Ammonia inhibition can be a major problem during anaerobic digestion of manure and municipal and industrial sludges. Anaerobic digestion is sensitive to higher NH3-N concentrations, which are inhibiting and at certain levels even toxic for methanogen bacteria. During anaerobic digestion, COD and BOD are converted for 60–80% into biogas. Nitrogen will only slightly be removed due to cell synthesis and due to hydrolysis most of the Kjehldahl nitrogen is converted to ammonium. Because of the reduction of COD in anaerobic digestion the effluent of the anaerobic treatment often results in a COD/N ratio which is too low for further aerobic nitrogen removal. The Nijhuis Ammonium Recovery system (NAR) can solve this problem and removes ammonium from digestate or other substrate for digestion. The NAR is a chemical process based on stripping of ammonia. The ammonium is recovered as ammonium sulphate, which can be used as a fertilizer for agricultural purposes. The NAR was proven to be a robust technology with a stable ammonium removal efficiency of 85–90% for anaerobic digested manure, municipal and other organic waste waters.

The cost effectiveness mainly depends on the concentration of NH3-N in the influent, scale of the installation, the availability of residual heat and the local value and market for ammonium sulphate. Above 2 g/l NH3-N the NAR system is competitive and at higher NH3-N concentrations more cost efficient compared to other state of the art nitrogen removal technologies and ranges between 1–3 €/kg N.

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