L. Tijhuis

Influence of detachment, substrate loading and reactor scale on the formation of biofilms in airlift reactors

For a stable and reliable operation of the biofilm airlift suspension reactor (BAS reactor) means to control biomass concentration, biofilm thickness and biofilm morphology are required. For this reason, the influence of applied detachment forces and surface substrate loading on the formation of heterotrophic biofilms in laboratory-scale BAS reactors was studied. Detachment forces were altered by variation of the initial bare carrier concentration or the superficial air velocity. In addition, the dynamics of biofilm formation during start-up of a full scale BAS reactor (300 m3) was monitored and compared with the laboratory-scale start-up (31). This study shows that the biofilm morphology and strength were influenced to a large extent by the surface substrate loading and applied detachment forces. A moderate surface substrate loading and a high detachment force yielded smooth and strong biofilms. The combination of a high surface substrate loading and low detachment forces did lead to rough biofilms, but did not lead to the expected high amount of biomass on the carrier, apparently because of the formation of weaker biofilms. The strength of the biofilms appeared to be related to the detachment forces applied during biofilm formation, in combination with the surface substrate loading. The biofilm morphology and biomass on carrier in the BAS reactor can be controlled using the carrier concentration, substrate loading rate and the superficial air velocity as parameters. The dynamics of biofilm formation during the start-up of a full-scale BAS reactor proved to be similar to heterotrophic biofilm formation in laboratory-scale reactors. This indicates that a model system on the laboratory scale can successfully be applied to predict dynamic phenomena in the full-scale reactor.

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

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.