BACKGROUND Sludge humic and fulvic acids (HAs and FAs) can be utilized as a source of organic fertilizer. After alkaline sludge pretreatment, recovery of HAs and FAs and the impact on subsequent anaerobic digestion were studied. RESULTS Total content of HAs and FAs in the raw sludge was 112.5 mg g(-1) total solids, and HAs constituted the major fraction. After the raw sludge was disintegrated with NaOH, an ultrafiltration membrane with a molecular weight cut-off of 30 000 D was chosen for humic recovery from the alkaline lysate. 22.9% of HAs and FAs in the raw sludge were recovered and their total concentration was 4083 mg L-1 in the retentate. HAs and FAs provided 30% of the organic carbon in the recovered retentate, which could be processed for humic fertilizer together with the other co-recovered macromolecular organic substances. After alkaline pretreatment and humic recovery, the biogas yield increased by 29.4-49.2% during subsequent sludge anaerobic digestion. Due to the recovery of humic substances and the enhanced organic biodegradation, the removal rate of sludge volatile solids was improved by about 70% compared with the control. CONCLUSION HAs, FAs and other macromolecular organic substances can be recovered from sludge by alkaline pretreatment and ultrafiltration separation, and subsequent anaerobic digestion improved significantly. (c) 2013 Society of Chemical Industry