The Paris Agreement concerning the global climate change implies that the operational era of wastewater treatment plants (WWTPs) towards carbon neutrality is coming. The narrow definition of carbon neutrality for WWTPs is regarded as energy self-sufficiency,which requires converting organic substance /excess sludge to renewable energy:methane (CH_4). However,the low energy conversion efficiency of excess sludge has been being a bottleneck of anaerobic digestion. Beyond pretreatment for excess sludge,adding waste iron scrap (WIS) to anaerobic digestion system is expected to become another breakthrough for enhancing the energy conversion efficiency,so that the purpose of"waste enhancing energy and waste becoming resource"could be realized. In this article,hydrogen evolution from iron corrosion and its effects on CH_4 production is firstly reviewed; then the roles of iron in reducing ORP,affecting biochemical /physiological characteristics of microorganisms and their enzyme activities are elaborated; finally,the environmental and economic issues of WIS-based anaerobic digestion are evaluated using the method of life cycle assessment (LCA).