Diffusion of Oxygen Through Activated Sludge Flocs: Experimental Measurement, Modeling, and Implications for Simultaneous Nitrification and Denitrification

Abstract

Diffusion of dissolved oxygen through activated sludge flocs was studied, as it represents a potential mechanism for simultaneous nitrification and denitrification in activated sludge systems. Dissolved oxygen profiles through six floc particles collected at different times from a full‐scale activated sludge plant demonstrated that that the dissolved oxygen concentration declines through all floc particles. For larger floc particles (2‐mm diameter and greater), the dissolved oxygen concentration reached near‐zero values at depths depending on process operating conditions. A mathematical model based on diffusion of dissolved oxygen, organic substrate (methanol), ammonia, nitrite, and nitrate through a spherical floc and consumption of dissolved oxygen by heterotrophs and autotrophs accurately predicted the dissolved oxygen profile and required adjustment of only one model parameter—the concentration of heterotrophs. A different dissolved oxygen decline pattern was exhibited for the smaller floc particles characterized, with the dissolved oxygen reaching a non‐zero plateau toward the center of the floc. This pattern was not reproduced with the mathematical model developed and suggests that additional mechanisms are responsible for the transport of dissolved oxygen into the center of these flocs. Implications of these results regarding the occurrence of simultaneous nitrification and denitrification include consideration of the factors that affect floc size and distribution (simultaneous nitrification and denitrification is maximized with larger floc particles), coupling of the International Water Association (London) activated models to predict activated sludge composition with diffusion models to consider intrafloc effects, and the effects of substrate diffusion on the apparent half‐saturation constant for various substrates in activated sludge systems.

From the affinity constant to the half-saturation index: Understanding conventional modeling concepts in novel wastewater treatment processes

“…As seen from Table 1, this is a value that exceeds significantly the normal range of reported K DO values for AOB. SND has been largely attributed to diffusional limitations of oxygen through the activated sludge flocs (dominating R2) (Daigger et al, 2007). In biofilm systems, maximum K DO values for AOB of 2 mg O 2 /L are commonly considered (Lackner and Smets, 2012); it is therefore logical to assume that values significantly higher than 2 mg O 2 /L are not due to diffusional effects but rather to other effects such as advection limitations in the bulk liquid.…”

Modelling the effect of biomass induced oxygen transfer limitations on the nitrogen removal performance of membrane bioreactor

“…It is often argued that floc size affects dissolved oxygen diffusion and smaller floc sizes induces faster diffusion of DO -nitrogen -into the floc matrix: Depending on the floc size, a part of the inner portion of the biomass sustains its metabolic functions using oxidized nitrogen (nitrate or nitrite) as the electron acceptor [16,23]. This argument is quite debatable, mainly because one cannot design an experimental setup where the floc size can be controlled and varied under selected operating conditions so that its effect can be properly assessed.…”

Factors affecting simultaneous nitrification and de-nitrification (SND) and its kinetics model in membrane bioreactor

“…For de-nitrification process, DO should be kept below 0.8 mg L −1 . High DO is unsuitable for SND, for DO will penetrate into the bio-flocs and anoxic zone is difficult to be formed [23].…”