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Sewage Sludge Treatment Scenarios: Techno-Economic Analyses of Energy and Phosphorus Recovery Focusing on Implementation Challenges
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-6141-7796
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Managing sewage sludge, an inevitable by-product of wastewater treatment processes rich in both contaminants and valuable resources, presents a dual challenge: ensuring pollution prevention by immobilizing or destroying contaminants, and facilitating resource recovery. Balancing these objectives is critical given the growing volumes of sewage sludge and the imperative to both protect the environment and recover valuable resources. The unknown risks of land application of sludge, the currently most common disposal method, make thermal conversion a promising alternative, as it enables energy recovery, the breakdown of potentially harmful organic compounds, and the formation of volume-reduced, sanitized products. Despite the technical feasibility, the commercial development of advanced recovery technologies has been slow. 

This thesis aims to expand knowledge on different sewage sludge treatment and disposal scenarios under varying conditions; thereby shedding light on implementation challenges, local opportunities, and the financial dynamics critical for phosphorus and energy recovery from the perspectives of wastewater treatment plants, investors, and policymakers. The aim is primarily addressed by performing techno-economic analysis of sewage sludge treatment scenarios, covering the entire sludge treatment process from sludge treatment to end products and disposal (Paper II-V). The techno-economic analysis is supplemented by a review of academic research on sewage sludge management from 1971 to 2019 (Paper I). 

Results from the analysis of sewage sludge management research (Paper I) show a narrow-focused perspective that often misses the broader, interconnected aspects of sewage sludge management, leading to research that, while detailed, fails to capture the complexity of the field. The investment viability of a new mono-/co-combustion plant for sewage sludge (Paper II) is highly conditional on heat, electricity, and fertilizer prices, and external financial support is often a crucial requirement. Cocombustion of sludge (in low ratios) with K-rich agricultural biomass requirement in and energy demand of a thermal dryer, and by yielding ash that contains phosphorus in a plant-available form. Utilizing existing heat facility (Paper III) and co-combustion to mitigate investment costs and energy demand in sludge management showed the potential to offer a cost-effective alternative to land application. However, the viability of co-combustion hinges on both a high heat market price (Paper II) and the proximity of affordable biomass resources (Paper III). Without these conditions, co-combustion may increase the financial burden of sludge management on wastewater treatment plants and policymakers. 

Retrofitting a wastewater treatment plant by integrating hydrothermal carbonization in sludge treatment (Paper IV), demonstrated good economic feasibility, primarily due to the avoided disposal costs, while also recovering phosphorus. However, integrating hydrothermal carbonization in a system designed for a thermal dryer may cause a significant reduction in electricity production.

Given the high moisture content of sludge and the low market prices for fertilizer, the potential revenue from energy and phosphorus recovery is inadequate to solely drive investment in advanced sewage sludge treatment technologies (Papers II-V). This issue is exacerbated by the fact that most wastewater treatment plants are small in scale. Collaborative sludge management across neighboring wastewater treatment plants (Paper V) increases phosphorus recovery capacity and leverages economies of scale, fostering investment in advanced technology. This strategy presents a significant opportunity to lower the treatment costs and offers a competitive alternative to land application, while encompassing energy and phosphorus recovery into the sludge treatment.

In conclusion, resource recovery and pollution prevention in sludge management is a complex task that necessitates simultaneous consideration of technical aspects, product quality, site-specific conditions, and profitability to ensure a comprehensive and viable approach. Leveraging local infrastructure and resources in sewage sludge management is crucial, highlighting the ecessity for strategies to be tailored to the local opportunities and limitations. Such an approach outperforms mono-combustion by eliminating the investment ensures that solutions are not only environmentally sustainable but also economically viable and socially acceptable. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Sewage sludge management, techno-economic analysis, phosphorus recovery, energy recovery, thermochemical treatment
National Category
Energy Engineering Energy Systems
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-105083ISBN: 978-91-8048-534-0 (print)ISBN: 978-91-8048-535-7 (electronic)OAI: oai:DiVA.org:ltu-105083DiVA, id: diva2:1851501
Public defence
2024-06-14, A117, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-04-15 Created: 2024-04-15 Last updated: 2024-05-24Bibliographically approved
List of papers
1. Fifty years of sewage sludge management research: Mapping researchers' motivations and concerns
Open this publication in new window or tab >>Fifty years of sewage sludge management research: Mapping researchers' motivations and concerns
2023 (English)In: Journal of Environmental Management, ISSN 0301-4797, E-ISSN 1095-8630, Vol. 325, article id 116412Article in journal (Refereed) Published
Abstract [en]

Sewage sludge management is torn between a desire for pollution prevention and reuse of a valuable resource. Reconciling these interests in sustainable management is a challenge for researchers. This study focuses on how research on sewage sludge management practices has evolved and scrutinizes how this research is interlinked with concerns and societal issues such as contaminants, economic efficiency, and legislation. Based on published academic papers on sewage sludge management between 1971 and 2019, this study found four trends in research focused on sewage sludge management: a decreasing interest in disposal (landfilling and sea dumping), a dominant interest in land application, a growing interest in sewage sludge as product, and a stable interest in energy recovery. Research on disposal focuses on increasing sludge volumes, legislative changes, and economic challenges with an interest in waste co-treatment. Research on land application concerns nutrient use and contaminants, mainly heavy metals. Research on sewage sludge as a product focuses on the extraction of certain resources and less on use of sewage sludge specifically. Research on energy recovery of sewage sludge focuses on volume reduction rather than contaminants. Two-thirds of the papers are detailed studies aiming to improve single technologies and assessing single risks or benefits. As management of sewage sludge is multifaceted, the narrow focus resulting from detailed studies promotes some concerns while excluding others. Therefore, this study highlights potential gaps such as the combination of nutrient use and disposal and energy recovery and nutrient use. 

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Land application, energy recovery, phosphorus, legislation, resource recovery, biosolid management
National Category
Energy Systems Environmental Management
Research subject
Energy Engineering; Waste Science and Technology
Identifiers
urn:nbn:se:ltu:diva-93660 (URN)10.1016/j.jenvman.2022.116412 (DOI)000877514100006 ()36274309 (PubMedID)2-s2.0-85140098755 (Scopus ID)
Funder
Swedish Energy Agency, P46028-1Swedish Research Council Formas, 2018-00194
Note

Validerad;2022;Nivå 2;2022-10-25 (hanlid);

Funder: Bio4Energy

Available from: 2022-10-20 Created: 2022-10-20 Last updated: 2024-04-15Bibliographically approved
2. Techno-Economic Analysis of Scenarios on Energy and Phosphorus Recovery from Mono- and Co-Combustion of Municipal Sewage Sludge
Open this publication in new window or tab >>Techno-Economic Analysis of Scenarios on Energy and Phosphorus Recovery from Mono- and Co-Combustion of Municipal Sewage Sludge
2022 (English)In: Sustainability, E-ISSN 2071-1050, Vol. 14, no 5, article id 2603Article in journal (Refereed) Published
Abstract [en]

This study evaluates the techno-economic feasibility of energy and phosphorus (P) fertilizer (PF) recovery from municipal sewage sludge (MSS) through incineration in new combustion plants. We evaluated the economic impact of five critical process design choices: (1) boiler type, (2) fuel (MSS mono-combustion/co-combustion with wheat straw), (3) production scale (10/100 MW), (4) products (heat, electricity, PF), and (5) ash destination. Aspen Plus modeling provided mass and energy balances of each technology scenario. The economic feasibility was evaluated by calculating the minimum selling price of the products, as well as the MSS gate fees required to reach profitability. The dependency on key boundary conditions (operating time, market prices, policy support) was also evaluated. The results showed a significant dependency on both energy and fertilizer market prices and on financial support in the form of an MSS gate fee. Heat was preferred over combined heat and power (CHP), which was feasible only on the largest scale (100 MW) at maximum annual operating time (8000 h/y). Co-combustion showed lower heat recovery cost (19–30 €/MWh) than mono-combustion (29–66 €/MWh) due to 25–35% lower energy demand and 17–25% higher fuel heating value. Co-combustion also showed promising performance for P recovery, as PF could be recovered without ash post-treatment and sold at a competitive price, and co-combustion could be applicable also in smaller cities. When implementing ash post-treatment, the final cost of ash-based PF was more than four times the price of commercial PF. In conclusion, investment in a new combustion plant for MSS treatment appears conditional to gate fees unless the boundary conditions would change significantly.

Place, publisher, year, edition, pages
MDPI, 2022
Keywords
municipal sewage sludge, energy recovery, phosphorus recovery, techno-economic analysis, mono-combustion, co-combustion
National Category
Energy Systems Environmental Management
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-89866 (URN)10.3390/su14052603 (DOI)000770990400001 ()2-s2.0-85125328366 (Scopus ID)
Funder
Swedish Research Council Formas, 2018-00194Bio4EnergySwedish Energy Agency
Note

Validerad;2022;Nivå 2;2022-03-25 (hanlid)

Available from: 2022-03-25 Created: 2022-03-25 Last updated: 2024-04-15Bibliographically approved
3. Economic viability of co-combusting sewage sludge with agricultural biomasses: a resource-efficient strategy for sludge treatment and phosphorus recovery in Sweden
Open this publication in new window or tab >>Economic viability of co-combusting sewage sludge with agricultural biomasses: a resource-efficient strategy for sludge treatment and phosphorus recovery in Sweden
(English)Manuscript (preprint) (Other academic)
National Category
Energy Systems Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-105081 (URN)
Available from: 2024-04-15 Created: 2024-04-15 Last updated: 2024-04-15
4. Introducing hydrothermal carbonization to sewage sludge treatment systems—a way of improving energy recovery and economic performance?
Open this publication in new window or tab >>Introducing hydrothermal carbonization to sewage sludge treatment systems—a way of improving energy recovery and economic performance?
2023 (English)In: Waste Management, ISSN 0956-053X, E-ISSN 1879-2456, Vol. 170, p. 131-143Article in journal (Refereed) Published
Abstract [en]

Hydrothermal carbonization (HTC) can mitigate the disposal costs of sewage sludge in a wastewater treatment plant. This study analyzes the impact of integrating HTC with anaerobic digestion (AD) and combustion from a combined energy and economic performance perspective. Net energy balance and investment opportunity are investigated for a number of technical scenarios considering i) different combinations of the technologies: AD + HTC, AD + thermal dryer + combustion, and AD + HTC + combustion, ii) different options for HTC process water treatment: wet oxidation (WO) + AD, and direct return to AD, and iii) different products: heat-only, heat and electricity, hydrochar, and phosphorus.

The results show trade-offs between investment cost, self-supplement of heat, and output electricity when WO is used. In AD + HTC, net heat output decreases compared to the reference plant, but avoided disposal costs and hydrochar revenue result in profitable investment when the process water is directly returned to the AD. Although HTC has a lower heat demand than the thermal dryer, replacing the thermal dryer with HTC is only possible when AD, HTC, and combustion are connected, or when WO covers HTC’s heat demand. HTC may impair the electricity production because of the necessity for a high-temperature heat source, whereas the thermal dryer can utilize a low-temperature heat source. In conclusion, energy advantages of HTC in AD + HTC + combustion are insufficient to provide a promising investment opportunity due to high investment costs of HTC. The investment opportunity improves by co-combustion of hydrochar and external sludge.

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Hydrothermal carbonization, Techno-economic analysis, Sewage sludge, Char, Thermal treatment
National Category
Energy Systems Environmental Management
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-93661 (URN)10.1016/j.wasman.2023.08.006 (DOI)001058081800001 ()37573718 (PubMedID)2-s2.0-85167813183 (Scopus ID)
Funder
Swedish Energy AgencySwedish Research Council Formas, 2018-00194Bio4Energy
Note

Validerad;2023;Nivå 2;2023-08-15 (joosat);

Licens fulltext: CC BY License;

This article has previously appeared as a manuscript in a thesis.

Available from: 2022-10-20 Created: 2022-10-20 Last updated: 2024-04-15Bibliographically approved
5. Economic feasibility and environmental impacts of phosphorus recovery in Swedish wastewater treatment plants
Open this publication in new window or tab >>Economic feasibility and environmental impacts of phosphorus recovery in Swedish wastewater treatment plants
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-105082 (URN)
Available from: 2024-04-15 Created: 2024-04-15 Last updated: 2024-04-15

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