TEMPERATURE EFFECTS ON ANAEROBIC DIGESTION OF CASSAVA PULP: THE CASE STUDY OF INDUSTRIAL APPLICATION

Cassava (Manihot esculenta) is one of the most important food crop in the world, especially in South Africa and South East Asia including Thailand  [1-3]. Thai cassava starch industry generates approximately 9.5 million tons per year of cassava pulp, with a moisture content of 80%. This byproduct is rich in organic compounds, predominantly starch (47.9–75.1%), followed by cellulose (4.11–25.8%), hemicellulose (4.20–16.8%), and lignin (1.2–8.2%) on a dry basis  [4]. In recent years, thermophilic anaerobic digestion has been implemented in Thailand for biogas production from cassava pulp. However, some biogas plants have reported low production efficiency due to a limited understanding of the effects of temperature on microbial activity. This study investigates the impact of temperature on microbial populations and biogas production performance.

This study examines biogas production from cassava pulp using a continuous stirred-tank reactor (CSTR). The process involves mixing 75 tons/day of cassava pulp with 300 m³/day of wastewater before feeding into two CSTR reactors (3,545 m³ each, 10-day retention). Heat Recovery Steam Generators (HRSG) regulate temperature, while screw press units separate solid residues for wastewater recycling. The produced biogas undergoes hydrogen sulfide removal and moisture elimination, ensuring its suitability for heat and electricity generation.

A study of temperatures affecting cassava pulp digestion was studied in CSTR lab-scale reactors. Five acrylic reactors (5 L) were inoculated with anaerobic sludge at 10 gVSS/L and operated at ambient to 50°C, adjusting temperatures incrementally. Cassava pulp, as a substrate, was fed daily at 3% TS concentration, maintaining a 60-day HRT in semi-continuous mode.  The organic loading rate of the digestor was 1.32±0.33 kg-TS/m³-d. Biogas production was measured via water displacement, with performance assessed using pH, alkalinity, and total volatile acid (TVA) monitoring.

The industrial anaerobic digestion was fed with cassava pulp at 75 tons/day. Two digesters were operated in a serial reactor setup, maintaining an OLR of 2.94 kg-TS/m³-d with a C/N ratio of 133. Performance evaluation at ambient temperature and 45°C was compared. The reactor operated with 2x HRT cycles to optimize microbial activity. The study analyzed biogas yield efficiency under varying operational conditions. While, the study of microbial population in anaerobic cassava pulp digestion used qRT-PCR to analyze microbial composition, starting with genomic DNA extraction via DNeasy PowerSoil Pro kit. DNA concentration was assessed using NanoPhotometer N60 Touch, followed by qRT-PCR quantification targeting functional genes and 16S rRNA. PCR conditions included 40 cycles, and analysis was performed with CFX96 Thermocycler, comparing results against standard plasmid curves.

After operating a 5 L reactor for six months, increasing the temperature from ambient conditions to 45°C enhanced organic acid production from cassava pulp by 10.53%, rising from 0.76 to 0.84 g-COD_TVA/g-TS_added. However, further increasing the temperature from 45°C to 50°C resulted in a decrease, with production efficiency dropping to 0.72 g-COD_TVA/g-TS_added. A similar trend was observed for methane production efficiency. Raising the temperature from ambient conditions to 45°C improved methane yield from 0.1 to 0.2 L-methane/g-TS_added. However, increasing the temperature to 50°C reduced methane production efficiency to 0.15 L-methane/g-TS_added.

Microbial population analysis revealed that increasing the temperature from ambient conditions to 45°C led to a higher proportion of Chloroflexi, with its highest abundance observed at this temperature. Chloroflexi plays a key role in carbohydrate degradation, producing monomers that serve as substrates for Spirocheatota, which subsequently convert these monomers into organic acids. This aligns with the peak organic acid production efficiency observed at 45°C. Additionally, Methanosaeta exhibited its highest proportion at 45°C, suggesting that methane production efficiency also reached its optimal level under these conditions.

Consequently, a temperature of 45°C was selected for application in a 7,000 m³ industrial biogas digester, resulting in an increase in solid digestion efficiency from 38.44% (ambient) to 41.33%. Moreover, biogas production efficiency improved from 359.09 m³/ton-TS-added (ambient) to 424.49 m³/ton-TS-added.