Low tech biochar production could be a highly effective nature-based solution for climate change mitigation in the developing world

To compare the climate change mitigation benefits of nature-based solutions for management of municipal green waste with conventional management. This study analyzed the carbon footprint of managing one ton of municipal green waste (MGW) in Lima Peru under 4 different scenarios: 1) Final disposal in authorized landfill, 2) Final disposal in informal landfill, 3) composting and 4) biochar production using a low-cost, low tech Kon-Tiki reactor. The results demonstrate the very clear potential for climate change mitigation from biochar production using low tech and therefore accessible technology in a typical developing world context. The carbon footprint of producing biochar was lower than for composting and biochar and compost both had carbon footprints significantly lower than landfilling. We argue that the standards used by nascent platforms for trading carbon removal credits generated by biochar should relax the technology requirement to favor engagement and participation of small-scale market participants in low-income countries. Waste management in the developing world presents significant challenges but often starts from a very low base which means there is large potential for reducing emissions, as well as for sequestering carbon.


Introduction
There has been significant progress in research towards developing biochar formulations that are optimised for specific purposes, such as heavy metal immobilization, carbon sequestration, agronomy, as an additive for animal feed etc. (Bolan et al. 2021). At the same time there is growing awareness among

Abstract
Aim To compare the climate change mitigation benefits of nature-based solutions for management of municipal green waste with conventional management. Methods This study analyzed the carbon footprint of managing one ton of municipal green waste (MGW) in Lima Peru under 4 different scenarios: 1) Final disposal in authorized landfill, 2) Final disposal in informal landfill, 3) composting and 4) biochar production using a low-cost, low tech Kon-Tiki reactor.

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Vol:. (1234567890) scientists and decision makers that simply curtailing greenhouse gas (GHG) emissions will be insufficient to avert a climate crisis (Ravi et al. 2016;Li et al. 2019;Rogelj et al. 2021). The amount of carbon in the earth's atmosphere has reached a level were investment in strategies and technologies that generate negative emissions, i.e., that draw down the level of carbon dioxide in the atmosphere, have become necessary (Alcalde et al. 2018;Mullingan et al. 2020;Carton et al. 2020;EBIC 2021). The possibilities for doing this are limited (Fuss et al. 2014;IPCC 2018).
Nevertheless, it is clear that biochar production and use is one such ready to implement technology / land management strategy that could generate significant amounts of negative emissions (Smith 2016). For this reason, we are now seeing the development of trading platforms for the buying and selling of carbon removal credits related to biochar production, i.e., Puro (2021) and Carbonfuture (2021).
Whilst such positive developments are to be welcomed, there is need for refinement of the general approach so that it is more inclusive and doesn't exclude, by design, participation from actors in the developing world. The above-mentioned trading platforms and standards tend to place an emphasis on using state-of-the-art technology for biochar production to minimize emissions during carbonization. This is perfectly reasonable and doable in a developed world context, were effective and advanced waste management systems and legislation are already in place. In the developing world, waste management often starts from a much lower base and a requirement for heavy upfront investment is likely to act as a barrier to investment and participation simply because the economic environment is higher risk in the developing world. Further, organic waste (or "biological" to differentiate from the label "organic") is often highly heterogenous. At points of aggregation for crops like coffee and cacao, volumes of waste may be small and seasonal. In these contexts, the requirement for significant upfront investment in state-of-the-art technology may act as a barrier to participation, even though significant emission reductions are possible without the aid of state-of-the-art technology.
The criteria for whether a project achieves negative emissions and whether it is eligible for carbon removal finance should ideally be anchored in something like the concept of additionality developed by the UN for REDD. In Perú in 2014 a total of 7.5 million metric tons of urban solid waste (which includes organics, construction waste, medical waste etc.) was generated; it is estimated that 47% of this waste ended up in informal (illegal) landfills and only 21% ended up in licenced landfills (Ziegler et al. 2018). If we focus exclusively on municipal green waste, the waste biomass obtained through the regular maintenance of urban green space, the figures are still large. For example, in Lima a mega city on the Peruvian coast, we see the generation of 1,382 tonnes of municipal green waste per month (SIGERSOL 2021). Only a small fraction of this is composted and the majority is destined for either formal or informal landfills were the green waste decomposes in an anoxic environment, liberating potent greenhouse gases such as methane and nitrous oxide (Eklund et al. 1998). In this context the production of biochar from municipal green waste in an artisanal biochar reactor such as a Kon-tiki kiln (Ithaka 2021) could be considered a significant improvement over the status quo. Here we calculate the carbon footprint of municipal green waste management using four different strategies: 1) final disposition in a licenced landfill, 2) final disposition in an informal landfill, 3) composting, and 4) biochar production using low cost, artisanal technology.

Materials and methods
Using one ton of Municipal Green Waste (MGW) as the unit of analysis we calculated the carbon footprint of managing municipal green waste in Lima, Peru under four different scenarios: 1) business as usual (licensed landfill), 2) business as usual (unlicensed landfill), 3) composting and 4) biochar production via a Kon-Tiki. In all four cases we assume that the carbon content of plant biomass is 50%, following IPCC (2014).
In all four scenarios the first significant emission of GHG comes from transportation of biomass. Lima is a megacity and transport distances vary depending on proximity to the urban core. Nevertheless, transport associated emissions are inevitable because landfill sites are located on the outskirts of the city, just beyond city limits, where the impact of waste management on residents is reduced. The informal landfills are even further from the urban core. Using the specific example of MGW from the municipality of Lurin on Lima's southern fringe we assume the emission of 340 g of CO 2 eq emitted per ton of municipal green waste per km in a medium sized truck (Seo et al. 2016). In this specific case changing from landfilling to either composting or biochar production reduces transport emissions (Table 1) because the biochar /compost operation is closer to the urban core than the landfill sites (Appendix s1).
The step after transporting is the processing of the waste. In the first and seconds scenarios, in which MGW is disposed of in formal and informal landfills there are many uncertainties around the amount of GHG released from the relevant biomass. Nevertheless, values are likely high since the anoxic conditions found in landfill favor the liberation of gases such as methane and nitrous oxide (Eklund et al. 1998). The emission of GHGs (carbon dioxide, methane and other trace gases and particulates (CO, NOx, PM 2.5 , PM 10 ) in percentage terms and the corresponding multipliers for converting these to CO 2 equivalents are from Babu et al. (2014), which were then applied to the baseline of 1 ton of MGW.
In the third scenario, in which the MGW is composted, we assume that 52% of the C contained in the waste biomass is retained in the compost, after Tiquia et al. (2002). For the remaining 48% C we assume 3% is lost as methane and 45% is liberated as CO 2 , based on Boldrin et al. (2009). Estimates of CO 2 eq from particulates and traces gases during composting are based on Van Haaren et al. (2010). For the production of compost, we also calculate the emissions that result from pumping the ground water needed for the composting process. Ideally compost should be 60% water by weight. We assume composting takes place over 60 days (after Zhu-Barker et al. 2016) although it is important to note that the actual time required for composting can be significantly shorter or longer depending on feedstock and management practices (Kuhlman 1990;Pace et al. 1995).
Emissions are not the only consideration with organics management, water usage is also critical especially in areas prone to drought. In Lima the rate of evaporation is 952.9 mm per annum (MIDAGRI n.d.). Thus, an area of 2m 2 , i.e., the area that 1000 kg of MGW would occupy, would require 317.6 L of water for optimal composting. In Lurin ground water is used and obtained using a 1 horsepower pump. This in turn requires 32.9 W of electricity which results in 0.013 kg of CO2eq. These calculations are based on data from the U.S Energy Information Administration (EIA 2020) who report that 0.42 kg of CO 2 eq are emitted per kWh. We assume the compost produced from a ton of MGW gets packaged in 10 laminated plastic bags ( Table 1) that have an emission of factor of 0.637 kgCO2eq/bag (Ma et al. 2019). Finally, once produced compost needs transportation to its place of use. We assume that any compost produced will be returned to the urban green spaces from which the waste biomass was collected. If we again assume emissions of 340 g of CO 2 eq emitted per ton, per km, for material transported in a medium sized truck (Seo et al. 2016), we arrive at a figure of 0.3 kg of CO 2 eq of additional transport emissions linked to managing one ton of MGW via composting. Finally, for the scenario in which MGW is used to produce biochar we assume that 30% of the C contained in the feedstock biomass is retained in the biochar produced, after Mohammadi et al. (2016). For the 70% of the C content of the feedstock which is lost during biochar production, we assume that 95.6% is liberated to the atmosphere in the form of CO 2 , 0.5% in the form of methane and 3.9% as other gases (CO, Volatile organic compounds, NOx, and particulates), based on the measurements of Cornelissen et al. (2016). Emission factors for converting methane and the other trace gases into CO 2 eqs are from Babu et al. (2014). We also calculated the emissions embodied in metal used to manufacture a Kontiki oven. For every ton of steel produced we assume that 1.85 tons of CO 2 eq are emitted to the atmosphere (Hoffmann et al. 2020). A Kon-Tiki kiln uses 200 kg of metal thus resulting in the emission 370 kg CO 2 eq. Every kiln has a life cycle of roughly 300 burns. Each individual burn consumes roughly a ton of MGW and produces around 320 kg of biochar. The 370 kg CO 2 eq embodied in the steel used to fabricate a Kontiki thus needs to be amortized across the biochar produced during the lifetime of the kiln. The result is a minor level of emissions from the metal embodied in the oven (Table 1). To quench 320 kg of biochar produced in a Kon-Tiki we assume requires 378.54 L of water (after McAvoy and Dettenmaier 2019).
We assume the biochar produced with a ton of MGW is packaged in 6 laminated plastic bags that have an emission of factor of 0.637 kgCO2eq/bag (Ma et al. 2019). Once produced the biochar needs to be transported to its final destination. In Peru the most likely final destination is san Martin, Peru's most important Cacao growing region. In san Martin biochar is being used to remediate soils with high levels of cadmium, in response to Regulation (EC) No 1881/2006 of the European Union which prohibits the import of cacao that contains more than 0.8 ppm of cadmium. The distance from Lima to san Martin is 630 km. We again assume 340 g of CO 2 eq emitted per ton of material, per km, in a medium sized truck (Seo et al. 2016). We assume a medium sized truck because the medium sized trucks that bring agricultural product to Lima generally return less than full and could be used to transport biochar. In this scenario the biochar produced from one ton of MGW would result in an additional 71.5 kg of CO 2 eq of transport related emissions per ton of MGW processed.

Results and discussion
The analysis of the carbon footprint across the 4 alternative strategies for managing MGW suggests that biochar should be the preferred treatment method from a carbon accounting perspective, followed by composting, followed by final disposition in licensed or unlicensed landfills (Table 1). If we extend the carbon foot print analysis to consider the fate of the carbon contained in the compost and biochar produced in scenarios 3 and 4 a slightly different picture emerges. The half-life of compost can be as low as 45 days (Araújo et al. 2020). Biochar on the other hand may persist in soil over millennial time scales (Spokas 2010), and may even promote the accumulation of new non-pyrogenic soil carbon (Weng et al. 2017). Thus, if we assume a half-life of compost equivalent to the figure reported in Araujo et al. 2020, then an additional 130 kg of CO2eq will be liberated to the atmosphere from compost after the said 45 days have elapsed. The half-life for biochar is measured in decades to millennia with minimal loss of carbon to the atmosphere. Thus, the carbon footprint of biochar is likely significantly improved relative to compost. However, in a real-world context both composting 1 3 Vol.: (0123456789) and biochar production should be considered synergistic for management of MGW. MGW is heterogenous containing components that are easy to compost (e.g., lawn clipping, leaves) and complicated to pyrolyze, and a woody component that is difficult to compost but relatively easy to pyrolyze (Fig. 1).
Governance over waste management practices in developing nations such as Peru is far from optimal. If heavy investment in state-of-the-art pyrolysis or gasification technology is required before biochar production becomes eligible for carbon credits, then very likely it will remain business as usual with the continuation of high rates of GHG emission from poorly managed landfills. We think the nascent standards for carbon removals from biochar should be expanded to include low-polluting, low-cost technologies to enable participation by actors in developing world contexts. The calculations in Table 1 demonstrate that every ton of MGW diverted from landfill to artisanal biochar production decreases CO 2 eq emissions to the atmosphere by one ton. Extrapolating to the amount of MGW produced each month in Lima, we arrive at a figure of 1300 tons CO 2 eq emissions mitigated per month. If we then consider waste management in all the other megacities in the developing world where waste management likely starts from the same low base the potential for climate change mitigation begins to look promising.
The development of the nascent trading platforms for negative emissions generated by biochar are a welcome development. We also agree that implementing state of the art technology is a laudable goal that would allow maximum control of production parameters such as temperature. This in turn would allow the optimization of biochar for specific purposes (Ippolito et al. 2020). However, given the significant emissions reductions of low-tech carbonization of MGW over current practices outlined in this analysis, we argue that low tech carbonization of organics should be financially incentivized to rapidly motivate waste management operators in the developing world to adopt carbonization as the preferred management strategy. Leveraging the emerging carbon removal markets could be an effective pathway to providing the needed financial incentives.
Acknowledgements The authors would like to thank the following people who contributed to the article by commenting on the first drafts: Funding Seed funding from the research office at the Universidad Cientifica del Sur (grant 027-2021-PRO99) is gratefully acknowledged.

Conflict of interest
The authors declare no conflicts of interest. Fig. 1 Municipal green waste is a heterogenous mixture that includes components (cut grass) for which composting is an ideal waste management strategy. There is also a woody component obtained via pruning trees and shrubs that would require additional processing (chipping) before composting becomes a viable strategy. The woody component is also likely to contain a high lignin content which will increase the time, cost and emissions of composting