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

Chapter 2
Literature Review
Anaerobic digestion is considered as an alternative option to manage and treat the organic
fraction of municipal solid waste (OFMSW). This process not only treats the organic waste
but also produces clean energy (biogas). The digestion residues (digestate) obtained from
the process can be used as soil amendment or even nutrient rich organic fertilizer
depending on its final quality. Based on the solid content of waste used in the process,
anaerobic digestion is of two types which are dry and wet anaerobic digestion. Dry
anaerobic digestion has got much attention due to its advantages of smaller reactor volume
requirement (higher organic loading rate), lesser water addition and lesser pretreatment
needed with higher volumetric biogas production rate as compared to wet digestion.
Moreover, high solid content of the digestate makes it simpler and easier to handle as
compared to liquid digestate of wet digestion that adds dewatering cost as well. Due to low
water content and small reactor volume, energy requirement for heating is less for dry
digester.
In this chapter, the process and problems of dry anaerobic digestion have been discussed.
Moreover, optimization of factors affecting dry anaerobic digestion has been discussed and
based on this, solutions of some problems in dry anaerobic digestion have been analyzed as
well as the developments in the process have been reviewed. Furthermore, characteristics
of digestate have been presented and the present management strategies for digestate have
been reviewed.
2.1 Introduction of Dry Anaerobic Digestion
In dry anaerobic digestion (high -solids digestion), the feedstock to be digested has total
solids (TS) content more than 15 %. In contrast, wet anaerobic digestion (low-solids
digestion) deals with diluted feedstock having TS content less than 15% (Li et al., 2011).
Dry anaerobic digestion technology emerged from research performed in 1980s that
documented higher biogas production rates by high-solid wastes fed without dilution.
Conventionally (1990 and earlier), wet anaerobic digestion used to be the main anaerobic
digestion technology for digestion of manures in vertical reactors requiring feed material
with less than 10% TS content (Forster-Carneiro et al., 2009). But then the trend of dry
anaerobic digestion technology increased so quickly that in late 1990s, total anaerobic
digestion capacity in Europe for treating OFMSW was equally divided between the wet
and dry anaerobic digestion as shown in Figure 2.1. The trend further changed and in 2006,
dry anaerobic digestion and wet anaerobic digestion provided 56% and 44% of the
capacity respectively (De Baere, 2006) . It became > 60% for dry digestion in 2010 as
shown in the same figure.
Dry anaerobic digestion is performed with organic fraction of municipal solid waste
(OFMSW) in both horizontal and vertical plug flow reactors. Apart from OFMSW, it can
also be conducted with straws and residues of crops, solid livestock waste (e.g. cow dung,
horse dung), food waste and dewatered sewage sludge as substrates (Mumme et al., 2010;
Kusch et al., 2008; Kim and Oh, 2011; Duan et al., 2012). According to Luning et al.
(2003), both the wet and dry anaerobic digestion processes can be considered as a proven
technology for the treatment of the OFMSW because the specific gas production by the full
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