Optimization of Anaerobic Digestion for Biogas Production from Fruit and Food Wastes - A Lab-Scale Study

Preliminary research has shown that through anaerobic digestion; vegetable, fruit and food wastes of high calorific contents can be transformed to a source of energy through the production of biogas in this day and age of energy insufficiencies. However, the process of anaerobic digestion has been observed to suffer from a rate limiting step which is hydrolysis.

 

The purposes of this study were to investigate adjusted modes of substrate pre-treatment (Mechanical, thermal and Chemical) and their role in maximizing the process of anaerobic digestion through speeding up hydrolysis and to compare production potentials of commonly available wastes in Nairobi for possible co-digestion in large scale production of biogas.

Four sets of experiments were carried out; each including collected waste samples of food (ugali+Rice),potato peels, banana peels and a mixture of the substrates mixed with rumen slurry in the ratio 3:1 under controlled mesophilic anaerobic conditions (37 ̊ C, pH 6.7-7.2) in the lab scale digester for a minimum retention time of seven days. The anaerobic environment was established by bubbling with nitrogen gas and the temperature generated using a hot plate and homogeneity established by periodic agitation

 

Three runs for each set up were carried out; first, the set ups the samples were initially chopped and left to stand in the open for two days (Mechanical pre –treatment), in the second additional heating to 70 ̊ C done (Thermal pre-treatment) and in the third set the samples were additionally treated with 2% NaOH (thermo Chemical pre - treatment).

 

Thermo-chemical pre-treatment was the most effective for speeding up hydrolysis with the    co digested substrates producing maximum biogas. The moisture content ranged between   67-83%. The pH reduced from 6.8-7.2 before digestion to 6.2-6.8 after digestion. The  desired C:N ratio was between 18:1 to 32:1 for Anaerobic Digestion. The gas produced was found to contain 63.89% methane, 33.12% CO2 and 3% other gases.

 

Moisture content, C:N ratio, and VS were found to have a  bearing on the gas production and with establishment of thermo chemical pre-treatment and co digestion as the best strategies for biogas production optimization from plant wastes, it is recommended that further research on the same be carried out and AD digesters on a larger scale embrace this technology.

 

Key Words: Anaerobic digestion, Hydrolysis, substrate Pre-treatment, Co-digestion, Mesophilic, C:N ratio.

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