INSTRUMENTATION FOR NUTRIENT REMOVAL

The use of instrumentation in standard nitrification plants has limited acceptance. Flow and dissolved oxygen (DO) measurements are well accepted. Most of the time only flow is included in an automatic control strategy. Mixed liquor suspended solids, pH and ORP are used at times. The use of chemical analyzers to determine the concentration of ammonia and their use in a control strategy remains an exception.When it is required to denitrify and remove phosphate, the complexity of the processes increases as well as the need for additional measurements such as nitrate and phosphate concentration. Depending on the plant configuration the measurement of BOD, TOC or Methanol concentration could also be beneficial.The combination of feed-forward and feed-back controls insures the optimization of the individual processes. Overall plant optimization, defined as the maintenance of high efficiency at the lowest possible cost, requires a full study of the limitations of the combined processes. That is outside the scope of this subject.Nitrification optimization implies the measurement and control of DO. The concentration of DO should be maintained within acceptable limits based on the ammonia concentration and the viability and concentration of the mixed liquor suspended solids. Lower ammonia loads can be nitrified with lower energy consumption.The cost of phosphate precipitation, in primary or secondary clarifiers, can be reduced if the addition is paced based on flow and adjusted based on a measured value.The nitrate concentration in a denitrification tank defines the recirculation rate as long as there is enough biomass, organic load and low DO. The knowledge of the incoming flow provides feed-forward capability to prevent upsets.This paper defines the available technologies and the use of the information for process optimization.