Does size matter: scaling a composting experiment

Abstract

Composting has been considered one of the simplest and most cost-effective methods forbiotreatment of oily soil, sludge and sediments. By nature, composting is a large-scaleprocess, where certain mass is needed to retain heat and moisture. In order to optimizecomposting, especially with oily wastes, various experiments may be necessary. To representthe composting process in small scale, in particular the magnitude and duration of temperatureprofiles, adequate scaling is required. Small-scale composting experiments were conducted inorder to be able to analyze the scaling-up effects of laboratory and pilot-scale experimentsinto 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 andsurface of each compost box, during a period of ten months. It was found, that the reactorswith a volume � 200L and a surface area to volume ratio (SA:V) 2': I 0: I, showed no differencebetween surface and centre temperature. The heat generated was lost to the surroundings at ahigher rate than could be sustained by the biomass. While the IO00L experiment with a SA:Vratio in the range of 6.0: I produced pronounced self heating. The results were in accordanceto the SA:V ratios and their relationship to heat generation and dissipation as shown inscientific literature. The results obtained, show that laboratory experiments with self-heatingreactors 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 simulatingfull scale composting processes in inexpensive self-heating reactors, it is suggested not toproceed with laboratory scale, but conduct properly insulated pilot-scale experiments withSA:V � 6.0:1.