Thermodynamic modelling of energy recovery options from digestate at waste water treatment plants

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Resource or Project Abstract

Stringent emission limits, population growth and increasing urbanisation continue to drive the advancement in wastewater treatment (WWT) technologies and waste management frameworks. Today in Ireland, over 96% of the sludge generated during WWT is spread on agricultural land; however, restrictions set by agricultural quality assurance schemes are encouraging the search for new alternatives. Plants with a capacity greater than 100,000 population equivalent (p.e.) can implement anaerobic digestion (AD) as a means of sludge treatment and energy recovery. Pilot and WWT plant scale studies have reported biogas yields between 4 and 10 GJ t-1 (1-3 kWh kg-1) of dry sludge through AD of municipal sewage sludge (Qiao et al., 2011). However, large scale biogas plants can consume about 40% of this energy for their operation, diminishing energy efficiency (Berglund and Börjesson, 2006). Thermal technologies for the conversion of either sewage sludge or digestate represent a potential route for both sludge volume reduction and energy recovery. In particular, sludge combustion and/or gasification could provide either thermal or chemical energy for combined Heat and Power (CHP) generation and could be readily integrated with WWT plants.

This project explores the state-of-the-art of combustion (incineration) and gasification technologies used for biomass and waste conversion. This study evaluates not only the technical performance of these technologies, but also investment and operational costs, and waste generation, treatment and valorisation through recovery of materials and chemicals. Using a pseudo-thermodynamic approach for modelling thermal conversion, the performance of combustion and gasification of sludge and digestate was evaluated under various operational conditions and solid material properties (moisture, composition). The model evaluated the technical performance of the thermal conversion processes, as well as the integration of energy carriers for power generation and heat recovery through different available technologies, such as steam and gas turbines, and combustion engines. In order to support local and governmental authorities in the consideration of these alternatives, different techno-economic indicators, including energy recovery efficiency, treatment costs, levelised cost of electricity generation and carbon footprint of the operation of the WWT-sludge management plant were included and thoroughly compared among the different potential process alternatives.

Gasification and AD-gasification integrated with CHP generation was technically feasible and offered means to reduce final waste disposal costs and improve energy efficiency of the WWT plant. The most efficient process concept for energy recovery used internal combustion engines to generate power from energy carriers produced from gasification and AD-gasification, i.e. biogas and syngas. Conditions under which electricity generation was maximised were met with extensive sludge pretreatment, i.e. drying, and thus with low heat recovery efficiencies. On the other hand, AD integrated with gasification gave a greater thermal recovery flexibility leading to conditions in which net surpluses of both electricity and heat were achieved. The combination of AD and gasification offered competitive costs of electricity generation (20-50c kWh-1), with low carbon footprint (<300 kg CO2 t-1 dry sludge). Internal combustion engines offer great flexibility and competitive power efficiencies at typical scales for energy recovery in WWT facilities (>10 MWe).
When gasification was used as the only sludge conversion additional energy for heat generation was required in the process. This additional heat was proposed to be generated via co-processing with renewable solid fuels and wastes, i.e. biomass, animal slurries, organic-fraction of municipal solid waste. Biomass rates between 0.8 and 1 times that of the sludge feed rate were required to meet energy demands at a reduced carbon footprint.

It is important to emphasise that the scale of the facility is vital in meeting sustainability criteria, especially in terms of operational and capital expenditures. The evaluation presented in this study was applied to the largest WWT scale in Ireland (1.6 Mp.e.), such as that of the Ringsend WWT plant, which currently produces approx. 85 t d-1 (tpd) of dry sludge (digestate). However, most existing anaerobic digestion facilities have capacities between 40,000 and 150,000 p.e., with sludge generation rates between 20 and 100 dry tpd, depending on the influent wastewater. Combustion engines offer sufficient flexibility to operate with the power capacities expected for these scales (>100 kWel). However, costs of installation can make the implementation of gasification challenging at small scales. Raw sludge generation rates under 130 tpd led to levelised costs of electricity generation slightly above the national costs fossil-based electricity (24c kWh-1). However, gasification and AD-gasification as sludge treatment was economically competitive with generation rates above 25 tpd. This scale challenge may still be overcome through other approaches that are suggested for future research. On-site thermal pretreatment can facilitate sludge transportation to a centralised facility, where energy recovery may offset overall treatment costs in terms of energy and carbon footprint. A centralised gasification facility would offer the possibility of implementing biomass and/or waste co-processing with greater economic and technical efficiencies, while reducing operational challenges. It is also important to note that sludge transportation and biomass co-processing will have additional energy penalties, transportation costs and carbon footprint, which must be taken into account in the evaluation of an optimal sludge transportation and treatment network at a county or national level. Biomass has direct and indirect carbon emissions linked to harvesting, use of fertilisers, land use change, importation, and transportation that were not considered in the present study. These may affect the carbon footprint of large scale plants with high biomass-to-sludge co-processing ratios.

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Contact Information for This Resource

Dr. Rory Monaghan
National University of Ireland Galway
Lecturer Mechanical Engineering
National University of Ireland Galway , Galway, , H91 HX31, Galway, Galway
Telephone: +353 (0)91 494256 (internal 4256)
e-mail: rory.monaghan@nuigalway.ie

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Access Information For This Resource

SAFER-Data Display URL http://erc.epa.ie/safer/iso19115/display?isoID=3146
Resource Keywordssludge, energy recovery, wastewater treatment, gasification, combustion, CHP, waste management
EPA/ERTDI/STRIVE Project Code2014-RE-DS-3
EPA/ERTDI/STRIVE Project ThemeEnvironmental Technologies
Resource Availability: Any User Can Download Files From This Resource
Public-Open
Limitations on the use of this ResourceData was sourced from open- and restricted access literature. Data can be reported upon referencing accurate source.
Number of Attached Files (Publicly and Openly Available for Download): 0
Project Start Date Wednesday 8th April 2015 (08-04-2015)
Earliest Recorded Date within any attached datasets or digital objects Monday 1st January 1996 (01-01-1996)
Most Recent Recorded Date within any attached datasets or digital objects Friday 8th April 2016 (08-04-2016)
Published on SAFERFriday 24th February 2017 (24-02-2017)
Date of Last EditFriday 24th February 2017 at 18:51:00 (24-02-2017)
Datasets or Files Updated On Friday 24th February 2017 at 18:48:33 (24-02-2017)

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Description of Geographical Characteristics of This Project or Dataset
This study evaluates not only the technical performance of these technologies, but also investment and operational costs, and waste generation, treatment and valorisation through recovery of materials and chemicals. Using a pseudo-thermodynamic approach for modelling thermal conversion, the performance of combustion and gasification of sludge and digestate was evaluated under various operational conditions and solid material properties (moisture, composition). The model evaluated the technical performance of the thermal conversion processes, as well as the integration of energy carriers for power generation and heat recovery through different available technologies, such as steam and gas turbines, and combustion engines. In order to support local and governmental authorities in the consideration of these alternatives, different techno-economic indicators, including energy recovery efficiency, treatment costs, levelised cost of electricity generation and carbon footprint of the operation of the WWT-sludge management plant were included and thoroughly compared among the different potential process alternatives.

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Supplementary Information About This Resource

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Lineage information about this project or dataset
The idea behind this project arose from the need to develop new means of improving energy efficiency and sufficiency in wastewater treatment plants via energy recovery from waste streams. Today in Ireland, over 96% of the sludge generated during WWT is spread on agricultural land; however, restrictions set by agricultural quality assurance schemes are encouraging the search for new alternatives. Thermal technologies for the conversion of either sewage sludge or anaerobic digestion residues represent a potential route for both waste volume reduction for disposal and energy recovery on-site. In particular, sludge combustion and/or gasification could provide either thermal or chemical energy for combined Heat and Power (CHP) generation that could be readily integrated with WWT plants. There was no previous project at a national/international level from which this project arose.
Supplementary Information
Chemical pseudo-equilibrium model for biomass gasification (Modelling tool)
Modelling tool built in Matlab R2015a and Cantera 2.2.1 that determines the product composition and syngas energy content obtained from the gasification of biomass and waste in a fluidised bed gasifier. This tool follows air gasification and uses as input the gasification temperature, equivalence ratio, biomass feed, CHO composition, and moisture/ash content.

Techno-economic evaluation tool for Anaerobic Digestion-Thermal conversion of sewage sludge in a Wastewater Treatment plant
This modelling tool generate performance indicators for a system following: a) Anaerobic digestion and/or dewatering of sewage sludge; b) Solids drying; c) Thermal conversion; d) Combined heat and power generation, CHP; e) Emissions collection/treatment/disposal. Different thermal conversion (combustion or gasification) and CHP technologies (combustion engine, steam or gas turbine) are considered. Default sludge and anaerobic digestion products are specified as a default; however, the user can adjust sludge properties, anaerobic digestion performance and the input of other biomass materials for co-digestion or thermal co-processing. Performance indicators include: a) Relative heat/power generation to the energy demands in the wastewater treatment facility (Energy coverage); b) process efficiency (heat recovery and electricity); c) Costs of operation and levelised cost of electricity generation, d) Carbon emissions due to plant operation.

WaSER WW - Modular tool for estimating the energy consumption in wastewater treatment plants (Modelling tool)
This user-friendly tool helps estimates the energy consumption (under average dry weather conditions) in a wastewater treatment plant by specifying the stages and technologies employed in the following treatment stages: a) primary sedimentation; b) biological sludge separation; c) nutrient removal; c) tertiary (disinfection) treatment; d) sludge dewatering, drying and/or anaerobic digestion with and without energy generation. The tool was created in Matlab R2015a; however, it does not require this software in order to be installed or used.

Publications include:

Energy recovery from thermal treatment of dewatered sludge in wastewater treatment plants. Q Yang, K Dussan, RFD Monaghan, X Zhan. 2016. Water Science & Technology 74 (3), 672-680

Integrated Thermal Conversion and Anaerobic Digestion for Sludge Management in Wastewater Treatment Plants. K Dussan, RFD Monaghan. 2016. Waste & Biomass Valorization Journal, under review.

Oral and poster presentations at national and international events include:

K Dussan, Q Yang, X Zhan, F Lane, RFD Monaghan. Anaerobic Digestion and Integrated Gasification: Energy Recovery within Wastewater Treatment Plants. 26th Irish Environmental Researchers? Colloquium ENVIRON 2016, Limerick, Ireland, March 2016

K Dussan, Q Yang, X Zhan, RFD Monaghan. Energy optimisation of waste gasification within wastewater treatment plants. 6th International Conference on Engineering for Waste and Biomass Valorisation, Albi, France, May 2016

K Dussan, Q Yang, X Zhan, RFD Monaghan. Thermodynamic evaluation of anaerobic digestion and integrated gasification for waste management and energy production within wastewater treatment plants. 24th European Biomass Conference and Exhibition, Amsterdam, The Netherlands, June 2016

K Dussan, RFD Monaghan. Towards Energy Self-Sufficient Wastewater Treatment for Ireland. EPA National Water Conference, Galway, Ireland, June 2016

Acknowledgments

Participants acknowledge the support given by the Ryan Institute, Marine and Renewable Energy Research Centre, Gas Networks Ireland and the Irish Research Council.

Special thanks to Fiona Lane (Irish Water), Mick Henry (EPA), Eamonn Merriman (EPA), Aisling O?Connor (EPA), Qingfeng Yang (NUIG), Dr. Xinmin Zhang (NUIG), Edelle Doherty (NUIG) and Dr. Eoghan Clifford (NUIG).
Links To Other Related Resources
  http://www.nuigalway.ie/therme/projects/old/epasludge/ (Opens in a new window)
  https://www.researchgate.net/profile/Karla_Dussan/publications (Opens in a new window)

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