Nutrient recovery from anaerobic sludge by membrane filtration: pilottests at a 2.5 MWebiogas plant

Abstract

Purpose Membrane filtration is recently applied to recover nutrients and dischargeable water from anaerobic sludge. The
purpose of this study is to quantify nutrient separation, membrane performance, and process stability and to increase the
economical applicability of the process by energetic optimisation.
Methods At the site of a 2.5 MWe
agricultural biogas plant, a membrane pilot plant was operated over a period of 7 months.
It consisted of a screw press separator, a decanter centrifuge, an ultrafiltration unit, and a three-stage reverse osmosis unit.
Mass and nutrient balances were generated by sampling and analysing every process stream. Process performance was analysed
by monitoring separation efficiencies, membrane flux, cleaning intervals, and energy demand.
Results Solid/liquid separation resulted in separation efficiencies of 70% for total solids and 80% for phosphorus. The solid
fraction contained high concentrations of organics and particle-ligated nutrients (20% TS, 8 kg t−1 Ntotal,
5.5 kg t−1 P2O5). The retentate of the reverse osmosis had high concentrations of dissolved ammonia and potassium (4 kg t−1 NH4–
N and 10 kg t−1 K2O). 38% of the sludge volume was recovered as clean water.
Conclusion The membrane pilot plant successfully produced a solid N/P-fertiliser, a liquid N/K-fertiliser and clean water.
The results contribute to a sound understanding and growing database for future adaption of the process chain. Hydrodynamic
optimisation within the pilot plant reduced the energy demand of the ultrafiltration step by 50%, which considerably
contributes to the economy of the process.