Optimization of plant-wide ammonia removal: bioaugmentation versus nitritation of centrate

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America's first large facility for total nitrogen removal from digested sludge dewatering liquor (centrate) has just been commissioned at the Winnipeg-North plant. Parallel sequencing batch reactors (SBR) achieve removal of 1600 kg of nitrogen through nitrification (ammonia conversion to nitrate) and denitrification (nitrate removal to nitrogen gas). The grant will address in full scale the potential of using the biomass produced in the SBR to augment the main stream high-purity oxygen activated sludge (HPOAS) reactors with nitrifying bacteria generated during nitrification of centrate in order to induce nitrification in the HPOAS reactors and contribute 'for free' to additional reduction of ammonia load in the whole plant. Bioaugmentation has been tried with mixed success in the Netherlands and Czech plants but there are a number of unknowns that need to be defined, e.g.: the amount of RAS returned; amounts of WAS produced in SBR; temperature differential between centrate SBR and HPOAS reactors; role of denitrification in restoring the alkalinity; role of dilution, beside temperature control; capacity of HPOAS to increase their own SRT before clarifiers fail. On the other hand the overall nitrogen removal can alternatively be accomplished using an incomplete ammonia oxidation to nitrite (nitritation) which uses only two-thirds of oxygen and by denitritation which saves a third of methanol used in denitrification, thus also improving the carbon footprint. The latter requires a change in the proposed operation of the SBR such that a higher temperature is used. Bioaugmentation effect will be reduced as the temperature differential is increased. The CRD project will examine both options in the whole plant, in the context of the target environmental effect of ammonia removal and associated energy and methanol saved, as Winnipeg-North is further upgraded to full nutrient removal. Planned phosphorus co-precipitation should help retain the nitrifiers in the HPOAS bioreactors. The results may save millions in new construction costs. The full scale results and simulation would be applicable to all treatment plants in Canada and wherever sludge is digested and the plant is required to remove nitrogen as well as phosphorus.