Fulvic acids (FA) were extd. from leachate of municipal land-fill and broth (concns. of 10 mg/L) and used for the prepn. of aq. mixts. with 1 g samples of active sludge enriched with archea and optionally with an antibiotic (1 mL/100 mL). Changes in the COD and total N concn. were examd. as a function of time of sludge exposure with FA or broth. The COD was reduced from 100% to 0% or 79% for samples with broth and FA after 3 days, resp., and remained const. for the sample with FA and the antibiotic. The concn. of N in the sample with FA increased approx. 3 times as a result of the impact of FA on microorganisms in the sludge, while in sample with broth was reduced to 0 mg/L.