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Oxidation Ditches - An Online Primer Need to know more? Send us an email or feel free to search our online series with our give away search engine: |
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The following approach can be used to estimate oxygen requirements
Ro = (1 - b * Yg ) * Rs + b * d * X , mg O2/L-hr
where:
Rs = rate of COD conversion, mg COD/L-hr (usually COD load * removal efficiency),
Ro = rate of oxygen uptake, mg/L-hr,
X = microorganism concentration, mgVSS/L
Yg = biomass yield coefficient, mass VSS/mg COD removed (usually 0.3 kg VSS/kg COD removed)
b = 1.42
d = endogenous decay rate (usually 0.1/day)
So for 42 mg COD/L-hr maximum COD loading rate ( = 1 g COD/L-d) and an assumed 2,000 mg/L VSS in the mixed liquor and an endogenous decay rate of 0.00417/hr (= 0.1/day), the oxygen uptake rate would be:
Ro = (1 - 1.42*0.3) * (42 * 90%) + 1.42 * 0.00417 * 2000 mg/L = 34 mg O2/L/hr
The aeration equipment would have to equal or exceed this rate to insure positive DO in the aeration basin. Of course, you will need to make sure you calculate the loading rates correctly in the zone of interest. For example, for plug flow type processes, the COD loading rate would be that in the first section of the basin.
Most manufacturers and equipment vendors have readily available software for preliminary designs and will generally assist you with proper equipment/unit selection including recommended/minimum/maximum depth, oxygen dispersion diameter, complete mix diameter and so on.
Aspiration type units provide good oxygen transfer but also cause a circular pattern of flow through the reactor. This circulation pattern is OK if the basin type requires circulation, such as oxidation ditches and facultative lagoons, but BNR reactors do not need this circulation. Aspiration type aeration devices also provide a high velocity jet that can cause erosion of the bottom or sides of the basin if the basin has a shallow depth or the unit is too close to the side of the berm.
A typical gear-driven, low speed mechanical surface aerator, a classic oxidation ditch paraphernalia, consists of an electric motor, gearbox, relatively large diameter rotors (say up to 10' or 3.2m), spool and mounting plate for pier-mounted units. A good quality, low speed unit can and should deliver say about 3.5 lbs O2/hp/hr in clean water. .
Most types and brands are suitable for AS applications, but each has its own best applications. For example, brush aerators are best for oxidation ditches while fixed diffusers and surface aerators are best for conventional AS systems. The key is to size the unit properly for each application. Once OTR characteristics are established, the sizing is fairly straightforward. Other factors include alpha factor, impact of floc size and settleability, impact on effluent TSS, etc. The key phrase is "if properly sized/selected."
Adequate contact must be provided between organic wastewater constituents and the microorganisms.
The volatile solids content of the mixed liquor in an activated sludge process typically is considered to represent the mass of microorganisms that is available for waste treatment. However, not all volatile solids are active microorganisms. Inert cell matter and volatile but nonbiodegradable influent solids also contribute to the mixed liquor volatile solids. Oxygen uptake occurs when active microorganisms consume biodegradable constituents of the wastewater. Respirometer-aided methods can provide oxygen uptake measurements which can be used to estimate actual active biomass in mixed liquors. Case studies show that mixed liquor volatile solids typically range from 5 to 25% active microorganisms.
We can't go much higher in MLVSS than about 2,000 mg/L and still get good oxygen transfer in the aeration basins.
For conventional activated sludge of average rate, i.e. medium rate, it is generally recommended approximately 50 lbBOD/1,000 cu.ft. as maximum. For process stability and better assurances of performance, [fine pore/fine bubble] diffused aeration systems favor the use of low f/m and that is generally restricted to about 10-15 lbBOD/1,000 cu.ft. This significantly lower rate sizing tip takes into account process recommendations (extended aeration) as well as diffuser technology old hands recommendations.
Design of the aeration tanks is also important for optimum efficiency. Fine bubble diffusers can work at 2.5 water depth but deeper basins will give greater efficiency and superior results on capital costs. Diffusers are directly dependent on liquid depth for their aggregate efficiency. As a result, if the basin depth is doubled, it will approximayely use the same horsepower but it will take only roughly one-half the number of required diffusers, i.e. capital cost of the diffusers is about 50%. Using deeper basins, e.g. 5m, and larger volumes offer much better assurances of performance. It must be said that one of the authors once witnessed a probably still existing wastewater treatment plant at an edible oil plant having a detention time of ... [only] ten (10) minutes.
Abattoir wastewaters, say from fowl is particularly difficult to deal with in that it contains a lot of protein and gut waste which is high in nitrogen. The protein is slow to degrade with strong odor potential, as rotting protein tends to have. Cadavarine for example is one of the organic chemicals produced by decaying flesh that gives it the ugly smell. Many of the abattoirs in the U.S. are using oversized and relatively inexpensive oxidation ditches to process these wastewaters.
For the purpose of rough calculations we can, whatever the units used, and using say COD as constituent, combine these two formulas:
Yn = Yo * ( 1 + 0.2 * Kd * SRT) / ( 1 + 1.2 * Kd * SRT)
Yn = bugs in tank * 100 / ( SRT * kgCOD/day * COD removal points)
Thus we have
bugs in tank * 100 / ( SRT * kgCOD/day * COD removal points) = Yo * ( 1 + 0.2 * Kd * SRT) / ( 1 + 1.2 * Kd * SRT)
Solving for one of the SRT we get the coveted iteration formula:
SRT = bugs in tank * 100 * ( 1 + 1.2 * Kd * SRT) / ( kgCOD/day * COD removal points * Yo * ( 1 + 0.2 * Kd * SRT) )
From now on it's just a matter of allowing Excel to perform circular reference iterations (you may have to enable this setting in your Excel version).
For those unfamiliar with iterative methods (a numerical analysis classic) one way to find a root of an equation is to solve for the variable and give iterative methods a try.
Were we to find a root for say
a * x^2 + b * x = (a * x + b) * x = 0
One possible iteration formula to try could be solving for x as follows:
x = 1 / (a * x + b)
While it may seem laughable to do it for this case the truth is that it works great for the not so immediate cases. With minor tweaks it can be used for UASBs, HAFs, BVFs, BNR TFs of all sorts.
It is not wise to use air stripping for nitrogen removal. Aerobic nitrification/denitrification is much more economical. Some possible treatment train may be as follows:
1. Anoxic/aerobic nitrification/denitrification in tanks-in-series arrangement (ie. AO2, Bardenpho, UCT, etc.)
2. Alum precipitation in an independent flocculation-clarifier from the aerobic process to remove phosphorus and to precipitate suspended solids (This separate clarifier avoids adding chemicals directly to the aeration system). Some powdered activated carbon may have to be added to remove color and soluble organics if color and additional COD removal is required.
3. Granular media filtration for final suspended solids removal
4. disinfection as needed.
The focus of nanobubble technology has been on methods for generation of nanobubbles and much less on applications. There are now some who are trying to commercialize the technology, but it is not clear concerning applications where it will offer advantages. With regard to wastewater treatment applications, historically the oxygen transfer (aeration) system has played the dual role of providing mixing energy to suspend biomass in the process. Nanobubbles, of course, do not do this. Thus, for nanobubble technology as it is being developed the applications will not be for traditional oxygen transfer in wastewater treatment systems.
