Influence of carrier type on nitrification in the moving-bed biofilm process

Two different types of carriers differing fundamentally in size, shape and structure were evaluated in parallel testing for nitrification potential using the moving-bed biofilm reactor (MBBR) technology. One of the carriers used was a cylindrical high-density polyethylene ring shaped carrier (AnoxKaldnes, K1 carrier) and the other was a spherical polyvinyl alcohol (PVA) gel bead shaped carrier (Kuraray, PVA-gel carrier). For each MBBR process, using artificial wastewater under autotrophic conditions, high maximal nitrification rates at 20°C were obtained. For the K1 carrier up to 27 mgNH₄-N/L.h (at 37% filling fraction) was found, corresponding to 49 mgNH₄-N/L.h at the recommended maximum filling fraction of 67%. This corresponds to a nitrification area rate of 3.5 gNH₄-N/m².d for the K1 carrier at 20°C. For the PVA-gel carrier up to 32 mgNH₄-N/L.h (at 9.6% filling fraction) was found, corresponding to 50.0 mg NH₄-N/L.h at the recommended maximum filling fraction of 15%. At the recommended filling fractions, the two carriers therefore required about the same reactor volume to reach the maximum observed nitrification rate. This presumption allowed us to estimate the effective specific surface area for the PVA gel carrier up to 2,500 m²/m³ versus 1,000 m²/m³ when only the outer surface is considered.