About 36,000 WWTP operating
in the EU use AD to reduce sludge reactivity (Bolzonella et al.,
2006) before disposal. About 10 million tonnes on dry basis of
sludge are produced in the EU each year, and its disposal amounts
to about 50% of the total operating costs of WWTP (Apples et al.,
2011). Due to the low organic matter and volatile solids (VS) concentration
of WMS originating from WWTP, the digester generally
operates at low organic loading rates (OLR) kgVS/m3 day