dry anaerobic digestion of municipal solid waste and digestate ...

amount of anaerobic fermentation residue. It has also been found in another study that low
application rate of digestates with a big time gap between application and planting
enhances their benefits as soil amendment as confirmed in lab by increased seed
germination with dilution of digestate and incubation (Abdullahi et al., 2008).
One of the major negative effects of digestate on plants is its toxicity. Digestates have been
found to be more phytotoxic than composts if applied directly on soil without any
treatment. However, phytotoxic effects can be mitigated by post-treatment (composting) of
digestate (Fuchs et al., 2008; Abdullahi et al., 2008).
2.12 Research Needs for the Dissertation
From this review, it can be noted that ammonia toxicity, VFA accumulation and
incomplete mixing are among the major problems of dry anaerobic digestion. In dry
anaerobic digestion, ammonia inhibition occurs at a lower TAN concentration as compared
to wet digestion. A high C/N ratio feed (having low N) causes a relatively slow microbial
growth and low biodegradation rate due to deficiency of nitrogen (and consequently less
production and accumulation of ammonia and VFA) and hence may alleviate the problem
of both ammonia and VFA accumulation in dry digestion. This can be done by increasing
or adding the fraction of low N (or high C/N ratio) waste. However, there is a maximum
limit of feed C/N ratio (i.e. 30) for the digestion process beyond which digestion is not
feasible. Thus the effect of feed C/N ratio higher than its maximum established limit (i.e.
30) on the performance of dry anaerobic digestion should be investigated in an attempt to
reduce the accumulation of ammonia.
Moreover, most of the previously performed research studies on dry anaerobic digestion
use lab-scale reactor (Table 2.5) with synthetic and well -homogenized feed having the
particle size of around 10 mm. Thus there is a need of research on full-scale or pilot-scale
dry anaerobic digesters operating at closer to the field conditions and optimizing their
continuous operation with practicable organic loading rate.
Furthermore, literature also shows the digested organic waste (digestate) still has certain
residual GHG emission potential, which depends on retention time and loading rate of
previous digestion. Also the stored digestate tends to emit methane to the atmosphere (if
not stored properly) and hence can contribute to the climate change. On the other hand, it
has certain amount of nutrients and organic matter, which could be useful if applied on
agricultural soils. Therefore, it is necessary to carefully analyze various digestate
management options based on their net GHG emission reductions under various scenarios,
so that the best scenario can be selected for digestate management and utilize its economic
value.
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