keywords: Methane gas, hydraulic retention time, chelating ligand, anaerobic digester
The need for alternative renewable energy sources has resulted in the use of methane gas as a renewable and environmentally friendly energy source as compared to non-renewable fossil fuels. Methane gas, which is the main constituent of biogas with fuel value, was generated from cow manure digester systems. In an attempt to improve methane gas yield; reduce hydraulic retention time, and decrease the concentration of hydrogen sulphide produced within the digester systems, ethylene diamine-N, N-diacetic acid, nitrilotri-acetic acid and diethylenetriamine-pentaacetic acid were introduced as chelating ligands. Several experiments and analyses were carried out which involved the use of the Biogas 5000 analyser, Flame Atomic Absorption Spectrophotometer; and other analytical tools. The results from these analyses showed that on addition of the chelating ligands, ethylene diamine-N, N-diacetic acid, nitrilotri-acetic acid and diethylenetriamine-pentaacetic acid, there was an increase in the methane gas yield ranging from 2 to 15% and a reduction in the production of hydrogen sulphide gas from 80 to 30%. Also, a reduction in hydraulic retention time was recorded from 50 to 25 days; and on further increase in chelating ligand concentration, the hydraulic retention time reduced from 25 to 19 days. Trace metals of Iron, Cobalt, and Nickel were present at a concentration that ranged from 0.001- 0.050 mg/L; these metals reacted with the chelating ligands introduced to form metal chelates. The formation of these metal chelates resulted in an increased bioavailability of essential nutrients, promoting growth and stability of the methane producing bacteria and the formation of elemental sulphur with the release of H+; thus, reducing hydrogen sulphide concentration. Also, the metal chelate formation resulted in the metal catalysis of the hydrolysis stage. Generally, the addition of chelating ligands to anaerobic digesters, decreases hydrogen sulphide concentration and hydraulic retention time, with a significantly increased in methane gas production.
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