Determination of external and internal mass transfer limitation in nitrifying microbial aggregates

Abstract

In this article we present a study of the effects of external and internal mass transfer limitation of oxygen in a nitrifying system. The oxygen uptake rates (OUR) were measured on both a macro-scale with a respirometric reactor using off-gas analysis (Titrimetric and Off-Gas Analysis (TOGA) sensor) and on a micro-scale with microsensors. These two methods provide independent, accurate measurements of the reaction rates and concentration profiles around and in the granules. The TOGA sensor and microsensor measurements showed a significant external mass transfer effect at low dissolved oxygen (DO) concentrations in the bulk liquid while it was insignificant at higher DO concentrations. The oxygen distribution with anaerobic or anoxic conditions in the center clearly shows major mass transfer limitation in the aggregate interior. The large drop in DO concentration of 22-80% between the bulk liquid and aggregate surface demonstrates that the external mass transfer resistance is also highly important. The maximum OUR even for floccular biomass was only attained at much higher DO concentrations (approximately 8 mg/L) than typically used in such systems. For granules, the DO required for maximal activity was estimated to be >20 mg/L, clearly indicating the effects of the major external and internal mass transfer limitations on the overall biomass activity. Smaller aggregates had a larger volumetric OUR indicating that the granules may have a lower activity in the interior part of the aggregate.

Simultaneous Biological Nutrient Removal: A State‐of‐the‐Art Review

“…Development of a microsensor that can be used to measure concentration gradients through biofilms and activated sludge flocs provided quantitative evidence of diffusion gradients in activated sludge flocs (Daigger et al, 2007;Li and Bishop, 2004, 2003a, 2003bSantegoeds et al, 1998;Schramm et al, 1999;Wilén et al, 2004). These observations confirmed that a low dissolved oxygen inner portion can develop in activated sludge flocs and that denitrification can occur in these zones.…”

“…Modeling of activated sludge flocs demonstrated that these effects could result because of the diffusion of substrates through the floc, resulting in depressed dissolved oxygen concentrations and reduced net nitrifier activity. Wilén et al (2004) Measures oxygen uptake rates and dissolved oxygen profiles for laboratory-scale flocculent and granular activated sludge systems using an off-gas respirometer and a microsensor. The results showed a significant external masstransfer boundary layer at low bulk dissolved oxygen concentrations, which decreased at elevated bulk dissolved oxygen concentrations.…”

Development and examination of a granular nitrogen-fixing wastewater treatment system

“…2 shows that both systems supported a diverse microbial community, likely a result of the varied environment within the granules [16] and the anticipated ubiquity of microorganisms capable of acetate degradation. Using microelectrode studies, Wilén et al [34] showed the rapid depletion of oxygen and presence of significant fractions of low/zero oxygen concentrations in granules observed at bulk liquid DO levels between 2 and 8 mg L À1 , and Tay et al [18] showed that obligate anaerobes can concentrate at depths of 0.8 mm below granule surfaces. The predicted oxygen profiles in granules of various radii are shown in Fig.…”

Experimentation and mathematical models for partial nitrification in aerobic granular sludge process

“…The DO concentration gradually decreases after penetrating into the granule's biofilm; therefore, the biomass growth occurs under various DO zones from low to high (Wilen et al, 2004). Fig.…”