Elsevier

Chemical Engineering Journal

Volume 178, 15 December 2011, Pages 138-145
Chemical Engineering Journal

Ammonia removal in anaerobic digestion by biogas stripping: An evaluation of process alternatives using a first order rate model based on experimental findings

https://doi.org/10.1016/j.cej.2011.10.027Get rights and content

Abstract

The feasibility of biogas stripping to remove ammonia in the anaerobic digestion of source segregated food waste was investigated. It was found in batch experiments that ammonia could be removed from digestate and that the removal followed 1st order kinetics with respect to total ammonia nitrogen concentration. Increasing temperature, biogas flow rate and initial pH all increased removal rates. Using kinetic data gathered in these experiments allowed the integration of ammonia stripping with an anaerobic digestion plant to be modelled for different configurations. Four scenarios were identified: post digestion, in situ, side-stream and pre-digestion ammonia removal relating to where in the process the ammonia stripping was performed. The modelling showed that in situ ammonia removal may be best able to reduce in-digester ammonia concentrations over a wide range of organic loading rates whereas pre-digestion showed most promise in terms of application due to the flexibility to control each part of the process separately. Further experimental work is required into these scenarios to confirm their viability.

Highlights

► The feasibility of ammonia stripping from digestate using biogas was demonstrated. ► Ammonia removal followed 1st order kinetics. ► Modelling of the integration of ammonia stripping and anaerobic digestion. ► Four integration scenarios analysed. ► Ammonia stripping can manage ammonia concentrations in anaerobic digestion.

Introduction

Food waste (FW) is a major waste stream and its disposal can be problematic due to its high moisture content, bulk density and biodegradability [1]. Furthermore it can contain pathogens, is subject to animal by-products regulations (ABPR) (EC 1069/2009) and in Europe its disposal to landfill is being phased out under the EU Landfill Directive (99/31/EC). When collected as a mixture with municipal and commercial wastes it can complicate or prevent recovery of other potentially valuable materials through contamination by contact with the FW. There is therefore increasing interest in the separate collection and management of source segregated food waste (SSFW) with an emphasis on further recovery of value from this material. The high moisture content and calorific value of FW make suitable for anaerobic digestion (AD) which produces biogas, a biofuel, and the nutrient content makes the digestate suitable to apply to agricultural land as a fertiliser [2].

Despite the advantages of anaerobic digestion (AD) as a treatment technology for SSFW there are potential operational problems due to the high proportion of proteinaceous material in the raw waste [3]. The anaerobic degradation of proteins leads to the release of ammonia which is a necessary nutrient for many organisms in the digester, but in elevated concentrations can be inhibitory, especially in its non-ionised form free ammonia (NH3) [4]. Most evidence suggests that the acetoclastic methanogens which degrade acetate and produce methane and carbon dioxide are more sensitive to free ammonia than hydrogenotrophic methanogens [5], [6], with inhibitory concentrations of total ammonia nitrogen (TAN) reported in the range of 0.76–4 g N l−1 [3], [4], [7], [8], [9].

Digesters fed on SSFW have been shown to exhibit symptoms of ammonia toxicity, but these problems may take a long time to cause process failure. The elevated ammonia/ammonium concentration provides buffering capacity that allows digester operation at higher concentrations of volatile fatty acids (VFA) than would be possible in a less buffered system [10], [11], [12]; it is only when this buffering capacity is broken by acid accumulation that the pH drops to a point where conditions are unfavourable for methanogenesis. This behaviour has also been seen in the digestion of other nitrogen-rich materials such as cattle slurry [13] and slaughterhouse waste [14]. It would be advantageous to be able to manage the ammonia concentrations in AD such that the problems of its accumulation are avoided, in turn preventing the build-up of VFA. Furthermore, the ability to manage the ammonia content of a digestate will allow higher rates of application in nitrogen sensitive zones under the EU Nitrates directive (91/676/EEC), and balancing of the overall nutrient composition.

Ammonia removal from wastewater and animal slurry has been well studied. Methods used include struvite precipitation [15]; biological ammonia oxidation and denitrification [16]; electrochemical conversion [17]; microwave [18] and ultrasound treatment [19]; and air stripping [20]. The latter is a proven concept for the removal and recovery of ammonia from slurries and wastes: air is blown through the liquid and the free ammonia transfers into the gas phase and can then be captured by absorption downstream. It has been found that the most rapid removal occurs at high temperature, high gas flow rate and high pH [20], [21], [22]. The importance of these parameters can be explained using pH equilibrium and phase change theories: pH modifies the ammonia removal behaviour on a chemical basis, by shifting the equilibrium between free ammonia, the volatile form, and ammonium salts which are non-volatile. Temperature alters the same equilibrium slightly, with an increase causing a greater fraction of free ammonia to be present in the digestate; and also has a physical effect in that it increases the saturated vapour pressure of the free ammonia, thus increasing the driving force which allows volatilisation into the gaseous form. Flow rate of gas has no chemical effect on the balance of free/ionic ammonia but instead changes the available surface interface between the liquid and gaseous phases within the stripping system, such that an increase in flow rate leads to an increase in reaction rate (in this case volatilisation and removal of ammonia).

The aim of this work was to investigate the possibility of in situ ammonia removal in the anaerobic digestion process itself, as it is during this stage that ammonia is released through hydrolysis and also the point where accumulation occurs leading to process inhibition. The most promising method for direct integration with an anaerobic digestion plant was thought to be gas stripping using biogas as the stripping gas. This has the potential advantage of removing ammonia whilst maintaining an anaerobic environment and keeping the carbon dioxide in equilibrium with the dissolved carbon in the digestate, thus avoiding pH change as demonstrated by De la Rubia et al. [23].

The paper presents data from ammonia stripping experiments using digestate from an anaerobic digester treating source segregated food waste [11]. The kinetic data was then used to model possible methods of integrating this ammonia management technology into an anaerobic digestion plant. The experiments and modelling in this work were carried out at three temperatures commonly associated with anaerobic digestion; 35 °C (mesophilic), 55 °C (thermophilic) and 70 °C (pasteurisation) with pH modified only when this could be feasibly done during the digestion process.

Section snippets

Batch removal of ammonia using biogas stripping from food waste digestate

Two digestates were used, both taken from an anaerobic digester treating source segregated food waste (Biocycle, Ludlow, UK). The first sample had a TAN concentration of approximately 8 g N l−1 in which the pH changed from 8.5 to 9.3 on storage between stripping experiments. The second sample had a TAN concentration of approximately 6 g N l−1 and a pH of 8.1–8.2.

Ammonia removal was carried out in a water jacketed heated glass column of total volume 3 l connected to a closed loop biogas recirculation

Batch removal of ammonia using biogas stripping from food waste digestate

The results of the ammonia stripping experiments are shown in Table 1. For each experimental run an ammonia removal time constant was calculated for using the measurements of TAN and fitting an exponential decay curve (i.e. 1st order kinetics): these are tabulated along with the respective correlation coefficients (r2). An example of this process is shown in Fig. 3 for experimental run 3.1, where the time coefficient of the equation is 0.031 and the reciprocal of this gives the ammonia removal

Conclusion

In all experimental runs it was possible to reduce the TAN concentration in the digestate by biogas stripping, which suggests that this technology may be viable as a tool for ammonia management in AD to relieve toxicity and manage digestate ammonia concentration. The kinetic of the TAN removal process was shown to approximate 1st order decay over a wide range of conditions of temperature and gas flow rate. At 35 and 55 °C the removal time constants were in the order of 600 h, whereas at 70 °C this

Acknowledgements

The authors wish to thank the UK Government Department for Environment, Food and Rural Affairs (Defra) and the European Union 7th Framework programme for the financial support to carry out this work through grant numbers WR1208 and 241334 (VALORGAS), respectively

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