Control of heterotrophic layer formation on nitrifying biofilms in a biofilm airlift suspension reactor

Abstract

A Biofilm Airlift Suspension (BAS) reactor was operated with nitrifying biofilm growth and heterotrophic suspended growth, simultaneously converting ammonium and acetate. Growth of heterotrophs in suspension decreases the diffusion limitation for the nitrifiers, and enlarges the nitrifying capacity of a biofilm reactor. Neither nitrifiers nor heterotrophs suffer from additional oxygen diffusion limitation when the heterotrophs grow in suspension. Control of the location of heterotrophic growth, either in suspension or in biofilms over the nitrifying biofilms, was possible by manipulation of the hydraulic retention time. A time delay for formation and disappearance of the heterotrophic biofilms of 10 to 15 days was observed. Surprisingly, it was found that in the presence of the heterotrophic layers the maximum specific activity on ammonia of the nitrifying biofilms increased. The reason for the increase in activity is unknown. The effect of heterotrophic biofilm formation on oxygen diffusion limitation for the nitrifiers is discussed. Some phenomena compensating the increased mass transfer resistance due to the growth of a heterotrophic layer are also presented.

Nitrifying and heterotrophic population dynamics in biofilm reactors: effects of hydraulic retention time and the presence of organic carbon

“…The mass accumulation rate for nitrifying biofilm of 0.03 kg m À3 d À1 , [21] excludes the chance of such a fast recovery by re-growth. Keeping in mind previously published data [2] our findings were unexpected since we assumed that reactor R 2 will, due to its longer retention time, favor suspended growth of the fast reproducing heterotrophs and thus allow for a higher biofilm-mediated nitrification during the presence of acetate.…”

“…This separation can be achieved by extending the reactor's hydraulic retention time. At comparatively long hydraulic retention times, the fast growing heterotrophic microorganisms (with reciprocal maximum specific growth rates smaller than the selected hydraulic retention time) grow mainly in suspension while the slow growing nitrifiers form biofilms [2]. However, in this study the reactor was operated at a high oxygen concentration of 6 mg l À1 , a condition which did also allow full nitrification in a conventional biofilm reactor (nitrifiers and heterotrophs in the biofilm) [2].…”

“…What remains to be discussed is (i) why a thicker heterotrophic biofilm is formed on top of the nitrifying biofilm in reactor R 2 compared to reactor R 1 and (ii) why heterotrophic biofilm formation and subsequent loss of nitrifying capacity was not observed in the airlift reactor of [2] after addition of organic carbon. Heterotrophic biofilm formation in reactor R 2 could be explained by increased liquid phase viscosity in this reactor due to the presence of extracellular biopolymers which accumulated compared to reactor R 1 due to the increased liquid retention time in reactor R 2 .…”

“…The competition between heterotrophic and nitrifying bacteria for substrates (oxygen and ammonia) and space in biofilms is of major practical importance and thus has been the subject of several previous studies (for example [1][2][3]). According to these investigations, competition in biofilms results in a stratified biofilm structure, the fast growing heterotrophic bacteria being placed in the outer layers, where both substrate concentration and detachment rate are high, while the slow growing nitrifying bacteria stay deeper inside the biofilm.…”

Nitrification in fixed-bed reactors treating saline wastewater

“…Besides that, the heterotrophic bacteria compete with nitrifying bacteria for optimal growth space on the carrier materials of fixed.bed reactors (Ohashi et al 1995, van Benthum et al 1997.…”