• Energy Research

Digestates from low-tech digesters need to be post-treated to ensure their safe agricultural reuse. This study evaluated, for the first time, vermifiltration as a post-treatment for the digestate from a low-tech digester implemented in a small-scale farm, treating cattle manure and cheese whey under psychrophilic conditions. Vermifiltration performance was monitored in terms of solids, organic matter, nutrients, and pathogens removal efficiency. In addition, the growth of earthworms (Eisenia foetida) and their role in the process was evaluated. Finally, the vermicompost and the effluent of the vermifilter were characterized in order to assess their potential reuse in agriculture. Vermifilters showed high removal efficiency of chemical oxygen demand (55-90%), total solids (60-80%), ammonium nitrogen (83-97%), and phosphate-P (28-49%). Earthworms effectively grew and reproduced on digestate (i.e. earthworms number increased by 183%), enhancing the vermifiltration performance, while reducing clogging and odour-related issues. Both the vermicompost and effluent produced complied with legislation limits established for soil improvers and wastewater for fertigation, respectively. Indeed, there was an absence of pathogens and non-detectable heavy metals concentrations. Vermifiltration may be thus considered a suitable post-treatment option for the digestate from low-tech digesters, allowing for its safe agricultural reuse and boosting the circular bioeconomy in small-scale farms.

Low-tech digesters are sustainable technologies to produce a clean fuel (biogas), which can be used for cooking and heating. Moreover, the degradation of organic waste in the digester also produces a liquid effluent (digestate), which can be reused in agriculture as a biofertilizer. The direct application of digestate to the soil may not be either feasible due to regulations or safe due to poor quality. The study aims to apply, for the first time, a multicriteria analysis to select the most suitable solution for the post-treatment and reuse of digestate from low-tech digesters implemented in small-scale farms. The post-treatments considered are: a degassing tank, a sand filter, a vermifilter, digestate recirculation, a facultative pond, and combinations of them. Ten criteria and twenty-one sub-criteria including technical, environmental and socio-economic aspects are defined to be weighted by experts. The analysis employs triangular fuzzy numbers to consider uncertainty and increase the robustness of the results, and it is validated using three case studies in Colombia. The alternatives are designed using data collected in-situ on the design and operation of the digesters and the quality of the digestates. Results confirm the robustness of the rankings and show that vermifiltration is the best alternative for the post-treatment of digestates from low-tech digesters implemented in small-scale farms. It is due to the fact that it produces a high-quality biofertilizer, it is easy to manage and it is implemented using local materials. Other prominent alternatives are sand filtration, recirculation and a degassing tank coupled with vermifiltration. Peer Reviewed Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.1 - Per a 2030, garantir l’accés universal a serveis d’energia assequibles, confiables i moderns Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.a - Per a 2030, augmentar la cooperació internacional per tal de facilitar l’accés a la investigació i a les tecnolo­gies energètiques no contaminants, incloses les fonts d’energia renovables, l’eficiència energètica i les tecnologies de combustibles fòssils avançades i menys contaminants, i promoure la inversió en infraestructures energètiques i tecnologies d’energia no contaminant Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.b - Per a 2030, ampliar la infraestructura i millorar la tecnologia per tal d’oferir serveis d’energia moderns i sos­tenibles per a tots els països en desenvolupament, en particular els països menys avançats, els petits estats insulars en desenvolupament i els països en desenvolupament sense litoral, d’acord amb els programes de suport respectius

This paper reviews the sector of waste-to-energy looking at the main processes and feedstock involved. Within this, incineration, gasification, pyrolysis, anaerobic digestion and hydrothermal liquefaction are named and discussed. Through the discussions and scrutiny, manure is highlighted as a significant source of ammonia, methane, and nitrogen oxides emission, estimated to be 40%, 22.5% and 28% respectively of the total UK's anthropogenic emissions. Manure, and indeed the pollution it poses, are shown to remain largely ignored. In waste to energy processing, manure is capable of providing biogas for a number of pathways including electricity generation. Anaerobic digestion is highlighted as a suitable process with the crucial capability of drastically reducing the pollution potential of manure and slurry compared to no processing, with up to 90% reduction in methane and 50% reduction in nitrogen oxide emissions. If the majority of the 90 million tonnes of manure and slurry in the UK were to be processed through biogas harvesting, this could have the potential of producing more than 1.615 TWh of electricity. As such, the economics and legislation surrounding the implementation of anaerobic digestion for manure and slurry are discussed. In the end, restraining factors that limit the implementation of anaerobic digesters on farms in the UK are discussed. These are found to be mainly capital costs, lack of grants, insufficiently high tariff systems, rather than low gas yields from manure and slurry.