Simulated Modelling, Design, and Performance Evaluation of a Pilot-Scale Trickling Filter System for Removal of Carbonaceous Pollutants from Domestic Wastewater

: The aim of the present study is to assess the wastewater treatment efﬁciency of a low-cost pilot-scale trickling ﬁlter (TF) system under a prevailing temperature range of 12 ◦ C–38 ◦ C. Operational data (both inﬂuent and efﬂuent) for 330 days were collected from the pilot-scale TF for various physicochemical and biological parameters. Average percentage reductions were observed in the ranges of 52–72, 51–73, 61–81, and 74–89% for BOD 5 , COD, TDS, and TSS, respectively, for the whole year except the winter season, where a 74–88% reduction was observed only for TSS, whilst BOD 5 , COD, and TDS demonstrated reductions in the ranges of 13–50, 13–49, and 23–61%, respectively. Furthermore, reductions of about 43–55% and 57–86% in fecal coliform count were observed after the 1st and 6th day of treatment, respectively, throughout study period. Moreover, the pilot-scale TF model was based on zero-order kinetics calibrated at 20 ◦ C using experimental BOD 5 data obtained in the month of October to calculate the k 20 value, which was further validated to determine the k t value for each BOD 5 experimental setup. The model resulted in more accurate measurements of the pilot-scale TF and could help to improve its ability to handle different types of wastewater in the future.


Introduction
Availability of fresh water has been decreasing continuously over the last few years, resulting in a severe water shortage throughout the world [1]. The increased urbanization, industrialization, and discharge of toxic hazardous wastes into freshwater streams has further resulted in the depletion of freshwater resources and risk to human health as well as aquatic life [2,3]. The treatment of already contaminated wastewater and its reuse could be the best possible remedy to restore existing water reservoirs. Unfortunately, the majority of developing and under-developed countries are facing a lack of proper strategies for the management of large amounts of wastewater discharged from various domestic and industrial sectors [3]. Wastewater treatment processes can be broadly characterized as physicochemical and biological methods [4,5]. Physicochemical processes involved with the passage of time, biofilms covering the stone media in TFs thicken, which may lead to a decrease in the specific activity associated with the substrate mass transfer limitation of microorganisms within the biofilm [26]. In other words, the activity of the biofilm is not dependent on fixed biomass, i.e., it increases with an increasing thickness of the biofilm up to a certain point (1-3 mm), defining the upper limit for what is termed as active biomass thickness; above this thickness point, the flow of nutrients to the biofilm is a limiting factor that differentiates active biomass from inactive biomass. Thus, in wastewater treatment plants, a steady, squeaky, and active biofilm would be required to optimally neutralize wastes [26]. Moreover, TFs produce a small amount of sludge during treatment, which is removed in the secondary clarifier to produce a quality effluent suitable for irrigation [27].
In many developing countries, it has been common practice for wastewater from the residential and recreational areas to be discharged directly into the natural environment and also be used for irrigation purposes, resulting in many water-borne diseases [28,29]. In addition, the high organic waste in wastewater may lead to eutrophication in the receiving water bodies, thereby decreasing the water quality and resulting in the accumulation of toxic compounds within vegetables and crops, which ultimately affects human life [15,30]. However, the treatment of wastewater is considered to be an essential component with respect to public health, locals' sense of community, and religious beliefs [31]. The safe use of treated water has simply been the use of retrieved water following use in the home, industry, or agriculture. To ensure that treated water is suitable for safe use in public and irrigation sectors, it must have low levels of total dissolved solids, pathogenic organisms, BOD 5 , and salt concentration. Furthermore, treated water allows a community to become less reliant on natural water reservoirs [32][33][34][35]. Tang et al. [36] also reported that treated water enhanced agricultural productivity by 10-30% compared to using untreated wastewater. Therefore, it is essential to develop a low-cost biological wastewater treatment facility to eradicate potential hazards from wastewater before its disposal into the natural environment.
In the current study, a pilot-scale stone media TF system was evaluated under varying environmental conditions. As Pakistan's temperature varies significantly during the year (especially in Islamabad), a humid subtropical climate exists with five different seasons, comprising winter (November, December, January, and February), spring (March and April), summer (May and June), rainy monsoon (July and August) and autumn (September and October). This work contributes to new knowledge in the area of novel TF application in specific climatic conditions. The present study is focused on a simplified approach to establishing design, operation, and management aspects of TF systems for low-cost wastewater treatment. Data was collected for carbonaceous and microbial loads for the safe use of effluent in different daily activities, i.e., ornamental, recreational, and horticulture purposes. Subsequently, a simplified zero-order kinetic model was developed to judge the productivity of the pilot-scale TF system with respect to the oxidation of carbonaceous compounds.

Experimental Setup and Operation
A pilot-scale TF system was installed in the workstation under shade located at the vicinity of the Department of Microbiology, Quaid-i-Azam University, Islamabad, Pakistan. The main body of the TF system was made up of stainless steel (diameter, 1.28 m; height, 2.29 m) to support the stones (trapezium shape) used as filter media. Each stone had the following dimensions: average height, 0.1 ± 0.02 m; base, 0.07 ± 0.01 m; top, 0.09 ± 0.01 m. The surface area of the stones was calculated using the standard equation, i.e., A (trapezium) = 0.5(base + top) × height. It was found that the average exposed surface area of one stone was 0.008 m 2 , calculated for the configuration of biofilm. A rotating arm distributor (diameter, 0.05 m; length, 0.73 m), with 28 small pores to distribute wastewater uniformly, was installed at the top of the filter bed. Electric pumps of 1.3 horse power were connected to the wastewater distribution system through a polyvinyl chloride (PVC) pipe system. To collect treated effluent and sludge, an underdrain system (diameter, 0.92; m; height, 0.37 m) was installed at the bottom of the TF system, with a total capacity of 0.24 m 3 . This was followed by a recirculation tank, which also assisted as a final clarifier, with a final effluent holding capacity of 0.267 m 3 . From there, the effluent was discharged to open lands for different activities. A schematic illustration of the treatment units is shown in Figure 1. The pilot-scale facility was operated in batch mode to treat about 0.195 m 3 (51.5 US gallons) of wastewater per day, with a hydraulic flow rate (Q) of 1.9293 m 3 /d and hydraulic loading (Q/A) of 1.51 m/d. For the development of biofilm, stones were collected from a nearby stream and were kept in a container containing untreated wastewater for about 1 week. Activated sludge was used for seeding in order to facilitate the process of biofilm development. The treatment of wastewater through the pilot-scale TF system was carried out in batch mode using standard protocols. Initially, about 1-2 h settling time was given to the wastewater in the influent feed tank (septic tank) in order to remove large suspended particles. Then, it was circulated and recirculated through the pilot-scale TF at a flow rate (Q) of 1.9293 m 3 /d for about 6 days, collecting the effluent after every 1st and 6th day of treatment. After the completion of one cycle, another cycle started with a new sample of wastewater following a similar scheme of treatment. This batch mode of treatment operation was followed for 11 months (August 2014-June 2015). The temperature was regularly monitored during whole experimental trial; it was in the range of 12 • C-38 • C.

Microbial Profiling of Wastewater Most Probable Number Test (MPN Index) for Fecal Coliforms
The Most Probable Number (MPN) test was performed for the examination and enumeration of fecal coliforms within influent and effluent samples. In this test, three sets of test tubes (with each set containing three test tubes filled with lactose broth along with inverted Durham tubes) were inoculated with influent and effluent samples and then incubated at 37 • C for 48 h. After incubation, test tubes with bubbles in the Durham tubes were considered to be positive, which were then streaked on nutrient agar plates (NA) and incubated at 37 • C for 24 h. After incubation, positive isolates of bacteria were confirmed by general microscopy techniques. The number of tubes that were considered positive for gas production were measured against the standard dilution table for the MPN index.

Determination of Physicochemical Parameters of Influent and Effluent Samples
To assess the efficiency of the pilot-scale TF system with respect to wastewater treatment, physicochemical parameters of influent and effluent samples-including biochemical oxygen demand (BOD 5 ), chemical oxygen demand (COD), total dissolved solid (TDS), and total suspended solid (TSS)-were determined in triplicate after every 1st and 6th day of treatment operation using standard protocols for water and wastewater analysis [35,36]. Moreover, for BOD 5 assessment, we used a dilution method in which four different types of reagents (phosphate buffer solution, magnesium sulphate solution, calcium chloride solution, and ferrous chloride solution) were used in a ratio of 1:1000. For COD assessment, different ranges of COD kits (0-40, 40-1500, and 1500-5000 mg/L) and instruments such as a thermoreactor and Spectroquant (Merck, Pharo 300) were used. The filtration assembly equipped with Whatman filter paper. A weighing balance, china crucibles, a hot plate, and oven were used for the assessment of TDS and TSS.

Model Expansion and Standardization
For the optimum design and operation of a wastewater treatment system, empirical modelling was considered to be an integral part. The modelling of wastewater treatment technologies is increasing day by day, and a number of theoretical, physical designs and systematic models are available in practice. In the current study, a basic systematic model was developed by assuming that the reduction of carbonaceous compounds (BOD 5 ) present in wastewater by a pilot-scale TF system was a single-phase process. Heterotrophic bacteria within the biofilm are most commonly involved in the degradation of organic compounds to obtain energy for their growth; as a result, the concentration of substrates in wastewater decreases with time during treatment. There are many complex dynamic models proposed by researchers [22,37]. These models are focused on describing highly complex biological processes using higher-order kinetic models (e.g., first-order or Monod kinetics) in order to simulate the process with high accuracy. However, they are difficult to apply in many real-life scenarios where the calibration of complex models may not be feasible. Therefore, a zero-order kinetic model was used to mathematically describe the nature of the pilot-scale TF system in this study. A zero-order kinetic model describes the process in a simplified way with less accuracy, but without the need for rigorous calibration exercises.
where S t denotes the final BOD 5 of one treatment unit at any time, S o represents the initial BOD 5 of the untreated wastewater, k t is a specific bioreactor coefficient at a specific temperature, and tis the time required (in days) for the treatment of wastewater. Moreover, X represents biomass that should remain unchanged during the steady state. For practical purposes, X was assumed to be constant. As reported by Naz et al. [22], the initial BOD 5 can be represented as the volumetric loading rate (O LR ): Substituting S o from Equation (5), This equation was used to mathematically describe the ability of the pilot-scale TF system to reduce carbonaceous compounds within wastewater. In this study, 11 months' data was collected by operating the pilot-scale TF system under a temperature range of 12 • C-38 • C; in this way, the model could be applied to all experimental results. The model calibration was done on experimental data obtained at 20 • C in order to determine the k 20 value using the equation k 20 = (S o -S t )/Xt, as shown in Figure 2, which was further validated for each experimental setup to produce a k t value for a particular temperature range.

Efficiency of Pilot-Scale TF for the Removal of Fecal Coliforms
Fecal coliforms are commonly used as an indicator of disease-causing pathogens in the aquatic environment. Influent from a municipality would contain large quantities of fecal coliforms, including pathogenic microorganisms in the range of 1100-5000 per 100 mL [38,39]. Furthermore, it has also been reported that influent containing large quantities of fecal coliforms, if not treated through treatment facilities, would be discharged as such into the surrounding natural environment and would be responsible for lifethreatening consequences [1].
In the present study, it was observed that the MPN index of influent samples was more than 1100 per 100 mL during the entire treatment operation, and the removal efficiency of fecal coliforms by the pilot-scale TF system varied between 43-55% and 57-86% after the 1st and 6th day of treatment, respectively ( Figure 3). Similar observations with respect to the reduction of coliforms by bench-scale stone media TF systems were found by Khan et al. [23]. A higher removal of pathogens was observed in the rainy monsoon (August), autumn (September, October), spring (March and April), and summer (May and June) seasons compared to the winter season (November-February) ( Table 1). A basic reason for this might be that during these seasons, the temperature was in the range of 20 • C-38 • C; therefore, the metabolic activities of microorganisms would be at their peak, and as a result, natural die-off processes and predation by protozoa would take place [21,40]. Another reason for pathogen removal during the treatment of wastewater by the pilot-scale TF system at 20 • C-38 • C might be extended retention time, reactor configuration, microbial competition for survival, and chemical interaction [41]. The removal efficiency of fecal coliforms by the reactor decreased to 43-47% in the winter season (10 • C-15 • C), which clearly indicated that fecal coliforms resisted inactivation when temperatures were lower than 20 • C. This resistance to inactivation was valid because this group of bacteria were mesophilic in nature, and the method to enumerate the fecal coliform bacteria includes incubation at 35 • C-37 • C. However, many of the pathogenic bacteria found in the wastewater were either bound to the solids matrix or trapped to form floc by absorption, coagulation, or precipitation processes that in turn provide better conditions for the survival of pathogenic microorganisms [42].  Shoushtarian and Negahban-Azar [43] also reported standard guidelines that treated water used for the irrigation of fodder crops must contain less than 200 fecal coliforms/100 mL, while Nnaji and Nnam [34] reported that the acceptable MPN index for irrigation with river water containing wastewater discharge was up to 1000 fecal coliforms/100 mL. In the current study, the MPN index of the treated water lay within standard limits; therefore, it is highly recommended that wastewater treatment plants should be designed in such a manner that not only reduces the organic pollution of rivers and streams but also reduces the load of pathogenic microorganisms. In the current study, the quality of domestic wastewater was examined in triplicates. It was dark grey in appearance and the average values of parameters such as BOD 5 (227.3 ± 23.4 mg/L), COD (334.3 ± 34.5 mg/L), TDS (537.3 ± 76.5 mg/L), and TSS (466.3 ± 45 mg/L) during the whole treatment operation showed considerable deviation from the standard limits as prescribed by Shoushtarian and Negahban-Azar [43], indicating a high level of contamination. Moreover, the BOD 5 /COD ratio for the whole period ranged from 0.64 to 0.72, with an average of 0.68. The high BOD 5 /COD ratio indicated a decline in DO concentration-the free oxygen in the water was utilized by microbes and organic compounds, leading to the failure of fish and other aquatic organisms to thrive. Based on these values, wastewater obtained from the residential area of QAU, Islamabad, has been considered as heavy-duty wastewater due to high loads of organic pollutants, as represented by high COD, BOD 5 values, total suspended solids (TSS), and total dissolved solids (TDS), as shown in Table 2. Therefore, a pilot-scale TF system was designed to treat highly contaminated domestic wastewater with high BOD in the range of 195.3-264.8 mg/L.

Post-Treatment Characterization of Wastewater
In the present study, it was observed that the pilot-scale facility worked in all seasons, but the highest average BOD 5 percentage reductions (68-71, 53-65, 67-72, and 70-74%) were observed in August (rainy monsoon season), September-October (autumn season), March-April (Spring season), and May-June (summer season), respectively, while in the winter season (November-February), it achieved removal of carbonaceous compounds in the range of 13-57% after Day 6 of treatment (Figure 4a). A similar pattern of percentage reduction was also noted for COD after Day 6 of treatment under different environmental conditions (Figure 4b). On the other hand, considerable average BOD 5 and COD percentage reductions were observed in May-June (29-37%), March-April (30-44%), August (32-34%), and September-October (8-29.5%), while very low percentage reductions were observed in November-February (5-32%) after Day 1 of treatment (see Figure 4a,b). A similar trend in removal proficiency for BOD 5 and COD had been reported previously, and the primary mechanism for BOD 5 and COD removal was found to be sedimentation, adsorption, and microbial digestion of organic compounds [14,44]. Furthermore, the TF system was found to be more efficient in the hot season (May-June), with a temperature range of 32 • C-38 • C, in comparison to the cold season (November-February), with temperatures lying in range of 10 • C-20 • C. A basic reason might be that the biochemical reactions are temperature dependent and the activity of the microorganism increases with the increase in temperature up to certain value, dropping with a decrease in temperature. A similar trend in BOD 5 and COD removal efficiency due to metabolic activities of microbial communities and seasonal variation had also been reported previously [45,46]. Furthermore, a high degree of reduction in TSS concentration (74-88%) during the entire treatment operation clearly indicated that TSS is a temperature-independent parameter, as shown in Figure 5a. Heavier particles, such as gravel and sand, often settle out in the primary sedimentation tank, while the remaining particles that do not settle out are termed colloidal or non-settleable solids. These non-settleable solids enter the reactor, where they are retained on the surface of the filter bed, and microbes present in the slime layer had sufficient contact time to utilize them as a source of nutrients [47,48]. In addition to this, Pungrasmi et al. [49] reported that continuous recirculation of wastewater over the surface of the filter bed at preeminent flow rates played a significant role in the removal of TSS from wastewater. A 1.9293 m 3 /day recirculation flow rate was maintained throughout the current study, which produced the highest percentage reductions in all seasons for TSS.  Another important parameter in water quality studies is TDS, although there is no health risk associated with it. However, the standard limit for TDS in water provisions reported by Khalid et al. [28] was 500 mg/L. In the case of TDS, the highest percentage reductions were obtained in March-April (73-80%), May-June (73-79%), August (58-74%), and September-October (61-75%). In the winter season (November-February), the concentration of TDS decreased in the range of 23-60% (Figure 5b). Reduction in TDS content with increasing hydraulic retention time (HRT) was also due to continuous recirculation of wastewater for an extended period of time. As a result, microorganisms within the biofilm had sufficient time to degrade dissolved organic components [41]. Furthermore, during their study, Khan et al. [23] reported 23 and 66% reductions in the TDS values after 24 and 48 h of recirculation, respectively, while using a stone media TF system integrated with a sand column filter. Furthermore, it was also observed that the influent TDS seemed to decrease as the temperature decreased; the reactor efficiency with respect to the removal of TDS also decreased in comparison with high temperature months. These differences might be a result of the varying domestic waste loads on the QAU campus, impacted by resident numbers, commercial activities, and sampling and analytical procedures.

Mathematical Model Applications
Modelling was considered to be an essential component to describe the reality of all processes occurring in human life [50]. The empirical model was designed using modelling data of BOD 5 obtained from experimental work done under different prevailing environmental temperature conditions. Such a model should be simple enough in order not to overly increase the level of intricacy and computational demand when coupled with other sub-models.
In the current study, the pilot-scale TF model was based on a simple zero-order kinetic model, which was calibrated at 20 • C using experimental BOD 5 data obtained in the month of October, to calculate the k 20 value, which was further validated to determine the k t value for each BOD 5 experimental setup performed under different temperature conditions ( Table 3). The value of k t obtained from experimental results showed better performance at six different temperature data sets: November (18 • C), December (12 • C), January (10 • C), March (23 • C), May (32 • C), and June (38 • C). In these different temperature data sets, the value of k t relates to higher removal of BOD 5 through the pilot-scale treatment system due to the excellent mass transfer coefficient, as shown in Figure 6. This type of simple model calibration of a zero-order kinetic model offers an advantage over complex and higher-order models which may describe the dynamics of the process more accurately. For design, operation, and maintenance points of view, this less accurate but easy-to-apply model offers a good alternative to complex models for low-cost TF systems for wastewater treatment. Key: OLR-organic loading rate; k t -specific bioreactor coefficient at a specific temperature. From zero-order kinetic modelling, it was observed that the value of k t was specific for each experimental data set. Furthermore, minor differences between the experimental data and the modelling ones were observed on some days, especially Day 1. A basic reason for this might be that in a fixed film system, microorganisms are attached to the surface; therefore, the supply of nutrients to the microbes in the slime layer might be restrained by bulk liquid and surface transport mechanisms. Normally, in fixed film growth systems, nutrients or organic substances present in wastewater are transported to the outermost surface of biofilm; from there, they diffuse into the biofilm for utilization. According to Fick's law and the Monod expression, there are different factors that affect the rate of substrate utilization within a biofilm, i.e., the transport of nutrients to the outer surface of the biofilm from the bulk liquid, the diffusion of nutrients from the outer surface into the biofilm, and the utilization kinetics within the biofilm. Beside this, there are some other factors which affect the efficiency of attached growth bioreactors, including hydraulic loading rates, temperature, recirculation flow rates, etc. In the literature, very few empirical models have been reported that can forecast the efficiency of a TF system, and among them, most models are based on steady state conditions. A major drawback of the steady state model is that it does not give sufficient information regarding biomass growth with time, void ratio of filter media, and recommended depth for the filter bed [51,52].
Moreover, modelling results describe about 21-45% removal of BOD 5 in low temperature range data sets (November, December, and January) and 67-90% in high temperature range data sets (March, May and June), as shown in Figure 6, which exhibited that the model for the pilot-scale TF system worked seamlessly in the removal of carbonaceous compounds from wastewater. Hvala et al. [51] also used an empirical model to evaluate the efficiency of wastewater treatment in terms of BOD 5 and observed a considerable reduction in the BOD 5 content. Furthermore, they concluded that bioreactor efficiency was greatly affected by changes in the organic and hydraulic loading rates. In an interpretation of this study, a significant correlation might exist in experimental as well as mathematical data for BOD 5 under different temperature conditions. Therefore, it gives more reliable and precise appraisals with regard to hydraulic load rates, biomass concentration, and their specific activity in wastewater research as well as the design of wastewater treatment plants. The model foretells that the percentage reduction of BOD 5 surpasses the maximum possible for the assumed size distribution of soluble organics. The key drawback of the existing model was that wastewater must have related features to those used for model calibration. This drawback not only incorporates the diffusion coefficients of dissolved organics but also microbial activities for these substrates. The model also assumes that recirculation of wastewater enhances the overall level of wastewater treatment efficiency by the pilot-scale TF system. Oliynyk and Kolpakova [53] also developed a mathematical model describing removal of BOD 5 from domestic wastewater by attached growth systems under many loading and design conditions. They reported that their simulated model was perfect to precisely predict percentage removal of BOD 5 as a function of hydraulic loading rate, recirculation rate, depth of filter bed, and microbial activity within the biofilm under different temperature conditions.

Conclusions
The wastewater treatment performance of the locally designed and constructed pilotscale TF system was good in all seasons apart from winter in terms of the removal of organic as well as microbial pollutants from domestic wastewater. It was concluded that in the rainy monsoon (August), autumn (September-October), spring (March-April), and summer (May-June) seasons, average percentage reductions were in the range of 52-72, 51-73, 61-81, and 74-89% for BOD 5 , COD, TDS, and TSS, respectively. However, in the winter season (November-February), an excellent percentage reduction was only observed for TSS (74-88%), while for other parameters such as BOD 5 , COD, and TDS, it was in the range of 13-50, 12-49 and 23-61%, respectively. Moreover, a substantial reduction in microbial content of the wastewater was also observed in all seasons of treatment due to nutrient depletion in the effluent samples, predation, and natural die off-processes. The MPN index of treated effluents decreased to significantly lower values in all the seasons. During the non-winter seasons, the TF system showed high potential for wastewater use in the irrigation sector, for which an MPN/100 mL of lower than 200 colonies is desired. This is particularly relevant to developing countries where water sources are under significant threat due to increasing pollution. The zero-order kinetic model provided a simplified mathematical interpretation of a composite set of biological, chemical, and physical interfaces that occur in TFs. Therefore, it was also concluded that the model resulted in a better understanding of pilot-scale TF design aspects and could help to improve their ability to handle different types of wastewater in the future, using such TFs at different scales. The results provide a simplified approach to assessing the potential application of TF systems in developing countries.