process

Solids resulting from wastewater are either applied to land as soil amendment and nutrient source, incinerated, or landfilled [31]. These solids are full of micropollutants that include organic chemicals originating from consumer-based products, which end up in the sewer drains during disposal (e.g., pharmaceuticals, antimicrobial compounds, personal care products, medicines and hormones, etc.) [32,33]. Therefore, the sewage sludges and wastewater treatment sludges and their by-products from these locations require new and innovative handling methods that alleviate environmental burdens, while producing value-added products, e.g., capacitors [34]. The thermal process of pyrolysis to biochars has been reported to combat some of these issues, for instance, the destruction of organic molecules, the destruction of microorganisms and the fixation of heavy metals, preventing them from leaching out into the soil.

3. The Pyrolysis Process

Pyrolysis is a relatively uncomplicated technology requiring no complex process plant compared with incineration and gasification and it is quite environmentally friendly [52]. Additional benefits include its ability to handle many waste types [53], while directly producing several product materials (for example, biochars, bio-oils and syngas) [54,55]. In terms of the limitations of the sustainability of the pyrolysis process for a treatment alternative, the issue for wet biosolids feedstock is a problem but it has been stated recently that the introduction of low-cost pre-drying solutions can improve the outcomes, prior to pyrolysis [40]. For example, solar drying methods are promising techniques as a low-cost drying pretreatment in arid environmental conditions, having plentiful sunlight and a low humidity environment. Alternatively, flue gases, available from the combustion of the bio-oils/biogas, in the temperature range 140–180 °C could achieve the same objective and even indirect heat exchange with the hot outlet pyrolysis gases. The pyrolysis process is classified into different types with the slow and fast pyrolysis being among the most commonly applied types (Figure 2).

Figure 2. Depiction of the pyrolysis of biosolids

4. Benefits of Pyrolysis

Pyrolysis is a relatively uncomplicated technology requiring no complex process plant compared with incineration and gasification and it is quite environmentally friendly [52]. Additional benefits include its ability to handle many waste types [53], while directly producing several product materials (for example, biochars, bio-oils and syngas) [54,55]. In terms of the limitations of the sustainability of the pyrolysis process for a treatment alternative, the issue for wet biosolids feedstock is a problem but it has been stated recently that the introduction of low-cost pre-drying solutions can improve the outcomes, prior to pyrolysis [40]. For example, solar drying methods are promising techniques as a low-cost drying pretreatment in arid environmental conditions, having plentiful sunlight and a low humidity environment. Alternatively, flue gases, available from the combustion of the bio-oils/biogas, in the temperature range 140–180 °C could achieve the same objective and even indirect heat exchange with the hot outlet pyrolysis gases.

This focused review paper aims to shed light on biochars produced from different pyrolysis methods and using different biosolids or treated sewage sludge. It should be noted that both types of sludges will be later referred to as bio-sludge as there seems to be no consistency in the literature regarding the use of these terminologies and types of sewage sludges—based on their level of treatment. Furthermore, the paper briefly discusses the available options for the recovery of energy from the pyrolysis products as reported in the literature. Both biogas and bio-oil for energy are useful byproducts of the biosludge pyrolysis process for the production of biochars. Finally, an overview of the role of biochars as a soil amendment additive is presented.

5. Limitations of the Pyrolysis Process

The pyrolysis process, whether fast or slow, is associated with a number of challenges which have been recently discussed in the work of [56] who has explored the use of wet biomass for bioenergy production. Briefly, the use of fast pyrolysis is typically associated with the requirements for dewatering or drying [40] stages, in addition to the resulting low yield and poor quality bio-oil. It also places significant pressure on vapor condensation and the gas cleaning system. Slow pyrolysis is also associated with a number of challenges that need remediation steps that are also outlined in the work of [56]. Such limitations include high energy requirements of the process in addition to the requirements for vapors separation and gas cleaning systems.

6. Treatment of Sludge Prior to Pyrolysis

Most of the studies focus on the pyrolysis process itself and the characterization of the end-products, while assigning minimal reference to the major pretreatment requirement of the sludge that was obtained prior to the pyrolysis. Mostly, these studies indicate that the biosludge was received in a dry form with certain characteristics [57,58]. Furthermore, some studies report further drying of the obtained sludge, such as the work of [59], who obtained their sludge from a printing and dyeing plant. In Poland, [60] obtained sewage sludge (SS) from two municipal wastewater treatment plants (WWTPs) based on MBT; mechanical and biological treatment. After the sewage sludge anaerobic digestion process in the WWTPs, samples of the sewage sludge were harvested in summer, at the final processing stage. Another treatment of sewage sludge was reported by [61] who implemented a triple oxidation ditch processing stage for the wastewater treatment step.

Another problematic sludge is oil sludge from oil refineries [62]. Initial efforts to make use of this sludge sought to convert it into useful resources such as lower molecule organic compounds and carbonaceous chars by the application of pyrolysis, but with very limited success [63]. In a later study, the oil sludge was the feedstock for the pyrolysis reaction which resulted in a unique product distribution and characteristics. Table 1 provides a summary of the recent work on the pyrolysis of treated sewage sludge and other biosludges. It should be noted that the review paper discusses the below-mentioned studies in more depth and technicality (the reader is welcome to consult with the review paper for more details).

Table 1. Previous studies on the pyrolysis of treated sewage sludge.

7. The Potential for Energy Recovery from the Pyrolysis Bio-Oil and Biogas

Although the focus of this review is on biochar production, it is important to take into account the two major by-products, biogas and bio-oil, because their effective utilization will have a tremendous benefit on the economic attractiveness of the pyrolysis process package [106]. Besides biochars, the pyrolysis products from biosolids include bio-oil and biogas, which are associated with the energy content that can be potentially recovered and utilized. Table 3 presents a summary of high calorific values or higher heating values (HHV) reported by several studies.

Table 3. HHV of bio-oil and biogas from the literature.

The study conducted by [112] report the HHV of bio-oil and char only, where HHV of 36–39 MJ/kg-oil was observed for the process. The HHVs were not influenced by the operating temperatures nor the sludge type as per the study. Another promising results were demonstrated by the work of [109] in which the energy content of biogas and bio-oil was always greater than the energy required for the pyrolysis study.

Woody and non-woody biomass materials were used in the production of bio-oil and biochar, with the investigation of HHV of both products [108]. The HHV of bio-oil was comparable to other studies mentioned in Table 2, with values in the range 12.19–22.32 MJ/kg. It is evident that the slight variations in the reported values for HHV are due to the type of biomass used, its composition and the type of pyrolysis used. Moreover, bio-oil HHV was influenced by the inert environment used with heating value increasing from 27.75 MJ/kg to 31.40 MJ/kg when using nitrogen and then hydrogen in the work of [107]. The results imply the importance of the type of gases in the subsequent values of HHV of bio-oil products.

The energy recovery from both streams comes with some challenges. For instance, the liquid oil is heterogenous with fractions that contain dissimilar HHV [113], in addition to being a corrosive material. It is, however, a very attractive source of energy when compared to both the gas and char products’ energy content. The inclusion of the by-products into the economic analysis can improve the economic effectiveness by 2 or 3 fold [106].

8. Benefits of Biosolids-Derived Biochars

It is crucial to distinguish between biochars produced from biosolids and those obtained from other types of biomass materials. There are commonalities in the uses of all types of biochar and as such the same benefits are expected from biosolids-derived biochars. From the results of previous work on biochars produced from other types of biomasses, the different biochar products were found successful for use in amending soils in commercial potting soil mixes, green roofs, and commercial agriculture [31]. However, the same benefits from biochars derived from biosolids require further research to determine its applicability, opening room for more research opportunities in the applied side of this type of research.

Furthermore, the addition of different biochars to soils promotes plant growth. This addition is associated with interesting biological and chemical changes such that a shift in rhizosphere microbial and fungal communities to more favorable compositions for plant growth is attained. The same changes can be achieved through the contribution of some chemicals to the soil-system that lead to similar increases in nutrient concentrations [31,115,116].