Aerator design

Aerators are designed primarily on their ability to transfer oxygen (O2) to the lagoon liquid. Of secondary importance is the ability of the aerator to mix or disperse the O2 throughout the lagoon. Where the aerator is intended for minimizing odors, complete mixing is not a consideration except as it relates to the surface area.

For the purpose of minimizing odors, aerators should transfer from 1 to 2 pounds of oxygen per pound of BOD5. Even a limited amount of oxygen transfer (as little as 1/3 lb O2 per lb BOD5) reduces the release of volatile acids and accompanying gases. For design purposes, use 1 pound of oxygen per pound of BOD5 unless local research indicates a higher value is needed.

Aerators are tested and rated according to their clean water transfer rate (CWTR) or laboratory transfer rate (LTR), whichever term is preferred. The resulting value is given for transfer at standard atmospheric pressure (14.7 psi), dissolved oxygen equal to 0 percent, and water at 20 °C. The actual transfer rate expected in field operation can be determined by using equation 10-8.

C sc where:

FTR = lb O2 per horsepower-hour transferred under field conditions CWTR = clean water transfer rate in lb per horsepower-hour transferred under standard laboratory conditions B = salinity-surface tension factor. It is the ration of the saturated concentration in the wastewater to that of clean water. Values range from 0.95 to 1.0. Cdc = O2 saturation concentration at design conditions of altitude and temperature (mg/L) from figures 10-26 and 10-27. DO = Average operating O2 concentration (mg/L). The recommended value of DO can vary from 1 to 3 depending on the reference material. A value of 1.5 should be considered a minimum. For areas where minimizing odors is particularly critical, a DO of 2 or more should be used. t = Design temperature (°C) O = Temperature correction factor; values range from 1.024 to 1.035. a = The ratio of the rate of O2 transfer in the wastewater to that of clean water. Generally taken as 0.75 for animal waste.

C =Saturation concentration of O, in clean sc 2

Unless local information supports using other values, the following values for calculating field transfer rates should be used: B=1.0, DO=1.5, 0=1.024, a=0.75, and

Figure 10-28 provides a quick solution to the term Ot-20, where O is equal to 1.024. Designs for both summer and winter temperatures are often necessary to determine the controlling (least) transfer rate.

Having calculated FTR, the next step is to determine horsepower requirements of aeration based on loading rates and FTR as calculated above. Horsepower requirements can be estimated using equation 10-9.

BOD5

FTR x HO

where: HP = Horsepower

BOD5 = 5-day biochemical oxygen demand loading of waste, lb/day HO = Hours of operation per day

Most lagoon systems should be designed on the basis of continual aerator operations.

The actual selection of aerator(s) is a subjective process and often depends on the availability of models in the particular area. In general, multiple small units are preferred to one large unit. The multiple units provide better coverage of the surface area as well as permit flexibility for the real possibility of equipment failure and reduced aeration.

Figure 10-27 Relation of dissolved oxygen saturation to elevation above mean sea level

1o,ooo w

5,000

Figure 10-27 Relation of dissolved oxygen saturation to elevation above mean sea level

5,000

60 70 80 90 100

Percent of dissolved oxygen saturation at mean sea level

60 70 80 90 100

Percent of dissolved oxygen saturation at mean sea level

Figure 10-26 Relation of dissolved oxygen saturation to water temperature (clean water at 20 "C and sea level)

Figure 10-28 Numeral values for Ot-20 at different temperatures where O=1.024

Figure 10-26 Relation of dissolved oxygen saturation to water temperature (clean water at 20 "C and sea level)

32 40 50 60 70

Water temperature

32 40 50 60 70

Water temperature

Figure 10-28 Numeral values for Ot-20 at different temperatures where O=1.024

32 40 50 60 70 80 Water temperature

32 40 50 60 70 80 Water temperature

Chapter lO

Agricultural Waste Management System Part 651

Component Design Agricultural Waste Management

Field Handbook

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