Low rate of anaerobic degradation together with inhibition problems ca
used by components such as milk fat and proteins were found to be reas
ons for restricted activity of high-rate anaerobic reactors operated w
ith dairy effluent. The present research was aimed to investigate the
influence of some relevant compounds, such as casein and milk fat and
their degradation compounds, on process efficiency. Methanogenic activ
ity and ATP content of the sludge were used as major follow up criteri
a for evaluating the influence of the tested compounds on process effi
ciency. It was found that casein was not degraded by anaerobic culture
s which were not pre-acclimated for this compound. On the other hand,
cultures acclimated to casein, utilized it very efficiently. Amino aci
ds typical for casein degradation were also used efficiently with no i
nhibitory effects under the test conditions and with no need for cultu
re pre-acclimatization. Milk fat was found to be a major cause for inh
ibition of methanogenic activity. It was found to have an immediate in
fluence on reducing gas production rate in reactors to which it was ad
ded. Milk fat was also found to have long term effects: sludge pre-exp
osed to milk fat exhibited reduced methanogenic activity when tested i
n medium free of milk fat. Milk fat also reduced the ATP concentration
in the sludge, indicating reduction of its total physiological activi
ty. Oleic acid, which is a major derivative of milk fat hydrolysis was
found to have inhibitory effects on methane production and on ATP con
centration. According to the results of the present research, it would
be advisable to treat dairy effluents by anaerobic digestion only aft
er reduction of the milk fat concentration below 100 mg/L, and after c
areful acclimatization of the digester culture to casein.