A biogas plant using livestock waste in which a methane fermentation process is applied is a useful facility for generating energy. The digestion liquid generated from the biogas plant as a residue has high potential for use as a crop fertilizer. However, high-density estrogens such as 17 beta-estradiol (E2) are included in livestock waste, and there is little information on the behavior of E2 in the digestion liquid. In this Study, a survey of E2 concentration at each process in a biogas plant using livestock waste was carried out. In addition, the efficiencies of E2 removal from the digestion liquid by activated carbon adsorption and soil infiltration were examined. The total concentration of E2 in raw livestock waste was reduced to 2 mu g/l after treatment, and the removal efficiency of E2 was about 80% for the plant. The methane fermentation process is important not only for the generation of methane but also for the removal of E2. The proportion of E2 conjugates comprising the total E2 concentration was 10% or less in all treated samples. In the plant, there is no likelihood of an increase in estrogen activity by the cleaving of E2 Conjugates. By carrying out activated carbon adsorption to remove E2 from the digestion liquid, a large portion of E2 was removed from the digestion liquid, but an E2 concentration of 0.5 mu g/l still remained in the treated digestion liquid. In contrast, it was possible to purify the digestion liquid to an E2 concentration of less than 0.002-0.011 mu g/l by soil infiltration. It is thus possible to utilize the digestion liquid as a fertilizer without causing aquatic environmental Pollution, but factors such as application rate, soil characteristics, and the E2 concentration of digestion liquid should be considered first. (C) 2008 Elsevier B.V. All rights reserved.