The feasibility of the so-called BIOFIX-process to treat municipal wastewater was investigated in a pilot-plant. The innovative element of this process is that carrier material to which a biofilm is attached is recirculated between a sorption reactor to take up COD from the wastewater and a reactor where this COD is used for (post-) denitrification. In between the sorption and denitrification reactor the wastewater passes two subsequent reactors for removal of the remaining COD and for nitrification, respectively. The results showed that COD uptake by the biofilm in the sorption reactor with a maximum of 34% of the influent load was far below expectations and did not meet the COD requirement for denitrification (50–60% uptake). Also, 9–21% of the influent load of ammonia was taken up by the biofilm. In the denitrification reactor this ammonia was released to the bulk and in this manner discharged with the effluent without having passed the nitrification reactor. Nitrification was inhibited by the presence of high concentrations of suspended solids (50–60 mgl–1) discharged from a COD removal reactor. Together these bottle-necks caused effluent concentrations which were well above the effluent demands and it therefore can be concluded that the BIOFIX-process is not a feasible process to treat domestic wastewater.

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