Exploitation of digestate: comparison between composting and hydrothermal processes

The increase of biowaste generation has reached critical levels in many Countries. The European legislation introduced the biowaste treatment and the organic recycling as central theme of its political agenda with the aim to promote the sustainable exploitation of this peculiar waste. The most utilized technologies applied to the biowaste treatment are based on the biological processes targeting to produce biogas or, more recently, biomethane to be used as fuel. The production of biomethane allows to produce a substitute of the fossil methane with a yield of about 0.07gCH4/gbiowaste; the remaining fractions are: waste coming from the pretreatment/refining steps, solid digestate or stabilized compost and leachate. The sustainable treatment of these fractions is a mandatory issue to threat the biowaste in a reliable and sustainable integrated process since their amount is more than 85% and the impact of their treatment on environment and economy of the overall treatment process can be quite relevant. This chapter is focused on a so called “smart facility” that integrate processes based on thermochemical processes with the biological one targeting to increase the overall sustainability, the flexibility regarding the input biowaste composition and the independency by the external factors affecting the waste trading.