### 1. Introduction

_{b}/V

_{r}) and different superficial velocities (U

_{g}) such as: for (i) V

_{b}/V

_{r}= 0.10, U

_{g}= 0.089 m/s, 0.095 m/s and 0.099 m/s (ii) V

_{b}/V

_{r}= 0.20, U

_{g}= 0.216 m/s, 0.220 m/s and 0.224 m/s (iii) V

_{b}/V

_{r}= 0.30, U

_{g}= 0.274 m/s, 0.278 m/s and 0.281 m/s. The study revealed that the flow behaviour of IFBBR approached plug flow condition for (V

_{b}/V

_{r}) ratio of 0.20 with U

_{g}of 0.220 m/s showing higher plug flow index and lower dispersion value compared to all others. The results were validated by carrying out degradation experiments in IFBBR using with same ratios of (V

_{b}/V

_{r}) and superficial velocities and reported the reductions in COD, TDS, TSS during the process of wastewater treatment. Thus, present study has been attempted to investigate the flow dynamic behavior of IFBBR and to compare with the experimental validation for the treatment of liquid biomedical pharmaceutical wastewater in the reactor. This is the novel approach which were not tried by any other researchers as per our knowledge and based on our literature studies.

### 2. Experimental Section

### 2.1. Experimental Set Up

^{3}as density and 1.714 x 10

^{3}m

^{−1}as surface area to volume ratio of particles.

### 2.3. Microorganism

*P.fluorescens*was chosen for the degradation of pharmaceutical wastewater since it has the potential to degrade all organics using it as the sole carbon and energy source. Primary culture was prepared by transferring microorganisms into feed medium containing mineral salt medium [20] and pharmaceutical biomedical wastewater which were then kept in incubator to attain steady state growth. Secondary culture was then developed from primary and was used as inoculum for the formation of biofilm onto polystyrene support particles in IFFBR [21].

### 2.4. Experimental Procedure

#### 2.4.1. RTD experiment

_{b}/V

_{r}) such as 0.10, 0.20, 0.30 flowing at different superficial air velocities, water was allowed to pass into the reactor. In the pulse input mode, 5 mL of HCl was injected into the reactor entrance in continuous operation. The time and conductivity of the water was noted at regular intervals of time (30 s) at the reactor outlet. The experiment was about to end when the conductivity reduced to the level of normal water. The experimental value of exit age distribution E(t) was determined to optimize the flow characterization and performance of the reactor.

#### 2.4.2. Pharmaceutical biomedical wastewater degradation studies in IFBBR

*P.fluorescens*for 2 days to encourage biofilm formation onto the support particles. The biomedical wastewater was then treated in IFBBR by varying the ratios of settled bed volume to reactor working volume with different superficial air velocities. The reactor was operated at room temperature with non-sterile condition. The treatment process was carried out until there was sufficient biodegradation of COD takes place inside the reactor. All the experiments were conducted in triplicate for each experimental condition to get the mean concordant value. Statistical analysis were done to find mean, variance standard deviation, standard error, etc. and incorporated in the figures of results and discussion section.

### 2.5. RTD Profiles and Its Design Parameters

*E(t)*curve can be evaluated [22]:

##### (5)

$$\left({\scriptstyle \frac{{\sigma}^{2}}{{{t}_{m}}^{2}}}\right)=2\hspace{0.17em}\left[{\scriptstyle \frac{D}{(uL)}}\right]-2\hspace{0.17em}{\left[{\scriptstyle \frac{D}{(uL)}}\right]}^{2}\hspace{0.17em}(1-{e}^{[(uL)/D]})$$*σ*

^{2}is the variance in min

^{2};

*t*

*is the mean residence time in min;*

_{m}*D*is the diffusion coefficient in (m

^{2}/s);

*u*is the fluid flow velocity in m/s;

*L*is the length of the reactor in m;

*V*

*is the reactor volume in*

_{r}*L*; and

*Q*is the volumetric flow rate of the fluid in (

*L*/

*h*).

### 2.6. Measurement and Analysis for Degradation Experiments

_{L}a), biofim thickness, biofilm dry density and pH. All the determinations were done according to standard procedures and methods [19] whereas the biofilm thickness and dry density of biofilm were estimated from the net weight of the bioparticles [24–26]. The low-density support particles form a packed bed appearance at the top of the reactor once they were fed to the reactor column. By supplying the superficial air velocity, the bottom layer of the packed bed starts to fluidize. Then upon increasing the air flow velocity (minimum fluidization velocity) the bed started first to fluidize downwards with uneven distribution in the reactor. Upon further increase in air velocity (critical fluidization velocity), the bed fluidize completely with uniform concentration of bio-particles throughout the reactor [27–29]. All the experimental runs were carried out with superficial air velocity (U

_{g}) which are equal to or greater than the critical fluidization velocity in IFBBR by varying the ratios of (V

_{b}/V

_{r}) with different U

_{g}. The average gas hold up can be calculated by using the relation [30]:

_{s,}V

_{r}are volume of solid particles and working volume of the reactor respectively; Z

_{f,}Z

_{i}are the aerated liquid level in the reactor column after fluidization and the initial liquid level before aeration, respectively.

### 3. Results and Discussion

_{b}/V

_{r}) ratio and superficial air velocities on fluid flow characteristics of the fluid inside the IFBBR. The performance of IFBBR was experimentally validated by investigating the biodegradation of pharmaceutical biomedical wastewater by varying the ratio of settled bed height to reactor volume (V

_{b}/V

_{r}) in the range of 0.10, 0.20 and 0.30 for various superficial air velocities U

_{g}for each (V

_{b}/V

_{r}) ratio. To optimize the ratio (V

_{b}/V

_{r}) and superficial air velocity U

_{g}with greater accuracy, experiments were performed for various ratios of (V

_{b}/V

_{r}) with three different superficial gas velocities U

_{g}(i.e., for (i) V

_{b}/V

_{r}= 0.10, U

_{g}= 0.089 m/s, 0.095 m/s and 0.099 m/s (ii) V

_{b}/V

_{r}= 0.20, U

_{g}= 0.216 m/s, 0.220 m/s and 0.224 m/s (iii) V

_{b}/V

_{r}= 0.30, U

_{g}= 0.274 m/s, 0.278 m/s and 0.281 m/s). All the experimental data were analyzed in triplicate and the removal efficiency of COD for different experimental runs was studied by conducting biodegradation experiments in IFBBR.

### 3.1. Effect of Superficial Air Flow Rates on Flow Dynamics

_{g}) for different (V

_{b}/V

_{r}) ratios such as 0.1, 0.2 and 0.3. The effect of flow velocities was found to be significant on the obtained RTDs and on the fluid flow behavior in the IFBBR. Fig. 2 shows the exit age distribution curve for various air flow velocities. The non-symmetrical curve shows the presence of short circuiting or bypassing along the reactor [31]. From Fig. 2, it was found that the peak points in E(t) curve reached the highest value of 0.85 for the superficial air velocity (U

_{g}) of 0.220 m/s for the (V

_{b}/V

_{r}) ratio of 0.2 which has good mixing condition and lack of short circuiting characteristics. Thus, the flow behavior for the air flow rate of 0.220 m/s have sufficient dispersion takes place between air and liquid phases with no dead zone and less short circuiting when compared to all other flow rates. The curve also approached near symmetrical for 0.220 m/s depicting much less short circuiting condition along the reactor length and thus the flow in IFBBR tends to act ideal under this condition.

^{2}) and dispersion (D) influence on the efficiency of the reactor to degrade the effluent. RTD data discussed above have been compared with the experimental results of the treatment studies of pharmaceutical based liquid biomedical wastewater, carried out with same set superficial air velocities and (V

_{b}/V

_{r}) ratio. The plug flow behavior for the optimized air flow velocity at 0.220 m/s with (V

_{b}/V

_{r}) of 0.20 from the RTD analysis matches well with the experimental degradation studies in IFBBR producing higher percentage of COD removal and are discussed in Sections 3.2 and 3.3.

### 3.2. Effect of Superficial Air Flow Velocity and (V_{b}/V_{r}) Ratio on COD Removal

_{b}/V

_{r}) with different superficial air velocities (U

_{g}) to treat pharmaceutical biomedical wastewater. The applied superficial air velocity (U

_{g}) were equal to or greater than the critical fluidization velocity below which the fluidization of bio-particles were not uniform throughout the reactor [29]. Experiments were carried for the (V

_{b}/V

_{r}) ratios of 0.10, 0.20 and 0.30 with its respective superficial air velocity, U

_{g.}

_{b}/V

_{r}) ratio. There found to be an optimal superficial air velocity at which the COD reduction attained high (92%) compared to all other air velocity. Table 3 shows optimal superficial air velocity, U

_{gm}with respect to COD reduction in IFBBR.

_{b}/V

_{r}) = 0.20 at its optimal superficial velocity, U

_{gm}= 0.220 m/s. An increase in COD removal with an increase in (V

_{b}/V

_{r}) from 0.10 to 0.20 can be attributed to the fact that for the increasing (V

_{b}/V

_{r}) there will be more biomass grown on large volume of particles resulting in higher degradation of biomedical wastewater. But with further increase in (V

_{b}/V

_{r}) from 0.2 to 0.3, larger volume of the reactor was occupied by support particles leading to higher solid hold up which worsen the phase mixing characteristics of the reactor resulting in decrease in treatment efficiency [32–34]. (V

_{b}/V

_{r}) ratio of 0.3 was found to be the critical value above which the fluidization of bio-particles was not possible and the particles would settle as a bed at the top of the reactor. Thus, (V

_{b}/V

_{r})

_{m}= 0.20, U

_{gm}= 0.220 m/s were found to be the optimal hydrodynamic operating parameters at which the COD reduction were found to reduced from 1,630 to 130 mg/L at a maximum with 92% removal efficiency. The results were well supported by the mass transfer studies in section 3.3

### 3.3. Effect of Superficial Air Velocity on Mass Transfer Coefficient (k_{L}a), Oxygen Transfer Rate (OTR) and Biofilm Characteristics for Various Ratios of (V_{b}/V_{r})

_{L}

*a*), OTR and biofilm characteristics for various (V

_{b}/V

_{r}) ratios. From the Table, it is evident that for the optimal superficial air velocity (U

_{gm}) there will be higher COD reduction in the treatment process due to higher volumetric mass transfer coefficient k

_{L}

*a*and oxygen transfer rate (OTR).

_{g}) and bubble size (d

_{b}) increases with increase in U

_{g}. ɛ

_{g}and d

_{b}play a vital role in increasing the interfacial area (

*a*), thereby the OTR and mass transfer coefficient (k

_{L}

*a*). If these parameters, have the relation of

*a*= 6ɛ

_{g}/d

_{b}, the increase in ɛ

_{g}produces higher

*a*, but the increase of bubble size d

_{b}decreases

*a*. When the increase rate of ɛ

_{g}dominates over that of d

_{b}, the k

_{L}

*a*increases. Because of these two competing effects, the increasing trend of k

_{L}

*a*becomes insignificant with increasing U

_{g}[24]. Thus at the optimal condition of U

_{g}, ɛ

_{g}dominates over the bubble size d

_{b}and have a significant effect in increasing the mass transfer rate and thereby the COD removal efficiency. At the optimum superficial velocity (U

_{gm}), it was found that a compact and stable biofilm structure was developed on the bioparticle. The bioparticle with compact and stable biofilm (L

_{f}at U

_{gm}) had highly dense biofilm with higher biofilm dry density, X

_{f}(0.2545 g/cm

^{3}) due to extra biomass growth in the base biofilm [35]. Above U

_{gm}, the biofilm thickness was found to be more having less biofilm dry density and the thickness could not be controlled by the detachment force anymore thereby decreasing the performance of the biomass inside the reactor.

### 3.4. Physiochemical Analysis of Treated Pharmaceutical Based Biomedical Wastewater

### 4. Conclusion

The research work investigated the flow dynamic characteristics of IFBBR for the treatment of liquid biomedical wastewater.

The RTD method was used to understand the flow dynamic behaviour of the reactor and the results showed that the plug flow index approached a higher value of 0.333 for the superficial air flow velocity of 0.220 m/s and (V

_{b}/V_{r}) of 0.2. Under this optimal condition, there exist sufficient dispersion between air and liquid phases inside the reactor with less occurrence of dead zone.The optimal plug flow behaviour from RTD analysis matches well with the experimental biomedical wastewater treatment studies in IFBBR producing higher percentage of removal of COD (92%), TDS (90%) and TSS (96%).

Mass transfer effects were also evaluated at the optimal operating conditions of IFBBR. For the optimal superficial air velocity (U

_{gm}) there will be higher COD reduction in the treatment process due to higher k_{L}*a*(1.732 min^{−1}) and OTR (0.0146 g/(L.min)).