Fluidized Immobilized Carbon Catalytic Oxidation Reactor for Treating Domestic Wastewater

Treating wastewater and reusing it have become normal in the present era since the scarcity of fresh water prevails in many parts of the world. There are numerous techniques for treating domestic wastewater, and many have been explored to advance the ones already in use. This study is taken up to explore the new technology called fluidized immobilized carbon catalytic oxidation (FICCO). Usually, both organic and inorganic materials are present in residential wastewater. In this study, catalyst-activated carbon produced from rice husk is added to a FICCO reactor to test the effectiveness of decreasing organic contaminants in wastewater. Six FICCO models were fabricated in this research study, and tests were conducted. The effectiveness of COD and BOD removal was investigated using six FICCO models with activated carbon made from rice husk as a catalyst and presented in this paper. The FICCO reactor was also used to treat organic contaminants such as surfactants, starch, oil, and protein with rice husk as activated carbon as a catalyst. Organic pollutants used the FICCO reactor; COD removal varied from 75.6% to 92.4%, BOD removal ranged from 74.9% to 89.5% at the optimum contact time, and catalyst rice husk-activated carbon application. The optimum catalyst dosage was 12 g per 620 ml of wastewater, which is the capacity of each reactor and a substantial reduction in sludge.


Introduction
Domestic wastewater (sewage), which typically contains 40 to 60% dissolved materials, 5 to 10% colloidal materials, and balance suspended materials, contributes signifcantly to pollution. Te need for wastewater treatment in India has grown due to the country's expanding industries and population. Tis sector will use the most treated wastewater for production and home purposes. Over many years, coagulation, focculation, and oxidation techniques have been developed to remove organic matter (expressed as BOD and COD) from wastewater produced by domestic and industrial processes. Even though the time needed for treatment is shorter, those procedures are expensive because they require the addition of chemicals and trained employees.
absorbed by the body [9]. Activated carbon granules or air stripping are frequently used to remove volatile organic compounds (VOCs) such as low molecular weight-chlorinated solvents and aromatics. Granular-activated carbon can reduce organic pollutants to undetectable ones [10].
To reduce organic pollutants in wastewater, it is suggested that a fuidized immobilized carbon catalytic oxidation (FICCO) reactor cell with low sludge generation be studied and evaluated. A complicated environment develops as the contaminated water stream passes over a thin layer of activated carbon, forming a mass transfer zone. Contaminants are absorbed by the surface of mesoporous-activated carbon (MAC), which is used for this purpose. Immobilized cell treatment technology enables productivity growth by lowering the output of surplus sludge. Te purpose of this study is to demonstrate the ability of the FICCO reactor system to remove dissolved organics such as BOD and COD from the residential sewage efuent in Adyar, Chennai.
Te COD and BOD removal efciency of the FICCO reactor was improved while adjusting the catalyst loading and time to determine the most efcient removal of organic pollutants. If rice husk carbon is employed, the investment cost of a sewage treatment plant would be greatly reduced. Tere would be a signifcant reduction in the operational cost of electrical energy use [11]. Te catalytic functionalization oxidation reactor (CFOR), heterogeneous-activated carbon Fenton catalytic oxidation (HAFCO), Fenton-activated carbon catalytic oxidation (FACCO), and fuidized immobilized cell carbon oxidation were some of the methods used by the authors. Combined catalytic reactors destroyed phenol with a 96.7% efciency, 100% of sulphide without releasing it into the environment, 95.3% of COD, and 99.2% of BOD.

Literature Survey on Fluidized Immobilized Carbon
Catalytic Oxidation Technology. Te removal of dissolved organics from home wastewater by an integrated oxidation process was proved by experimental tests using UV-visible and Fourier-transform infrared spectroscopy [12]. Te most recent studies on biochar and hydrochar manufacturing, characterisation, and prospective applications have been examined. Te efects of reaction temperature, feedstock types, and thermochemical conversion modes were contrasted with biochar. Tere was also discussion on the benefts and difculties of encouraging the use of biochar in energy and the environment, such as adsorbents, catalytic precursors, soil amendment, anaerobic digestion and composting, and electrochemical energy storage materials. Te authors also discussed the future directions for biochar research and development [13].
To produce various silicon-based chemicals and active carbon, rice husks (RHs) are processed [14] and evaluated critically. Te impacts of various process parameters on the pyrolysis stage, the efect of physical, chemical, and thermal treatments, activating conditions, and the mechanisms of activated carbon consolidation were all discovered as alternative processing methods. An acidic stream is necessary to neutralise the efuent during biological wastewater treatment when pH is neutral. As a result, a very deadly gas called hydrogen sulphide is released. Tis work removed COD from wastewater and sulphide in lime sulphide liquor using the heterogeneous peroxide oxidation (HPO) method. Wastewater's sulphide was catalytically oxidised to sulphate by cleaving protein molecules, and the ammonia level was raised by roughly 78.05% [15].
Te authors discussed the fuidized bed reactor's uses in wastewater treatment [16] and highlighted the crucial design and operational factors that afect the reactor's performance. Fluidized bed reactors for liquid-solid and gas-liquid-solid processes, all efective methods for treating wastewater, were examined, as was the applicability of these processes in advanced oxidation, biological, and adsorption processes. A wastewater treatment system that is both energy-and moneyefcient is necessary for the industry to adopt cleaner manufacturing. Tere were some recommendations for potential research topics at the end of the review. Te properties of the sorbent texture and the amount removed do not appear to be directly related. Te order in which phenol absorption rises is 2-CP, P, Cr 4-NP 2, 4-DNP 2, 4-DCP.
Te authors [17] talked about various ways to make activated carbon (AC) from rice husk (RH) precursors as well as its uses. Applications for the environment, catalysis, and energy have all been noted. Future chances for development and research were also analyzed. Overall, it was found that RH-derived AC holds much promise for various uses that can be further researched on a practical scale, i.e., for prospective industrial services. In a review of recent studies on activated conversions of emerging contaminants, metal-free techniques have been highlighted as one of the most signifcant advancements. At the end, cutting-edge coupling procedures and some prospects were presented [18].
Te methods for removing (or degrading) emerging trace organic contaminants are described in this paper. Alkylphenols and their polyethoxylated derivatives, which are thought to interact with the wildlife's hormonal system, were the subject of the study, which concentrated on one type of endocrine disruptor. A fascinating technique for treating alkylphenol is photocatalytic oxidation since it can achieve entire mineralisation [19]. Tis study demonstrates the high efciency of the wet air oxidation method for converting the majority of contaminants to carbon dioxide. It has been shown that energy net generation typically occurs throughout a continuous process. Tis research is considered the frst step for creating a large-scale wet air oxidation unit [20].
Worldwide production of maize husk and rice husk (MS/ RH) makes them desirable and economically advantageous feedstocks for large-scale biochar manufacturing for environmental remediation. As a result, the authors [21] thoroughly describe the preparation, characterisation, and environmental remediation of pure and composite MS/RH biochar. Other wastewater treatment research studies that have used an experimental laboratory setting include those by Ahmad et al. [22], Bond et al. [23], Zhang et al. [13], Watanabe [24], Snowden et al. [25], Liu et al. [26], Liu et al. [27], Kennedy et al. [28], Hasler et al. [29], and Van Haandel et al. [30][31][32]. As demonstrated by the preceding method, FICCO and rice husk wastewater treatment is a novel technique, which is why this study is being conducted.

Objectives of the Study.
It is learnt from the above literature survey on these lines that there was a scope for exploration of a new technique to improve the treatment efciency. In this study, triangle-shaped sheets with diferent angles were introduced at the top for sufcient catalyst retention in the reactor to study its efects on removal effciencies of COD and BOD from domestic wastewater. Tis research's main objective is to design and test experimentally to fnd COD and BOD removal efciency at the optimum contact time and catalyst dosage. Furthermore, the removal of COD and BOD caused by individual organic pollutants such as surfactants, starch, oil, and proteins separately by artifcial addition was explored.

Collection of Domestic Wastewater (Sewage) and Rice
Husk. Te sewage treatment facility (STP), Adyar, Chennai, provided wastewater, and Table 1 lists its basic characteristics. To create activated carbon, rice husk, a raw material acquired from the agricultural sector, was extensively cleaned with water to eliminate dust. Later, a 600 μm sieve was used to flter dry samples.

Preparation of Rice Husk-Activated Carbon.
Porous carbon was produced via precarbonisation and chemical activation. Te rice husk was heated to 400°C for four hours during the precarbonisation stage and then allowed to cool naturally to ambient temperature. As a result of this procedure, precarbonised carbon was produced (PCC). Precarbonised carbon was stimulated by subsequent chemical activation. 250 g of an aqueous solution containing 85% H 3 PO 4 was mixed by weight with 50 g of precarbonised carbon during the chemical activation process. Te precarbonised carbon to chemical triggering agent ratio was determined at 4.2 pH. At 85°C for 4 hours, precarbonised carbon and the chemical activator were well mixed. Precarbonised carbon slurry was mixed and vacuum-dried for 24 hours at 110°C. Te samples were then heated to 700°C for 1 hour at a 100 mL/min fow rate in a vertical cylindrical furnace with regular air. Activated carbon was cooled, repeatedly rinsed with hot water until the pH was neutral, and then washed with cold water to remove any phosphorus compounds that could have remained. To create the fnished product, 110°C drying was applied to the washed samples.

FICCO Reactor Models.
Fluidized immobilized carbon catalytic oxidation (FICCO) reactors, which have a 9 cm square plan and a 12 cm height, were made from acrylic sheets. Triangles were fxed at the top to ensure smooth fow and efcient operation of the fuidized system for wastewater treatment. Tese reactors were constructed with 620 mL working volumes and 740 mL reactor volumes with an aeration provision. Te fuid bed was kept in place, and air circulation was made possible by using the air distribution system. Figure 1 depicts the models that were created. A typical 3D view of the outline diagram of the FICCO model with a triangle slope at the top and side by using the settling tank is shown in Figure 2. Each reactor provided with a diferent angled triangle slope at the top is shown in Figures 3(a)-3(f ).

Operation of FICCO Reactors.
During the conduction of experiments with FICCO reactors, initial COD and BOD concentrations are maintained constant. At the same time, the optimum contact time and carbon dosage are used for the performance studies on the efectiveness of COD and BOD removal.
Wastewater enters the reactor from the bottom and overfows when it reaches the triangle's top surface, spilling into the adjacent tank. Triangles are barriers to keeping activated carbon in the reactor for a predetermined time by changing the fow rate. To obtain greater efciency in removing COD and BOD, activated carbon must slide down and remain in the reactor for a long period on the smooth surface provided by the slope of the triangles at the top. Te air distribution system is designed to maintain the fuid bed while allowing air to circulate for oxidation.

Results and Discussion
Experimental research made use of FICCO reactor models (Figures 3(a)-3(f )). Te catalyst's optimum dosage and contact time for domestic wastewater were obtained to continue the study and validate the fndings.

Performance Studies Using FICCO Reactors.
Initially, performance studies using FICCO reactors were conducted for the removal efciency of contaminants without the addition of activated carbon. Raw sewage and FICCOtreated wastewater were analyzed for characteristics including pH, sulphates, chlorides, COD, BOD, total suspended solids, and oil and grease, as shown in Table 1.
As presented in Table 1, without catalyst-activated carbon, BOD and COD removal efciencies are 53.65 and 53.74, respectively. Te performance of this will be improved with the use of catalyst. Initially, the optimum dose of the catalyst prepared using rice husks and the optimum contact time required for removal of BOD and COD were experimentally observed, and the same was used in the research work to explore various options such as diferent triangle shapes provided at the top of FICCO models 3A to 3F. Table 1 that without an activated carbon catalyst, the average BOD and COD reduction efciency through this experiment is only about 53.65% and 53.74%, respectively. Tus, the signifcance of catalyst loading is Journal of Environmental and Public Health 3 elucidated with experimental studies. Te efect of catalyst mass loading on COD during the catalytic oxidation of domestic wastewater is shown in Figure 4. Te average percentage of COD reduced rose from 22% to around 90% as catalyst loading was increased from 1 to 12 g per 620 ml while the contact time was kept constant at 150 minutes. Tere was no increase in percentage COD removal as catalyst mass loading was increased from 12 to 15 g per 620 ml. Under current operating conditions, a catalyst concentration of 12 g per 620 ml can thus be deemed optimal.

Optimum Contact Time. COD and BOD removal studies
were conducted with an optimum 12 g per 620 ml dosage to determine the optimum contact time. Figure 5 illustrates the proportion of COD and BOD removed with and without the catalyst as a function of time. Te ratio of COD removed with and without motivation grew steeply for the frst 3 hours and then gradually for the next 2 hours, reaching 57% and 92% from 54% to 90%, which is meagre. Te proportion of BOD removed was similar to that of BOD released at about 44% and 82% in 3 hours and reached nearly 46% and 84%, respectively, in the next 2 hours, which is a meagre change. Hence, 3 hours is considered the optimum contact time. So the optimum contact time is considered for further studies as 180 minutes, since the removal efciency is not signifcant after 3 hours of the contact time.

Performance of the FICCO Reactor with Catalyst-Activated Carbon Made from Rice Husks.
Each FICCO reactor is equipped with a triangle slope, as shown in Figures 3(a) to 3(f ). Initially, the constant fow was applied to FICCO reactors, with a continuous concentration of COD and BOD to all A to F reactors. Ten, aeration was started with an immobilized cell reactor system with appropriate aeration. At an optimum hydraulic retention time (HRT) of 3 hours, an inlet fow containing a predetermined concentration of COD and BOD was continually supplied into the reactor and analyzed for inlet and outlet results. Table 2 lists the COD and BOD removal efciency of each of the six reactors conducted with the predetermined optimum dose of activated carbon and the optimum contact time. Te enhanced average percentage removal of COD and BOD compared to reactors without the catalyst and triangle slopes confrmed the robust working of the FICCO reactor with those arrangements. Te results improved from around 53% to about 84% on average. It is observed from Table 2 that the maximum and minimum removal efciencies of COD are 92.4% and 75.6%, respectively. Te maximum and minimum BOD removal efciencies are 89.5% and 74.9%, respectively. Te average of all six FICCO reactors' efciency in COD removal is 84.13%, and in BOD, it is 80.71%. Tis is higher than in the results obtained without the catalyst, which are presented in Table 1. In Table 1, COD and BOD removal efciencies without the addition of catalyst-activated carbon are 53.74% and 53.65%, respectively. With the addition of catalyst-activated carbon made from the rice husk, the removal efciency of COD increased by about 30.39% and that of BOD by approximately 27.06%, which is a substantial amount of removal. Another observation is that the FICCO reactor shown in Figure 3(c) with a 45°triangle-shaped slope has shown higher efciency than the other slope angles of triangles. Tis can be attributed to the angle at which catalyst-activated carbon retains sufcient time before leaving the settling tank, which facilitates a slightly higher adsorption rate.

Performance of the FICCO Reactor for the Removal of Specific Organic Pollutants
Further studies were explored using the FICCO reactor, especially "3C" type, which has a 45°slope triangle shape at the top of the reactor, giving the maximum efciency in removing COD and BOD. Individual organic pollutants,  such as surfactants, starch, oil, and proteins, were added to water, and the performance of the FICCO reactor was studied. Te results of studies are presented in Sections 4.1 to 4.4, and also, sludge characteristics are also analyzed and presented in Section 4.5.

Performance of the FICCO Reactor for Removal of
Surfactants. Te efect of anionic surfactant concentration on the performance of the FICCO reactor was investigated. Wastewater at a surfactant concentration of 23.8 mg/L was introduced into the reactor and operated for a week at an optimum catalyst dosage of 12 mg per 620 ml and a contact time of 3 hours, and every day samples from the input and outlet were collected and analyzed. Te results are shown in Figure 6. Te maximum removal efciencies of COD and BOD were found to be 82.7% and 81.5%, respectively, which is shown in the graph (Figure 4). Te average COD and BOD removal efciencies were 76.37% and 74.97%, respectively. Te average outlet concentration of surfactants was 4.8 mg/ L. Anionic surfactants are readily biodegradable under aerobic conditions as opposed to anaerobic conditions, at which they are unlikely to be degraded.

Performance of the FICCO Reactor for Removal of Starch.
Te efect on the performance of the FICCO reactor was explored with the addition of starch. Te reactor was operated with the same optimum catalyst dose of 12 g per 620 ml and at an optimum contact time of 3 hours. Experiments were conducted for about a week, and daily samples of the input and output were collected and analyzed whose results are shown in Figure 7. Te initial starch content was 32.1 mg/L in wastewater. Te average outlet concentration of starch after treatment was 5 mg/L. Te maximum COD and BOD removal efciencies were found to be 88.1% and 87.7%, respectively. Te average COD and BOD removal efciencies were 76% and 74.2%, respectively. Tis clearly shows that the aeration process with a catalyst in this reactor can reduce the starch content substantially within a short period.

Performance of the FICCO Reactor for Removal of Oil.
Te performance of the FICCO reactor was studied with the concentration of adding oil in addition to surfactants and starch separately. Te reactor was operated with the same optimum catalyst dose of 12 g per 620 ml and at an optimum contact time of 3 hours. Experiments were conducted for about a week, and daily samples of the input and output were collected and analyzed whose results are shown in Figure 8. Te wastewater's infuent concentration of oil was 45 mg/L. Te average efuent concentration of oil using the FICCO reactor was found to be 6 mg/L. Te maximum COD and BOD removal efciencies were 85.2% and 84.1%, respectively. Te average COD and BOD removal efciencies were calculated to be 75.7% and 72.64%, respectively.

Performance of the FICCO Reactor for the Removal of
Protein. Te performance of the FICCO reactor was studied with the concentration of adding protein in addition to surfactants, starch, and oil separately. Te reactor was operated with the same optimum catalyst dose of 12 g per 620 ml and at an optimum contact time of 3 hours. Experiments were conducted for about a week, and daily input and output samples were collected and analyzed, as shown in Figure 9. Te wastewater's infuent concentration of protein was 33.7 mg/L. Te average efuent protein concentration using the FICCO reactor was 4.27 mg/L. Te maximum COD and BOD removal efciencies were 85.5% and 77.1%, respectively, for

Sludge Characteristics of Treated Organic Pollutants.
Te sludge production and its characteristics for all the given four conditions of adding a surfactant, starch, oil, and protein are presented in Table 3. After 7 days of continuous running, sludge was collected using the reactor, and the experiments on sludge characterisation were studied. It is observed from Table 3 that sludge produced per 35 litres of organic pollutant surfactant, starch, oil, and protein is 720 ml, 730 ml, 643 ml, and 1210 ml, respectively. It is a meagre amount compared to the volume of wastewater treated. Hence, disposal costs, including transportation costs, reduce substantially.   Journal of Environmental and Public Health 7

. Conclusions
Using a fuidized immobilized carbon catalytic oxidation reactor with aeration and activated carbon made from rice husks efectively lowers the quantity of organic contaminants found in domestic wastewater. With carbon-based rice husk-activated carbon, the average removal efciencies of COD and BOD content from domestic sewage were observed to be 84.13% and 80.71%, respectively, which have improved substantially from 53.74% to 53.65%, respectively. Almost 30% improvement is seen from the results after adding catalyst-activated carbon made from rice husks. Sludge generation was also reduced substantially; only 60% of typical sewage treatment plants in the FICCO treatment facility produced sludge output. Hence, the carbon-based rice husk signifcantly reduces the construction expenditure and operational costs of the sewage treatment facility. With a 3-hour contact period and an appropriate dosage of 12 g per 620 ml of the efuent, the FICCO reactor gives a substantial 30% more efciency in removing COD and BOD when compared without catalyst-activated carbon. In addition to providing the catalyst with the proper dose and contact duration, equipping the reactor with a 45°slope triangle makes it simple to keep catalyst-activated carbon for adequate time to use it for organic contamination adsorption efectively.
Wastewater after FICCO reactor treatment can be let into an algal pond, where it might undergo additional treatment through an algal cycle before being released into the environment or reused. Finally, it is concluded that with rice husk-based catalyst-activated carbon and the FICCO reactor model, the overall removal efciency of COD and BOD is substantial. Hence, this can be employed for treating domestic wastewater cost efectively and efciently where space constraints exist since this type of treatment requires less space and takes only 3 hours of contact.

Data Availability
Te data used to support the fndings of this study are available from the author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.