In airlift reactors the biomass is immobilized on small (200-300 μm) carrier particles, suspended in the water phase. If domestic wastewater is treated in these systems, the soluble and colloidal compounds are generally removed with an efficiency of 75±5%. However, the overall treatment results are negatively affected by the high amount of suspended material in the effluent. Different fractions in influent and effluent were analyzed using microscopic techniques and chemical analyses to collect more information about the behavior of suspended solids. Settlable (Se), centrifugable (C), filterable (F) and soluble (So) fractions were obtained by sequential settling, centrifugation and filtration. After correction for sorbed soluble compounds the fractions C, F, and So contributed 30%, 20% and 50% respectively to the COD of presettled wastewater. During periods when biofilms were compact/smooth and the population of protozoa in the reactor was small, size and composition of the C-fraction only slightly changed when passing the reactor. The removal efficiency of this fraction increased from less than 10% to 15-25% if large populations of protozoa were present, which were able to consume more than 99% of the free cells. Additionally, a small amount of settlable compounds (< 20 mg COD/l) was formed in the reactor. This amount increased to 40-60 mg COD/l as soon as the surface of the films changed from smooth to very irregular due to growth of “streamers” and/or filamentous bacteria. Retention of this suspended biomass under these circumstances resulted in the development of an activated sludge process under these circumstances. The advantages of the biofilm process were thus partly lost.

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