Evaluating Nitrifier Kinetics in the Context of Various Substrate Limitation Scenarios

Abstract

Novel aeration strategies are popularized for reducing the aeration demand of nitrification systems without compromising the effluent. Two separate dissolved oxygen (DO) intensification strategies - low DO setpoint and ammonia-based aeration control (ABAC) – are evaluated to determine how ammonia oxidizer kinetics change due to limitations between oxygen and or ammonia. While successful nitrification has been proven within these configurations, few studies have investigated the effect of both the oxygen affinity (K_DO) and the ammonia affinity (K_(NH_X )) within the same context. Low DO systems tend to discount the K_(NH_X ) due to their proclivity towards complete nitrification. However, with the emergence of ABAC and other operational strategies that require an ammonia residual, determining the ammonia removal rate when the reaction kinetics are first order is imperative for estimating treatment efficiency in process modelling scenarios. The Hampton Roads Sanitation Districts (HRSD) pilot process at the Virginia Initiative Plant (VIP) consisted of a continuously fed 3.8 Lpm A/O process with one continuously stirred anaerobic reactor followed by five continuously stirred aerobic reactors in series. The operating DO setpoint was incrementally decreased from 2.0 mg/L to 0.2 mg/L while maintaining a constant temperature and solids retention time (SRT) of 8.5 days. The ammonia oxidizer K_DO was positively correlated with the operating DO. This K_DO decrease was due to a community shift towards comammox Nitrospira clade A which comprised up to >80% of the nitrifier community at a DO setpoint of 0.2 mg/L. The ABAC study operated with an SRT < 5.5 days and observed a K_(NH_X ) shift once an effluent ammonia residual became established. The changes in K_DO and K_(NH_X ) are attributed to kinetic selection as in reference to the low DO study and the poor coefficient correlation with floc size. Multiple substrate limitation models were applied to confirm the observed K_DO and K_(NH_X ) readings with in-situ ammonia profiles. The multiplicative model does best to describe both the ammonia removal profile and the effluent ammonia concentration recorded in the pilot study.