Low-pH Ammonia Oxidation at a Biological Nutrient Removal Facility

McAlpine Creek Wastewater Management Facility (MCWWMF), owned and operated by Charlotte Mecklenburg Utilities (CMU) of Charlotte, North Carolina, is permitted to treat 64 million gallons per day (mgd). Along with MCWWMF, three CMU treatment plants discharge effluent into the Catawba River just upstream of the South Carolina border. CMU worked with North and South Carolina regulators agreeing to accept a National Pollutant Discharge Elimination System (NPDES) permit limit for phosphorus set on the same basis as that for South Carolina dischargers: 1 mg/L TP. The same permit requires that the facility meet an ammonia-N limit of 1.0 mg/L in summer and 1.9 mg/L in winter months. The facility expansion and conversion to Enhanced Biological Phosphorus Removal (EBPR) is complete and the facility is operational at this time. The facility expansion also included real-time monitoring and feed-back loop control of the phosphate, nitrogen, DO, pH and solids, allowing the facility to add ferric chloride if needed to meet discharge requirements at all times. Monitoring data during startup showed that the pH values were much lower than typically needed for the ammonia oxidizing bacteria; i.e.; nitrifiers. Corresponding performance values were evaluated and the change in the population activity showed that even at pH values of less then 7.0, as low as 5.7, the system was able to continue to nitrify. EBPR functions were found to be more sensitive to low pH values. A short-term study was conducted to establish PO4-P, NH3-N, NO3-N, pH and DO profiles in the aeration basins. The results were then utilized to develop a control strategy for the daily operations, with the goal of maintaining sustainable ammonia and phosphorus removal.

The limitations of nitrification (i.e.; the stepwise ammonia oxidation to nitrite and then to nitrate) under aerated conditions has been well established for the wastewater treatment practice since early 1970s (Grady et al., 1999). The nitrogen in the raw wastewater comprising mainly of ammonia and organic nitrogen in the form of amino acids, proteins and other bound forms is biologically converted to ammonia nitrogen (i.e.; ammonification) prior to being available for anabolic uptake and oxidation. Autotrophic bacteria responsible from nitrification can be found in sediments, wastewater treatment facilities, fresh water, soils and marine environments. As these environments impose different sets of breeding conditions, different groups find chance to proliferate under each.

Nitrifiying organisms at wastewater treatment plants grow under a set of boundary conditions because of their sensitivity relative to the heterotrophic organisms. Most important ones of these conditions are temperature, pH and dissolved oxygen concentration. Also given the slow growth rate and low biomass yield of the nitrifying bacteria, these boundary conditions become even more important to maintain a healthy population to achieve the necessary treatment at a facility. In addition to these, Nitrosomonas and Nitrobacter, the two main genera of nitrifying bacteria have intricate inhibitory impacts on each other because of the sensitivity of each to total ammonia and nitrite levels. pH therefore is a unique parameter that can impact nitrification directly and indirectly, because shifts in pH also mean shifts in the relative concentrations of the substrate for the nitrifying organisms. Figure 1 illustrates the effect of pH on the activity of ammonia oxidizing bacteria (WEF, 1998). The data presented in this figure was gathered from twelve different studies, demonstrating the pH range for uninhibited growth (i.e.; 7.5 to 8.5). It also shows that outside this pH range the autothrophic activity, mainly that of Nitrosomonas become a fraction of that within the pH range of 7.5 to 8.5.

Princic et al. (1998), and Tarre and Green (2004) investigated low-pH nitrification kinetics and underlying population dynamics. Each study was able to observe nitrification at pH values of 6.0 and below. They have both shown that acclimation in the form of shifts in the dominant nitrifying populations, especially below pH 7.0 allows continued ammonia oxidation at a wider pH range then typically considered, although proceeds at a much lower rate at low pH values

This paper deals with a unique situation experienced at a full scale biological nutrient removal facility that resulted in well-sustained nitrification under otherwise inhibitory conditions. The observed conditions and gradual transitions that took place at the facility further emphasizes the importance of acclimation in biological systems.

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