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Integrated sewage sludge treatment scenarios – techno-economic analysis on energy and phosphorus recovery
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-6141-7796
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Sewage sludge is a by-product of wastewater treatment that simultaneously gathers contaminants, valuable organic matter, and nutrients. The treatment of the increasing amount of sewage sludge is important from both pollution prevention and resource recovery perspectives as i) large shares of mineral phosphorus, listed as a critical raw material, terminate in the sewage sludge, and ii) energy recovery from sewage sludge can cover the energy-intensive demand of the treatment process. Previous research has identified sewage sludge combustion as a suitable treatment approach as it both addresses contaminant destruction and paves the way for efficient phosphorus recovery from the sewage sludge ash. The commercial development of this practice has, however, been slow. Therefore, this thesis aims to investigate the challenges in sustainable sewage sludge management, and to, in more detail, identify the economic viability of energy and phosphorus recovery from sewage sludge through combustion.

The thesis’ aim is divided into two objectives addressed in three papers. First, to investigate how different aspects of sewage sludge management, such as contaminants, economic efficiency, technical aspects, and legislation, evolve and interact. This has been done by a review of sewage sludge management research over fifty years (Paper I). Second, to investigate the economic viability of simultaneous energy and phosphorus recovery from sewage sludge by comparing different technology and market scenarios. This has been done for i) new sewage sludge mono-/co-combustion plants (Paper II), and ii) the integration of treatment technologies, mainly anaerobic digestion, hydrothermal carbonization, and combustion, in an existing wastewater treatment plant (Paper III). 

Results from the analysis of sewage sludge management research (Paper I) show a narrow-focused perspective that often excludes inseparable aspects such as combination of economic consideration and advanced extraction technology. The investment viability of a new mono-/co-combustion of sewage sludge (Paper II) is highly conditional on heat, electricity, and fertilizer price, and external financial support is often a crucial requirement. Sewage sludge co-combustion with potassium-rich biomasses improves sewage sludge quality and forms usable ash as fertilizer without further need for phosphorus recovery technology. In this case, the economic feasibility of the process is independent of usable ash revenue, which stimulates a competitive selling price for the ash, thereby improving the marketing of sludge-based fertilizer. Avoided disposal costs of sewage sludge for a retrofitted wastewater treatment plant by introducing hydrothermal carbonization (Paper III) shows good economic feasibility while recovering phosphorus. Integrating anaerobic digestion, hydrothermal carbonization, and combustion may also improve investment incentives by improving energy outputs and phosphorus recovery. The economic feasibility is contingent on product (hydrochar, heat, electricity) prices and sensitive to added equipment costs, and costs for sludge transportation and disposal. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords [en]
Sewage sludge management, techno-economic analysis, phosphorus recovery, thermochemical treatment
National Category
Energy Systems
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-93662ISBN: 978-91-8048-196-0 (print)ISBN: 978-91-8048-197-7 (electronic)OAI: oai:DiVA.org:ltu-93662DiVA, id: diva2:1705062
Presentation
2022-12-16, A117, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-10-21 Created: 2022-10-20 Last updated: 2023-09-05Bibliographically 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. 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

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Bagheri, Marzieh

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