Water and nutrients recovering from livestock manure by membrane processes

Anaerobic digestion (AD) in a biogas plant is a well proven process in which livestock effluents break down naturally in the absence of oxygen to produce two valuable products: biogas and digestate. Three successive steps of membrane separation (UF, NF, RO) were carried out on digestate liquid fractions in order to recover molecules of interest and fresh water. Results demonstrate the capacity of a 300 kD filtration cut‐off to clarify and disinfect anaerobic digestion liquid phase, no suspended solids were detected in the permeate while more than 80 % of organic matters were removed. This clarification step allowed the use of downstream desalinating steps (NF, RO and NF/RO) which contribute to the achievement of fresh water production and ionic compounds of interest (phosphates, ammonium, potassium, magnesium…) concentration. NF and RO step allowed a total discoloration of treated water with a high removal of dissolved organic matter DOM (more than 90 %). The ion retention depends on the considered ion and the origin and composition of UF digestate. Nevertheless, a direct use of High Pressure Reverse Osmosis HPRO downstream UF step allowed an important retention of ions whatever the origin of digestate. The addition of RO downstream the NF step (NF/RO), increased the total salt rejection opening the door to a water reuse on site. The fresh water quality and the RO flux were higher for UF/NF/RO than UF/RO proving the interest for such a UF/NF/RO coupling.