Abstract: Elodea nuttalli,Potamogeton maackianus and Alternanthera philoxeroides were crushed,pressed and then filtrated to get filtrate of hydrophyte as fermentation material.Effects of varying levels of anaerobic sludge and additional N on liquid state fermentation were studied.The results show that highest biogas production（863.2 mL）in which CH₄ content was 720～750 mL·L^-1 in produced biogas and COD_Cr removal rate（90%） were reached within 10 days after 200 mL·L^-1 of anaerobic sludge was added.Biogas production from anaerobic fermentation of press filtrate of Elodea nuttalli,Potamogeton maackianus and Alternanthera philoxeroides was increased by 2%,10% and 30%,respectively,when 1 g·kg^-1 N was （added）.

Key words: hydrophyte, press filtrate, anaerobic fermentation, nitrogen, biogas