Using Step-Feed to Improve Secondary Effluent Ammonia Control

Pima County Regional Wastewater Reclamation Department (Tucson, AZ) was facing a mandate from the Arizona Department of Environmental Quality (ADEQ) to reduce the amount of total nitrogen and ammonia in system effluent and to pass the whole effluent toxicity test, as well as increase the treatment capacity to handle the County’s growing population. To meet this challenge, the County developed and maintained a Regional Optimization Master Plan (ROMP) for wastewater treatment facilities required through the year 2030. The ROMP Final Report, published in November 2007, recommended the implementation of a number of capital improvements necessary to achieve regulatory compliance and meet the anticipated capacity requirements for the service area. One of these was the construction of a new facility, the Agua Nueva WRF, adjacent to an aging facility that would be decommissioned after the new plant was placed into operation.The DBO delivery approach used by Pima County to deliver the project encouraged the development of low net-present value approaches, which included the minimization of both energy and chemical usage. Additionally the very stringent disinfection requirements (4 of 7 days non-detect for E. Coli), in combination with a concern over NDMA and THM formation, led to the selection of chloramination for effluent disinfection. In order to minimize operating costs, and minimize the need for supplemental ammonia in disinfection, an innovative secondary effluent ammonia control system was developed. This system uses a combination of the standard approach of adjusting operating dissolved oxygen levels in the aerobic zones with a system that adjusts the step-feed fractions to the various passes. The goal of the step-feed control system is to provide the last pass of the bioreactor with a constant mass of ammonia, i.e. base loading.The use of primary effluent and bioreactor on-line ammonia monitoring to control the feed splits and DO setpoints for the Aqua Nueva WRF bioreactors was shown to enhance effluent ammonia control, reduce chemical usage and reduce power usage. It was found that a combination of factors in the raw sewage, which include a tripling of ammonia load from minimum to maximum on a diurnal basis, rapid increases in the ammonia load in the morning hours, and influent nitrification inhibition, exceeded the control authority of this control system at ANWRF.However the ANWRF feed-forward step-feed system was able to significantly improve the control of effluent ammonia and meet the design intent of the DBO project. This shows that step-feeding a bioreactor provides an additional level of control authority above that possible with DO control alone.