Editorial: Design, operation and economics of large wastewater treatment plants

Design, operation and economics of large wastewater treatment plantsThe Conferences on the Design, Operation and Economics of Large Wastewater Treatment Plants (LWWTP) are the most traditional specialized conferences organized by IWA specialist groups.The first conference (at that time called 'workshop') was held in Vienna in 1971 and then consecutively every four years in Vienna, until 1987 when the conference began to be held in Budapest (1987, 1999, 2011) and Prague (1991, 2003, 2015) as well.The LWWTP Conference has always been a good place for meeting with leading experts in the field from Europe, USA and Canada, Australia, South Africa, Latin America, South-East Asia and from many other parts of the world.The 2020 conference was planned to be held in Vienna, to celebrate the 50 year anniversary of this great event.However, due to the start of the Corona pandemic in early 2020, the conference was initially postponed to the first half of 2021 and later completely cancelled.With all of the travel restrictions, the organizing committee felt that it would not be possible to run the LWWTP2021 conference as a truly international event, bringing together experts from all around the world.By the time the decision was made to cancel the conference, most presenters had already submitted extended abstracts for their presentations and posters, so it was proposed to use these papers to prepare a Water Science and Technology special issue.This special issue contains some of the best papers that were intended as oral presentations for the LWWTP2020, as well as some additional papers that were submitted especially for the special issue.The fully peer reviewed papers are mainly based on operational data and research centred around large-scale applications of innovative technologies and approaches.The published papers also reflect the most recent topics connected with the design and operation of LWWTPs, as data acquisition for design purposes, application of mathematical models (especially computational fluid dynamics models), nitrogen removal optimization and financial sustainability.Emerging topics of interest are also examined, such as the removal of micropollutants and microplastics, greenhouse gas emissions, activated sludge granulation and the effect of street thawing salt on plants.In addition, to adequately celebrate the 50-year anniversary of the specialist group and its conference, two papers focus on the historical development of wastewater treatment processes in this period, with a special focus on the conference series.From those review articles, it becomes clear that one of the strengths of the specialist group and the conference is the strong connection between practitioners and researchers from around the globe, who are focused on the economical design and operation of large wastewater treatment plants.