Large scale composting model

One way to treat the organic wastes accordingly to the environmental policies is to develop biological treatment likecomposting. Nevertheless, this development largely relies on the quality of the final product and as a consequence onthe quality of the biological activity during the treatment. Favourable conditions (oxygen concentration, temperatureand moisture content) in the waste bed largely contribute to the establishment of a good aerobic biological activity andguarantee the organic matter stabilisation with limitation and control of odorous and greenhouse effect gaseousemissions.

Further Authors:
G.Duchalais - Terralys, Gargenville
F. Henon - Université de Toulouse

Several approaches (0D biochemical reducing, see Pommier et al.2007, effective 1D modelling couplingtransport and biochemical) have been made to understand the behaviour of such systems. In this paper we will present a2D numerical model using Darcy scale equations for heat and mass transport coupled with a biochemical reactivescheme. Then, we will solve that system (using experimental measurements on reactivity and transport coefficients)with a commercial code (COMSOL TM). The model described here is based on the biological model presented inTrémier et al 2005 coupled with an upscale transport model detailed in Hénon 2008 which takes into account the majorcomponents of the gas phase: N2, O2, CO2 and also H2O. This is a crucial point because of:
• The reaction rate, depending on the moisture content (humidity comes from the initial condition of the sludge butalso from the reactive scheme because reactions produce water),
• heat content, very sensitive to the evaporation rate in the sludge.
It has been shown in Pujol et al 2011 that the impact of drying could be important on the reactivity but also that thepseudo component air could not be sufficient to represent the drying in the sludge.The process studied was a closed reactor composting process (180 m3 rectangular box) with positive forced aeration.The air was blown from the bottom of the reactor, via two ventilation pipes. In the upper part of the reactor, air wassucked and led to a biofilter treatment system. The treated waste was a mixture of sewage sludge and bulking agent thatwas composted during four weeks without turning. Several informations were recorded during the treatment liketemperature evolutions at different locations (see Henon et al. 2009 for more details about the temperature recording).We have validated this code by comparing the temperatures obtained through the simulations with those recordedduring the experiments.After this step of validation and a discussion on final composition of the organic matter in the experiments compared tothe ones estimated by simulations, we have used this numerical model as an optimization tool. Modifying the initial,boundary and operating conditions we have been able to determine the best conditions to this particular composting process. A whole set of conditions is discussed in the paper.

Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2012 (Juni 2012)
Seiten: 9
Preis inkl. MwSt.: € 9,00
Autor: Grald Debenest
Anne Tremier
Michel Quintard
Jean-Luc Martel

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