Enhanced Effects of Tourmaline on Moving Bed Biofilm Reactor–Based Partial Nitrification Process

This paper compares two different carriers, polyurethane foam (PU) and polyurethane foam loaded with tourmaline (TPU), for the partial nitrification (PN) of supernatant from an anaerobic sludge digester using moving bed biofilm reactors (MBBRs). In parallel laboratory tests, the TPU reactor finished start-up in 51 days while the PU reactor took 67 days. In the TPU reactor, the ${\mathrm{NH}}_4^{+}\text{-}\mathrm{N}$ removal efficiency, total nitrogen removal efficiency, and nitrite accumulation ratio were 83.8%, 55.6%, and 75.4%, respectively. In response to the hydraulic shocks, the recovery was observed in 20 days for the TPU reactor and 34 days for the PU reactor, respectively, showing that the TPU reactor had a good resistance to the hydraulic shock loading. The biofilm formed in the TPU and PU carriers was comprised of a network of spherical and bacillus communities, which were surrounded by the extracellular polymeric substances (EPS). The biofilm structure in the PU biofilm was less dense with cell-free voids compared to that in the TPU biofilm. The real-time polymerase chain reaction (RT-PCR) data showed a trend similar to the biomass results in that the TPU biofilm had a faster growing rate of bacteria concentration than the PU biofilm. Free ammonia (FA) and free nitrous acid (FNA) levels fell within the inhibition range for nitrite-oxidizing bacteria (NOB) during the whole operation. The inhibition mechanism of NOB might be due to the coexistence of FA and FNA. This study revealed the potential of a partial nitrification process to treat supernatant from an anaerobic sludge digester by combining tourmaline and biofilm.