Organic waste such as food scraps or animal manure can be converted into energy when microbes feed on it and, in the absence of oxygen, produce biogas. Biogas can be used for heating and cooking for example. Some organic material called digestate is left after biogas production and it is a good fertilizer in agriculture that can replace mineral fertilizer. The solid fraction of digestate can be composted before using it. However, digestate processing and use can result in emissions of greenhouse gases, often larger than from the use of mineral fertilizers. NIBIO has previously found that greenhouse gas emissions during composting can be a problem as well.
In India, rice is the major crop, and important in peoples diet. Rice cultivation produces more greenhouses gases than other crops, because rice is cultivated in flooded soils where almost no oxygen is present. The microbes involved in biogas production are probably similar to those in flooded soils, because they do not need oxygen to survive. In this project, questions regarding greenhouse gas emissions from digestate application to flooded soils are being investigated. Spring and summer 2019, 4 students from the Indian partner organisation, KIIT, have carried out research for their master thesis at NIBIO. They have investigated the effect of applying digestate to flooded and non-flooded soil on emissions of methane, nitrous oxide and carbon dioxide, as well the fertilizer effect of digestate. They have also assessed the microbial communities in soil, how populations of microbes responsible for methane and nitrous oxide production respond to digestate applications. An experiment comparing greenhouse gas emissions from composting digestate and the undigested food waste has also been carried out. The results show almost 12 times higher emissions, particularly of methane, from digestate composting than from composting the food waste it was made from. When microbes in digestate were killed before composting, methane emissions were almost eliminated. This indicates that the microbial community adapted to high methane, that is biogas, production was the reason for the high emissions. We also found that heating only to 70C worked equally well, and this treatment could be recommended. The results suggest that digestate needs to be treated differently from other waste, but anaerobic digestion is still a favourable way to treat organic waste because it produces biogas that can replace fossil fuels.
The corona crisis has severely limited the interaction between the Norwegian and Indian partners, and consequently the development of ties between partners. Visits to India have been carried out, but mid-term meeting in Norway has not been possible. Field work intended in India has also been severely curtailed. However, lab work in Norway has proceeded to plan, and some work intended to be done in India has been taken over and carried out in Norway instead.
RICEDIG was a research project with collaboration with India.
Outcomes and impact on different levels:
Scientific: In this project we have studied digestate in rice paddy soil, waterlogged and not, and during composting. We have assessed greenhouse gas (GHG) emissions and microbial community. Results indicate that at least during composting, the microbial community imported with the digestate is important for GHG emissions. In waterlogged soil it changes the timing of emissions, but not magnitude.
Practical: Based on the results we recommend some mitigation measures to reduce methane emission when digestate is composted. A possible mitigation option is heating to 70C. We therefore suggest doing a hygenisation treatment after anaerobic digestion, instead of on the feedstock before it. In rice cultivation mitigation is not needed, but digestate should be stored covered before use.
Collaboration: Contacts with researchers and associations interested in biogas in India have been established, and six Indian students have done project work in Norway.
Production of biogas from agricultural residues and kitchen waste is one of the best ways to reduce the global warming footprint of agriculture and the food chain. Digestates are residues from biogas production, rich in nutrients but potentially inducing greenhouse gas (GHG) emissions when processed, stored and applied to soil. Composting of digestate might reduce emissions once in soil, but there may be emissions from the composting process, as NIBIOs ongoing research indicate. In India, rice grown in flooded soil is an important source of methane. Biogas production is currently promoted in India as well, but there is little research on the effect of digestate and digestate products on GHG emissions. Ricedig will investigate GHG emissions from applications of digestate and digestate products to flooded soil/rice cultivation and digestate composting. A large number of digestates and digestate products will be compared in an incubation experiment. Digestate and the corresponding substrate will be compared in a composting trial, and digestate and composted digestate will be compared in a field experiment. Microbial diversity will be assessed in all products and experiment, particularly methanogenic diversity. From this it will be possible to understand GHG, particularly methane emission from flooded soil when digestate is applied, and how digestate treatment, particularly composting affects the global warming potential.