Effect of slow biodegradable substrate addition on biofilm structure and reactor performance in two MBBRs filled with different support media

In this study, two moving-bed biofilm reactors (MBBR 1 and MBBR 2) filled with different size of carrier media (Kaldnes K 1 and Kaldnes K 1 micro, respectively) were subjected to soluble (sugar and sodium acetate (Ac)) substrate and mixture of soluble and particulate (particulate potato starch (PS)) substrate in a very high organic loading rate (12 kgCOD/m 3·d) at different temperatures (26 and 15°C, in MBBR 1 and MBBR 2, respectively). The effects of carrier type and substrate on biofilm structure and reactor performance have been studied. Starch was removed by adsorption at the biofilm surface and hydrolyzed which caused substrate gradient in MBBR 1, however, hydrolyzed uniformly within biofilm in MBBR 2. The biofilm of MBBR 1 was irregular due to filamentous structure growth due to the substrate gradient, while, it was regular in MBBR 2 due to uniform distribution of substrate. The performance of both MBBRs in ammonium, COD and TN removal decreased significantly when the amount of small particles in the reactor increased owing to feeding by starch, which led to biomass density decline. The type of media affected the quantity and distribution of attached biomass, which in turn influenced the activity of specific microbial functional groups in the biofilm. The biofilm in MBBR 2 was thicker and consequently nitrogen removal by denitrification was much higher. The lower temperature did not affect negatively the reactor performance in MBBR 2.