Nitrification, Denitrification and Biological Phosphate Removal in Sequencing Batch Reactors Treating Piggery Wastewater

Abstract

In Italy, because of the high concentration of animals in many agricultural areas, not all piggery manure can be landspread. Considering their chemical characteristics - high concentration in organics and nutrients -processes providing the removal of COD, N & P have to be adopted. Other aspects have to be considered with regard to cost saving and simple management. In order to study a simple andreliable treatment system, two 51 bench-scale SBRs were built. Each reactor treated 500 ml/d of clarified (after centrifugation) piggery wastewater. A cycle with the following alternating phases of reaction was chosen: first denitrification and phosphorus release, first oxidation-nitrification, second denitrification and second oxidation-nitrification. The systems showed good flexibility and enhanced removal of COD, N and P was easily reached modifying the duration of each phase. The best efficiency in N removal has been noticed in the reactor in which the feeding distribution was done in the two denitrification phases, allowing a better use of the organic substrate for the denitrifying bacteria. This confirmed that the limiting factor, in the removal of N from piggery wastewater, is represented by the too high ratio TKN/COD. Anyway good removals of COD (~93%), N (88-93%) and P(~95%) were obtained in both reactors. A relevant concentration of unbiodegradable COD - at the adopted HRT, SRT and reaction temperature - was still present in the effluent of both reactors (~300 mg/l). Track studies allowed the evaluation of the nitrification and denitrification kinetic constants. Their values were higher than the ones reported in the literature. No inhibition of the nitrifying activity due to high concentration of N-NH4 (>80 mg N/l), was observed. Specific studies on biological phosphate removal have been carried out. In spite of the high concentration of P in the biomass and the low concentration of P in the effluent, showing a surplus biological P removal, no P release was observed; this might have been masked by biologically induced chemical precipitation. In fact, piggery wastewaters are rich in metals and a high concentration of metal-bound P was analytically observed. However, a high concentration of polyphosphates has been analytically determined in the biomass. This should confirm the presence in the microbial community of bacteria performing the surplus P removal.