Erratum to “Carbon neutrality: An achievable goal for sustainable wastewater treatment plants” [Water Res. 87 (2015 Dec 15) 413e415] Xiaodi Hao a, * , Damien Batstone b , Jeremy S. Guest c a Beijing University of Civil Engineering and Architecture, China b The University of Queensland, Australia c University of Illinois at Urbana-Champaign, USA The publisher regrets to inform that there were major changes which were missed during the preparation of the final version of the Editorial article entitled “Carbon neutrality: an achievable goal for sustainable wastewater treatment plants”. The correct version of the Editorial with all changes implemented has been given below. The publisher would like to apologise for any inconvenience caused. Editorial Carbon neutrality: an achievable goal for sustainable wastewater treatment plants Carbon neutrality is a key issue to enable sustainable wastewater treatment plants (WWTPs) around the world. Both Europe and America began making steps towards carbon-neutral operation of WWTPs some years ago, and it has been proposed that their goal of carbon-neutral operation can be achieved by 2030. For example, the Dutch STOWA established a route map in 2008 for resource and energy recovery from WWTPs and proposed a new guiding concept for future WWTPs: NEWs (Nutrient þ Energy þ Water factories). Many studies and practical trials have been used to ascertain the feasibility of energy self-sufficiency by recovering energy from wastewater to offset on-site energy demand, and these initiatives have supported the related goal of mitigating greenhouse gas (GHG) emissions over the lifetime of a WWTP. Indeed, some energy-neutral operations have emerged at a handful of European and American WWTPs, but progress towards carbon- neutrality is less well established. In practice, carbon neutrality is often equated with energy neutrality. Research and development on low energy treatment and energy recovery from wastewater has a broad knowledge base, including the recovery of organic energy from influent carbon and excess sludge, co- substrate digestion, heat recovery, and biomass incineration. Beyond energy, however, treatment plants also incur GHG emissions from the treatment process itself (e.g., fugitive N 2 O or CH 4 emissions) and from resource consumption (e.g., chemicals dosing, concrete). Thus, a portfolio of solutions addressing energy consumption, energy recovery/production, and other direct and indirect sources of GHG emissions must be leveraged to establish WWTPs as carbon-neutral entities. Under the circumstances, the editorial board of Water Research decided to open a window especially for dealing with the topic of carbon neutrality, and thus this special issue (SI) was proposed. The SI aimed to discuss new concepts and ideas crucial to developing energy- efficient treatment technologies designed to both save and recover energy for operating WWTPs. From about 50 submissions, 13 articles were selected for publication after peer reviews, ranging from potentials of energy recovery and co-substrate digestion to development of new processes and design methodologies quantifying and navigating trade-offs across multiple dimensions of sustainability. Potentials of energy recovery from wastewater treatment and/or wastewater heat Excess sludge is definitely an important energy source to be recovered via anaerobic digestion. However, the amount of excess sludge depends heavily on the influent organic (carbon source: COD) concentrations. In some cases, carbon sources are insufficient, and barely meet the needs of nutrient removal, and thus energy neutrality cannot be achieved, or is incompatible with conventional nutrient removal. Anaerobic digesters generally have surplus capacity (about 20% in Germany), which is available for co-substrate digestion along with excess sludge. This is highlighted by a full-scale study in Austria demonstrating the use of existing infrastructure by addition of organic wastes (organic fraction of municipal waste) to anaerobic digesters to improve the energy balance of a WWTP substantially, resulting in “1 þ 1>2” in terms of biogas production and solids reduction (Aichinger et al.). The results reveal that organic co-substrate addition of up to 94% of the organic sludge load resulted in tripling the biogas production and that at an organic co-substrate addition DOI of original article: http://dx.doi.org/10.1016/j.watres.2015.11.043. * Corresponding author. E-mail address: haoxiaodi@bucea.edu.cn (X. Hao). Contents lists available at ScienceDirect Water Research journal homepage: www.elsevier.com/locate/watres http://dx.doi.org/10.1016/j.watres.2016.01.030 0043-1354/© 2016 Published by Elsevier Ltd. Water Research 92 (2016) 284e286