Control of gas emissions during composting of municipal solid waste, municipal biowaste and industrial biowaste

The environmental quality of composting means both the capacity to produce composts of high environmental and agronomic quality, and no health or environmental impact of the process.

The degradation of organic matter during the composting process is known to release various gases in the atmosphere. In order to better control and reduce the emissions of greenhouse gases (GHG) and odorous gases during composting, VEOLIA Environnement has developed a pilot unit that reproduces the entire composting process in controlled conditions. Also, gas analyzers continuously evaluate the main volatile compounds.
The aim of the study is to monitor the emissions of greenhouse gases and odorous gases at pilot-scale during the composting of 3 types of wastes and to determine the key factors that can mitigate these emissions.
Municipal solid waste (MSW), municipal biowaste (MBIO), and industrial biowaste (IBIO) were collected on industrial plants and separately composted in 160L insulated reactors. The composting process was the same for each reactor: controlled aeration with a proper and dry air blowing system with continuous flows monitoring during 14 weeks and mechanical turnings at 3 and 6 weeks.
The temperatures were continuously monitored in the core of the compost. The gaseous effluent were collected and quantified. Every 80 minutes, the emission of carbon dioxide (CO2), ammonia (NH3), hydrogen sulphide (H2S), sulphur compounds, nitrous oxide (N2O) were quantified and every day total hydrocarbon (THC) analysis was carried out. Initial mixtures and composts were sampled after 3, 6 and 14 weeks of composting for physico-chemical measurements and estimation of organic matter stabilization (Van Soest fractionation and carbon mineralization in soil-compost mixtures).
Both temperature (60-70°C during 4 weeks) and oxygen conditions (5 to 18% in the matrix) indicated that composting was carried out in satisfactory conditions in each reactor. During the first 2 weeks, a strong O2 consumption along with a CO2 emission was observed, resulting from the high microbiological activity (active phase). It was explained by the large proportion of easily biodegradable organic matter in the initial mixtures as confirmed by the results of organic matter characterization. After 3 weeks of composting, both the high temperatures and the decrease of the biodegradable fraction induced a slow down of the microbial activity (stabilisation phase). Then temperature and O2 consumption decreased.
NH3 and sulphur compounds were emitted only during the active phase with the 3 wastes. For NH3, emissions were earlier for MSW and MBIO than for IBIO. This could be explained by its initial pH value (7.1 and 6 respectively) and its high temperature at the beginning of the process. Nevertheless, for MBIO with high initial nitrogen content, cumulative flux of NH3 was six times larger than for MSW (2.33 g NH3.kg-1 initial dry matter and 0.35 g NH3.kg-1 initial dry matter respectively). The majority of detected sulphur compounds were DMS and CH3SH.
N2O was only emitted during the stabilisation phase for MBIO and IBIO (0.5 g N2O.kg-1 initial dry matter), and not detected for MSW. The larger nitrogen content of MBIO and IBIO at the beginning of stabilization phase (6 weeks) and the lower temperatures during the stabilization phase (< 45°C, incomplete nitrification/denitrification reactions) could explain these differences.
These first results showed that quality and quantity of gases emission during composting were strongly connected to the type of waste. It also pointed out that controlling some key parameters (temperature, initial N content) by adapting the composting process to the type of waste is essential to ensure low and controlled environmental impacts.



Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2008 (Oktober 2008)
Seiten: 9
Preis inkl. MwSt.: € 8,00
Autor: Hélène Bacheley
Cédric Francou
M. Chevallier
M. Poitrenaud

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