Investigation of Oxygen Profile in a Nitrifying Moving Bed Biofilm Process : Theory and Validation of a Mathematical Model

Investigation of oxygen profile in a nitrifying Moving Bed Biofilm

Reactor; Theory and validation of a mathematical model

Alma Mašić

_{, Jessica Bengtsson}

_{, Niels Chr. Overgaard}

_,

Magnus Christensson

_,

_{Anders Heyden}

Several mathematical models can be used to reproduce and predict

the behaviour of biofilms. Depending on conditions like

substrate concentrations or temperature, there are many models representing the variety of processes. The AnoxKaldnes

MBBR technology is well-established

for wastewater treatment where a bacterial biofilm

grows on the protective surfaces of

suspended carriers [1]. Previous studies on the Kaldnes

_{K1 carrier have shown that the nitrification rate was close to first}

order kinetics with respect to DO when oxygen limiting conditions were established [2]. Thus, higher DO gives higher

nitrification rates but at the expense of increased energy consumption. The flat shaped BiofilmChip

_{P was developed to}

increase the available protective surface area and to allow good

conditions for transport of substrates into the biofilm.

We present the oxygen profile of the biofilm within the BiofilmChip

_{P carriers based on two different methodologies:}

Firstly, the oxygen profile was measured

on actual carriers that have been used in a Moving Bed Biofilm

Reactor and

secondly, the oxygen profile was estimated, based on a mathematical model.

* E-mail: alma.masic@mah.se 1AppliedMathematicsGroup, MalmöUniversity, Sweden

2AnoxKaldnes AB, Sweden

Estimation of oxygen profile:

mathematical modelling

Introduction

Conclusions

[1] Rusten B. and Ødegaard H. (2007). Design and operation of nutrient removal

plants for very low effluent concentrations. Proc. Nutrient Removal 2007, p.1307-1331.

[2]Hem L. J, Rusten, B, Ødegaard H. (1994) Nitrification in a moving bed biofilm reactor, Wat. Res. 28:1425-1433

[3] Lu, R, Yu, T. (2002) Fabrication and evaluation of an oxygen microelectrode

applicable to environmental engineering and Science, J. Environ. Eng. Sci. 1:225-235

[4]WannerO, GujerW. (1986). A multispecies biofilm model. Biotech. Bioeng. 28:314-328

[5]Wäsche, S, Horn H, Hempel, C. H. (2002). Influence of growth conditions on biofilm development and mass transfer at the bulk/biofilm interface, Wat. Res. 36(19): 4775-4784

References

Measurements of oxygen profile

A lab-scale Moving Bed Biofilm Reactor, MBBR, (7 l) with

BiofilmChip™ P was operated for nitrification using a synthetic medium containing NH4Cl without any organic substrates. The

medium flow was controlled to achieve an effluent concentration of 5-8 mg NH4-N/l, DO being 5 mg/l at 10˚C. After more than a year of

continuous operation, carriers were taken out for microelectrode measurements in a tube flow cell.

Carriers from the reactor were placed inside a long plastic tube cell, completely enclosed by the surrounding wall, to ensure that all flow would pass through the holes of the carriers. The same medium composition, as for the continuously operated MBBR, was circulated through the test tube. DO and temperature of the medium were controlled in a small jacked glass in connection with the test tube. Measuring of the oxygen profile in situ the nitrifying biofilm was made

We used the 1-D mathematical model proposed by [4] to estimate the oxygen profile. The model has been adapted to the specific wastewater treatment process, which contains two kinds of autotrophic bacteria (ammonium and nitrite oxidizers), inert matter and four substrates (oxygen, ammonium, nitrite and nitrate). Mathematically this results in a system of differential equations. Assuming the diffusive components to be in a quasi-steady state at all times, the substrate fields Sk are found as the solution of a

boundary value problem of coupled ordinary diff. equations

The volume fractions fi of the biomass componentsare tracked

over time by solving a system of hyperbolic equations

Both systems are furthermore coupled to each other. The thickness of the biofilm is found by integrating a simple first order differential equation with an erosion term which is proportionalto the square of the biofilm thickness [4].

Numerically, a built-in Matlab function (bvp4c) was used to solve the boundary value problem for the substrates. The hyperbolic equations for the volume fractions were solved by time discretization and a simple finite difference upwind scheme. The system was solved until steady state was reached.

The oxygen profile from the microelectrode measurements indicates the thickness of the boundary layer and the thickness of biofilm, penetrated by oxygen. The horizontal part of the profile shows the oxygen concentration in the bulk phase. The boundary layer starts where the current or DO begins to drop and lasts until the linear region where the biofilm begins. The part of the biofilm penetrated by oxygen starts at the linear region and ends where the oxygen drops to zero, i.e. where the current is constant [5].

A measured oxygen profile of the biofilm on BiofilmChipTM _{P is} shown in figure 2. The oxygen concentration in the bulk phase was about 4.8 mg/l which decreased to 3.7 mg/l over the boundary layer. The oxygen was

The measured oxygen profile corresponded with the simulation to a significant extent, comparing the thickness of biofilm and slope. Since the model neglects the boundary layer, we assume that the concentration at the biofilm surface is the same as in the bulk phase. This results in differences between the measured and simulated curve, as can be seen in figure 2. Oxygen was not completely depleted in the simulation, probably due to a thin biofilm and the choice of bulk substrate concentrations.

Microelectrode measurements are a useful technique for evaluation of mathematical models of biofilms. Our work proposes a new and more accurate model that also comprises the boundary layer.

Figure 2.Measured [solid] and simulated [dashed] oxygen profile.

Results

depleted at biofilm depth of 120 μm. The simulated oxygen profile indicates the biofilm penetrated by oxygen (i.e. the boundary layer was not simulated).

with a Clark-type oxygen microelectrode [3]. The sensor was inserted into the biofilm, positioned with a micromanipulator (figure 1), enable to positioning with a precision of 10 μm in z-direction. The DO concentration was measured in situ along the depth of the biofilm at different