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The reaction of monochloramine and hydroxylamine: implications for ammonia–oxidizing bacteria in chloraminated drinking water
Citation:
Wahman, D. AND G. Speitel. The reaction of monochloramine and hydroxylamine: implications for ammonia–oxidizing bacteria in chloraminated drinking water. Presented at 2013 Water Quality Technology Conference, Long Beach, CA, November 03 - 07, 2013.
Impact/Purpose:
To investigate the relevance of the reaction between monochloramine and hydroxylamine under chloraminated drinking water conditions.
Description:
Drinking water chloramine use may promote ammonia–oxidizing bacteria (AOB) growth because of naturally occurring ammonia, residual ammonia remaining from chloramine formation, and ammonia released from chloramine decay and demand. A rapid chloramine residual loss is often associated with the onset of nitrification that may result in noncompliance with existing regulations; therefore, understanding nitrification and its control in drinking water is of practical importance. For biological ammonia oxidation (1st nitrification step), AOB use two enzymes in two consecutive reactions: (1) ammonia monooxygenase (AMO) catalyzes ammonia oxidation to hydroxylamine and (2) hydroxylamine oxidoreductase catalyzes hydroxylamine oxidation to nitrite, supplying electrons back to AMO for ammonia oxidation. Chloramine also inactivates AOB but rates vary, depending on inactivation criterion (metabolic activity, culturability, cell membrane integrity). One mechanism for the fastest inactivation (metabolic activity basis) would be the direct monochloramine/hydroxylamine reaction (MHR). Because biological hydroxylamine oxidation (AOB’s 2nd reaction step) supplies the electrons required for biological ammonia oxidation (AOB’s 1st reaction step), the MHR provides a direct mechanism for monochloramine to inhibit AOB, essentially short-circuiting AOB through hydroxylamine consumption. In addition, the MHR provides another monochloramine demand and ammonia release mechanism. Therefore, the MHR provides two competing AOB metabolic impacts (inhibition through hydroxylamine consumption and promotion through ammonia release), requiring evaluation to understand the MHR’s potential importance on AOB in chloraminated drinking water (CDW). The present study conducted a detailed literature evaluation and proposed a MHR scheme (MHRS) under relevant CDW conditions (reactant concentrations, air saturation, and pH 7–9). Next, the MHRS was experimentally evaluated at a mid–range pH (8.3) and somewhat lower pH (7.7). Then, the experimental data was simulated using published rate constants for the MHRS implemented in an AQUASIM kinetic model, leading to refinement of the literature reported MHRS rate constants that were poorly or variably defined. Finally, using the updated MHRS, simulations investigated the MHRS relevance on AOB activity under representative CDW conditions. Overall, these simulations indicated that the MHRS kinetics are relevant under CDW conditions and provide a possible additional mechanism for AOB inhibition, monochloramine demand, and subsequent ammonia release. Based on utility experience, it is recommended to maintain greater than a 2 mg/L chloramine residual at the CDW distribution system entrance and 1.5 mg/L at all distribution sites. Even though the simulations conducted herein were simplifications, the minimum chloramine requirement may be related, in part, to the impact on ammonia oxidation resulting from the direct reaction of monochloramine and hydroxylamine.
URLs/Downloads:
http://www.awwa.org/portals/0/files/education/conferences/wqtc/conference-program/resources/57.htm