Rice is the staple food of the people of Bangladesh. Burning and landfilling of carbon-rich rice straw (RS) causes greenhouse gas emissions. On the other hand, cow dung (CD) and poultry droppings (PD) produced from the livestock sector in Bangladesh could be a potential threat to the environment if the wastes are not properly managed. However, anaerobic co-digestion of RS with CD and PD could be an effective means of biogas generation. Therefore, co-digestion of CD and PD with carbon-rich RS was conducted in batch assay at seven different mixing ratios (100:0, 90:10, 70:30, 50:50, 30:70, 10:90, 0:100) separately. Mesophilic condition (35 °C) was maintained for 92 days of digestion time to investigate biogas production potential and find out optimal mixing ratios of both co-digestion sets. Co-digestion of CD and RS at 70:30 ratio significantly showed maximum biogas yield (441.7 ± 54.1 ml/gVS). Additionally, an increase in biogas yield in this ratio was 212.11 % and 38.10 % compared to mono-digestion of CD and RS, respectively. Another co-digestion set of PD with RS showed highest biogas yield (344.8 ± 22.3 ml/gVS) at 90:10 ratio. The 90:10 ratio of PD and RS improved biogas yield by 173.16 % and 7.8 % as compared to mono-digestion of PD and RS, respectively. Co-digestion of RS with CD and PD had a statistically significant effect (P ≤ 0.05) on biogas production. Furthermore, kinetic modelling outcomes suggested the modified Gompertz model as ideal for forecasting biomethane production over time in both cases. The findings of this study will help in the implementation of ACoD at the field level.

Anaerobic co-digestion (ACoD) of cow dung (CD) and maize cob (MC) may be envisaged as the best way to enhance biomethane formation and production of nutrient-enriched fertilizer for the implementation of a circular bio-economic system. The study aimed to find out the optimum ratio for the highest biogas production to produce heat and energy and also the generation of nutrient-enriched organic fertilizer to use in crop land. A batch study was carried out for 99 days in an incubator maintaining 35 ± 1 °C temperature for seven different test groups of CD and MC (100:0, 90:10, 70:30, 50:50, 30:70, 10:90, and 0:100). The highest biogas production (356.6 ± 21.2 mL/gVS) was at 50:50 ratio with 138.05% and 32.02% increments compared to the digestion of CD and MC alone, respectively. Kinetic modeling showed the best fit using a Logistic model to evaluate ACoD of CD and MC mathematically. ACoD of available CD and MC in Bangladesh could produce 716.63 GWh/yr electricity for consumption and a large volume of nitrogen-enriched fertilizer to use in nitrogen-deficit soil. There was no significant difference in nutrient enrichment among different test groups. Awareness about ACoD technology and proper use of digestate might bring this technology to field-level utilization and thus help to implement the circular bio-economic concept through zero waste generation.

Solar drying is an easy and environmentally friendly way of food preservation. The use of a solar dryer for multicrop may also make this technology cost‐effective. In fact, the performance evaluation of a solar dryer for different fruits and vegetables promotes the usability of a dryer for crops. Hence, in this study, a newly developed natural convection solar dryer has been evaluated for drying cabbage. Cabbage samples were pretreated with a brine solution (with 5% [ w / w ] NaCl) followed by blanching. Pretreated samples were dried both in the dryer and open sun. Control samples were also dried in the solar dryer and open sun. The cabbage was dried in single and multilayer arrangements, maintaining thin layer drying thickness. Temperature distribution in and outside of the dryer was also investigated during the experiment, both spatially and temporally. The acceptability of dried cabbage was checked using a 9‐point hedonic scale test. The experimental value of the moisture ratio was fitted with four different thin layer drying models—Newton, Page, modified Page, and Henderson. The drying air temperature consistently exceeded the ambient temperature by approximately 1°C–35°C, depending on the intensity of solar radiation. Drying efficiency for single‐layer and multilayer arrangements were 25.25% and 43.57%, respectively. Hence, multilayer drying of cabbage in the dryer can be an effective means of drying with higher efficiency. From the acceptability test, it was found that solar dried pretreated cabbage was mostly liked. The best fitted model was the Page model for both single and multilayer drying. Performance evaluation for multilayer drying of other fruits and vegetables might increase this dryer’s usability.