Upgrading of a wastewater treatment plant with a hybrid moving bed biofilm reactor (MBBR)

The wastewater treatment plant of Porto Tolle (RO, Italy) was originally projected for 2200 person equivalent (p.e.) and it was made of a pumping station, an activated sludge oxidation tank (395 m3), a settler (315 m3), and two sludge drying beds. Other units were not yet in use in 2008: a fine screen, a sand and grit removal unit, a new settler (570 m3), a disinfection tank and a sludge thickener. Effective hydraulic load was 245% higher, organic load was 46% lower and nitrogen load was 39% higher than project values. Moreover, higher pollutant loads and more strict emission limits for nitrogen were expected. So the plant was upgraded: the old settler was divided into a sector of 180 m3 that was converted into a predenitrification tank, and a sector of 100 m3 that was converted into a hybrid MBBR tank filled with 50% AnoxKaldnesTM K3 carriers; the new settler was connected to the hybrid MBBR, and the other units were started. Biofilm growth was observed two months after plant restarting, its concentration reached 1.1 gTS/m2 (0.26 kgTS/m3), while activated sludge concentration was 2.0–2.8 kgTSS/m3 in all the period of study. The upgraded plant treats 1587 m3/d wastewater with 57 kgCOD/d, 23 kgBOD/d and 13.3 kgN/d, and has a significant residual capacity; the effluent respects all emission limits.