Exploring drying kinetics and fate of nutrients in thermal digestion of solid organic waste

Optimisation of process parameters of a thermal digester for the rapid conversion of food waste into value-added soil conditioner

A novel thermal digester for converting food waste (FW) into nutrient-rich soil conditioner was designed and explored. The process variables, that is, temperature, the volume of the digestion chamber and the rotational speed of the digester were optimised using response surface methodology (RSM). The study revealed that the digester temperature of 150°C and rotational speed of 40 RPM required minimum time (180 minutes) for attaining the equilibrium moisture with a minimum energy consumption of 0.218 kWh kg-1. The process resulted in 80 ± 2.5% reduction in total volume of the FW. Detailed characterisation revealed that the end product was comparable to the organic fertiliser as per the Fertiliser Association of India norms. The digestion helps in breakdown of cellulose content of FW into hemicellulose which supports formation of primary and secondary walls, seed storage carbohydrates, and facilitates plant growth. 1H-Nuclear magnetic resonance (1H-NMR) spectra of the end product revealed mineralisation of organics during digestion. Decrease in ultraviolet (UV) absorbance value at 280 nm also revealed the humification of the end product. X-ray diffraction (XRD) analysis disclosed extremely low crystallinity and non-recalcitrant nature of the end product. A low humification index value (HI-3.43), high fertilising index (FI-4.8), and clean index (CI-5.0) revealed that the end product could safely be utilised as an organic fertiliser. The cost-benefit analysis revealed that thermal digestion technique is profitable and economically viable with benefit-cost ratio (BCR) of 1.35. The study offers a unique approach for the rapid and hassle-free production of value-added soil conditioner from FW.

Investigating the dynamics of ammonia volatilisation and the role of additives in thermal digestion of food waste

Averting nutrient volatilisation in thermal treatment of organic waste is a challenging task. The dynamics of ammonia volatilisation and the role of additives in preventing the losses of nutrients in thermal digestion of food waste (FW) were explored. The experimental trials were performed in a convective dehydrator at different combinations of temperatures and airflow velocities. The study dictated that ammonia volatilisation rate increased with increase in temperature and airflow velocity. The losses reached to its peak during the initial drying period and then gradually declined in the falling rate period. An artificial intelligence-based random forest model was explored to precisely predict the ammonia losses during the drying process. The SEM-EDX images confirmed enhanced N (2.25%) in the alum treated end product compared to blank (N - 1.8%) and thus reveals alum induced mineralization of nutrients. Higher intensities of the N containing compounds peaks observed in FTIR spectra also supported the mineralization of nitrogen. XRD analysis indicated formation of stable ammonium compounds in the sample digested with alum. Cost benefit analysis of the alum aided digestion revealed that it enhances the nutrient retention and overall cost of N in the end product by ₹626/tonne. The study revealed high potential of alum in reducing the ammonia volatilisation and enhancing the agronomical value of nutrients in the thermal digestion process.