Family farmers in Brazil could diversify their sources of income and improve agriculture practices by adopting circular economy principles on their farms. Closed-loop technological systems can be used to manage organic waste and produce fertiliser and biogas thereby generating revenue. Anaerobic Digestion (AD) is a proven technology that can produce digestate (i.e. fertiliser) and biogas from organic waste, although digestate application in soil and crops without treatment can have adverse effects. However, in practice, there is a lack of knowledge about the benefits of recycling organic waste in farming communities in Brazil. Therefore, the main aim of this paper is to provide conceptual design configurations of closed-loop systems that manage organic waste and generate revenue for small farms in Brazil. A literature review of selected technologies and interviews with Brazilian family farmers were used to inform the components of the proposed conceptual designs. The proposed designs are based on circular economy principles, incorporating AD, pyrolysis for biochar, hydroponics and vermifiltration in various configurations. A complete closed-loop system consisting of a 7.5 m³ digester, pyrolysis unit, a combined hydroponic and vermifilter unit and a shredder is estimated to cost around USD$1600 (R$ 6600). The flexibility of the proposed systems has the potential to increase resilience and income for small-scale farmers, whilst encouraging good practices for waste management. The conceptual designs can be used as a basis for further research and development of small-scale organic waste management solutions in Brazil. GRAPHICAL ABSTRACT

Anaerobic digestion (AD) systems are prominent in low- and middle-income countries (LMICs), although their implementation within organic farms to enhance resource efficiency and “close the loop” has been limited. This paper thus reviewed existing literature on the potential of AD technologies as part of closed-loop rural family farming communities in LMICs. Data from eleven existing case studies matching this criterion was then collated to understand practical considerations of implementing and maintaining viable AD systems for small farmers. The case studies analyzed indicate that most, if not all, of the biogas produced in the AD process is used for household purposes such as cooking, lighting and heating. The AD systems are either based on the fixed biogas dome or the floating drum design, although the tubular flexible balloon model is mentioned as a low-cost alternative. Future research opportunities in this topic include studying the applicability of recommendations offered across different geographies, consideration of long-term sustainability and impact of biodigester technology, and sociocultural factors such as community ownership and indigenous practices.

Innovative breeds of sugar cane yield up to 2.5 times as much organic matter as conventional breeds, resulting in a great potential for biogas production. The use of biogas production as a complementary solution to conventional and second-generation ethanol production in Brazil may increase the energy produced per hectare in the sugarcane sector. Herein, it was demonstrated that through ensiling, energy cane can be conserved for six months; the stored cane can then be fed into a continuous biogas process. This approach is necessary to achieve year-round biogas production at an industrial scale. Batch tests revealed specific biogas potentials between 400 and 600 L_N/kg_VS for both the ensiled and non-ensiled energy cane, and the specific biogas potential of a continuous biogas process fed with ensiled energy cane was in the same range. Peak biogas losses through ensiling of up to 27% after six months were observed. Finally, compared with second-generation ethanol production using energy cane, the results indicated that biogas production from energy cane may lead to higher energy yields per hectare, with an average energy yield of up to 162 MWh/ha. Finally, the Farm²CBG concept is introduced, showing an approach for decentralized biogas production.