The COVID-19 has been lasting for almost two years and has caused enormous changes in every aspect of our lives and work. The pandemic also challenged the field of scientific research and conferences. Scientific conferences which serve as a high‐impact international platform bring together researchers and multi-stakeholders from academia, government and private sectors to share the knowledge and experiences for the transformation towards a sustainable society. However, most of the conferences had to transform to an on-line form due to the mobility restrictions against the pandemic. The usual face-to-face lectures and discussions become supplemented by online audio, video, or even typing Q/A. It is evident that this fully on-line form can the smooth communication and effectiveness of the events.

At the starting point of organising the conferences (January 2021), which are the 24th annual conference on Process Integration for Energy Saving and Pollution Reduction – PRES'21, and the follow-up SPIL Scientific Conference on Energy, Water, Emission, Waste in Industry and Cities, it was challenging to foresee the situation with the pandemic after about 10 months and decide to be fully on-line or a regular face-to-face conference. According to statistical data from the previous conference, more than 400 participants from more than 50 countries would be there. It would be impossible to set up the conference fitting the situation of all international participants. After rounds and rounds of discussions within the organising committee, the PRES conference president, Professor Jiří Jaromír Klemeš, announced to held the conference in a hybrid form, which is on-line conference for most of the international participants who are not able to travel, and with a limited number of on-site (face to face) spaces for participants who being able to manage to come to Brno. The organisation was quite challenging due to the frequent changes in infection situations and measures against the COVID-19. Even in this tough time, the conferences were finally successfully held both on-line, and on-site at the Hotel International Brno, the PRES’21 Conference from 31st October to 3th of November, and SPIL conference from 4th to 5th November 2021.

Thanks to all the effort, supports and contributions from our PRES Conference President Prof Jiří Jaromir Klemeš, colleagues, collaborators, and all participants, the PRES Conference received a total number of 513 abstracts with 1,022 authors from 67 countries worldwide. Among these, 50 presentations with more than a hundred participants were delivered at the conference venue in Brno, with strict measures against COVID-19. In addition, ten invited lectures from distinguished professors and researchers, from world top universities and institutes, were also presented at both on-line and on-site conference. Prof Ignacio E. Grossmann from the Carnegie Mellon, a top US university, delivered the lecture titled “Optimal Synthesis and Planning of Sustainable Chemical Processes and Energy Systems”. Prof Santanu Bandyopadhyay from the Indian Institute of Technology Bombay, and one of the pioneering researchers in Pinch Analysis, contributed with a lecture “Incorporating Uncertainties in Pinch Analysis”. Prof Raf Dewil, who is a leading expert in wastewater treatment, and the Editor in Chief of the Journal of Environmental Management, presented the plenary lecture with the title of “Energy Positive Wastewater Treatment: Creating Value from an Underexploited Resource to reveal the water-energy nexus developments”.

The conference also invited distinguished researchers from world top universities as Guest Speakers of Honour, including Prof Donald Huisingh from University of Tennessee, Prof Qiuwang Wang from Xi’an Jiaotong University, Prof Pen-Chi Chiang from National Taiwan University, Professor Stratos Pistikopoulos from Texas A&M University, Prof Soteris Kalogirou from Cyprus University of Technology and Prof André Bardow from ETH Zurich. In addition, there are 397 inspiring presentations on the virtual conference website, 1,354 comments and questions from the on-line participants. The lectures from Prof Grossmann received more than 400 views, which is right after the opening presentation from Prof Klemeš (543 views). Discussions are still continuing after the official ending of the conference, and we hope that will lead to many fruitful collaborations. This hybrid conference with all the presentations downloaded and commented enabled those who were interested “to be present” even if they missed them due to the impossibility of monitoring them in real time.

The SPIL’21 Scientific conference, which followed the PRES conference, to reduce the environmental footprints for travelling, had also been successfully organised in hybrid form with the leading organisation of Dr Yee Van Fan. The conference hosted on-site more than 70 participants with 30 presentations at the conference venue and totalling 303 registered delegates. The conference started with a Welcome address by Vice-Dean Jiří Hlinka, and Opening Presentation by Prof Jiří Jaromír Klemeš (Head of SPIL), and the first invited lecture by Prof Petr Stehlík. A total of 14 invited lectures were presented to the participants, of which seven were delivered on-site. Prof Sandro Nižetić from the University of Split, Croatia, delivered his lecture with the title “A pathway to improve performance and applicability of photovoltaic technologies in energy transition”, sharing his visions in the future development of photovoltaic technologies in future energy supply. Prof Ms Aoife M. Foley, a leading researcher from Queen’s University Belfast and the Trinity Colleague, UK, and Editor-in-Chief of the high impacted journal Renewable & Sustainable Energy Reviews (IF 14.982) delivered the lecture titled “Green Energy Systems by 2050 for a ‘net’ Zero Carbon future”. Prof Fengqi You (Cornell University, US) and Prof Paweł Ocłoń (Cracow University of Technology, Poland) were another two invited plenary speakers of SPIL’21. A total of 106 presentations have been delivered on the virtual conference platform raising 1,380 comments and questions. The opening presentation of SPIL achievements was viewed on-line by 342 times. Guest Speakers of Honour included Prof André Bardow (ETH Zurich), Prof Dimitrios Gerogiorgis (The University of Edinburgh), Prof Iqbal M Mujtaba (University of Bradford), Prof Ignacio E. Grossmann (Carnegie Mellon University), Prof Petr Stehlik (VUT Brno), Prof Ferenc Friedler (Széchenyi István University), Prof Qiuwang Wang (Xi’an Jiaotong University), Prof Robin Smith (The University of Manchester), Prof Zdravko Kravanja (University of Maribor) and Dr Farooq Sher (Nottingham Trent University).

It has been a great challenge as well as a great experience for the whole organising team to manage the conferences in hybrid form for the first time. This is probably a new trend for academic conferences in the long term, not only because of the pandemic, but also considering the ability and feasibility of researchers to travel to the conference venue, a hybrid form can provide more possibilities for researchers from all around the world to learn, share, and inspire new knowledge. From the gained experience also benefited a recently accepted paper by NATURE Communications Journal "Trend of virtual and hybrid conferences since COVID-19 effectively mitigate climate change" co-authored by Professor Fengqi You, Roxanne E. and Michael J. Zak Professor at Cornell University, Ithaca, State New York, OS, his collaborators Yanqiu Tao, Debbie Steckel and Prof Jiří Klemeš, SPIL, FME, VUT Brno.