Process Biochemistry 34 (1999) 601 – 612 The role of periodic agitation and water addition in managing moisture limitations during high-solids aerobic decomposition L.P. Walker a, *, T.D. Nock a , J.M. Gossett b , J.S. VanderGheynst c a Department of Agricultural and Biological Engineering, Riley -Robb Hall, Ithaca, NY 14853 -5701, USA b School of Ciil and Enironmental Engineering, Cornell Uniersity, Ithaca, NY 14853, USA c Department of Biological and Agricultural Engineering, Bainer Hall, Uniersity of California, Dais, CA 95616 -5295, USA Received 18 September 1998; accepted 18 October 1998 Abstract The results of this study were obtained from a pilot-scale experimental system designed to mimic an agitated bed composting process. This choice of scale represents a compromise between a better controlled and reproducible bench-scale system-and a reality-based full-scale system. Two substrates were studied: a synthetic food waste (dry dog food) and anaerobically digested, polymer – dewatered biosolids. The goal was to evaluate the degree to which periodic agitation: (1) lessened spatial gradients in moisture and temperature; and (2) thereby improved sustained degradation rates and cumulative extent of biodegradation. Coefficient of variation for cumulative O 2 measurements in the three replications of the 55% dog food and wood chips experiment with an aeration rate of 100 l min -1 were of the order of 25–30% which was consistent with other bench and pilot-scale studies. Drying occurred in both static-bed and agitated-bed studies. However, moisture gradients in the agitated-bed were smaller than in the static-bed experiments. Also, drying of the solids matrix increased with increased aeration rates for all the experiments. For the 45% dog food and wood chips experiments, a moisture content of 30% was reached in 180–300 h for aeration rates of 50–100 l min -1 , respectively; while for the 55% dog food and wood chips experiments, 30% was reached in 320–480 h for aeration rates of 50–100 l min -1 . The results of the water addition study showed that adding water three times per week resulted in a cumulative O 2 uptake at 622 g O 2 kg -1 of TS at 496 h, a 48% increase over no water addition for an additional 96 h of decomposition, and a 26% increase over adding water once a week. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: High solids; Decomposition; Aerobic; Agitation; Moisture; Composting www.elsevier.com/locate/procbio 1. Introduction There are three broad classes of composting technol- ogy: windrows, aerated static piles and in-vessel pro- cesses [1,2]. The differences between these classes of technology are largely based on how intensely the het- erogeneous, polyphasic environment is managed to achieve the desired goal. Windrows are the simplest and the least intensively managed, while in-vessel processes are more sophisticated and intensively managed [1 – 6]. The two process variables that are most frequently used to manage this natural process are aeration rate and mixing frequency [1,6 – 11]. Generally, the aeration rate required for heat re- moval is much greater than either stoichiometric or final drying requirements, [12,13] and the enthalpy change associated with the mass transport of water from the solids matrix to the air is the major mecha- nism for heat removal. This evaporation of water from the organic matrix can represent as much as 90% of the heat loss from the process [12]. In addition, cooling by aeration leads to high rates of moisture removal from the organic matrix [2,14,15]. If severe enough, this moisture loss can result in a significant reduction in the rate of decomposition. In analysis by Haug [2], decom- position rates were shown to be reduced 24–60% when the moisture content of the matrix is reduced from 55 to 45% and 45 to 35%, respectively. Static-bed and daily mixed in-vessel reactors often exhibit spatial gradients in key state variables (tempera- * Corresponding author. Tel.: +1-607-2552465; fax: +1-607- 2554080. 0032-9592/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII:S0032-9592(98)00122-8