Membrane Technology
Brochure Digestate Dewatering by Membranes Ver 6
Digestate can be efficiently dewatered using specialised membrane technology. However, digestate is high in solids, typically 10% and this requires the careful selection of membranes. For biogas operators who have no waste heat from a CHP, membrane technology offers a solution to concentrate the digestate and recover the nutrients.
The concentrated digestate can reach 26% of the original volume and contains all of the nutrients and solids that were in the raw feed. The ammonia is as Ammonium NH4+ and will not off-gas to the atmosphere during storage.
The process flow diagram below, Drg PS1107, shows steps required to process and concentrate digestate or livestock slurry achieving a 74% reduction in volume. The first Reverse Osmosis membrane is a Low Fouling type specially designed for high solids liquids. It uses an induced vortex on the membrane surface to physically lift the solids from the membrane allowing the permeate to flow. The permeate is solids free and can be further processed in conventional spiral wound Reverse Osmosis membranes to polish the water and remove the ammonia to very low levels.
The output products are:
i) Clean Water for discharge to land or water course subject to consent.
ii) Concentrated liquid, 26% of the original press water volume from the screw press separator containing all of the solids, biomass and nutrients with the Ammonia as Ammonium NH4+. For storage, land spreading or drying and pelletising of CHP heat is available.
iii) Separated, stackable solids from the screw press separator pre-treatment for storage and land spreading
If heat is available from a CHP, then further drying and pelletising can be achieved.
It maybe beneficial to partially return the concentrate in ii) above to the digester to increase the biomass concentration which in turn will allow for an increased substrate feed rate and an increases biogas yield. Careful control of the ammonia will be needed.
Membrane Processing System for Digestate showing Outputs
The outputs of separated solids from the screw press, the concentrate & permeate from the membrane system vary according to the solids loading of the raw feed. The above drawing PS 1137 gives estimates for these outputs for a range of raw feeds at different solids levels.