Does size matter: scaling a composting experiment

Abstract

Composting has been considered one of the simplest and most cost-effective methods for
biotreatment of oily soil, sludge and sediments. By nature, composting is a large-scale
process, where certain mass is needed to retain heat and moisture. In order to optimize
composting, especially with oily wastes, various experiments may be necessary. To represent
the composting process in small scale, in particular the magnitude and duration of temperature
profiles, adequate scaling is required. Small-scale composting experiments were conducted in
order to be able to analyze the scaling-up effects of laboratory and pilot-scale experiments
into full-scale composting. Four naturally ventilated box reactors of different volumes: 2L,
20L, 200L, and lO00L were used. The compost mixture consisted of oily sediments, sawdust,
and peat. The temperature of all compost mixtures was recorded daily at the centre and
surface of each compost box, during a period of ten months. It was found, that the reactors
with a volume � 200L and a surface area to volume ratio (SA:V) 2': I 0: I, showed no difference
between surface and centre temperature. The heat generated was lost to the surroundings at a
higher rate than could be sustained by the biomass. While the IO00L experiment with a SA:V
ratio in the range of 6.0: I produced pronounced self heating. The results were in accordance
to the SA:V ratios and their relationship to heat generation and dissipation as shown in
scientific literature. The results obtained, show that laboratory experiments with self-heating
reactors of SA:V ratio 2':I 0: I containing oily-sludge should not be used to simulate full scale,
since the results are impossible to verify. In order to carry out reliable experiments simulating
full scale composting processes in inexpensive self-heating reactors, it is suggested not to
proceed with laboratory scale, but conduct properly insulated pilot-scale experiments with
SA:V � 6.0:1.