SIMULTANIOUS NITRIFICATION AND DENITRIFICATION IN ONE-STAGE ACTIVATED SLUDGE SYSTEMS

A structured model of suspended-growth biological processes is used to identify the conditions under which simultaneous denitrification and nitrification occur reliably at steady state for a single-reactor system. Systematic variation of the dissolved oxygen concentration demonstrates that a D.O. “window” for major denitrification with nitrification occurs for approximately 0.1-0.75 mg O2/L; the maximum denitrification is at D.O. = 0.18 mg/L, for which 28% of the input COD is transferred to N2. The greatest denitrification occurs for an influent COD:TKN ratio of 6.4 mgCOD/mgN. Higher ratios send more N to wasted biomass, while lower ratios have too little COD to consume all the NO3−-N. Operation at a low D.O. needed to maximize denitrification raises the minimum SRT to allow nitrification to about 7 days; denitrification commences as soon as nitrification provides NO3−-N. Effluent soluble COD and TKN increase somewhat when D.O. and NO3− are simultaneously at low concentrations.