Biogas production from kitchen waste co-digested with selected municipal solid waste and its purification using soda ash from lake Natron and eggshell waste

Abstract

The amount of kitchen and municipal waste increases along with consumption standards and urbanization, thus, most cities face the problem of managing it. It can be transformed into energy, addressing the twin challenges of waste management and energy insecurity. The presence of impurities in biogas limits its application hence purification needed. This study investigated the potential of kitchen waste co-digested with municipal solid waste for biogas production and purification using soda ash and eggshells. The specific objectives were to: characterize kitchen waste and municipal solid waste for biogas production; optimize the biogas production through co-digestion; assess the adsorption ability of soda ash and eggshells sorbents; evaluate their regeneration and reuse; and modeling of fixed bed adsorption studies. Standard methods characterized kitchen and municipal solid waste for moisture content, volatile solids, and total solids. Biogas was collected via water displacement method, and digestion temperatures were 20-40°C. Brunauer-Emmett-Teller, Barrett-Joyner- Halenda, and quanta chrome NOVA 4200 methods were used for sorbent characterization. Biogas purification was conducted using soda ash from Lake Natron and eggshell waste. The effect of particle size (280-400μm), sorbent mass (25-75g), flow rate (0.03-0.04m 3 /h), and calcination temperature (750-900°C) for eggshells was studied. Regeneration was done via soda ash exposure in air, for 1, 5, and 7 days, eggshells were re-calcinated under 750°C. The data were fitted into kinetic and breakthrough models. The results indicated that cabbage contained 96.36±1.73% volatile solid produced a biogas yield of 800±8.8mL within 10 days, while cooked rice had an 83.00±1.49% volatile solid, produced biogas 2821±31.03mL within 28 days. Furthermore, co-digestion of kitchen and municipal solid waste showed that a mixing ratio of 1:1 produced the highest biogas yield (2907± 32mL). The lowest yield of 2907±32mL was obtained at 20°C while the highest yield of 4963±54.6mL was obtained at 40°C. Regarding pH, the yield was 2808±31mL at pH 6.5 and 7810±86mL at pH 7.3, indicating a 178.1% increase in biogas yield. Samples sieved at 280μm, 75g, and a flow rate of 0.03m 3 /h perform best with a removal efficiency of 94%, and sorption capacity (SC)of 0.02g/100g for soda ash while eggshells calcined at 850°C, had a RE of 83%, SC of 5.0g/100g. Regeneration for 7 days of exposure showed the highest RE of 90%. Meanwhile, in the first cycle, regenerated eggshells showed a RE of 79.8% and an SC of 4.97g/100g. The experimental data fitted well to the Freundlich for both H 2 S and CO 2 removal with a range of 0<1/n<1. Breakthrough studies showed that the data for carbon dioxide was best fitted to the Thomas model with R 2 0.94-0.99 while for H 2 S removal Yoon-Nelson model was the best with R 2 0.93-0.98. The CO 2 and H 2 S uptake were fitted well to the intra-particle model. In conclusion, cooked rice waste could be mono-digested while others require co-digestion to increase yield. The high RE and SC obtained show that soda ash and eggshells are promising materials for biogas purification. The study recommends the use of bio-waste in biogas production to address the twin challenges of waste management and energy insecurity.