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INTRODUCTION
Anaerobic digestion of protein-rich organic wastes faces a process instability issue at high organic loading rates due to accumulation of ammonia and subsequent inhibition of methanogens (Rajagopal et al. 2013; Yenigun & Demirel 2013; Cook et al. 2017; Amha et al. 2018). Recently, a process of vacuum thermal stripping coupled with acid absorption was invented to recover ammonia from digestate and ammonia-rich wastewater (Ukwuani & Tao 2016). It appears to be the most efficient ammonia recovery process, stripping ≥93% of ammonia out of dairy manure digestate and swine manure in 1–3 h (Ukwuani & Tao 2016; Zhang et al. 2017). Ammonia mass transfer through the liquid-gas interface is most efficient in a boiling feed (Tao & Ukwuani 2015). Applying vacuum to a stripping vessel reduces the boiling temperature, thus reducing energy consumption for heating. Moreover, it overcomes several technical challenges to the ammonia recovery processes (Ukwuani & Tao 2016). For example, it allows feed with high concentrations of solids; the digestate remains anaerobic after vacuum thermal stripping and is safe to be recirculated to the digesters. After vacuum thermal stripping, solids solubilization is enhanced by the low-temperature thermal and alkaline treatment (Kinnunen & Rintala 2016; Lv et al. 2017; Nazari et al. 2017). When the ammonia recovery process is installed in a recirculation line of an anaerobic digester, the concentration of ammonia in the digestate is reduced and consequently the digester can be loaded at a higher rate while maintaining stable operation as demonstrated by Tao et al. (2017) through in-digester ammonia removal. Meanwhile, recirculation of stripped digestate to the digester transforms the energy-intensive thermal stripping process into a net energy neutral technology because of heat recycle (Anwar & Tao 2016).
When applying vacuum to an enclosed stripper with heated digestate or wastewater, the feed boils at a temperature below the normal boiling point of water. The optimum boiling point for stripping ammonia has been determined to be 65 °C and 25–28 kPa (Ukwuani & Tao 2016). Vapor bubbles form in the feed when vapor pressure overcomes the surrounding pressure, triggering water vaporization and ammonia mass transfer from feed to vapor bubbles and finally out of the feed. The vacuum is maintained by a vacuum pump connected to a gas absorption column holding a...