Combined anaerobic and activated sludge anoxic/oxic treatment for piggery wastewater

Intensification of pig production in Brittany (France) led to high quantity of manure to be managed. In this context, a process combining anaerobic digestion and anoxic/oxic treatment was developed. The aim was to partially convert organic matter (OM) into a valuable energy, at the same time respecting the environmental constraints as regards nitrogen and reducing the energy costs at the farm scale. However, OM content of digested pig slurry is insufficient to allow a further complete denitrification of the mineral nitrogen content. Hence, three configurations were developed to manage the OM requirement and achieve denitrification such that (i) a fraction of the raw wastewater bypasses the digester and feed directly the anoxic/oxic reactor (Phase1), (ii) in addition to bypass, a recirculation was done from oxic reactor to digester (Phase 2 and 3), (iii) neither bypass nor recirculation (Phase 4). Partial nitrification (PN) was applied by regulating oxygen inflow time in Phase 3 and 4. Thus, the combined process allowed a removal of about 38-52% of CODt, 79-88% of CODs, 66-75% of TKN and 98-99% of NH4 +N concentrations. Anaerobic digester was able to produce 5.9 Nm3 of CH4/m3 of slurry added. Due to PN, the oxygen and OM requirements respectively for nitrification and denitrification were reduced. Comparison between each configuration was intrinsically difficult to make due to the difference observed in the slurry characterisation in each phase. However, a raw influent bypass between 20-30% combined with PN is suggested for piggery wastewater treatment in France, as the interest of recirculation from oxic reactor to digester is very limited using a CSTR.