Maintenance of sufficient methanogenic populations is critical for stable p
erformance of anaerobic systems. The usually monitored parameters like COD
removal, VFA levels, quantity and composition of biogas produced etc., do n
ot reflect the composition of biomass under varied operational/environmenta
l conditions. The change in biomass composition in terms of relative popula
tion levels of methanogens has been indirectly assessed through methanogeni
c activity tests using two different substrates at equivalent COD load. The
results of activity tests have been well correlated with the performance r
esults of the bench-scale upflow anaerobic filter. This clearly suggests th
e use of the methanogenic activity test to monitor biomass composition alon
g with usually monitored parameters for obtaining a better insight into the
reactor stability and performance.