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

digestates. Mechanically, liquid-solid separation can be done by use of screw extractor
separator, rotary screen separator (Bauer et al., 2009; Gioelli et al., 2011), filter press and
centrifugation.
2.10.2 Direct land application of liquid digestate
The C/N ratio of digestate should be less than 20 to be fit for land application (Wood,
2008). Palm (2008) stated that this liquid part of digestate, if applied to soild, can give a 90
% yield of that given by mineral fertilizer. The digestate may also be spread directly onto
farmland as slurry. However, its application has been limited to maximum amount of 170
kg N/ha/y by the EU Nitrate Directive in 1999 (Al Seadi et al., 2001).
Load of the nutrients is one of the concerns in recycling anaerobic digestion digestate on
farmland because of danger of nitrate leaching and overloading of phosphorous leading to
surface and ground water pollution. Lack of stability of fresh organic waste is also a
concern. Storage and application of digestate, are therefore, important in this case. Cost of
digestate fertilizer increases because of additional storage and it also increases emission
during storage. To regulate this nutrient loading, limits have been set in different countries
as given in the Table 2.11.
Table 2.11 Regulations of Nutrient Loading on Agricultural Land
Country Nutrient Load
(kg N/ha/y)
Required Storage
Austria 100 6 months
Denmark 170 (cattle)
140 (pig)
30 kg P/ha/y
7 ton DM/ha/y
9
Italy 170-500 3-6
UK 250-500 4
Modified from Al Seadi et al., 2001
2.10.3 Aerobic post-treatment of solid digestate and its effects on quality
Aerobic post-treatment of the anaerobically digested waste or digestate bring about
changes in pH, solids content and C/N ratio as shown in Figure 2.11. In this study, aeration
was carried out intermittently for 5 h on a daily basis for the treatment. The figure shows
that pH increased between 6.7 and 7.8 during the aeration period.
Figure 2.11 also shows that aeration brings about decrease in VS (%TS) possibly due to
aerobic decomposition of organic matter remaining at the end of anaerobic decomposition.
The decrease in C/N may be an indication of the breakdown of organic matter during this
period. Beyond the 40th day, the C/N ratio becomes considerably less variable due
probably to the depletion of most of the readily biodegradable organic compounds. pH and
C/N therefore seem to be good indicators for assessing compost maturity and stability.
Figure 2.11 also indicates that the compost was fully stabilized on the 40th day.
Thus, an improved aeration frequency can bring about stability in a shorter period. At the
end of the 70 days, the composition of the stabilised compost was found to be pH 7.8; TVS
= 45% and C:N = 12.7 that are recommended values for a compost to use.
40