Impact of hydraulic retention time (HRT) and anti-foaming chemicals on biofilm development in moving-bed biofilm reactor (MBBR) carriers.

This experimental thesis, conducted over a total duration of six months at the AnoxKaldnes Company located in Lund (Sweden), aims to first verify, through a pilot plant, the possible impact of two antifoam agents such as silicone fluids and hydrophobic silica on a water treatment plant. In particular, it was observed how these substances would affect the reduction of soluble COD and the growth of biofilm on Anox KTM 5 supports. Through an in-depth study of a series of parameters and microscopic analyses, it was found that these antifoam chemicals did not compromise the reduction of soluble COD. Furthermore, as predicted by the literature, it was observed that hydrophobic silica had a negative impact on biofilm growth, thereby slowing it down compared to the growth observed with the second antifoam substance considered (silicone fluids), without compromising the percentage of degradability related to soluble COD. At a later stage, with a new pilot plant, it was analysed how the variation of a fundamental parameter such as the hydraulic retention time (HRT) would compromise the reduction of soluble COD and the growth of biofilm itself, also finding in this case no correlation between the different HRTs used and the degradation of sCOD, due to the fact that all three systems considered with different HRTs led to a high percentage of contaminant removal (i.e., soluble COD). A different finding was observed when considering the biofilm that developed in the three Moving Bed Biofilm Reactor (MBBR) systems, which led to the preference for an HRT of 10 hours for medium-short operational times.