GHG emissions during the composting process as a function of the aeration strategy

Composting is an aerobic thermophilic biodegradation process that requires oxygen to stabilize the organic wastes and optimal moisture content for the microorganisms development (Haug, 1993). Several parameters as C/N ratio, material porosity, moisture content and aeration rules for the oxygen supply must be analyzed and controlled to optimize the process development. Frequently, aeration is defined as the most important factor for the process performance.

Further Author:
C. Maulini

In general, the main aeration systems used are: forced aeration, physical turning and natural convection. Different aeration strategies of the first system have been proposed with the common goal of optimizing the biological activity along the process. For this reason, most aeration regulations are based on the temperature and/or oxygen content measurements. Often, the airflow supply is also regulated by means of fixed cycles. Recently, the feasibility of oxygen supply according to the biological activity evolution (measured as Oxygen Uptake Rate (OUR)) has been studied by Puyuelo et al. (2010). Briefly, it consists on finding, for every moment of the process, the airflow that increases the oxygen uptake rate through the comparison of consecutive OUR values together with the airflow applied. In order to compare the process performance and the greenhouse gases emissions as a function of the aeration strategy, three pilot composting trials with the same organic waste but different aeration systems were undertaken. Oxygen, aeration cycles and the new OUR control were the strategies studied. Temperature control was not undertaken since the results obtained in a previous work (Puyuelo et al., 2010) showed highest energy consumption. The experiments were carried out during 20 days and temperature, oxygen content and airflow supplied were continuously measured. Daily, a sample of exhaust gases was taken to quantify by gas chromatography the N2O, CH4 and global Volatile Organic Compounds (VOCs) content. Different VOCs were also identified by mass chromatography. Ammonia content was measured once a day with a specific sensor. Moreover, the stability degree of initial and final samples obtained was determined by the Dynamic Respirometric Index. Similar temperatures profiles were achieved in all experiments although the oxygen content was oscillating during the most part of the oxygen and cyclic controls. The progressive airflow evolution of the OUR control avoided this severe oxygen changes. In addition, this system achieved the highest oxygen uptake during the process and most stable endproduct. Regarding the GHG emissions, the gradual airflow changes also prevented the high peaks detected under oxygen and cyclic control, when maximum airflow was applied. In Table 1 the global emissions of each contaminant are summarized. As observed, the lowest global emissions were detected by means of the OUR control. The lowest CH4 emission showed that the systems with intermittent airflow increased the anoxic zones. Maximum VOCs measures were around 1400 mg m-3 for oxygen and cyclic controls and they were below 700 mg m-3 during the OUR control. On the contrast, ammonia and N2O measures could not be compared since no clear evolution was observed. All measurements permitted to demonstrate that the OUR control optimizes the biological activity, the material stabilization and reduces the gaseous emissions associated to the composting process.



Copyright: © European Compost Network ECN e.V.
Source: Orbit 2012 (Juni 2012)
Pages: 8
Price: € 8,00
Autor: B. Puyuelo
Teresa Gea
Dr. Antoni Sánchez

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