Characterising and modelling free ammonia and ammonium inhibition in anaerobic systems

Abstract

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH) and ammonium ion (NH), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH and NH inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NHHCO and NHCl without pH adjustment, rather than by using NHCl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R > 0.99) the joint inhibition of NH and NH on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KI) and upper limit (concentration where inhibition is complete; KI). The threshold inhibition function also identifies the 50% inhibition concentration (KI) at the midpoint of KI and KI. Finally, experimental and model results show that at pH 7.3-7.7 and TAN concentrations above 2000 mgN·L, both NH and NH contribute significantly to overall inhibition.

Modified anaerobic digestion model No.1 (ADM1) for modeling anaerobic digestion process at different ammonium concentrations

“…The most widely accepted mechanism explaining ammonia inhibition claims that elevated ammonia levels result in the change in intracellular pH, increase in maintenance energy requirement, depletion of intracellular potassium, and inhibition of specific enzyme reactions (Wittmann, Zeng, & Deckwer, 1996). Inhibiting ammonia concentrations ranging from 1.7 to 14 g/L, which causes obvious reduction in MP, are discovered by different researchers due to the differences in inocula, temperature, organic loading, and other factors (Astals, Peces, Batstone, Jensen, & Tait, 2018;Polizzi, Alatriste-Mondragón, & Munz, 2018). An excessive ammonia concentration can inhibit the acetogenesis and methanogenesis resulting in the accumulation of volatile fatty acids (VFA), mainly acetic acid and propionic acid (Fotidis, Karakashev, & Angelidaki, 2013;Rajagopal, Massé, & Singh, 2013;Yenigün & Demirel, 2013).…”

Ammonia Inhibition of Anaerobic Volatile Fatty Acid Degrading Microbial Communities

“…Furthermore, the shift toward more syntrophic acetate oxidation (SAO) instead of acetoclastic methanogenesis at elevated ammonia concentrations has received much attention (Schnürer and Nordberg, 2008 ; Werner et al, 2014 ; Luo et al, 2016 ). Commonly, free ammonia is thought responsible for ammonia inhibition because it can diffuse into the cells (Rajagopal et al, 2013 ), but also the ammonia ion is thought to cause inhibition (Astals et al, 2018 ). The concentration of free ammonia depends on total ammonia nitrogen (TAN) concentration, pH, and temperature.…”

Approaches to the mitigation of ammonia inhibition during anaerobic digestion – a review

“…The effect of pH and temperature increase does not only retard the growth of methanogens but also affects the production rate of biogas during anaerobic digestion (Xie et al 2015). For example, at pH range of 7.3-7.7 and TAN concentration of 2,000 mgNL À1 , it was observed that both NH 3 and NH 4 þ induce the inhibition process largely as per both experimental and model results (Astals et al 2018). Generally, during the thermophilic condition in which the operating temperature is above 40°C, there is an accumulation of fatty acids, which inhibits the growth of methanogens resulting into low biogas production (Jena et al 2017).…”

“…Alkaline pH range between 7.0-7.4.-Mesophilic temperature (35°C À 45°C) Astals et al (2018)Wang et al (2019);Zinatizadeh & Mirghorayshi (2019);Mpofu et al (2020) C/N ratio adjustment -C/N ratio ranging between 15/1 and 30/1 is convenient for reducing ammonia inhibition.Xu et al (2016); Li et al (2019) Air stripping -Temperature above 45°C for volatilisation of ammonia.…”