Moving Bed Biofilm Reactors (MBBRs) are a popular sewage treatment technology that utilizes microorganisms to remove organic and inorganic pollutants. The capacity of an MBBR tank plays a crucial role in the efficiency of the treatment process.          
Let us discuss how to calculate the MBBR tank capacity in Sewage Treatment Plant (STP).

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Importance of MBBR Tank   

There are several reasons why MBBR tanks are essential in wastewater treatment:

High Treatment Efficiency: MBBR tanks are highly efficient in treating wastewater due to their unique design. The tanks contain a large number of plastic media carriers that provide a large surface area for bacteria to grow and form biofilm. This increases the rate of biological reactions, leading to faster and more efficient wastewater treatment.      

Low Space Requirement: MBBR tanks require less space compared to other conventional wastewater treatment technologies, making them ideal for areas with limited space.

Easy to operate: MBBR tanks are relatively easy to operate and maintain. They require minimal operator attention and have low energy and chemical requirements, making them a cost-effective option.           

Robust Design: MBBR tanks are designed to be durable and withstand harsh conditions. They are made from high-quality materials that can resist corrosion and are not affected by fluctuations in temperature.       

Versatility: MBBR tanks can be used for a variety of applications, including municipal wastewater treatment, industrial wastewater treatment, and the treatment of contaminated groundwater.

Retention Time in STP          

Retention time is the amount of time sewage remains in the treatment system. It is an important parameter that determines the efficiency of the treatment process.

In an MBBR system, the retention time is the time taken for sewage to flow through the tank, allowing the microorganisms to remove the pollutants. Formula for retention time in STP is as follows:

Retention Time = Volume of MBBR tank / Flow rate of sewage

How to calculate MBBR tank capacity for MBBR based STP?

The capacity of a Moving Bed Biofilm Reactor tank is determined by the retention time, the flow rate of sewage, and the volume of the tank.       

The following formula is used to calculate the MBBR tank capacity:

MBBR Tank Capacity = Flow rate of sewage x Retention Time

Where,           

Flow rate of sewage = Volume of sewage generated per day / 24 hours  

Retention Time = Volume of MBBR tank / Flow rate of sewage

An Example:  

Let's assume that the volume of sewage generated per day is 50,000 liters and the retention time required is 6 hours. The flow rate of sewage would be:   

Flow rate of sewage = 50,000 / 24 = 2,083.33 liters/hour  

Now, using the retention time formula, we can calculate the required MBBR tank capacity:        

Retention Time = Volume of MBBR tank / Flow rate of sewage    

6 hours = Volume of MBBR tank / 2,083.33 liters/hour     

Volume of MBBR tank = 6 x 2,083.33 = 12,500 liters

Therefore, the required MBBR tank capacity for a retention time of 6 hours and a flow rate of 2,083.33 liters/hour is 12,500 liters.

Conclusion     

In conclusion, the MBBR tank capacity is a critical parameter in the design and operation of an MBBR system. The retention time in the STP is a key factor in determining the tank capacity. Using theformula and calculation method described above, we can accurately determine the required MBBR tank capacity for a given retention time and flow rate of sewage. This information is essential foreffective MBBR system design and operation, ensuring optimal treatment efficiency and sewage quality.

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This experimental study investigates the possibility of removing low concentrations of pollutants (organic matters, nitrogen, and phosphorus) from treated sewage using an MBBR polishing system for the purpose of using the treated water as a water resource.

The temperature of the treated wastewater was consistent at approximately 30 °C. The aeration agitator was kept running throughout the study, to achieve complete mixing and homogeneous wastewater. The specifications of the treated wastewater as the MBBR system&#;s influent were as shown in Table 4

In this work, treated domestic sewage, with very low concentrations of pollutants, was used as the feed. Real treated sewage was used because it is easier to biodegrade than synthetic wastewater and because it contains ubiquitous microbial community. The raw sewage from the facility was first treated by using an aerobic STP designed for a work capacity of population equivalent (p.e.). However, the STP was not fully operational yet and, at present, caters to only approximately 100 people. This meant that the effluent (treated sewage) had low concentrations of pollutants.

The 3-D EMM protects sensitive bacteria, such as nitrifiers, from being washed-out. The oxygen profile of biofilm layers enabled the survival of anaerobic, anoxic, and aerobic microorganisms in one system. Therefore, the system could perform both oxidative and reductive reactions, such as nitrification-denitrification and azo-bond cleavage, thus yielding excellent water-polishing capabilities.

In this study, the density of the 3-D EMM was adjusted so that these plastic media would be suspended in the tank. This was because the treated wastewater had a lower density (1.02&#;1.04 g/cm 3 ) than the EMM. Consequently, to reduce the media density, a small ball of low-density polyethylene (used for packaging) was placed inside each EMM in order to suspend it or to make it float.

The 3-D EMM carries numerous advantages. For example, the medium&#;s lighter density allows lower liquid superficial velocity, which suits a longer hydraulic retention time (HRT) and reducing aero pumping energy. This medium is manufactured using HDPE material and is appropriate for a long service life that allows microorganisms to grow and attach. In this study, the bulk medium volume was 5% of the reactor volume, as the EMM medium has a specific structure and size, as well as a higher surface area, compared to other media.

Envirosource Multimedia (EMM, Malaysia)&#;the three-dimensional (3-D) HDPE medium&#;was used as a biofilm carrier in this study. The EMM is a cylinder that is perforated on both sides. Figure 2 and Table 3 show the structure and the physical properties of the medium, respectively.

The gravity overflow allowed the excess mixed-liquor to flow into a 2.54 cm T-connector, providing equalized atmospheric pressure, before exiting the system via a 2.54 cm clear tubing, vertically downward to the external clarifier. The influent was pumped from a sewage treatment plant (STP) final clarifier through a 2.50-cm internal diameter polyvinyl chloride (PVC) tube, connected to pump head tubes by fittings to the source tank.

The source tank was a 550-L cylindrical vessel with 1.5 cm thickness located above the MBBR using a stand. This gave the liquid some height to flow under gravity through a 2.54 cm diameter pipe into the MBBR. The stand and the elevation along the source tank&#;s cylindrical sidewall for the gravity overflow port were chosen to provide a 550-L operational volume for the vessel.

The pumps also provided sufficient mixing and kept the biomass suspended. Additionally, a stable base was used to support the reactor. The aeration system provided coarse bubbles and kept the media in circulation. The up-flow configuration was chosen, due to the fact that it could treat high influent flow rates and had a longer operating cycle. It could also reduce the &#;smelly water&#; issue at the top of the reactor when the air reacted with the treated effluent [ 23 ].

The bottom section of the MBBR was the most vital part, as this was where both the air distributor and the perforation barrier were installed. The bottom section had four fine bubble walls and air stone tube diffusers mounted on it. Diffusers were placed at an equal distance from the center to facilitate equal distribution. Outside the reactor, these diffusers were connected to a central air supply with rubber tubes. This connection made it easy to orient the air supply pipes around the reactor.

A variable flow rate (gravity-dependent) ( Table 2 ) was used for the reactor, without a pump and using a hand valve, which enabled the flow rates to be adjusted. The MBBR inlet pipe was occasionally flushed to remove built-up solids and biofilm inside the pipe. This step enabled the researcher to maintain a continuous flow of treated wastewater supply at all times. The MBBR had two sampling ports, which were located on the MBBR inlet line and outlet line.

The MBBR polishing system consisted of MBBR and a source tank, as shown in Figure 1 . The MBBR was a cylindrical column with an inside diameter of 0.75 m and a total height of 1.2 m, in which 1.13 m was the effective reactor height. In this study, non-exposure to light was important for reducing phototropic organism proliferation in the MBBR, such as algae. Therefore, the reactors were made from a black and thick (1.5 cm) high density polyethylene (HDPE) material in order to enable operation in darkness, except for periods of sampling or reactor maintenance. The MBBR and the source tank were both closed at the top using a HDPE convex cover.

A fine diffuser was used for the MBBR aeration and mixing. The air was diffused from the bottom of the reactor at a constant aeration rate of 15 L/min at all stages in order to supply oxygen to the microbial mass to facilitate biological activity and mix the carriers. A dissolved oxygen (DO) meter probe (ODEON, Caudan, France) was utilized to periodically check the DO levels in the reactor. If necessary, the air flow rate was adjusted to maintain an adequate DO concentration, which was maintained at 5&#;7 mg/L to minimize adverse effects on nitrification, and the temperature was maintained at 27 °C to 30 °C.

Afterwards, the MBBR was continuously fed with treated wastewater at 24-h HRT, to minimize shock load and to avoid the wash-out of the initial bacteria. After three weeks, noticeable biofilm growth was observed. The MBBR was then sequentially operated at different HRTs (please refer to Table 2 ) by changing the HRT once a steady state had been obtained. Toet et al. [ 1 ] additionally evaluated the pollutant-removal performance of a surface-flow wetland system for polishing tertiary effluent from an STP at HRT of 0.3, 0.8, 2.3, and 9.3 days.

The MBBR polishing system was designed and set up to operate in an outdoor facility with an open environment. It was decided that the complete research work must be carried out under realistic conditions. Effluent from the aerobic STP clarifier (please refer to Figure 1 ) was used as the influent for MBBR. Firstly, the MBBR polishing system was assembled and tested for any leakage, to ensure that the system functioned properly. Then, the MBBR was filled with treated sewage for the seeding process.

2.3. Analytical Methods

After attaining a steady state, the sample matrix from the MBBR was tested for various physicochemical and microbiological parameters. This sample matrix was divided into two parts: the MBBR influent and the MBBR effluent. To maintain sampling consistency, the wastewater samples were taken from 10.00 a.m. to 11.00 a.m. on sampling days. All samples were collected in sterile mL polyethylene bottles, which had been soaked and cleaned prior to sampling.

5 samples were collected in black bottles. The analysis was carried out in the Environmental Laboratory of the Civil Engineering Department, Universiti Kebangsaan Malaysia (UKM). Most tests were carried out in triplicate and filtered through a 0.45 μm filter before testing, to reduce the effect of suspended solids on the measured values.

The samples were then kept in ice storage and processed at the lab. The BODsamples were collected in black bottles. The analysis was carried out in the Environmental Laboratory of the Civil Engineering Department, Universiti Kebangsaan Malaysia (UKM). Most tests were carried out in triplicate and filtered through a 0.45 μm filter before testing, to reduce the effect of suspended solids on the measured values. Table 5 lists the parameters and methods used.

5, COD, NH3-N, nitrate&#;nitrogen (NO3-N), pH, DO, TSS, and total phosphorus. The methods used for the analyses were the Standard Method for the Examination of Water and Wastewater from the American Public Health Association (APHA) [

The analysed physicochemical parameters focused in this study were BOD, COD, NH-N, nitrate&#;nitrogen (NO-N), pH, DO, TSS, and total phosphorus. The methods used for the analyses were the Standard Method for the Examination of Water and Wastewater from the American Public Health Association (APHA) [ 24 ], along with the HACH methods, as described by the Water Analysis Handbook [ 25 ].

The pH was continuously recorded using a pH probe (ODEON, Caudan, France), whereas for the temperature, an ODEON range open x model, temperature sensor probe with automatic temperature correction, was used. The DO in MBBR reactor was maintained above 2.0 mg/L throughout the study period using an ODEON range open x model, dissolved oxygen sensor. To determine the attached solids fixed in EMM carriers (include attached biomass), three pieces of the PE carriers were taken out of the reactor and kept in three separate beakers with milli-q water.

The beakers were inserted into an ultrasonic cleaner, POWER SONIC 405 (Hwashin Technology Co., Seoul, Korea) until the attached solids and biomass on the carriers were slugged off from the carriers. Then, the solution of biomass and milli-q water was filtered through a GFC Whatman&#;s 0.45 μm filter paper. The retained solid residue on the filter paper was dried by placing inside an oven at 105 °C for 1 h, followed by desiccation for 20 min, and finally weighted to calculate the mixed liquor suspended solid (MLSS). The main details of the cadmium reduction method (spectrophotometer HACH DR , Method ) and spectrophotometry measurement is that 10 mL of the sample was used. The reagent was NitraVer 5 Nitrate Reagent Powder Pillow.

For the microbiological parameters, the coliform-forming analysis was conducted on the effluent and influent of MBBR as a requirement for the water quality assessment (please refer to Table 5 ). Aside from that, the identification of bacterial communities inside the MBBR was additionally performed by extracting the biofilm attached on the selected 3-D EMM, and the samples were then sent to the external molecular laboratory to run the 16S rRNA gene sequencing analysis. Biofilm thickness and the morphology of the biofilm attached to the 3-D EMM were also investigated via scanning electron microscopy (SEM) at the SEM Laboratory, UKM.

The 3-D EMM was randomly selected from the reactor at each phase of the study. For sample preparation, the 3-D EMM was cut without detaching the biofilm from the medium itself, to enable the visualization of all sections of the media and the respective attached biofilm. Images were captured at random locations on the biofilm medium, and a minimum of 20 thickness measurements total&#;per medium&#;were acquired and analyzed for each experimental phase.

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