1. Introduction
Many studies have been conducted in pilot-scale reactors that
enable easier tracking of the composting process than in full-scale
plants (Mason and Milke, 2005). Large-scale reactors (10–300 l)
frequently involve a self-heating phase, during which the compost
temperature exceeds 60 C, depending solely on microbial heat
production and ensuring a well-conducted composting process.
In such conditions, the simulation of the thermodynamic regime,
including the thermophilic phase, cooling and maturation phases,
should enable reproduction of many other parameters of full-scale
composting systems, including biological activity and metabolic
capacities (Ryckeboer et al., 2003; Sanz et al., 2006), moisture
and water vapour transport, oxygen status and temperature
(Mason and Milke, 2005).
Small-scale reactors (<10 l) have also been employed because
they are easier to handle, less expensive and easier to control than
large-scale reactors or full-scale systems (Petiot and de Guardia,
2004). They have been used to evaluate substrate compostability
(Hu et al., 2009) or process suitability (Körner et al., 2003), define
parameters for mathematical models (Sánchez Arias et al., 2011)
and investigate the fate of specific compounds (Zenjari et al.,
2006). Indeed, the miniaturisation of the process is required when
the behaviour of pollutants is studied using radiolabeled chemicals
due to the limited amount of necessary materials and the ability to
control output gazes (Reid et al., 2002). Nevertheless, the experimental
simulation of the composting at a small scale is not obvious
because the mass of the organic matter involved in the process
may not be large enough to reproduce heat generation and transfer
and the resulting thermal inertia of full-scale systems (Mason and
Milke, 2005). A small size of reactor may also limit potential sampling
during the entire process (Hesnawi and McCartney, 2006). In
small-volume reactors, a rapid decrease of temperature is usually
observed because of the limited amounts of organic substrates
and heat losses, contrasting with the slow and gradual decline in
temperature of full-scale composting (Petiot and de Guardia,
2004).
On the other hand, very few studies have evaluated the realism
of the composting process at a small scale, comparing the biochemical
properties of organic matter before and after composting
(Michel et al., 1995). Moreover, composting experiments at a small
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