Treatment costs of ammonia recovery from biogas digestate by air stripping analyzed by process simulation

Ammonia can be recovered from liquid biogas digestate as ammonium sulfate by air stripping. A full-scale plant treating 30,000 kg h−1 of digestate was simulated in Aspen Plus.The classical sequence stripper–absorber was modified by introducing a flash drum to reduce the buffer capacity. The stripped digestate was mixed with NaOH up to a pH value of 9 and fed to the stripping column using air at 90 °C. The air flow rate was optimized considering the possible recycle from the absorption column. The complete process was modeled using the process simulator Aspen Plus considering a recovery of ammonia equal to 95%. The total capital investment was evaluated by means of Aspen Plus Process Economic Analyzer in about 5.5 million US$, and the costs of energy and chemicals were quantified in less than 1 million US$ year−1. A benefit of 1.6 US$ for m3 of digestate was obtained considering an ammonium sulfate price of 522 US$ ton−1.