Raw sludge was pretreated, and the separated sludge liquid was used as substrate in a continuous operated up-flow anaerobic sludge blanket reactor to produce volatile fatty acids (VFAs). The highest VFA productivity of continuous fermentation with sludge liquid at an organic loading rate (OLR) of 10.0 kg COD/m(3)/d was about 5.0-fold and 4.0-fold higher than batch and semi-continuous fermentation with pretreated sludge slurry, respectively. Moreover, the liquid fermentation with an OLR of 10.0 kg COD/m3/d consumed the least energy, which was about 10.57% and 12.12% of batch and semi-continuous sludge fermentation, respectively. When combined with online nitrogen and phosphorus recovery, VFA production further increased by 20.67% and struvite recovery efficiency reached 1.98 +/- 0.28 g/g PO43-. The process showed high VFA production, low energy consumption and good nutrients recovery by continuous liquid anaerobic fermentation, significantly increasing the economic potential of VFA production from waste activated sludge.