Response Surface Methodology for Optimization of Rotating Biological Contactor Combined With An External Membrane Filtration

Sharjeel Waqas Universiti Teknologi Petronas: Universiti Teknologi PETRONAS Noor dza Yub Harun Technology University of Petronas: Universiti Teknologi PETRONAS Muhammad Roil Bilad (  roil.bilad@ubd.edu.bn ) Universiti Brunei Darussalam https://orcid.org/0000-0001-7292-6046 Tau k Samsuri Universitas Pendidikan Indonesia Nik Abdul Hadi Nordin Technology University of Petronas: Universiti Teknologi PETRONAS


Introduction
Biological methods have been widely adapted in wastewater treatment with the advantages of more cost-e cient, smaller footprint requirements, higher speci c biomass activities, and sustainable and environmentally friendly (Liu et al., 2021;Fito et al., 2019). Stringent e uent standards imposed by regulatory authorities highlighting the need for the development of a sustainable and environmentally friendly treatment process (Ashraf et al., 2021;Preisner, 2020). Rotating biological contactors (RBC) rely on the rotating party submerged disks to grow bio lm that is kept in constant motion for energye cient aeration. The longer sludge retentions time (SRT) able the growth of the nitrifying community throughout the bio lm, to perform complete nitri cation (Šíma et al., 2016). In RBC, su cient microbial growth on the rotating disks exploits the bene t of high e uent quality and treatment e ciency. The RBC bioreactor has been utilized to achieve steady-state nitri cation round the year and to improve removal e ciency and treatment capacity due to the liberty to multiplicate the mixed liquor suspended solids (MLSS). RBC operates at a high microbial concentration that allows a higher organic loading rate (Vasiliadou et al., 2016). Like in the membrane bioreactor, RBC can further be extended by incorporating membrane ltration for sludge separation. Recently, various RBC con gurations appear as a promising alternative to the traditional treatment processes and they have been applied for the treatment of both municipal and industrial wastewaters (Waqas and Bilad, 2019).
Membrane integrated RBC has been explored to enhance the physical and biological operation of the bioreactor.
Membrane integrated RBC employs membrane technology as post-treatment eliminating the need for a settling tank. The operational parameters, SRT, hydraulic retention time (HRT), and disk rotational speed have a considerable in uence on microbial activity and membrane fouling potential . These operational parameters can alter microorganism properties and helps in optimizing the system's performance. Recent studies have shown that short HRT results in a high organic loading rate which could increase the viscosity and sludge concentration as well as increase the lamentous bacteria growth (Deng et al., 2016). Recent studies have depicted that disk rotational speed, HRT, and SRT in uence the biodegradation process and micro-pollutant removal in membrane systems (Waqas et al., 2020a;Gkotsis et al., 2017).
The relationship and optimization of the operational parameters can help to increase the performance e ciency of the bioreactor. Hence, the design of experiments (DOE) is an excellent choice to optimize the process parameters. The DOE acquires fewer experiments and statistically predictable with highly reliable and e cient results. Response surface methodology (RSM) is an excellent example of such an approach in enhancing the performance e ciency of bioreactors and can study the behavior of various parameters simultaneously (Abdulgader et al., 2020). RSM is an empirical statistical technique that can investigate mathematical modeling to comprehend the mutual relationship of various process parameters on the response variable. The quantitative data is generated from the design of experiments and analysis of regression models and operational conditions can result in high-end performance (Belgada et al., 2020).
Biological wastewater treatment incorporating membrane separation has been the focus of research worldwide (Baneshi et al., 2020;Sheth et al., 2021;Bezirgiannidis et al., 2018). Some studies have focused on the optimization of the operating conditions of the membrane process. Askari et al., (Askari et al., 2018) studied RSM for the examination of the effect of process conditions on the NF membrane removal e ciency. The operational parameters, disk rotational speed, HRT, and SRT have a signi cant in uent on microbial community concentration, biological performance, and membrane fouling propensity (Nuansawan et al., 2016;Deng et al., 2016). Disk rotational speed, HRT, and SRT can alter the extracellular polymeric substances secretion, sludge settling characteristics, and mixed liquor properties. However, no research work has been documented to optimize process parameters in RBC integrated with external membrane bioreactor using polysulfone membrane.
In this study, RSM modeling and optimization technique was utilized to investigate the relationship between different variables via establishing the predicted models. Box-Behnken design (BBD) through RSM was employed to examine the effect of three operational parameters (disk rotational speed, HRT, and SRT) on membrane permeability. The objective of this study is to investigate the relationship between operational parameters (disk rotational speed, HRT, and SRT) and the response parameter of permeability and to nd the optimal condition of the process by using RSM. During experimentation, different operational parameters (disk rotational speed, HRT, and SRT) values were altered through variable speed shaft motor, an increase of organic loading rate and sludge wastage rate, respectively and performance of RBC combined with external membrane ltration (RBC-ME) bioreactor was analyzed. The optimization of membrane incorporated wastewater treatment process improves membrane permeability and reduces the operational cost of the process.

Wastewater preparation and bioreactor acclimatization
The lab-scale RBC-ME bioreactor was fed with the synthetic wastewater prepared by blending food leftover as suggested in our previous study (Waqas et al., 2020c) and achieved in uent wastewater concentrations are summarized in Table 1. Table 1 In uent characteristics for the RBC-ME bioreactor employing the polysulfone membrane in the post-treatment.

Component
In uent

Bioreactor set-up
The RBC-ME bioreactor consisted of a feed wastewater tank, RBC bioreactor and external membrane vessel was constructed in-house as shown in Fig. 1. The feed wastewater tank of capacity 42 L was used as a storage vessel to supply a constant supply of wastewater to the RBC bioreactor. The feed tank was facilitated with a mechanical stirrer to maintain the uniform concentration of wastewater throughout the storage vessel. The RBC bioreactor consisted of a 25×25×30 cm tank fabricated from poly (methyl methacrylate) and had a working volume of 6.5 L. The bioreactor was equipped with 5 rotating disks of 1.8 cm thickness and 18 cm diameter mounted on a stainless-steel shaft. The shaft was attached to a DC motor that rotates the disks. The disks were also fabricated from poly (methyl methacrylate) equivalent to a net surface area of 2034 cm 2 and placed inside the RBC bioreactor at 40% submergence. The disks were attached to polyurethane sheets (1.22-1.27 g/cm 3 density) to colonize the microbial population. The at sheet membrane module was placed after the RBC bioreactor acting as a post-treatment. The RBC-ME system did not include a settling tank (typically a part of a conventional RBC unit), which was replaced with a membrane ltration tank. The permeate was collected from the ltration cell and its volume was measured regularly.

Bioreactor operation
The lab-scale RBC-ME bioreactor was inoculated from the activated sludge obtained from the nearly full-scale domestic wastewater treatment plant. The bioreactor was operated for 45 days, divided into two phases. During the rst phase, the bioreactor was operated on for 17 days to fully acclimatize the bio lm. The feed wastewater organic loading rate was kept constant at 19 g chemical oxygen demand (COD)/m 2 d and bio lm formed at the rotating disks was observed for any changes. The membrane panel was placed into the system after bio lm acclimatization and to study the impact of membrane permeability performance under various parameters.

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The experimental investigation was done for three operating parameters (disk rotational speed, HRT, and SRT). The disk rotational speed was set from 30 to 50 rpm, HRT was set from 9 to 15 h and SRT was set from 5 to 15 days. The disk rotational speed was increase with the variable speed DC motor; ow rate was increased to increase the HRT while SRT was varied by wasting a portion of the reactor volume each day.

Analytical methods
COD, total nitrogen (TN), ammonia, and nitrate were measured using the speci c Hach digestion solution (HACH, Loveland, USA) for each compound. The solution was diluted to fall into the range of the digestion vials being used for the study. The values were determined through Hach DR3900 Spectrophotometer (HACH, Loveland, USA). Hach 2100Q portable turbidimeter (HACH, Loveland, USA) and Hach HQ411D benchtop PH/MV meter (HACH, Loveland, USA) were used to determine turbidity and pH, respectively (APHA, 1997).

Determination of ltration performance
The low pressure for ltration not only reduces the energy cost, but it also is less susceptible to membrane fouling and maintains sustainable ux as reported elsewhere (Bilad et al., 2020). The membrane permeability (L), L/m 2 h bar) was calculated by using Eq. 1.
where V is the volume of permeance (L), ∆P transmembrane pressure (bar), t ltration time (h), and A membrane area (m 2 ).    Table 3 were determined after the acclimatization time, showing a stable biological performance as depicted from steady removal e ciencies. The synthetic domestic wastewater used as the RBC feed contained a high number of organics. Therefore, carbonaceous bacteria undertaken the removal of the organics were expected to dominate in the bio lm. The high abundance of carbonaceous bacteria biodegraded the readily available substrate (organic pollutants). The RBC-ME bioreactor demonstrated excellent biological removal e ciencies in terms of COD, ammonia, TN, and turbidity (Table 3). The results show signi cantly higher removal e ciencies for the COD, TN, ammonia, and turbidity of 87.6 ± 2.7, 45.2 ± 2.6, 98.5 ± 0.07, and 97.8 ± 0.2, respectively. The higher e ciency for COD and ammonia can be attributed to the high microbial activity of carbonaceous and ammonia oxidizing bacteria (Waqas et al., 2021c;). An increase in e uent nitrate concentration (1.8 ± 0.2) is due to poor anoxic conditions within the bio lm resulting in a low concentration of nitrate oxidizing bacteria. In the attached growth system, the relative position of the bio lm to the surface affects the oxygen concentration, being the lowest (anoxic) in the deeper part. It can be speculated that the formed bio lm on the substrate media was still too thin to exert the required anoxic condition for denitri cation. Higher removal e ciency for turbidity was achieved by the RBC-ME bioreactor due to the membrane separation. The results showed the turbidity value substantially diminished from 14.6 ± 0.1 NTU to 0.32 ± 0.03 NTU in the RBC-ME attributes to 97.8 ± 0.2% removal e ciencies ( Table 3). Consequently, the biological performance can be enhanced by increasing the biomass SRT.

RSM model optimization
Full fractional three-factor BBD was applied to investigate the effects of three independent parameters to model the steady-state membrane permeability. The results of the BBD model are described in common ANOVA (Table 5)  Eqs. 1 and 2 were used to predict the steady-state membrane permeability listed in  A diagnostic plot such as the predicted versus actual values shown in Fig. 2 supports adjudicating the model satisfactoriness visually. Figure 2 indicates an adequate agreement between real data and the ones obtained from the models. A model normally can be considered reproducible if its coe cient of variance (CV) is less than 10%. The quadratic model represents a CV value of 0.8978% indicating good reproducibility of the model.

Process analysis
Membrane permeability is affected by operational parameters, that can exist in neutral, positive, or negative con gurations. Those parameters can also signi cantly affect biological performance and e uent quality. Therefore, the study of the in uence of these parameters on the steady-state membrane permeability can help to optimize the whole process (biodegradation and permeability). The dependency of the operational parameters on the membrane permeability is presented in Fig. 3   However, a less signi cant effect was found for the HRT because of the higher organics ratio and lower nitrogen concentration (Huang et al., 2011). All the organics are readily degraded and do not require higher HRT resulting in good performance even at low HRTs. Removal e ciencies decrease when moving away from these points, which means either a decrease or increase in any of the tested variables results in the decline of the permeability. The interaction between HRT and SRT as shown in Fig. 3c and f reveals that higher HRT and SRT increases, as also reported elsewhere (Berkessa et al., 2018). With the increase of both HRT and SRT from 12 h and 5 d, respectively, an increase in permeability is observed. Indeed, the membrane permeability further increases at higher HRT and SRT. HRT seems just slightly affect permeability. However, high SRT results in e cient settling of sludge and subsequently higher membrane permeability (Mannina et al., 2018). Overall, high values of both HRT and SRT favor higher permeability.

Process optimization
The optimal condition of three process variables of disk rotational speed, HRT, and SRT for maximum steady-state permeability was examined based on the desirability function. Parameter optimization was performed to determine the highest particular point, which ampli es the signi cant function i.e. steady-state permeability.
As shown in Fig. 4, the optimal conditions for maximum permeability (144.6 L/m 2 h bar) were found at a disk rotational speed of 36.1 rpm, HRT of 18 h, and SRT of 14.9 d, as shown in Fig. 4 and Table 7. The excellent agreement between predicted and experimental results con rms model validation to simulate the steady-state permeability.  Table 8 shows the experimental and model predictive values for the optimum condition. The steady-state permeability response of both the experimental and model values is in close agreement.

Conclusions
This research applies RSM to optimize the biological and ltration performance of an RBC coupled with external membrane ltration. The RBC-ME bioreactor was a successful biological treatment process to achieve a high biological removal e ciency. The RBC exhibits 87.6 ± 2.7% of COD, 45.2 ± 2.6% TN, 98.5 ± 0.07% ammonia, and 97.8 ± 0.2% turbidity removal e ciencies. The RSM results demonstrated the effects of the operating parameters as well as their interactive effects on permeability as the response. At higher disk rotational speeds (> 35 rpm), the permeability decreased due to the higher shear rate and shredding of bio lm ocs. At higher HRT and SRT, higher permeability was obtained. By applying RSM, the optimum region for the bioreactor operating conditions was located. The optimum conditions obtained were 144.6 L/m 2 h bar permeability at disk rotational speed 36.1 rpm, 18 h HRT, and 14.9 d SRT. The results demonstrated good agreement amongst experimental and model predictions. It is evident from the current study that RSM is an e cient statistical optimization approach that can help to distinguish between the most important operational parameters at the cost of minimum time and effort. The development of the membrane integrated RBC system can signi cantly enhance the e uent quality to satisfy the stringent regulations and can serve as a promising alternative for decentralized application for the development of a sustainable environment.

Declarations Data Availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Figure 1
Schematic diagram of RBC-ME con guration.

Figure 2
Design-expert plot; predicted vs. actual values plot for steady-state permeability.

Figure 4
Process optimization values of the operational parameters at maximum steady-state membrane permeability.