Formation and Detachment of Biofilms and Granules in a Nitrifying Biofilm Airlift Suspension Reactor

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Abstract
The influence of the bare basalt carrier concentration on nitrifying biofilm formation in biofilm airlift suspension (BAS) reactors was studied. The time needed to develop fully covered biofilm particles strongly increased with an increasing initial carrier concentration. This was caused by the increased shear and subsequent higher detachment. During startup the diameter of the biofilm particles increased, leading to a higher biomass concentration, larger biofilm surface area, and lower surface‐specific substrate load. Eventually a “steady state” thickness is obtained, where surface‐specific growth and detachment are equal. The biofilm density decreased with a decreasing bare basalt concentration. The steady state biomass concentration was not very much influenced by the initial carrier concentration. Up to 75% of the produced biomass was retained in the reactors. Maximum biomass retention was found when the initial bare basalt concentration was in between 10 and 30 g·L−1. The continuous detachment of biofilm fragments leads, upon growth of these small fragments, to the formation of granules (biofilms without a carrier). The granules have average density and size equal to those of the biofilm particles. The granules are formed, if the detached biofilm fragments are large enough to be retained by the three‐phase separator on top of the BAS reactor. Since the superficial liquid velocity in the three‐phase separator is scale dependent, granule formation will be influenced by reactor scale, hydraulic load, and dimensions of the three‐phase separator.