Aeration efficiency

Figs. 8 and 9 Time taken for VFA to reach 0.65 and 0.23 g/1 during storage after treatment against aeration energy, assuming an aerator efficiency of 1 kg 02/kWh. 

The minimum energy requirement for treating separated pig slurry to control odour was 110 Wh pig place-1 day-1, assuming an aerator efficiency of 1kg 02/kWh input. 

Probably the major problem in the scale-np of fermentation processes lies in maintaining aeration efficiency to aerobic cultnres. Aeration is normally achieved by sparging with air and the rate determining step in a non-viscons fermentation is the... 

Not much is known on induced aeration via hollow shafts. Aldrich and van Deventer [57] report a decrease in aeration efficiency with increasing wg, dp and ps. 

The presence of anions such as Cl , Br , I , and amino acids or other complexing agents significantly decreases the reduction rate and consequently the aeration efficiency. 

Even for the simple stirred, aerated fermenter, there is no one single solution for the scale-up of aeration-agitation which can be applied with high probability of success for all fermentation processes. Scale-up methods based on aeration efficiency (kio) or power consumption/unit volume have become the standard practice in the fermentation field. 

Here we have apparent discrepancies between the results of two studies. The organisms employed differ markedly in oxygen and nutrient requirements as well as in antibiotic production. Furthermore, the morphological changes arising from agitation result in broths markedly different in their physical characteristics, and it is also extremely difQcult to compare the aeration efficiencies at each stage of two fermentations. 

Figure 5. Aeration efficiencies of various gas-liquid contacting devices (air-electrolyte systems). Reproduced, with permission, from Ref. 38. Copyright 1981,...

To minimize the specific power input the aerator efficiency has to be maximized. This holds true for primary dispersion. To minimize the specific power input P/Vr, the coalescence rate has to be minimized as well. Different strategies for minimizing the coalescence rate were considered in ref. 5. 

Fermentation media may exhibit a highly viscous non-Newtonian flow behavior (1,2) resulting in low aeration efficiencies (3). For studying the volumetric mass transfer coefficients (kj a) under such conditions solutions of sodium carboxymethylcellulose (CMC) are often used as model media. In bubble columns such investiga-... 

Figure 3.27. Typical plot of data of surface aerator efficiencies expressed as modified oxygen transfer number versus Froude number iVpr. The shaded area includes the range of different tested types. (Adapted from Zlokamik, 1979.)...

It is also interesting to use Figure 6 to make a comparison of different aeration devices on the basis of energy-efficiency. From equation 6 and assuming a constant driving force... 

Aerator type Materials of constmction Oxygen transfer efficiency, OTE, % Oxygen transfer rate, OTR, g/(W-h)... 

Static tube aerators are economically attractive and have a high transfer efficiency. They are weU-suited for lagoon appHcations. On the other hand, they are poor mixers and are not recommended for use when the sludge concentration is over 3000 mg/L. 

Horizontally Mixing Aspirator Aerators. An aerator using a horizontally mixing aspirator has a marine propeller, submerged under water, attached to a soHd or a hoUow shaft. The other end of the shaft is out of the water and attached to an electric motor. When the propeller is rotated at high velocity, at either 1800 or 3600 rpm, a pressure drop develops around the propeller. Air is then aspirated under the water and mixed with the water, and moved out. This type of aerator, shown ia Figure 3g, is very efficient ia mixing wastewater. 

Aerated Lagoons. Aerated lagoons are 2—5 mhquid depth depending on the aeration system and detention times are 2—10 days. They are mainly used because of their efficiency in removing BOD from textile effluents (2). 

Temperature, pH, and feed rate are often measured and controlled. Dissolved oxygen (DO) can be controlled using aeration, agitation, pressure, and/or feed rate. Oxygen consumption and carbon dioxide formation can be measured in the outgoing air to provide insight into the metaboHc status of the microorganism. No rehable on-line measurement exists for biomass, substrate, or products. Most optimization is based on empirical methods simulation of quantitative models may provide more efficient optimization of fermentation. 

There is no standard aeration-tank shape or size. Aeration tanks can be round, square, or rectangular. Shallow aeration tanks are more difficult to mix than deeper tanks. Yet aeration-tank depths have ranged from 0.6 m (2 ft) to 18 m (60 ft). The oxidation-ditch systems tend to be shallow, while some high-rate diffused-aeration systems have used veiy deep tanks to provide more efficient oxygen transfer. 

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