Advanced modelling to determine free ammonia concentrations during (hyper-)thermophilic anaerobic digestion in high strength wastewaters

Marinus J Moerland,Harry Bruning,Cees J.N Buisman,Miriam H.A Van Eekert

doi:10.1016/j.jece.2021.106724

Abstract

Anaerobic digestion is an attractive treatment technology for concentrated waste streams. However, high ammonia concentrations cause inhibition of methanogenesis, especially when operated at elevated temperatures like (hyper-)thermophilic (55 and 70 °C) anaerobic digestion. These emerging (hyper-)thermophilic technologies are beneficial due to high conversion rates and pathogen removal, but are more susceptible for ammonia toxicity as consequence of a temperature-induced pKa shift. Determination of NH3-N (free ammonia nitrogen (FAN); toxic form) concentrations is conventionally based on an equilibrium model and the total ammonia nitrogen concentration (TAN). However, the conventional equilibrium model overestimates the FAN concentration and therefore we developed an Ionic Activity Model which takes the ionic strength and organic matter interactions into account. The difference between the two models could mainly be attributed to the high ionic strength of the waste stream, whereas interactions with organic matter had a smaller effect. Based on this Ionic Activity Model and batch experiments at hyper-thermophilic conditions, we found that acetoclastic methanogenesis was completely inhibited at FAN concentrations exceeding 588 mg/L, whereas hydrogenotrophic methanogenesis could produce methane up to 925 mg/L. During thermophilic and hyper-thermophilic black water treatment, the ionic strength and organic matter interactions resulted in NH3 concentrations below the inhibitory threshold.

Highlights

Domestic wastewater is an important source of nutrients, of which toilet water holds the main fraction
The BW is collected in a concentrated manner through source separation with vacuum toilets which results in a nitrogen-rich stream containing high concentrations of total ammonia nitrogen (TAN) and free NH3-N (Free ammonia nitrogen; free ammonia concentration (FAN)), whose toxicity could compromise thethermophilic anaerobic digestion process performance [33]
The Equilibrium Model overestimates the NH3 concentration compared to the Ionic Activity Model, as was already demonstrated by Hafner and Bisogni [14] and Capson-Tojo et al [7]

Summary

Introduction

Domestic wastewater is an important source of nutrients, of which toilet water (black water; BW) holds the main fraction. Separation at the source is a novel approach in sanitation which is preferred for efficient energy and nutrient recovery from domestic wastewater [5]. Anaerobic treatment of concentrated BW at thermophilic conditions already showed successful pathogen elimina tion [22] along with 70% CODtotal removal and 62% methanisation based on CODtotal load during treatment in an UASB at a retention time of 8.7 days and organic loading rate of 3 kgCOD/m3/day [23]. The BW is collected in a concentrated manner through source separation with vacuum toilets which results in a nitrogen-rich stream containing high concentrations of total ammonia nitrogen (TAN) and free NH3-N (Free ammonia nitrogen; FAN), whose toxicity could compromise the (hyper-)thermophilic anaerobic digestion process performance [33]. The effect of ionic strength (total wur.nl (M.H.A. van Eekert)

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