Fabrication and Optimization of an Effective Anaerobic Digester for Biogas Production Using Vegetable Wastes From Wakulima and Kangemi Markets in Nairobi County, Kenya

Abstract

This work focuses on generation of biogas and voltage from market wastes inoculated with abattoir wastes. The market wastes were analyzed for proximate and ultimate composition using standard techniques. Bacterial studies of the inoculum involved microbial counts, isolation in anaerobic conditions and bio-chemical analysis. Biogas production was done at psychlophilic, mesophilic and thermophilic conditions using market wastes. The influence of acidic and alkaline waste pretreatments, pH, temperature, C: N ratio, inoculum to substrate ratio and proximate properties was also investigated. Biogas upgrade was studied using zeolite rocks, desulphurizer, maize cobs, steel wire and worn out tyres cartridges. A portable digester was fabricated which incorporated agitation, pH monitoring and temperature regulation mechanis with an Arduino-based automatic biogas leakage detection and mitigation measures. A 1450 L Ferro-cement and a 14000 L bricks pilot scale digesters were constructed. Bio-slurry was employed in vegetable and maize farming. Finally, waste conversion to electricity was studied using microbial fuel cell technology at optimized conditions. The results obtained in this research show that the microbial counts in rumen fluid and cow dung were 3.15±0.01 * 1010 cfu/mL and 1.50 ±0.02* 10 10 cfu/mL respectively. The volatile solids were found to be 81.69±1.52 and 73.50±2.20% of the total solids while the C: N ratio was 29.62±0.51 and 17.06±0.50 in rumen fluid and cow dung respectively. Thermophilic biogas production was highest in waste mixtures at 4700 mL for the 1.5 L reactor capacity. The thermochemical pretreatment results in more cumulative biogas production at 6200 mL, followed by thermal at 4900 mL and then chemical pretreatments at 3750 mL for 500 g mixed fruits and vegetable market wastes for 500 mL -1500 mL digester capacity. The optimal pH observed in this study was 6.70 – 7.23. Biogas production was highly dependent on proximate properties like moisture, carbohydrates, fat and protein levels. The best working range for C: N ratio was 19 – 30, with higher levels significantly reducing biogas production. The biochemical methane potential studies revealed that generated biogas was 1000 to 3500 mL/g.VS with CH4 levels of 56 – 60%. The measured level of raw biogas was viii 227ppm H2S, >20% CO2 and 52-56% CH4. The most efficient upgrade material was zeolite rocks with upgrade levels of 89 – 93% methane. The total removal for zeolite was observed to be 75% for CO2 and 95.34% for H2S. A re-engineered digester with automatic loading, agitation and pH and temperature regulation mechanisms was fabricated and biogas yields studied from the pilot scale studied. A portable biogas safety device was designed and developed using Arduino micro-controller. The device alerts the user in the event of excess smoke or fire breakout via a call or SMS using the SIM900 GSM module. Microbial fuel cell technology was employed in direct conversion of market wastes to electricity. The results obtained from the MFC indicated that voltage recovered increased with time. On average, avocado and watermelon produced 0.357V and 0.009V, respectively. The power density generated was 0.060856 to 22.53043 μW/M2, while the current density was 0.751315 to 63.11044 mA/m2. Clostridium Spp., Proteus and rumen fluid generated 0.622 V, 0.465 V and 0.759V, respectively. The data obtained from varying MFC operating parameters indicate that 6.6668 * 10-3 m2 electrode S/A produced 0.00399 m2 and 0.01331 m2 voltage and power, respectively. Tomato wastes generated 0.385 V, 0.038 mA and 0.01463 Mw, voltage, current and power, respectively across 45 KΩ resistor. Anaerobic digestion and microbial fuel cells technologies are recommended for market and abattoir wastes management. Keywords: Arduino, Biogas, Bio-methane, Market wastes, Microbial fuel cells