The value of digestate for better recycling of nutrients in smallholder agriculture

Project number: SMP-19074
Requesting country: Kenya
Year: 2019
Budget: € 40.000
Project leader: Wolter Elbersen /
Knowledge institute: Wageningen University & Research
Project partners: Kind of Green Consulting, Biomass Research, SNV / Africa Biogas Partnership Programme, Kenya Biogas Programme

The value of digestate for better recycling of nutrients in smallholder agriculture

The number of anaerobic digesters in Sub-Saharan Africa is increasing rapidly at the moment, in part due to substantial efforts such as SNV’s Africa Biogas Partnership Programme and Dutch businesses The ABPP project constructed over 50,000 biodigesters in multiple countries including Kenya

Biogas produced in micro-digesters has economic, productivity, health, climate and sustainability benefits: it is a clean cooking fuel that replaces firewood, charcoal and kerosene. The bioslurry, ordigestate, that comes out of biogas systems is rich in nutrients and can be applied directly to crops and vegetables to improve yield output and contribute to soil structure. It is also applied to treat plant diseases and to repel insects.

Bioslurry can potentially replace chemical fertilizers, which saves money and benefits the environment. As fertilizers and animal manure are scarce in Sub Saharan Africa, it is imperative to make the best possible use of bioslurry. Treatment in digesters will affect the fertilizer value of the substrate (e.g. animal manure, organic residues) since the partial breakdown of organic materials increases nutrient availability. Other advantages of bioslurry include its physical structure (being more even), its contribution to combat acidification (often having a higher pH than the substrate), reduced numbers of contaminants. However, there is a huge variation in bioslurry composition and expert knowledge on its application amongst African smallholders still is lacking. One of the main problems is the lack of clear and safe recommendations for bioslurry application in the field.

To address this problem, in this project a desk study will be carried out to search the literature for knowledge that describes the influence of anaerobic digestion on the plant nutrition value and fertilizer effect of different feedstocks (animal manures, crop residues, food waste, etc.). The obtained knowledge will be compiled into factsheets and simple calculation rules. These rules will enable Kind of Green Consulting and Biomass Research B.V. to develop a mobile tool that allows farmers to keep track of the quality of the bioslurry, given the types of feedstock and the conditions in the digester, and provides advice on the approximate quantities that should be applied on crops such as maize, coffee, fruits and vegetables.