MBBR or Moving Bed Bioreactor is a cost-effective wastewater treatment method. MBBR is a highly effective biological cleaning process that combines traditional activated sludge, with a biofilm medium.
In aeration and anoxic tanks, large volumes of microorganism floating media are used in the MBBR process. Organic matter is consumed by microorganisms. The medium increases the surface area available for biological microorganisms, to attach to and grow on. By increasing the surface area, the tank requires less space to treat the wastewater.
Let’s see how aeration requirement is calculated in MBBR.
Design of MBBR media
To better understand the MBBR Media design, consider the following example:
Question: How will an MBBR reactor treat a wastewater flow of 1 MGD (million gallons per day) containing 100 mg/L BOD?
Assuming a removal efficiency of 90-95%,
Design SALR (Surface Area Loading Rate) value= 7.5g/m2 /day
BOD loading rate = 1MGD x100 (Mg/L) x 8.3 (Lb/mg/MG/L)
= 830 (lb/day) *453.59 (g/lb)
=376479.7 g BOD/day
Required Surface area = (376479.7 g BOD/day)/7.5g/m2 /day = 50197.2 m2
How to calculate aeration requirements in MBBR?
We must first determine our oxygen demand before we can calculate the oxygen transfer rate. Different factors must be considered depending on the wastewater treatment target, to calculate the actual oxygen requirement (AOR).
1. Water influents are not consistent and fluctuate between daily minimum, average, and peak flows. To calculate the amount of aeration required, design guidelines recommend dividing the average day of the peak month, by 24 hours.
2. The amount of oxygen required for microorganisms to decompose organic material, is approximately 0.9 to 1.3 lb O2 per lb BOD.
3. Total nitrogen reduction occurs when nitrifying bacteria oxidize ammonium (NH4+) to nitrite (NO2-) and then to nitrate (NO3-). The bacteria require approximately 4.6lb of O2 per pound of ammonium for this process.
Let’s learn the calculation with example
Assume a wastewater treatment plant's influent has an average day, peak month flow of 100,000GPD, 150ppm BOD, and 40ppm ammonium. The daily BOD loading is approximately 125 pounds per day, and the daily ammonium loading is approximately 30 pounds per day.
As a result, the AOR per day is 125lbs * 1.1 + 30lbs * 4.6 = 275.5 lb oxygen.
Finally, the AOR must be adjusted to account for environmental factors such as temperature, pressure, oxygen saturation, and so on. The correcting factor for wastewater conditions is typically between 0.3 and 0.5, depending on the aeration equipment. The standard oxygen requirement is the corrected AOR value (SOR). In our example, we use a factor of 0.4, yielding a SOR of 688lbs O2 per day.
We now know how much oxygen must be present in the water per day, for biological processes to occur. To meet the calculated daily oxygen requirement, we must still transfer gaseous oxygen into the water. Different products are developed by equipment manufacturers, to dissolve gaseous oxygen in wastewater. Depending on the product technology, oxygen transfer rates range from 0.5 to 2.5% per ft. of water depth.
Oxygen transfer rate or OTR
The oxygen transfer rate (OTR) is affected by a number of factors, including temperature, pressure, oxygen saturation, and, most importantly, the alpha-factor, which describes the difference between clean water and wastewater.
Assume we have 15ft of water depth, which equals an OTR efficiency of 1.5% * 15 = 22.5%. As a result, the aeration equipment must produce 3,057lb of oxygen per day.
Finally, because the aeration equipment does not provide pure oxygen, the required amount of oxygen must be converted into the total required air amount. Because, air contains 21% oxygen, dividing our oxygen number of 3,057lb by 0.21 results in a total required air amount per day of 14,557lbs or 194,000ft3.
If you have any questions about wastewater treatment plants, or if you would like to discuss your specific needs, please contact the Netsol Water Solutions team right away.
Our team is an expert in water treatment and can help you tailor a solution to your specific requirements.We collaborate with our clients to design a customized wastewater treatment solution, which meets your specific requirements, as well as an on-going service plan to completely maintain your WWTP.
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