Co-compostage : Etude du procédé et de ses impacts environnementaux

Context

As far as they arise from elaboration or sorting processes clearly described, an important part of the organic wastes of industrial and/or urban origin can also be considered as not harmful. This authorises to consider valorisation chains like composting and biomethanisation.In their original state organic wastes are not however all compostable as such. We must mix them in order to stimulate their co-composting : co-composting will consist in gathering different organic materials within each one the balance of constituent is not optimal and to associate them in proportions adapted so as to realize an optimal bio transformation.If the process is at present well know, interrogations subsist. What’s about the way of organic matters association in order to stimulate the process and what’s about the process environmental impacts (greenhouse gases (GHG) emission, filtering capacity against organic contaminants, impact on bioavailability of heavy metals)? What about hygienisation effect on pathogens and agronomic interests of products?

Objectives

The purpose of this project is to develop control and tools to optimise co-composting process and to evaluate its environmental impacts.Studies are intended to :· Specify with raw materials composition (C, N content) the right associations for good composting. Near Infrared spectrometry (NIRS) is used for this part of the study. · Specify (1) the greenhouse gases fluxes from composting, (2) the filtering capacity against organic micro-contaminants and pathogens and (3) the impact of this process on heavy metals bioavailability.

All information collected from these studies will allow to write a “good practices in composting” code in order to increase compost quality and reduce its environmental negative effects.

Results obtained

Development of NIRS allows to determine parameters like dry matter, organic nitrogen, total carbon and organic matter content. The main difficulty is the lack of homogeneity of fresh samples. At present NIRS allows the determination of these parameters in 72 hours (including drying). Studies about agronomic value of products show the difficulty to determine with precision the kinetic of organic nitrogen mineralisation after compost application. Mineralisation is strongly dependent of pedo-climatic conditions. However we could estimate that 20% of the total nitrogen is available the year of application. From the co-composting environmental impacts point of view, different points can be highlighted. (1) Taking into account, GHG emissions, the first results concern a matter balance during composting of different organic materials. This study shows a decrease of 50% of the initial weight. Lost gases are composed for 75% of water vapour, 15 % of carbon dioxide and methane and about 1% of nitrogen. Te results of more precise follow up are under analysis. (2) From the trace heavy metals, without the Cd exception, a re-concentration was observed during the composting process in link to total mass reduction due, as underlined here above, to water and carbon losses. Nevertheless different elements groups can be identified. So, in average, As and Co contents had been multiplied by two. These concentrations increased, in average, of 70 % for the Cr and the Hg of more than 30 % for the Cu and the Pb, and of around 20% for the Zn. Large variations around these means can be highlighted in the four composting processes followed up. In a general way, we observed a concentration increase till the end of the second month of the process before to underline a relative stabilisation. What’s about the co-composting impact on the evolution of the fraction of the exchangeable trace heavy metals ? This fraction reflects the proportion of these heavy metals susceptible to be mobilised by the crops fertilised with such co-compost. Once again, different trace heavy metals groups had been idendified on the basis of their exchangeable fraction that was, in average :* lower than 5 % [1.5 to 4.3 %] for the Cr;* lower than 20 % for the As [7 to 26 %]; the Co [9 to 36 %], the Cu [14 to 26 %], the Hg [0 to 36 %] and the Ni [7 to 30 %];* lower than 50 % for the three other trace heavy metals that are the Cd [22 to 70 %], the Pb [36 to 64 %] and the Zn [33 to 58 %].Co-composting process did not modify this classification in deep. It had led to :* an increase of the exchangeable Cd and Zn fractions;* a reduction of the Co, Cr and Ni exchangeable proportions, probably immobilised in more or less complex and stabilised humic molecules;* a relative statut quo for the As, Cu, Hg and Pb. Due to re-concentration of these trace heavy metals during co-composting process and due to the low evolution of their exchangeable fractions, it is important to pay attention to the quality, to the low initial content in trace heavy metals, of the raw materials dedicated to this process. (3)The filtering capacity, of co-composting process, against organic micro-contaminants has been evaluated in two compost piles with initial concentrations of, respectively, 2.5 and 5 mg/ kg of DM. In order to take into account the diversity of the molecules potentially meet within this family, the following groups were analysed : the Fluoranthen (for its low bio-degradability) and the 6 of Borneff (Benzo(a)pyrene, Benzo(b)fluoranthen, Benzo(ghi)perylen, Benzo(k)fluoranthen, Fluoranthen and Indeno(1,2,3-cd)pyrene. The results highlighted the good filtering caracity of co-composting process face to these molecules. Indeed, mass balances underlined a degradation of the initial quantities by 19.3, 42.1 and 35.9 %, respectively for the six of Borneff, le Fluoranthen and the total amount of organic micro-cotaminants, in the first trial. In the second trial, these degradations were higher with, respectively, 39.0, 66.2 and 45.1 % of the initial contents. (4)The filtering capacity of co-composting process towards pathogen micro-organisms was also followed up during to co-composting process. In this context, 5 groups were took into account : total coliforms, Enterococcus, Escherichia coli, sulfito-reductor Clostridium spores and the salmonellas. Co-composting process had led to a significant reduction of the density of all these groups and particularly for Escherichia coli and sulfito-reductor Clostridium spores. Nevertheless a given contamination level remains at the end of the process.Here we can underline that the filtering process continues once the co-compost is spread on the field through the concurrence occurring with the soil micro-organisms that are better fitted to this surrounding type.