Haridas et al. [42] operated a Buoyant filter bioreactor (BFBR)packed with polystyrene balls and biogas was reintroduced tothe reactor to fluidize the bed material and prevent the clog-ging. COD removal efficiency was above 85% during the treatmentof simulated milk wastewater at OLRs applied. Increase in OLRenhanced biogas production, and methane yield was 0.37 m3kg−1CODremovedat OLR of 10 kg COD m−3d−1which was 100% close totheoretical value. Scum formation inside the BFBR was observedduring the increment of OLR; however, accumulated scum wasdegraded when wastewater feeding stopped. Comparing the CODperformance