Trace metals are known to affect the yield of biogas
independent of the feedstock. This is because of their effect on
the microbial contents of the digester during the anaerobic
digestion [47-49]. Demirel and Scherer [50] argued that there
is no direct formula for optimum composition of trace metals
needed for maximum biogas production. Our result shows that
for rice husk, Ni2+ (100 ppm) gave 300 mL/day while Zn2+
(100 ppm) gave 110 mL/day. For cow dung, Ni2+ gave 30
mL/day while Zn2+ gave 74 mL/day. No significant effect was
observed with Fe2+ in both RH and CD respectively. Table 3
gives the effect of trace metals on the digester
performance/biogas yield for different/same or different
feedstock according to the literature.