Composting in small laboratory pilots: Performance and reproducibility

Abstract

Small-scale reactors (<10 l) have been employed in composting research, but few attempts have assessed the performance of composting considering the transformations of organic matter. Moreover, composting at small scales is often performed by imposing a fixed temperature, thus creating artificial conditions, and the reproducibility of composting has rarely been reported. The objectives of this study are to design an innovative small-scale composting device safeguarding self-heating to drive the composting process and to assess the performance and reproducibility of composting in small-scale pilots. The experimental setup included six 4-l reactors used for composting a mixture of sewage sludge and green wastes. The performance of the process was assessed by monitoring the temperature, O₂ consumption and CO₂ emissions, and characterising the biochemical evolution of organic matter. A good reproducibility was found for the six replicates with coefficients of variation for all parameters generally lower than 19%. An intense self-heating ensured the existence of a spontaneous thermophilic phase in all reactors. The average loss of total organic matter (TOM) was 46% of the initial content. Compared to the initial mixture, the hot water soluble fraction decreased by 62%, the hemicellulose-like fraction by 68%, the cellulose-like fraction by 50% and the lignin-like fractions by 12% in the final compost. The TOM losses, compost stabilisation and evolution of the biochemical fractions were similar to observed in large reactors or on-site experiments, excluding the lignin degradation, which was less important than in full-scale systems. The reproducibility of the process and the quality of the final compost make it possible to propose the use of this experimental device for research requiring a mass reduction of the initial composted waste mixtures.

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 scale are often performed imposing a fixed temperature throughout the process that may create artificial or unrealistic conditions (Mason and Milke, 2005). Finally, the reproducibility of reactor-scale composting has been rarely reported (Schloss et al., 2000, Petiot and de Guardia, 2004), although it conditions the potential use of the results for comparing different composting mixtures or conditions.

Thus, this work seeks to design an experimental new device to perform realistic composting while reducing the pilot volume and the mass of composted wastes and to assess the performance and the reproducibility of this small-scale composting system regarding the chemical and biochemical transformations of organic matter. A small-scale reactor system that enables self-heating in six parallel replicates was created, and it was tested using a classical waste mixture of sewage sludge and green waste.