Upgrading of a wastewater treatment plant with hybrid MBBR

The wastewater treatment plant of Porto Tolle (RO, Italy) was originally projected for 2200 p.e.; expected loads were 550 m3/d, 264 kgCOD/d, 132 kgBOD/d and 26 kgN/d. Emission limits were stated by the Regional Plan for Water Resanation and were: TSS 35 mg/L, COD 125 mg/L, BOD 25 mg/L, NH4 + 20 mg/L, NO2-N 1 mg/L, NO3-N 30 mg/L, P 10 mg/L. The plant was originally made of a pumping station, a circular activated sludge oxidation tank (395 m3) aerated by a 1480 Nm3/h blower and fine bubble diffusers, a sedimentation unit in the external circular crown (315 m3), and two drying beds for sludge (fig. 1A). Other units were built but not yet in use in 2008: fine screen, sand and grit removal, a circular settler (570 m3), a disinfection tank with peracetic acid and a sludge thickener. According to average data of 2007, the plant treated 1898 m3/d with 35 mg/L TSS, 75 mg/L COD, 37 mg/L BOD, 19.1 mg/L TKN; hydraulic load was 245% higher, organic load was 46% lower and nitrogen load was 39% higher than project values. Wastewater was (and is) very dilute because of infiltrations of rain waters and clean water from the river Po. Even if the plant respected emission limits, the regional environmental law was near to be modified with more strict emission limits for nitrogen, the plant had no denitrification, and higher pollutant loads were expected for following years. So it was decided to modify the plant and to start units not yet in use; the choice to convert part of existing volume into a hybrid MBBR was justified with the possibility of reducing activated sludge concentration. So the circular crown was divided into two sections (fig. 1B): a sector of 180 m3 was converted into a predenitrification tank, a sector of 100 m3 was converted into a hybrid MBBR oxidation tank. This reactor was filled with 50% AnoxKaldnesTM K3 carriers (tab. 1) and aerated by a new 1000 Nm3/h blower and medium bubbles air diffusers