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

1.1 Background
Chapter 1
Introduction
Global solid waste generation is increasing day by day, not only because of growing
population, but also due to improved standard of living. About 40-60% of the household
waste is biodegradable in Asia, most of which is usually disposed by landfilling or open
dumping. Improper handling and disposal of the gigantic amount of solid waste seriously
affects the air, land and water environments and human health. It is also contributing to
climate change by releasing methane and carbon dioxide. There is a pressing need to
manage it from the time of creation to its safe disposal. Incineration may also not be
suitable for organic waste because of low calorific value due to its high moisture content.
Moreover, it causes air pollution and also requires high capital and operating cost.
Therefore anaerobic digestion and composting could be alternative options to treat organic
waste.
Anaerobic digestion is widely being practiced as major treatment option for disposal of
organic municipal solid waste on par with composting technology. Anaerobic digestion
mainly combines with the energy recovery benefits, green house gas mitigation and
produces stable end products, which can be further upgraded as compost for land
application (Forster-Carneiro et al. 2008; Walker et al. 2009). In general, anaerobic
digestion systems are broadly categorized under wet (20 %
total solids), mesophilic (35 -40 o C) or thermophilic (> 55 o C), batch or continuous and
single or two stage systems (Fdez-Guelfo et al. 2010; Forster-Carneiro et al. 2008; Yabu et
al. 2011).
Dry anaerobic digestion offers several advantages over wet digestion process like, lesser
water addition, smaller reactor volume, technical simplicity in design due to plug flow
movement of substrate and no mechanical devices required inside the reactor for mixing
and easy handling of digested residues (Guendouz et al. 2010; Yabu et al. 2011). Similarly,
better process conversion efficiency and maximum net energy gains are reported especially
with the thermophilic operations of dry anaerobic digestions systems (Fdez-Guelfo et al.
2010; Forster-Carneiro et al. 2008). For example, the Dranco and Kompogas processes are
single stage, dry, thermophilic systems, which have been commercialized in Europe and
other parts of the world.
Ammonia-N accumulation is, however, identified as a major issue with dry thermophilic
anaerobic digestion systems, which can affect the overall methane yield. Generally, the
OFMSW is characterized with the average of 4% of protein content, a major source of
nitrogen, which is removed via ammonification process and accumulated as ammonia-N
(Jokela and Rintala, 2003). Also, the chances of ammonia-N accumulation are higher, if
the feedstock is mixed up with the large portions of food processing waste and animal
waste from slaughter houses. Even though, the protein degradation process is found to be
very slow, the released ammonia-N tend to accumulate in anaerobic digesters because of
leachate recycling and there is no mechanism to remove it except by leachate removal or
leaching.
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