In the most organized countries, anaerobic digestion is nowadays the most common way to treat sludge and/or livestock manure, producing energy from renewable sources. Anaerobic digestion process determines transformations of nitrogen compounds but the total nitrogen content of digestate and, hence, nitrogen load results unvaried. Where surplus nitrogen is a problem (undersized wastewater treatment plants, nitrate pollution), dedicated solutions must be implemented. This paper presents a critical analysis of anaerobic digester supernatant treatments. The main treatments are discussed under the main items: (a) biological treatments, (b) physical, chemical and thermal process. Performances of the treatment process are given, discussing advantages and drawbacks.