We estimate that the French residual biomass baseline (that is, the current environmental performances of residual biomass management in France) is generating 18.4 ± 2.7 MtCO2-eq.y− 1 (GWP100), 255 ± 35 ktN-eq.y− 1 (marine eutrophication) and 12,300 ± 800 diseases i per year due to particulate matter formation (Fig. 1). For climate change, the residual biomass baseline is equivalent to around 4% of total territorial emissions (418 MtCO2-eq.y− 1; Haut Conseil pour le climat, 2021). Yet, such environmental performances and associated uncertainties rely on few streams and management practices. For example, crop residues and manure management not only generate around 90% of the total environmental impacts (for the three impact categories), but also contribute to around 90% of the total uncertainty (Fig. 1). Indeed, the present residual biomass baseline is a consequence of both (i) the differentiated potential and composition among the identified residual streams and (ii) the disparate environmental performances of the various management practices currently in place: both aspects contributing to the overall uncertainties.
While the overall theoretical potential (THP) of the residual biomasses for France is estimated to 1975 PJ.y− 1 (ranging 1664–2401), crop residues make up around 67%, followed by manure at 13% and PFR at 8% (Table 1). The predominance of crop residues and manure in the THP is also true in terms of nitrogen as these represent together around 90% of total N (737 ktN.y− 1 for crop residue and 894 ktN.y− 1 for available manure and slurry; Table 2). In turn, most of the residual biomass THP (ca. 85%; either accounted as wet weight, dry matter or nitrogen) is currently left to decay or spread on land (after storage) because this is the main end-of-life of crop residues, PFR and livestock effluents (Fig. 2). Indeed, manure and slurry remain majorly (ca. at 88%) conventionally stored and spread on land, while 60–70% of crop residues are directly ploughed and the remaining 30–40% return to land as part of animal litter. Accordingly, over the simulations, crop residues ploughing alone was found to generate around 21% of total GHG emissions, 27% of total marine eutrophication impacts but neglectable impacts on particulate matter formation. Overall, the most impactful current management practice was found to be the conventional storage and spreading of cow manure and slurry, by cumulating 50% of total GHG emissions, 45% of marine eutrophication impacts and 62% of total particulate matter formation. To identify the hotspots and the potential leverage points to set up bioeconomy strategies, the results for each stream are further detailed.
Table 1
– Theoretical potential of residual biomass for France
| MtDM.y− 1 | Mtww.y− 1 | PJ.y− 1 |
Streams | Sub-streams | Value | Low | High | Value | Low | High | Value | Low | High |
Primary Forestry Residues | | 8.4 | 4.4 | 14.0 | 10.1 | 5.3 | 16.9 | 159 | 83 | 264 |
Crop Residues | | 72.3 | 62.0 | 82.5 | 83.4 | 71.6 | 95.2 | 1330 | 1142 | 1518 |
Pruning Residues | Orchard | 0.92 | 0.7 | 1.2 | 1.3 | 1.0 | 1.7 | 17 | 13 | 21 |
Viticulture | 1.3 | 1.1 | 1.4 | 1.8 | 1.5 | 2.1 | 23 | 23 | 23 |
Manure and slurry | Cow | 11.1 | 9.5 | 12.4 | 97.3 | 83.4 | 109 | 208 | 178 | 232 |
Pig | 0.74 | 0.6 | 0.82 | 8.2 | 7.2 | 9.2 | 14 | 12 | 16 |
Poultry | 2.2 | 2.1 | 2.4 | 7.4 | 6.9 | 8.0 | 34 | 32 | 38 |
Green Waste | | 2.9 | 2.9 | 6.9 | 4.3 | 4.3 | 10.1 | 53 | 53 | 126 |
Sewage Sludge | | 0.8 | 0.8 | 1.3 | 4.2 | 4.2 | 6.6 | 15 | 15 | 23 |
Household Organic Waste | | 2.0 | 1.8 | 2.3 | 4.8 | 4.4 | 5.7 | 38 | 35 | 45 |
Agro-Industrial Waste | | 4.4 | 3.9 | 4.8 | 5.5 | 4.9 | 6.0 | 86 | 79 | 96 |
Total | | 107.1 | 89.9 | 130.0 | 228.3 | 194.7 | 270.4 | 1975 | 1664 | 2401 |
France-wise aggregated amounts of residual biomass streams available for the bioeconomy, estimated with the uncertainty range. Background data and calculations in the data repository.
3.1. Uncertain induced effects of crop residues on national agronomic practices
Albeit crop residues are one of the predominant streams foreseen to supply the bioeconomy, the environmental impacts associated with their current management practices remain uncertain. This stream alone is the greatest contributor to the total uncertainty regarding climate change and marine eutrophication categories for the THP (Fig. 1). Indeed, the current impact of crop residues management were estimated to lie around 4.7 ± 2.2 MtCO2-eq.y− 1 (Standard deviation across the Monte Carlo simulations; SD) but the values ranged from − 2.7 to 12.4 MtCO2-eq.y− 1. A variance of the same relative amplitude was also estimated for marine eutrophication. Corresponding uncertainties are mostly due to the estimation of the environmental impacts, as the quantities of crop residues estimated through the different RPR ratios only display a ± 15% relative variation which is in line with previous studies (see SI). In turn, the environmental consequences of returning crop residues to soil, either directly (ploughing) or after being used as animal bedding (as part of the litter; Fig. 2) depend mostly on the net balance between (i) the influences on fertilizing practices, (ii) long-term induced soil organic carbon (SOC) changes, and (iii) the level of land emissions. Deriving nationally-representative data to estimate such effects is challenging, as modifications of crop residues management are part of agronomic planning and are therefore rarely changed while keeping all other factors equal. In our estimation, a large part of the variance associated with the LCA results of crop residues (either ploughed or used as animal bedding) stem from the uncertainty related to the average land emission factors (EF). For climate change, the main driver is N2O EF (both direct and indirect), while the contribution of crop residues to SOC levels comes afterwards (here, GWP100 indicator is used). For marine eutrophication, land NO3 EF drives the impacts of crop residues decay. Despite displaying a similar variation, we estimate that using crop residues as animal bedding generates less climate change and marine eutrophication impacts than direct land ploughing (in average − 40 kgCO2-eq.tww−1 and − 0.66 kgN-eq.tww−1; see SI). This is mostly because, in France, ploughing crop residues induces a net requirement in mineral fertilizer to compensate for their high C:N ratio. This results in an average application of an extra + 10.5 kgN.ha− 1 to facilitate the degradation of straw into humus and ensure enough available mineral N for the next culture. This means that the 472 ktN.y− 1 embedded in crop residues ploughed on land require an extra 86 ktN.y− 1 mineral N application to achieve target C:N ratio (Table 2). On the other hand, crop residues which return to land as part of animal litter (i.e. manure and straw mixture) already display a favorable C:N ratio, and are therefore accounted for in the fertilizing planning (based on N balance), hence substituting the use of mineral fertilizer. Overall, the induced environmental benefits largely overpass the additional transportation and handling requirements of crop residues when used for animal bedding compared to their direct ploughing on land (under conventional valorization distances). Albeit generating an additional requirement of mineral fertilizer for the next culture, ploughing crop residues nevertheless contributes to N soils stocks and should in theory influence agronomic practices towards a lower use of mineral N in the long-term perspective. Yet, in front of the lack of information of how this effect is actually accounted for by land holders in practice, this was not included to derive the present residual biomass baseline (Fig. 1). While the phosphorous (P) content of crop residues is limited, it is important to note that crop residues return to soil are likely largely contributing to total potassium (K) needs as these are equivalent to ca. 936 ktK2O.y− 1 while current consumption of mineral K lies around 500 ktK2O.y− 1 (Table 2).
Table 2
– Estimated NPK nutrients balance of residual biomass streams and current role in fertilizing practices
Streams | Management | N balance (ktN.y− 1) | P balance (ktP2O5.y− 1) | K balance (ktK2O.y− 1) |
Embedded | Transfer (%) | MFE (%) | As fertilizer | Embedded | Transfer (%) | MFE (%) | As fertilizer | Embedded | Transfer (%) | MFE (%) | As fertilizer |
Crop residues | Animal bed. | 262 | 100% | 24% | 62 | 53 | 100% | 85% | 45 | 371 | 100% | 90% | 334 |
Ploughing | 472 | 100% | * | -86 | 95 | 100% | 90% | 86 | 668 | 100% | 90% | 602 |
Cow manure | Spreading | 445 | 30% | 24% | 32 | 148 | 100% | 85% | 125 | 511 | 100% | 90% | 460 |
Composting | 30 | 41% | 30% | 4 | 10 | 100% | 90% | 9 | 34 | 100% | 90% | 31 |
AD | 20 | 92% | 56% | 10 | 7 | 100% | 85% | 6 | 23 | 100% | 85% | 19 |
Cow & pig slurry | Spreading | 227 | 36% | 54% | 44 | 69 | 100% | 85% | 58 | 175 | 100% | 90% | 158 |
AD | 21 | 88% | 56% | 11 | 6 | 100% | 85% | 6 | 16 | 100% | 85% | 14 |
Aerobic treat. | 19 | 29% | 56% | 3 | 7 | 100% | 85% | 5 | 14 | 100% | 90% | 13 |
Poultry manure | Spreading | 118 | 30% | 27% | 9 | 71 | 100% | 85% | 61 | 66 | 100% | 90% | 59 |
Composting | 15 | 48% | 28% | 2 | 9 | 100% | 90% | 8 | 8 | 100% | 90% | 7 |
Green Waste | Composting | 29 | 43% | 9% | 1 | 6 | 95% | 40% | 2 | 18 | 95% | 90% | 16 |
AD | 3 | 94% | 56% | 1.5 | 1 | 100% | 85% | 0.5 | 2 | 100% | 85% | 1.5 |
Sewage Sludge | Spreading | 14 | 44% | 27% | 2 | 1 | 72% | 80% | 1 | 1 | 69% | 80% | 0.5 |
Composting | 18 | 43% | 20% | 1.5 | 2 | 95% | 90% | 1.5 | 1 | 95% | 90% | 1 |
AD | 13 | 91% | 56% | 7 | 1 | 100% | 85% | 1 | 1 | 100% | 85% | 0.5 |
Household Organic Waste | Composting | 35 | 43% | 10% | 1.5 | 15 | 95% | 40% | 6 | 21 | 95% | 90% | 18 |
AD | 4 | 93% | 56% | 2 | 2 | 100% | 85% | 1.5 | 2 | 100% | 85% | 2 |
Agro-Industrial Waste | Spreading | 4 | 100% | 27% | 1 | 2 | 100% | 80% | 1 | 3 | 100% | 80% | 3 |
Composting | 7 | 43% | 8% | 0.2 | 3 | 95% | 40% | 1 | 7 | 95% | 90% | 6 |
Total | | 1753 | 107 | 505 | 424 | 1943 | 1744 |
Current consumption of mineral fertilizers** | | 2080 | | 435 | | 503 |
* : Considering that the COMIFER method is nationally applied. **: Based on (FAO, 2019). “Embedded” columns correspond to the estimated nutrients present within the residual streams (baseline value). “Transfer” columns display the net nutrient balance at gate of the management practice (in-out ratio, baseline value). MFE stands for “Mineral fertilizer equivalent”, which represents the share of the nutrients present in the material which is returned to the land (e.g. digestate, compost, etc.) actually substituting the use of mineral fertilizers. For nitrogen, the MFE was estimated through Brockmann et al., (2018) equation and the values displayed is the average calculated over the Monte-Carlo iterations for each case. For phosphorous (P) and potassium (K), MFE were derived from a benchmark of the values usually recommended by the Chambers of Agriculture. The present estimated role of residual-based P & K on fertilizing practices were certainly overestimated here as (i) no considerations on soils saturations were accounted for (e.g. concentration of organic fertilizers in some regions) and (ii) it is unclear if at the national level such MFE guidelines accounting of P and K are strictly followed in practice. Uncertainties are not represented here, and only the streams and management practices linked with fertilizing services are displayed. Background data and calculation are available in the SI, rounding was applied here for tractability, which explains the possible unmatching balances.
3.2. Current manure management is the largest contributor to the residual biomass baseline
Current manure management causes around 60–80% of the environmental impacts while representing respectively 13% and 49% of the THP in terms of energy and nitrogen (combined effects of bovine, swine and poultry sectors). Most comes from current cow excreta management, which represent 80% of total available livestock excreta while the remaining part is shared between pig and poultry production (Table 1). While at the national scale poultry generates ca. three times as much manure than pig production on an energy basis, these are rather equivalent in terms of excreted nitrogen (around 130 ktN.y− 1 each). The uncertainty related to manure potential estimation (± 13% and ± 12% for respectively cow and pig excreta) is due to the uncertainty on the daily manure generated per head per specie and stage of life in average for France (particularly by dairy cows and sows; see SI). This leads to a total manure THP ranging 203–286 PJ.y− 1 and 778–997 ktN.y− 1, hence the uncertainty on the national manure potential contributes to the same extent, or even more to the total uncertainty than the uncertainty on unitary manure management environmental impacts (Fig. 1). This is because a large share of the impacts of conventional manure and slurry storage and spreading (representing ca. 85–90% of current uses; Table 1) directly relates to the amounts of volatile solids (VS; to estimate for e.g. storage methane emissions) and nitrogen (sizing all N-related emissions such as N2O and NH3). N2O alone drives around 50% of the climate change impacts of conventional manure and slurry management, while CH4 storage emissions contributes to ca. 20–25% of total GHG emissions for solid manure management but more than 50% for slurry management. NH3 emissions are almost the only contributor to particulate matter formation of these pathways, while marine eutrophication impacts are dominated by nitrates leaching. Again, the uncertainties on the EF are the main contributors to the uncertainty on the unitary LCA results, particularly the ones influencing the N balance along the storage and land spreading of manure and slurry. In turn, uncertainties on the climate change performances of the different poultry manure management options are larger than for the rest of animal excreta (SI), due to its higher N concentration on a wet weight basis (ex-animal: 18 kgN.tww−1 for poultry against 13 and 7 respectively for pig and cow).
In average, the implementation of composting or aerobic treatments (for respectively manure and slurry) generate net environmental benefits compared to conventional storage and spreading, but these are not widely implemented: ca. 6–10% of cow and poultry manure is currently composted (mostly on open-air platforms) in France, and 5–10% of slurries are aerated. Both treatments help volatilizing part of the VS and N, hence limiting subsequent emissions at the storage and land spreading phases compared to the situation where these are not implemented. For example, we estimate that implementing aerated treatments instead of conventional storage can save ca. 80–110 kgCO2-eq.tww−1 and 0.6–1.2 kgN-eq.tww−1 for slurry management. Similarly, the share of manure and slurry currently valorized through anaerobic digestion (AD) remains low (ca. 5% of these streams), albeit this pathway appears as the best option to reduce the impacts associated with manure management (SI). Indeed, AD implementation is estimated to save around 170–200 kgCO2-eq.tww−1 for slurry compared to conventional management and 70 kgCO2-eq.tww−1 for manure, mostly by (i) substituting marginal energy supplies with the generated biogas and (ii) limiting N losses prior land spreading. Indeed, as much as 60–70% of livestock effluent N is estimated to be lost under conventional storage systems in average, while this volatilization is limited to ca. 10% when AD treatments are implemented (Table 2). Yet, manure and slurry AD mostly generates net environmental impacts which are mostly due to the magnitude of land N emissions (e.g. NH3, N2O, NOx, etc.) for N-rich streams, offsetting the benefits of fertilizer substitution because of limited MFE (as in Javourez et al., 2022). In addition, a large share of the biogas produced (ca. 40%v in France) remains valorized in-situ through combined heat & power production (CHP), hence (i) not always substituting a heat service (e.g. summer time), (ii) substituting a low-temperature (LT) heat service which would have been else provided with an already efficient system (e.g. heat pump) and (iii), not reusing the CO2 fraction of the biogas. Overall, livestock effluents current management is estimated to save up to 278 ktP2O5.y− 1 and 761 ktK2O.y− 1 (Table 2).
3.3. Current management of woody residues might already provide net climate change benefits
Primary forestry residues (PFR) represent around 8% of the overall residual biomass THP albeit the estimated available volumes display a large uncertainty (83–263 PJ.y− 1; Table 1). These are currently largely left to decay in-situ while a small share (0–4% of total) is locally burned without energy recovery (Fig. 2). The particulate matter emissions associated with this small share being burned nevertheless represent a third of total particulate matter formation impacts associated with current PFR management, the rest stemming from decay NH3. Current climate change performances of leaving PFR on the ground after forestry operations is also highly uncertain (Fig. 1). Indeed, such performances stem from the balance between the long-term PFR carbon sequestration potential against CH4 and N2O decay emissions, which are characteristics tightly related with local pedoclimatic and forestry management conditions (see SI). Accordingly, we estimate that the average effects at the national level currently range from − 50 to + 100 kgCO2 − eq.tww−1 (-0.6 to 1.1 MtCO2-eq.y− 1 nationally aggregated; SD over the Monte Carlo simulations), meaning that comprehensive spatially-explicit assessments are required to refine the bioeconomy environmental threshold when it comes to mobilizing PFR. Of similar biochemical composition, pruning residues are estimated to represent a potential of 36–45 PJ.y− 1 (ca. 2% of total). But up to 11% is estimated to still being burned in-situ (e.g. vineyards pruning) hence contributing to the same extent as crop residues to total particulate matter formation (Fig. 1). The share of pruning residues (estimated to 0–11%) already used for space heating in domestic furnaces is likely saving ca. 90–150 kgCO2 − eq.tww−1 compared to their direct decay on land because these substitute marginal domestic heat supplies (Table 1; SI). Corresponding uncertainty mostly arise from insufficient knowledge about average domestic furnace combustion efficiency and heat use practices.
3.4. Municipal and industrial biowaste management offer limited room for improvements with conventional management practices.
Together, sewage sludge, green waste and household biowaste only represent around 6–7% of the THP either in terms of wet weight, dry weight, energy or nitrogen (Table 1; Table 2). These also cumulate around 7% of total GHG emissions, but only 2–3% of total marine eutrophication and particulate matter formation. Accordingly, albeit displaying a relatively high uncertainty on the available amounts (102–193 PJ.y− 1 estimated here) and unitary impacts (in average ± 35% for the climate change category over the different pathways), the contribution of these streams to the total uncertainty is also almost negligible (Fig. 1). Composting is the valorization pathway the most implemented for green/garden waste and household organic waste (ca. 80–90% of the share already collected separately; Fig. 2), while the distribution of sewage sludge along the different management pathways is less clear (SI). This is because sludge accounting and corresponding post-treatment operations are relative to the technical solutions of the specific wastewater treatment plant and its (sometimes subjective) boundaries. For example, some facilities report sludge volumes before anaerobic digestion treatment while others reporting the final amount of digested sludge.
Composting of municipal biowaste is usually developed in France in dedicated confined facilities (as opposed to cow manure open-air composting; section 3.3), with an endeavor to limit fugitive emissions. Yet, composting GHG emissions remain dominated by N2O generated at each stage (composting, maturation and land spreading). Similarly, marine eutrophication impacts are mostly driven by nitrates leaching when spreading. Overall, the composting environmental performances for these two impact categories are sensitive to the factors influencing N balances and the actual mineral fertilizer equivalent (MFE) considered for compost within fertilizing campaigns (usually low for such organic streams, here estimated to ca. 8–20%; Table 2). Yet, all unit operation considered, composting green residues generates GHG emissions of the same magnitude as when used as mulch (ca. 80 kgCO2-eq.tww−1). This is because green compost has a low MFE-N which is offset by the associated utilities and emissions. The same applies for sewage sludge: conventional (limed) treatment and composting mostly perform similarly (ca. 75 kgCO2-eq.tww−1). Yet for both cases, composting induces net marine eutrophication savings compared to direct spreading or used as mulch of respectively sewage sludge and green waste. Notably, the reuse of municipal biowaste in agronomy, despite initially hosting around 116 ktN.y− 1, is estimated to only contribute to substitute ca. 16.5 ktN.y− 1 of mineral fertilizer application.
Except for landfilling, the climate impacts of the different sludge management options are driven by the differentiated N2O emission among the different unit operations workflows. This means that sludge incineration is part of the best climate mitigation option, as it limits such emissions: incineration is estimated to save around 40 kgCO2 − eq.tww−1 in average compared to composting or lime treatment, and 115 kgCO2 − eq.tww−1 compared to anaerobic digestion (SI). For the same reasons (i.e. lower N-related emissions), incineration was also found as the best performing pathway for sludge regarding marine eutrophication. This is because for most organic streams, albeit uncertain, the nitrates leaching EF mostly offsets the benefits of substituting mineral N fertilizing for this impact category. These effects also apply for household biowaste, where incineration notably remains the best performing pathway regarding climate change. This was estimated considering current energy recovery systems in place within French incineration plants (only 60% are currently implementing CHP recovery), and also considering that a large share of produced heat is likely not substituting anything (see SI). Yet, incineration emissions compliant with EU regulation were assumed across all facilities in France. On the other hand, AD valorization was found the second worst option after landfilling for these municipal streams, which is partly due to the relatively high content of sulfur in sewage sludge and household biowaste, leading to important biogas detoxification requirements (not the case for green waste). Net benefits of AD are not ensured neither for marine eutrophication as ammoniacal nitrogen (TAN) is increased after the digestion. As a result, digestate field emissions offset the benefits of avoiding the fabrication and spreading of mineral fertilizers as nitrates leaching is still present.
Importantly, the present household biowaste performances represent the near-term situation (2024) when source-segregated biowaste collection is nationally implemented in France (Ministères Écologie Énergie Territoires, 2022). In this context, we estimate household biowaste management to generate 0.43 ± 0.14 MtCO2 − eq.tww−1 (Fig. 1) while current (here, 2020) situation is generating around 0.96 ± 0.39 MtCO2 − eq.tww−1. This 50% gain is mostly achieved by phasing out biowaste landfilling (currently estimated to represent ca. 21% of French biowaste end-of-life). Yet, according to our modeling, diverting household biowaste from landfills towards incineration would have generated even more gains in terms of GHG emissions than the foreseen situation where most of biowaste will end in composting facilities. Yet, the specific case of household biowaste is further discussed in the section 4.2.
The relative performances of incineration and composting as highlighted for household biowaste also apply for agro-industrial waste. This stream is an aggregation of various sub-streams summing 4–5% of total THP (Table 1), including the only reported organic waste stream currently entering a high-value valorization chain: recycling of paper and cardboard. These explain the net GHG emissions savings of agro-industrial waste (ca. -0.25 MtCO2-eq.y− 1, yet ranging − 0.69 to + 0.20; Fig. 1). The relatively large uncertainty is due to average landfilling and recycling environmental performances, here approximated through direct Monte-Carlo simulations on ecoinvent proxies (SI).