Optimization of the experimental design of a pilot plant for ammonia vacuum degasification for ammonium sulfate production

Vacuum degasification (VD) of ammonia from waste streams and subsequent production of nitrogen fertilizer can be one element of the effort towards closing the nitrogen (N) cycle and thus avoiding emissions harmful to the environment. This master’s thesis comprises a literature research for suitable substrates and laboratory experiments for the optimization of a design and experimental design of a VD pilot plant for ammonia recovery. Eight streams among the top 20 food waste streams in Europe and their associated streams, all digestates from digestion or co-digestion of animal waste streams, agricultural digestates and manures were identified as suitable substrates for N-recovery with VD. During 120 min of VD at pH 9.0, 190 mbar and 60 °C a total ammonia-N (TAN) removal rate of 0.81 ± 0.03 was achieved with an NaOH load of about 80 ml (L substrate)-1 in biogas digestate. The TAN removal rate during VD at 190 mbar and 60°C was dependent on pH with a dose response function. For efficient ammonia removal pH = 9.0 was necessary. Evidence for an ammonia volatilization inhibition at pH = 8.5 not explicable by the ammonia dissociation was found. At 158 % of the boiling pressure, 69 % of the TAN removal rate at boiling pressure was reached. Air stripping the hot substrate for 60 min lowered the NaOH load for maintaining pH 9.0 during VD by 30 %. Electrical conductivity (EC) and pH during the degasification treatment did not correlate. Alkaline hydrolysis could be the reason for pH decrease during VD at pH 9.5. In the pilot plant a pH sensor and a possibility to adjust the pH continuously should be installed. The pH for VD experiments at 60°C should be in the range 8.75–9.5. Pressures below the boiling pressure should not be excluded. Experiments with CO2 stripping should be conducted to exhaust the potential for NaOH saving.

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