Optimization of small-scale low-cost anaerobic digestion system: Model-based analysis

In this research thesis, the modelling of biogas production through the anaerobic digestion (AD) process is investigated, with a key focus on the modelling of low cost small -scale plastic bag types digester, which has a potential application in the rural communities of the developing countries. The present work investigated the thermal modelling of the plastic bag digester and its integration with simplified anaerobic digestion models to assess the feasibility and to develop operating protocols. In order to develop the AD process model, a parameter estimation method was developed, and this was used to describe the methane production of from food waste, green waste and pig manure, using data obtained from batch and semi-continuous experiments. It was found that in the case of a semi-continuous process, the model structure, rather than kinetic parameter values or kinetic equation used, determines the ability of the models to describe the degradation of food waste and green waste. For food waste, inhibition also played an important role in the model’s ability to reproduce methane production data obtained experimentally. Further, it was found the kinetic parameter values obtained from the batch and semi-continuous processes are different. The kinetic parameter values or the best kinetic equation obtained from the parameter estimation method were used as input in the biochemical model, which is combined with a thermal model to simulate the performance of different designs of the plastic bag digester in Port Harcourt In Nigeria and Cuzco in Peru. Three designs of the digester were model; CASE 1 simple plastic bag digester without greenhouse cover, CASE 2 plastic bag digester with a greenhouse cover, and CASE 3 plastic bag digester with greenhouse cover and solar heating system. It was found that the addition of external heating sources such as greenhouse and solar heating system in CASE 2 and CASE 3 in Port Harcourt in Nigeria and Cuzco in Peru, has a positive impact on the performance of the digester (improved slurry temperature) compared with CASE 1. However, CASE 3 is not suitable in Port Harcourt, in Nigeria, since the difference in the slurry temperature in CASE 3 and CASE 2 is 1.48 oC, while the model predicts the suitability of CASE 3 in Cuzco, in Peru. The combined thermal model (CASE 2) and the biochemical model (3R) was used to model the operational protocol for small scale digester in different climate of developing countries. The protocol was developed for mono and co-digestion processes. It was found that minimum HRT (Hydraulic retention time) and maximum OLR (Organic loading rate) of a specific feedstock is different given the difference in. For the co-digestion process, the substrate characteristics and the substrate mix ratio have a strong influence on minimum HRT and maximum OLR