However, physico-chemical pretreatment technology is limited
due to the cost of the coagulants, the energy required for
adding and mixing coagulants, and the increased sludge handling.
Anaerobic digesters such as up-flow anaerobic sludge blanket and
hydrolytic up-flow sludge bed systems have been proved alternative
pretreatments for CWs. Álvarez et al. (2008) reviewed performances
of 16 CWs combined with anaerobic digesters and found
high removal of COD (74 ± 12%) and BOD (83 ± 9%).