The present Special Issue consists of twenty nine papers presented during the HERAKLION 2019 7th International Conference on Sustainable Solid Waste Management (https://www.heraklion2019.uest.gr) that was organised in Heraklion, Crete in Greece starting on 26th and ending on 29th June 2019. It is also noted that more than 700 participants from all over the world traveled to Heraklion to participate.

The HERAKLION 2019 Conference was a very successful and actually the largest conference event of this conference series following the previous conferences of this series, in ATHENS in 2012 and 2014 respectively, in TINOS in 2015, in CYPRUS in 2016, in ATHENS in 2017 & in NAXOS in 2018. The last conference event in Crete brought closer the participants working in solid waste treatment and management with emphasis on biorefineries and generally circular economy. The conference gave emphasis on energy from waste and the large number of site visits for the application of circular economy in agriculture & livestock production. The HERAKLION 2019 conference agenda was indeed rich, providing a great variety of waste related research works presented through more than 500 presentations within the 26 sessions with oral presentations and the large poster sessions.

High-quality papers of the HERAKLION 2019 Conference related to composting, biogas and anaerobic digestion, valorization of waste materials, biorefineries and recent advances in recycling were selected by the Scientific Committee to be included after improvement and normal review process in this WAVE Special Issue. The papers present scientific attempts in order to improve the use of potential waste materials and resources via different processes, set up biorefineries and enhance circular economy in different fields.

Last but not least, the two Guest Editors would like once more to thank the WAVE Editor-in-Chief, Prof. Ange Nzihou, who offered us the chance to make the sixth WAVE Special Issue including high quality papers from our conference series on sustainable solid waste management. Being delighted to see this journal has an increasing impact factor every year, we are confident that our conference series has also contributed over the years to the WAVE increasing prestige and interest for waste experts and scientists worldwide.