Mass and heat balances for biological nitrogen removal in an activated sludge process: to couple or not to couple?

Abstract

ABSTRACT Models adapt constantly, usually increasing the degree of detail describing physical phenomena. In water resource recovery facilities, models based on mass and/or heat balances have been used to describe and improve operation. While both mass and heat balances have proven their worth individually, the question arises to which extent their coupling, which entails increased model complexity, warrants the supposedly more precise simulation results. In order to answer this question, the need for and effects of coupling mass and heat balances in modelling studies were evaluated in this work for a biological nitrogen removal process treating highly concentrated wastewater. This evaluation consisted on assessing the effect of the coupling of mass and heat balances on the prediction of: (1) nitrogen removal efficiency; (2) temperature; (3) heat recovery. In general, mass balances are sufficient for evaluating nitrogen removal efficiency and effluent nitrogen concentrations. If one desires to evaluate the effect of temperature changes (e.g. daily, weekly, seasonally) on nitrogen removal efficiency, the use of temperature profiles as an input variable to a mass balance-based model is recommended over the coupling of mass and heat balances. In terms of temperature prediction, considering a constant biological heat generation term in the heat balance model provides sufficient information – i.e. without the coupling of mass and heat balances. Also, for evaluating the heat recovery potential of the system, constant biological heat generation values provide valuable information, at least under normal operating conditions, i.e. when the solids retention time is large enough to maintain nitrification.