Pyrolysis of Sewage Sludge: Unlocking the Hidden Potential for Valorization and Carbon Sequestration ^†

Abstract

Sewage sludge management remains a critical environmental and economic challenge due to high volumes, transport requirements, and landfill restrictions. Pyrolysis offers a promising alternative by reducing sludge mass and producing biochar with potential for soil fertility enhancement and long-term carbon sequestration. This study integrates physicochemical characterization of Portuguese wastewater treatment plant sludges with experimental drying data and literature-based pyrolysis yields to estimate mass reduction, energy requirements, and carbon retention. A simplified life cycle comparison highlights potential reductions in greenhouse gas emissions, human toxicity, and land use, while also suggesting significant economic savings from avoided transport and landfill disposal.

1. Introduction

Wastewater treatment plants (WWTPs) generate large volumes of sewage sludge as an inevitable by-product of urban and industrial effluent treatment. In Portugal alone, it is estimated that over 416,000 tonnes of dry sewage sludge are produced annually, posing significant environmental and economic challenges due to traditional disposal, particularly landfilling, and transport requirements [1]. Transportation and landfill fees account for a substantial part of the operational costs of wastewater management, which ultimately fall on municipalities and end users [1,2].

Sewage sludge is characterized by high moisture content, high ash fractions, and relevant amounts of nitrogen, sulfur, and heavy metals, which limit its direct valorization pathways [3]. Nevertheless, thermochemical conversion, and specifically pyrolysis, has gained attention as an alternative capable of addressing both waste reduction and valorization needs. By promoting mass reduction and generating a stable carbon-rich solid (biochar), pyrolysis is emerging as a potential local solution that reduces the amount of sludge requiring transport and disposal [4,5].

Sewage sludge pyrolysis typically yields 25–45% biochar (dry basis). This biochar contains ~50–60% fixed carbon, supporting long-term sequestration [4,5,6]. Using a factor of 0.22 kg CO₂ per kilogram of biomass input [7], processing 400,000 tonnes of dry feedstock would sequester ~88,000 tonnes of CO₂ annually through soil application.

Sewage sludge biochar may also enhance soil fertility, water and nutrient retention, and stabilize carbon for long-term sequestration [5], though its agronomic use depends on meeting regulatory limits for heavy metals and organic contaminants [3].

Pyrolysis differs from incineration and gasification mainly by operating at lower temperatures (350–550 °C versus 800–1000 °C for incineration and 700–1300 °C for gasification) [8]. This reduces energy demand and limits emissions of NOx, SOx, and dioxins relative to incineration. Importantly, pyrolysis recovers carbon as biochar—a stable, carbon-rich solid—while incineration yields low-value ash and gasification converts most carbon to syngas [8]. Overall, pyrolysis offers a favorable balance between environmental safety, residue valorization, and resource recovery.

This study aims to provide a preliminary assessment of sewage sludge pyrolysis in the Portuguese context, integrating experimental drying data with bibliographic yields from sludge pyrolysis studies. A simplified life cycle comparison is also proposed, contrasting the current disposal route (landfill + transport) with a local pyrolysis scenario, in order to explore the potential environmental and economic benefits.

2. Materials and Methods

Sewage sludge samples were obtained from three Portuguese wastewater treatment plants (WWTPs), namely Serzedo, Ponte da Baia, and Santo Emilião. Their physicochemical characterization—including moisture content, ash, volatile matter, fixed carbon, elemental composition (C, H, N, S, O), and higher heating value (HHV)—was previously determined and published in [3]. These data provide the baseline for evaluating the energy potential and constraints associated with the sludge.

Oven-drying tests at 105 °C were performed for two sludges (Serzedo and Ponte da Baía) to quantify pre-treatment moisture removal and derive lower-bound energy requirements for water evaporation. The Santo Emilião sludge sample was received air-dried and could not be characterized. These experimental results were combined with literature-reported slow pyrolysis data for sewage sludge, typically 400–700 °C, 20–60 min residence times, 5–15 °C·min⁻¹ heating rates, and inert N₂ atmospheres [4,5,6,9,10], to build a theoretical valorisation framework.

The analysis followed three complementary steps:

• Pyrolysis performance estimation—bibliographic yields were coupled with the sludge compositions to estimate possible ranges of biochar production and carbon retention.
• Energy pre-treatment assessment—energy consumption for drying was calculated per kg of wet sludge, allowing comparison with the energy content of the dried material.
• Life cycle perspective—a simplified LCA approach was implemented in openLCA using the freely available ELCD database for background processes, comparing two scenarios: (i) current practice of sludge transportation and landfilling and (ii) local pyrolysis with valorisation of the produced biochar. The assessment emphasized avoided transport emissions and potential carbon sequestration in soils.

3. Results and Discussion

3.1. Drying Efficiency and Energy Demand

Drying tests revealed that approximately 69% (Serzedo) and 69.6% (Ponte da Baia) of the initial wet sludge mass was removed as water. Assuming a latent heat of evaporation of 2257 kJ/kg [11], the theoretical minimum energy required for drying is 4.66 MJ (Serzedo) and 4.20 MJ (Ponte da Baia) per batch—values that represent a lower bound neglecting system inefficiencies. These results provide a quantitative basis for evaluating pre-treatment energy demand in decentralized sludge valorisation systems.

3.2. Physicochemical Characterization: From Sludge to Biochar

The three WWTP sludges show high ash (≈20–30%), modest fixed carbon (~10–12%), and non-negligible N (~6–9%) and S (~0.7–0.8%) fractions, which constrain direct energy valorisation and motivate thermochemical conversion [3]. The literature consistently reports that sludge-derived biochar exhibits increased fixed carbon and higher HHV (often ≥30 MJ kg⁻¹) relative to the parent sludge, with yields decreasing as pyrolysis temperature rises, while carbon concentration and aromaticity increase [4,5]. These trends suggest substantial enhancement of fuel quality and valorisation potential post-pyrolysis.

3.3. Estimated Mass Reduction and Biochar Potential

Using typical dry-basis biochar yields of 25–45% for sewage sludge pyrolysis [4,5,9], applying these to the dried masses measured here (~0.92 kg Serzedo; ~0.82 kg Ponte da Baía) gives ~0.23–0.41 kg (Serzedo) and ~0.21–0.37 kg (Ponte) of biochar per batch. Against the wet input, this corresponds to ~86–92% reduction in transport mass. Such ranges agree with review-level reports that show yield declines with temperature while fixed carbon retention increases, improving carbon stability for soil use [5,6]. Transport costs alone typically vary between 13 and 30 EUR/t of wet sludge, accounting for 16–18% of the overall service cost that also includes loading, recovery/disposal, and analysis [10].

3.4. Simplified Life Cycle Assessment (LCA) Comparison

A preliminary LCA framework compared the current route—transport and landfilling of dewatered sludge—with a local pyrolysis scenario producing biochar for soil application. Three impact categories were considered: global warming potential (GWP100), human toxicity potential (HTP), and land use. The literature shows that sludge pyrolysis can achieve net negative GWP values due to carbon sequestration in biochar (e.g., −78 to −182 kg CO₂-eq per tonne of wet sludge, equivalent to −339 to −791 kg CO₂-eq/t DS at 23% TS) [4,9]. By contrast, landfill-based routes exhibit positive footprints of ~700–830 kg CO₂-eq/t DS, largely driven by methane generation [12]. Regarding HTP, pyrolysis immobilizes heavy metals and destroys organic micropollutants, leading to substantially lower toxicity indicators than direct land application or incineration [5]. Landfilling prolongs land occupation, whereas pyrolysis with biochar application reduces land use pressure through carbon storage and potential crop-yield gains [13]. Indeed, adding biochar to agricultural soils has been shown to boost yields beyond long-term trends, effectively reducing land required per unit of production [14].

4. Conclusions, Limitations, and Future Prospects

This preliminary assessment highlights the potential of sewage sludge pyrolysis as a decentralized route to reduce disposal costs, lower emissions, and enable long-term carbon sequestration. Results remain theoretical, and future work should include experimental validation and compliance with local regulations on biochar quality.