Kinetics of nitrification in a fixed biofilm reactor using dewatered sludge-fly ash composite ceramic particle as a supporting medium

Abstract

A mathematical model system was derived to describe the kinetics of ammonium nitrification in a fixed biofilm reactor using dewatered sludge-fly ash composite ceramic particle as a supporting medium. The model incorporates diffusive mass transport and Monod kinetics. The model was solved using a combination of the orthogonal collocation method and Gear’s method. A batch test was conducted to observe the nitrification of ammonium-nitrogen (${\text{NH}}_{4}^{ + }$-N) and the growth of nitrifying biomass. The compositions of nitrifying bacterial community in the batch kinetic test were analyzed using PCR–DGGE method. The experimental results show that the most staining intensity abundance of bands occurred on day 2.75 with the highest biomass concentration of 46.5 mg/L. Chemostat kinetic tests were performed independently to evaluate the biokinetic parameters used in the model prediction. In the column test, the removal efficiency of ${\text{NH}}_{4}^{ + }$-N was approximately 96 % while the concentration of suspended nitrifying biomass was approximately 16 mg VSS/L and model-predicted biofilm thickness reached up to 0.21 cm in the steady state. The profiles of denaturing gradient gel electrophoresis (DGGE) of different microbial communities demonstrated that indigenous nitrifying bacteria (Nitrospira and Nitrobacter) existed and were the dominant species in the fixed biofilm process.

Appendices

Appendix 1: Conceptual basis of biofilm

A schematic diagram represented the ammonium utilization by attached and suspended nitrifying biomasses in the biological reactor is illustrated in Fig. 10. For the attached biomass, the ammonium concentration profile is established in the biofilm due to the diffusional resistance to the transport of the ammonium. In the biofilm reactor, the attached biomass accumulates to form a biofilm and sheared-off biofilm mass is assumed to be in dispersed form, increasing the suspended biomass concentration (Tsai et al. 2005). The shear loss of attached biomass into the flowing liquid increased the amount of suspended biomass in the bulk liquid.

Fig. 10

Schematic diagram of ammonium utilization by nitrifying biomass

Appendix 2: Bioreactor setup

The detailed design of a fixed biofilm reactor packing with dewater sludge-fly ash ceramic particle is presented in Fig. 11. The reactor was inoculated by charging an 80 mL nitrifying biomass with 4.8 mg VSS/L. The fixed biofilm reactor with a high recycle flow rate (Q _r /Q = 25) for maintaining a completely mixed stirred tank reactor was applied to conduct the experiments of ammonium nitrification. The use of a completely mixed flow reactor greatly simplifies the mathematical description of a fixed biofilm reactor because ${\text{NH}}_{4}^{ + }$-N concentration profiles along the reactor need not to be considered. The reactor was fed with an initial ${\text{NH}}_{4}^{ + }$-N concentration of 120 mg/L at 15 L/day, which yield a hydraulic residence time of 5.6 h. The working volume of reactor was 3.5 L. The dewatered sludge-fly ash ceramic particles were places in the fixed biofilm reactor between two plates used for fixation. Dissolved oxygen (DO) was controlled at a value above 4.5 mg/L for nitrification by an adjustable air pump. The air supply was sterilized by filtration through polytetrafluoroethylene (PTFE) membranes with a pore size of 0.2 μm. The pH was controlled at 7.2 ± 0.1 by adding ${\text{HPO}}_{4}^{ 2- }$/${{{{\text{H}}_{2}}{\text{PO}}}}_{4}^{ 2- }$ in the feed solution (Hem et al. 1994).

Fig. 11

Schematic diagram of the experimental set-up for nitrification in column studies

Appendix 3

a :

The specific surface area of bed (L⁻¹)

α :

The conversion factor for the utilization of ${\text{NH}}_{4}^{ + }$ to nitrate (M_s ${\text{M}}_{s}^{-1}$)

ϕ_b :

Association parameters

μ:

Secific growth rate of nitrifying biomass (T⁻¹)

A:

Total surface area of ceramic particles (L²)

b:

Decay coefficient of nitrifying biomass (T⁻¹)

b_s :

Shear-loss coefficient of nitrifying biomass (T⁻¹)

D_f :

Diffusion coefficient of ammonium in biofilm (L²T⁻¹)

d_p :

Diameter of the ceramic particle (L)

D_w :

Diffusion coefficient of ammonium in bulk liquid (L²T⁻¹)

J_f :

Flux of ammonium from bulk liquid into nitrifying biofilm (M_sL⁻²T⁻¹)

K:

Monod maximum specific utilization rate of ammonium (M_sM_xT⁻¹)

K_f :

Liquid film transfer coefficient of ammonium (LT⁻¹)

K_s :

Monod half-velocity coefficient of ammonium (M_sL⁻³)

L_f :

Thickness of nitrifying biofilm (L)

L_f0 :

Initial thickness of nitrifying biofilm (L)

M:

Mass in general

M_b :

Molecular weight of solvent (M)

M_s :

Mass of ${\text{NH}}_{4}^{ + }$

M_x :

Mass of nitrifying biomass

Q:

Influent flow rate (L³T^-1)

R_e :

Reynolds number

S:

Effluent concentration of ${\text{NH}}_{4}^{ + }$ in the aeration tank (M_sL⁻³)

S₀ :

Influent concentration of ${\text{NH}}_{4}^{ + }$ in the aeration tank (M_sL⁻³)

S_b :

Concentration of ${\text{NH}}_{4}^{ + }$ in bulk liquid (M_sL⁻³)

S_b0 :

concentration of ${\text{NH}}_{4}^{ + }$ in the feed (M_sL⁻³)

S_c :

Schmidt number

S_f :

Concentration of ${\text{NH}}_{4}^{ + }$ in biofilm (M_sL⁻³)

S_n :

The nitrate concentration in the effluent (M_sL⁻³)

S_s :

Concentration of ${\text{NH}}_{4}^{ + }$ at liquid/biofilm interface (M_sL^-3)

T:

Absolute temperature (K)

T:

Time (T)

U:

Specific rate of nitrification (M_s ${\text{M}}_{x}^{ -1 }$T^-1)

V:

Effective working volume of reactor (L³)

V_b :

Molar volume of solute as liquid at its normal boiling point (L³mol⁻¹)

V_s :

Superficial flow velocity through column (LT⁻¹)

X:

biomass concentration in the aeration tank (M_xL⁻³)

X_b :

concentration of nitrifying biomass in bulk liquid (M_xL⁻³)

X_b0 :

initial concentration of nitrifying biomass in bulk liquid (M_xL⁻³)

X_f :

Density of nitrifying biofilm (M_xL⁻³)

Y:

Growth yield of nitrifying biomass (M_xM_s ^-1)

Z_f :

Radial distance in biofilm (L)

ε:

Reactor porosity (dimensionless)

θ_c :

Sludge age (T)

θ:

Hydraulic residence time (T)

μ_b :

Absolute viscosity of solvent (ML⁻¹T⁻¹)

μ_max :

Monod maximum specific growth rate (T⁻¹)

μ_w :

Viscosity of water (L²T⁻¹)

υ:

Kinematic viscosity (L²T⁻¹)