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

Sand drying bed (SDB) is simple, easy to operate and needs low energy for its operation
than mechanical dewatering systems. One circular SDB with a radius and height of 30 and
80 cm respectively was used. The detailed design with top and cross-sectional views of the
bed has been presented in Figures 3.8 and 3.9 respectively. It consists of a 15 cm thick
layer each of coarse, medium and fine gravels. The first layer (15 cm depth) of the bed
from bottom consist of course gravels, followed by other layers of medium and then fine
gravels (15 cm depth each). A perforated central pipe of 2.5 cm diameter with screening
net, placed longitudinally in the center collects the filtrate percolated through the graded
gravel. The slope to the pipe is (6/30*100) 20 percent. Approximately 25 cm layer of
digestate can be placed over the bed, whereas freeboard is 10 cm.
Figure 3.7 Plastic drums for storage of digestate
Digestate dewatering by use of sand bed mostly happens by two mechanisms, i.e., drainage
or downward percolation of water and water evaporation from the surface of digestate.
The dying period (retention time) of digestate was different for different runs. It was in the
range of 2-5 days achieving a TS content of more than 50%. It depended on the
characteristics of digestate and weather conditions. The removal of dewatered digestate
from SDB was done with a shovel and the dewatered digestate was used for curing and
further analysis.
3.4.3 Curing of dewatered digestate
Curing is aerobic treatment of digestate passively (without the use of energy). In this study,
it was done by spreading the digestate with high TS (55%) in thin layer of 5 cm in a tray. It
was done for a period of 30 days under ambient conditions ( at 30-33°C). Dewatering of
digestate was needed to increase its TS before curing.
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