The effects of thermal pretreatment (90, 120, 140 and 160 degrees C) on the morphology (organic and inorganic nitrogen) and distribution properties (in solid phase, liquid phase and gas phase) of nitrogen in kitchen waste (KW) and on anaerobic digestion performance were investigated. The results show that thermal pretreatment could efficiently enhance the solubilisation of organic nitrogen compounds in KW, especially at high temperatures and long heating durations. Approximately 3.0-47.9% of organic nitrogen in KW decreases in total nitrogen content was obtained in the solid phase after thermal pretreatment. Higher biogas production and biodegradability of organics (in terms of the removal rate of soluble chemical oxygen demand, total organic nitrogen, and volatile solids) during subsequent anaerobic digestion were observed compared with the levels for untreated KW. An overall economic analysis indicates that the most profitable pretreatment process was achieved at 90 and 120 degrees C for treatment time of 30 and 15 min respectively, with a net potential profit (2-8 (sic) ton(-1) kW). (C) 2015 Elsevier Ltd. All rights reserved.