Anaerobic sludges taken from 16 different biogas plants were analyzed with respect to their rheological characteristics. All sludges showed temperature‐dependent shear‐thinning behavior with viscosities from 900 – 6000 mPa s at 20 °C. Nevertheless, the liquid fraction of the anaerobic sludges also revealed temperature‐dependent, shear‐thinning behavior with viscosities well above water viscosity (2 – 40 mPa s at 20 °C). The rheological behavior of the liquid phase could be linked to organic fractions, i.e., proteins and polysaccharides. Shear‐thinning and temperature‐dependent behavior was modeled by the power‐law equation and the Arrhenius law, respectively.
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 MW e 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 N total , 5.5 kg t −1 P 2 O 5). The retentate of the reverse osmosis had high concentrations of dissolved ammonia and potassium (4 kg t −1 NH 4-N and 10 kg t −1 K 2 O). 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.
Membrane filtration of biological suspensions is frequently limited by fouling. This mechanism is well understood for ultrafiltration of activated sludge in membrane bioreactors. A rather young application of ultrafiltration is the recovery of nutrients from anaerobic digestates, e.g., from agricultural biogas plants. A process chain of solid/liquid separation, ultrafiltration, and reverse osmoses separates the digestate into different products: an organic N-P-fertilizer (solid digestate), a recirculate (UF retentate), a liquid N-K-fertilizer (RO retentate) and water. Despite the preceding particle removal, high crossflow velocities are required in the ultrafiltration step to overcome fouling. This leads to high operation costs of the ultrafiltration step and often limits the economical application of the complete process chain. In this study, under-stoichiometric ozone treatment of the ultrafiltration feed stream is investigated. Ozone treatment reduced the biopolymer concentration and apparent viscosity of different digestate centrates. Permeabilities of centrate treated with ozone were higher than without ozone treatment. In a laboratory test rig and in a pilot plant operated at the site of two full scale biogas plants, ultrafiltration flux could be improved by 50–80% by ozonation. Nutrient concentrations in the fertilizer products were not affected by ozone treatment.
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