Thermophilic anaerobic digestion of food waste

Abstract

There is a requirement in the European Union to divert organic wastes from landfill because of the risk of methane emissions. One alternative is anaerobic digestion of organic wastes, such as food waste, to stabilise them whilst at the same time recovering the energy from them. One problem with this approach is that the high nitrogen content of food waste may lead to ammonia inhibition. A solution to this has been found for mesophilic digestion but had not been attempted in thermophilic digestion where ammonia toxicity is known to be more acute.
The work was carried out in laboratory-scale semi-continuous digesters operated over long time periods to provide maximum opportunity for acclimatisation, and in duplicate to give an indication of reproducibility. A series of experimental runs were undertaken at thermophilic temperatures to assess the influence of trace element (TE) addition on the digestion process. These were carried out at organic loading rates (OLR) of 2, 3 and 4 g volatile solids (VS) l-1 day-1 against unsupplemented controls at OLR 2 g VS l-1 day-1. Although TE addition could offset the accumulation of VFA which occurred in response to an increasing concentration of total ammonical nitrogen (TAN), it could not prevent this. The high alkalinity resulting from ammonia, however, allowed the digesters to continue to produce methane until VFA had accumulated to high concentrations before eventual failure due to a rapid drop in pH.
To determine the threshold inhibitory ammonia concentration in thermophilic digestion, one pair of digesters was run on synthetic low nitrogen food waste (low-N food waste) at an OLR 2 g VS l-1 day-1 and compared to a control pair running on domestic food waste at the same loading. All four digesters received TE supplementation. The digesters fed with low-N food waste showed consistently stable performance with pH ~7.4, IA/PA ratio ~0.4-0.5, SMP 0.39 l CH4 g-1 VS, 52-55% CH4, total VFA <500 mg l-1 and 88% VS destruction whereas the controls showed signs of failure after 112 days and irreversible VFA accumulation at a TAN concentration >3.5 g N l-1. One of the low N digesters was supplemented with urea slowly and one by a shock dose: both showed signs of VFA accumulation at TAN >2.5 gl-1 and, again, an irreversible trend in propionic acid build-up when TAN >4 g N l-1. Long term operation showed meta-stable conditions when the digesters were operated at TAN between 2.5 - 3.5g l-1 with oscillations in VFA (especially propionic acid) concentration.
Mesophilic digestion at 37oC with TE addition showed very stable performance with pH ~8, IA/PA ? 0.3, SMP ~0.48 l CH4 g-1 VS, 55-60% CH4, total VFA < 300 mg l-1 and VS destruction ~75-78% with a final total ammonia nitrogen (TAN) concentration of 4.5 g N l-1. As the temperature in digesters was raised from 35 to 43 oC in 1oC steps a change in performance was noted when the temperature reached 40 oC. Above this temperature VFA concentrations rose above 4000 mg l-1 and biogas and methane production fluctuated. It is probable that the higher temperature increased the concentration of free ammonia nitrogen (FAN) to ~800 g N l-1 at the measured TAN concentration ~5.5-6.0 g N l-1 and this was sufficient be inhibitory even with TE dosing.
Fluorescent in situ hybridisation (FISH) was used to identify the methanogenic populations in some of the trials over selected time periods. This showed changes in population structure both in relation to temperature (mesophilic or thermophilic) and also in response to increasing concentrations of TAN. At high TAN concentrations Methanomicrobiales was the dominant methanogenic group and under mesophilic conditions this proved to be extremely ammonia tolerant. A 14C radio-labelling assay confirmed the dominant pathway to methane formation was by the hydrogenotrophic route which reflected the known metabolic pathway of this methanogen.
It was concluded that thermophilic digestion of source segregated domestic food waste would lead to instability and failure of the process unless measures were introduced to reduce the TAN concentration to < 3.5 g N l-1, and preferably to < 2.5 g N l-1.
Keywords: Anaerobic digestion, food waste, biogas, VFA accumulation, ammonia inhibition, Fluorescent in situ Hybridisation (FISH)

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ORCID for C.J. Banks: orcid.org/0000-0001-6795-814X

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