Environmental Processes

, Volume 5, Issue 3, pp 667–681 | Cite as

Unit Energy Consumption as Benchmark to Select Energy Positive Retrofitting Strategies for Finnish Wastewater Treatment Plants (WWTPs): a Case Study of Mikkeli WWTP

  • Khum Gurung
  • Walter Z. Tang
  • Mika Sillanpää
Technical Note


Retrofitting municipal wastewater treatment plants (WWTPs) to energy positive is a major challenge faced by many water utilities. Selection of innovative technologies to achieve retrofitting goals is critical for capital improvement programs in WWTPs. This paper aims to provide a statistical analysis method of unit energy consumption in conventional Finnish WWTPs, presenting Mikkeli WWTP as a case study. The average energy consumption at Finnish WWTPs was quantified as a mean of 0.49 kWh/m3 with a standard deviation of 0.197. The statistical analysis showed that the total energy consumption in Finnish WWTPs are positively correlated with inflow rate and sludge production. However, the unit energy consumption decreases with increasing plant capacity. The energy benchmarking of Mikkeli WWTP confirmed the energy gap of 0.11 kWh/kg COD in electricity. The major energy saving potentials are attributed to secondary treatment, screening and grit removal, and influent pump stations. A plausible innovative retrofitting strategy comprising four emerging energy-neutral or positive technologies is proposed to maximally harness the chemical energy content in wastewater: enhanced primary sedimentation, staged anaerobic fludized membrane bioreactor (SAF-MBR) with completely autotrophic nitrogen removal over nitrite process (CANON), and co-digestion of sludge with organic food-waste. The net energy balance of emerging technologies showed a maximum energy saving potential of 1.26 kWh/kg COD, which could be sufficient to overcome the energy gap of Mikkeli WWTP, providing net positive energy surplus of 1.15 kWh/kg COD.


Municipal WWTPs Energy consumptions Energy benchmarking Innovative energy positive retrofitting strategies 



Authors would like to thank B. Anne, R. Risto, T. Esko, and P. Paula for providing the necessary database of energy consumptions in Mikkeli WWTP.

Compliance with Ethical Standards

Competing Interests

The authors declare that they have no conflict of interests in relation to this work.


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© The Author(s) 2018
corrected publication 2018

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

Authors and Affiliations

  1. 1.Laboratory of Green Chemistry, School of Engineering ScienceLappeenranta University of TechnologyMikkeliFinland
  2. 2.Department of Civil and Environmental EngineeringFlorida International UniversityMiamiUSA

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