Technical and financial evaluation of an emerging thermochemical technology for sustainable management of municipal wastewater sludge

Historically in South Africa, municipal wastewater sludge has been viewed as a waste product because of its potential to contain high levels of contaminants such as pathogens and other pollutants. In this regard, wastewater sludge has largely been disposed of (South Africa Department of Environmental Affairs 2019), with its management mostly implemented for compliance with the Department of Water and Sanitation (DWS) legislative and regulatory requirements. However, with the current increasing population growth and levels of urbanization resulting in more municipal wastewater sludge being produced, it is becoming increasingly unsustainable to dispose of wastewater sludge. Furthermore, stricter regulations regarding landfill disposal also require that other options for managing municipal wastewater sludge be investigated.

Currently, there is a general consensus that municipal wastewater sludge is a potential source of valuable resources and energy (Musvoto et al. 2018). However, the beneficiation of municipal wastewater sludge must ensure the recovery and reuse of valuable components with minimal negative impacts of the municipal wastewater sludge or residues from sludge treatment (Rulkens 2008). Although recommendations for the beneficial use of sludge were made in the new DWS regulations, there are very few wastewater treatment plants (WWTPs) currently designed for energy and/or other valuable resource recovery from municipal wastewater sludge (Herselman & Moodley 2009). This is despite energy and resource recovery having long been identified as essential for the long-term sustainability of WWTPs (Marx et al. 2004).

Generally, biological and thermochemical processes are the two primary pathways to achieve energy recovery from municipal wastewater sludge (Shanmugam et al. 2022; Osman et al. 2023). WWTPs currently utilize proven and well-established technologies based on, inter alia, (i) thermal hydrolysis, (ii) gasification, and (iii) pyrolysis (Oladejo et al. 2019). A study by Lacroix et al. (2014) has investigated coupling anaerobic digestion (AD) and gasification for municipal wastewater sludge management to recover energy resources, concluding the energy requirements reduction of up to 35%. Several other studies have been conducted to demonstrate improved energy efficiency and recovery of WWTPs (van der Merwe-Botha et al. 2016; Zvimba & Musvoto 2020; Zvimba et al. 2021). However, the implementation of findings has been challenging due to a lack of expertise, and technically and financially feasible technologies.

Hydrothermal carbonization (HTC) is a novel thermochemical process that converts wet organic matter, under moderate temperature and pressure, into a multi-use hydrochar potentially useful as a biofuel (Berge et al. 2011; Liu et al. 2013; Gerner et al. 2021). The HTC technology eliminates the need for an energy-intensive pre-drying step due to its ability to process organic waste in the presence of water. The HTC process further produce hydrochar from widely available raw materials such as wastewater sludge, agricultural waste, microalgae, and manure (Nadarajah et al. 2021). Due to its multiple applications, hydrochar has gained popularity as an economically and environmentally friendly material (Padhye et al. 2022). The application of HTC extends to tertiary wastewater treatment, where hydrochar effectively adsorbs heavy metals, organic compounds, and anions, thereby improving treated effluent quality. However, challenges remain, such as optimizing the process for diverse raw materials, managing economic costs, and addressing environmental and social impacts, with significant application of the process limited by its current low technology readiness level. By developing catalysts enhancing reaction mechanisms, the HTC can significantly contribute towards achieving carbon neutrality goals and fostering sustainable development.

In order to provide wastewater services with more technological options for sustainable resource recovery, this study evaluated the technical and financial feasibility of an improved version of the HTC process, the enhanced hydrothermal polymerization (EHTP) technology as an emerging sludge-to-energy thermochemical technology that can be implemented for sustainable municipal wastewater sludge management. The EHTP process is a catalysed, wet, sub-critical water thermochemical conversion process that processes biomass into a solid hydrochar.

The EHTP technology was implemented to process municipal wastewater sludge from a full-scale wastewater treatment plant (WWTP). The selected plant handles about 155 ML/day average dry weather flow (ADWF) generating an estimated 60 t/day municipal wastewater sludge. The WWTP consist of four modules, with each module having a primary settling tank (PST), activated sludge bioreactor, and secondary settling tank (SST). The waste-activated sludge (WAS) is obtained through dissolved air flotation (DAF) thickeners and then mixed with primary sludge (PS) from the PST prior to AD. The digested sludge (DS) is divided into two portions; one portion is dewatered using belt filter presses (BFP) while the other portion is dried through open-air sludge paddies. Dried sludge from the paddies is composted onsite and the compost together with the BFP sludge is collected by local farmers at no charge.

The performance of the EHTP technology was evaluated for processing of municipal wastewater sludge (20% dry solids feed content) for two scenarios (i) greenfield installation of the technology to treat 50 t/day dry sludge of combined PS and WAS mixed in a 1:1 ratio and (ii) retrofitted emerging technology into an existing conventional mesophilic AD plant treating 35 t/day dry DS mixed with screenings (S) in a 1:1 ratio. The waste generated from the process was disposed of according to the current practice at the case study plant site utilizing processed sludge/by-products in the agricultural sector and disposing to the nearest landfill and paying a tipping fee. An oxygen bomb calorimetric test was used to determine the gross calorific values or higher heating values (HHV). Furthermore, a Mettler TGA/DSC1 was used for conducting a modified ASTM E1131 proximate analysis (moisture, volatiles, fixed carbon, and ash contents) for both the feedstocks and hydrochar.

The financial evaluation was determined using the cost benefit analysis (CBA) approach as described by Mullins et al. (2014). The CBA was applied using hyperbolic discounting to evaluate the long-term environmental impacts of the projects. The CBA was applied through net present value (NPV) in order to determine and compare the life cycle costs and benefits of each technology technical evaluation scenario. This was based on two assumptions that the hydrochar could be of beneficial use or disposed to landfill. Four financial models were considered for evaluation, and these included (i) 100% equity, (ii) 100% loan, (iii) 45% grant and 55% equity, and (v) 45% grant and 55% loan.

Overall, from Figure 2, it can be seen that processing about 1.5 t dry sludge using the EHTP technology produces about 1 t of hydrochar, and this translates into transforming 14–19 GJ/t feedstock into approximately 17–24 GJ/t hydrochar. The use of the proprietary catalyst lowers the temperature, pressure, and energy required while the cellulose molecular structure undergoes a complex alteration during the conversion process resulting in a gain of 3–5 GJ/t. Lucian & Fiori (2017), have described such a conversion process as characterized by complex pathways that are believed to include hydrolysis, dehydration, decarboxylation, aromatization, and re-condensation. The results further indicated that all microbial life forms were eliminated as shown by 0 counts of E. coli, ova, and other spores. However, a high concentration of total chemical oxygen demand (TCOD) in the supernatant was observed, though this was much lower than the total influent load into the WWTP. The ortho P and ammonia concentrations were quite negligible in the supernatant suggesting their assimilation into the hydrochar solid phase, thereby confirming the potential use of hydrochar as a fertilizer.

To further understand the ratio of combustible to incombustible constituents in the feedstock and hydrochar, proximate analysis was carried out to determine the volatile matter, fixed carbon and ash contents for both the municipal wastewater sludge feedstocks and the hydrochar.

Figure 3 shows the proximate analysis data indicating the quality characteristics of the feedstock sludge and hydrochar.

Generally, from Figure 3, it can be seen that the feedstock contains higher volatile matter compared to the hydrochar. The presence of volatile matter is also significant in the DS and WAS feedstock, thereby confirming the low energy content for the hydrochar generated from the respective feedstocks as given in Figure 2. Figure 3 further illustrates the highest fixed carbon for the PS and WAS product, and this is quite consistent with a significantly high energy content for the PS and WAS generated hydrochar given in Figure 2. From Figure 3, the hydrochar derived from DS had the lowest fixed carbon content than the corresponding feedstock, hence the lower energy content for the DS derived hydrochar given in Figure 3. This is attributed to carbon depletion during the AD process.

A financial evaluation of preliminary designs for a greenfield installation processing 50 t/day combined PS and WAS and a 35 t/day retrofit processing DS with screenings (S) was carried out. The financial evaluation outcomes for these two scenarios using CBA and NPV as the decision criteria are shown in Table 2. From Table 2, it can be seen that the beneficial use of ash when the hydrochar is utilized for energy generation is more financially favourable compared to the disposal of ash to landfill option. This was based on the higher NPV values for different financial models for greenfield PS and WAS processing. A higher NPV value is indicative of a favourable financial model. Although the only financial model to yield a positive NPV was the 45% subsidy and 55% loan for both landfill disposal and beneficial use of ash, it is important for water utilities to conduct a detailed financial modelling before committing to a specific technology choice, municipal wastewater sludge disposal, or beneficial use scenario.

For the retrofit processing of 35 t/day of DS combined with screenings, it was noted that utilizing ash for beneficial purposes resulted in lower NPV figures compared to disposal into a landfill. However, among all the financial models assessed, it was found that those relying solely on 100% equity investment yielded negative NPV. Consequently, retrofitting AD at WWTPs with emerging EHTP technology appears financially viable, and this approach would potentially reduce municipal wastewater sludge disposal to landfill while supporting energy generation from the hydrochar, thereby augmenting the biogas energy generation initiative. Overall, integrating the EHTP with existing AD processes presents an opportunity to prevent infrastructure redundancy through the proposed EHTP retrofit.

Table 3 gives a summary of the CBA outcomes for the proposed greenfield installation processing 50 t/day of (i) combined PS and WAS, and (ii) combined PS, WAS, and screenings (S) using the EHTP technology compared to the application of advanced thermal hydrolysis–mesophilic anaerobic digestion (TH–MAD) and conventional MAD. It can be seen that under the chosen financing model, the EHTP technology consistently displayed the highest and positive NPV across all scenarios. This suggests that employing the EHTP technology as an emerging thermochemical process for managing municipal wastewater sludge at a new site presents a favourable financial scenario. Conversely, traditional AD proved to be the least financially viable option, even when generated biogas is considered for combined heat and power (CHP). Furthermore, when comparing conventional MAD with advanced TH–MAD, the latter emerges as a more financially feasible and attractive option. However, it is important to note that the financial benefits are also dependent on WWTP size. In this regard, previous studies have indicated that WWTPs with influent flows of 15 ML/day or less may not produce adequate municipal wastewater sludge to permit the installation of advanced AD with CHP generation capabilities (Shen et al. 2015; Musvoto et al. 2018). Generally, the beneficial use of municipal wastewater sludge is considered a more financially feasible and attractive option compared to the disposal of municipal wastewater sludge to landfills for all three technologies.

A review of the widely used thermochemical and biochemical conversion processes has established that the well-established advanced TH–MAD and the emerging EHTP technology generally offer several benefits compared to conventional mesophilic AD. The benefits include, inter alia, the generation of more energy resulting in a financially more appealing scenario as demonstrated by the financial evaluations conducted under this study. Moreover, the technologies significantly reduce the amount of municipal wastewater sludge, potentially lowering disposal costs for municipal wastewater sludge that does not meet compliance standards for beneficial use. The processed municipal wastewater sludge is also easier to dewater, achieving higher dry solids concentration, which further decreases the volume requiring disposal. Sludge from advanced TH–MAD plants typically reaches about 30% dry solids after dewatering. In contrast, the EHTP-produced hydrochar is hydrophobic, allowing it to be dewatered to a similar level without the need for polyelectrolytes, which is cost-effective, and also results in sterile sludge classified as EPA Class A (USEPA 2020).

Based on the current study, the EHTP technology represents the latest generation of emerging biomass-to-energy technologies that can be utilized for generating energy from municipal wastewater sludge. In this regard, the EHTP technology exhibits, inter alia, the following specific advantages over the widely used thermochemical and biochemical conversion processes for municipal wastewater sludge management:

• Provides a unified solution for managing both municipal wastewater sludge and screenings at a WWTP.

• Allows for integration with existing technologies such as AD, offering opportunities for further energy recovery from digested sludge and screenings.

• Produces hydrochar with various applications beyond serving as a green biofuel.

• Releases less gas (1–5%) with only traces of methane as most organics remain in solid form resulting in low greenhouse gas (GHG) emissions (Liu et al. 2024).

• Produces process wastewater requiring less complex pre-treatment, which can be discharged into the main liquid treatment process of the WWTP.

• Financially more feasible and favourable compared to other thermochemical and biochemical conversion processes.

Furthermore, due to the capability to process various types of biomasses to produce a versatile hydrochar and its compatibility with other technologies, the EHTP technology offers a viable pathway for implementing circular economy principles within the water and wastewater service sectors.

Based on the current study, the technical evaluation of a pilot-scale emerging thermochemical technology processing municipal wastewater sludge into a multi-use hydrochar presented several technical advantages over the widely used and established thermochemical and biochemical conversion processes. These include inter alia, the provision of a unified solution for managing both municipal wastewater sludge and screenings at a WWTP, including volarization of the sludge into valuable resources. Furthermore, the technology offers potential for integration with existing wastewater services technologies such as AD, thereby offering opportunities for further improved resource recovery from DS and screenings. The financial evaluation based on the proposed preliminary designs of full-scale greenfield and retrofit installations processing 50 and 35 t/day, respectively, suggests that the emerging EHTP technology is the most financially viable option compared to other widely used and well-established thermochemical and biochemical conversion processes. Overall, the current study findings suggest the significant application potential of the emerging EHTP technology for municipal wastewater sludge management, potentially contributing towards climate change mitigation. However, further studies that explore the technology's full life cycle would be required to understand the full-scale application of this emerging technology.

This study was financially supported by the Water Research Commission under project K5//2475/3.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.