Biogas plants designed for the further processing of organic waste, liquid manure or renewable raw materials produce digestate that needs to be disposed of. If this waste is not used for agricultural purposes, it is usually separated into solid and liquid phases by means of screw presses or centrifuges (decanters). The resulting material consists of solids (containing high amounts of phosphate) and liquid substrate containing high amounts of nitrogen.
Where possible, the liquid phase is reused as process water. Surplus material can be used as nutritious fertiliser that provides reduced odour and improved plant availability. If there are no appropriate agricultural areas in the vicinity of the biogas plant, major expenses for storage and transport to remote sewage farms may occur.
For the discharge of liquid digestates into water bodies or public sewage systems, legal limits apply. These values can only be achieved by implementing certain technical processes. In practical applications, membrane technologies have proven to be particularly suitable for this process. By using process combinations such as ultrafiltration (UF) and reverse osmosis (RO), the liquid digestate is separated into two material flows at a ratio of 2:1. The separation is done using approx. 2/3 permeate (purified water) on one side and approx. 1/3 concentrate on the other side. The permeate can be discharged and the concentrate can be used as a high-quality liquid fertiliser.