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Thermodynamic modelling of energy recovery options from digestate at wastewater treatment plants

Researcher: Dr. Karla Dussan



Environmental Protection Agency Ireland

Grant 2014-RE-DS-3

Starting date


Finishing date


Summary and objectives

This work proposes to build a thermodynamic model for all sub-processes in the recovery of energy by thermochemical means from AD digestate at wastewater treatment plants. The model will include balance submodels for: (1) anaerobic digestion (simplified), (2) digestate pre-treatment and/or drying, (3) energy conversion via (3a) combustion, (3b) gasification or (3c) pyrolysis, (4) energy integration with other WWT processes, (5) air emissions control, (6) waste product handling, (7) recovery of valuable non-energy resources, and (8) subsequent production of (8a) heat, (8b) electricity and/or (8c) fuel. The work will not focus on the use of energy recovery for domestic wastewater treatment systems (DWWTS), due to the small size of these systems and the impracticality of implementing energy recovery.


To review and assess the technical feasibility of thermochemical processes for energy recovery from AD digestate:

  1. Review of relevant AD digestate properties, energy requirements at WWTs and technologies available for energy recovery from AD digestate.
  2. Creation of physics-based computer model to quantify the production of useful energy, valuable non-energy products, air emissions and waste products from thermochemical conversion of AD digestate.
  3. Comparison of process performance of available technologies: most feasible in Irish contexts.
  4. Recommendations on future research and demonstration priorities aimed at reducing energy demand and GHG emissions at WWT plants.

Projected research outputs

  • Reviews of wastewater treatment and anaerobic digestion technologies used in Ireland and their energy consumption as electricity and heat.
  • Thermodynamic-based models of the thermal conversion (pyrolysis, combustion, gasification) of sludge digestates and the potential electricity/heat offset within wastewater treatment plants.
  • Feasibility of the implementation of thermal conversion technologies.

Conferences proceedings

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, 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

Yang Q, Dussan K, Monaghan RFD, Zhan X. Energy recovery from thermal treatment of dewatered sludge in wastewater treatment plants. 1st IWA Resource Recovery Conference, Ghent, Belgium Aug-Sep, 2015

Journal publications

Thermodynamic Modelling of Energy Recovery Options from Digestate at Wastewater Treatment Plants. K Dussan, RFD Monaghan. 2017. Environmental Protection Agency of Ireland, Report Number 2016

Integrated Thermal Conversion and Anaerobic Digestion for Sludge Management in Wastewater Treatment Plants. K Dussan, RFD Monaghan. 2017. Waste and Biomass Valorization. DOI: 10.1007/s12649-016-9812-x

Energy recovery from thermal treatment of dewatered sludge in wastewater treatment plants. Q Yang, K Dussan, RFD Monaghan, X Zhan. 2016. Water Science & Technology. 2016. DOI: 10.2166/wst.2016.251 


EU Urban Wastewater Directive

EPA Ireland: Environmental indicators

EPA Ireland: Urban Wastewater Treatment


Why using thermal conversion for sludge management?


Nearly 55,000 tons of sewage sludge are generated every year in Ireland (1). Management of this waste (dewatering, drying, transportation, disposal) consumes a significant amount of energy that affects negatively the environmental impact of wastewater treatment. Transportation to disposal site (landfilling) can increment over 15% the fossil energy used by the treatment operations.

Currently, only 27% of the wastewater treatment facilities treat sewage sludge through anaerobic digestion (2). This biological process converts 30-60% of the organic material in sewage sludge producing biogas. Biogas is a renewable green fuel that can fuel the operations of the wastewater treatment facility. However, this process does not eliminate sludge completely.

In a typical wastewater treatment plant generating 210 tons of dry sewage sludge, over 300 tons of wet digestate (50% dry matter) need to be transported every day to disposal or final use site.


Thermal conversion of sludge and digestates offer an attractive alternative that offers two products: dry and reduced inorganic waste and energy! In the scenario considered above, less than 80 tons of waste will require transportation after thermal conversion, over 70% savings in energy spent in sludge management. At the same time, the wastewater treatment facility would become energy-independent, and even an energy generation site.

What are the advantages of technologies such as combustion, pyrolysis and gasification?



  • Energy efficient processes: These technologies offer diverse and flexible alternatives adaptable to different necessities of energy consumption and process scale.
  • Managing hazardous wastes: Toxic materials and heavy metals commonly found in wastes can be trapped in inert wastes that can later be used in industrial processes or disposed of in a cheaper and more environmental way.
  • Reduction of the environmental impact: By generating energy from wastes, the carbon footprint of wastewater treatment is decreased. Likewise, the final waste is an stabilised and inodorous material that upon environmental guidelines, poses no threat to water, air and soil.