Anaerobic digestion is an efficient way of using a wet fraction of municipal solid waste (MSW) for energy purposes as it can produce biogas. The moisture content of the digestate after application of mechanical dewatering is still high, and the amount of heat needed for drying is significant. Hydrothermal carbonization (HTC) is a process that can potentially offer great benefits by improved mechanical dewatering and valorisation of the digestate into a better-quality solid fuel. However, such valorisation produces liquid by-product effluent, rich in organic compounds. Membrane separation could be used to treat such effluent and densify concentration of the organic compounds, while at the same time to facilitate recovery of clean water in the permeate. This work presents the results of investigation, performed using polymeric nanofiltration (NF) membranes. The study showed that membrane separation keeps significant fraction of inorganics in the retentate. Such concentration significantly increases the biomethane potential (BMP) of such effluent as well as the energy that could be theoretically recovered in microbial fuel cell, using the concentrated retentate.