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How to design aerated lagoons for surface aerators?

      First, we should know the energy requirement for mixing for mechanical aerators.
As with surface aerators, the depth and width of the aeration tanks depend on aerator size. Typically, normal water depth is from 1.2~3.6 meter. If the draft tube mixer be used, the depth can up to about 10.5 meter.

Basically, the air requirements for good mixing varies from 20~30 m3/min/1000m3.
The power requirements for maintaining a completely mixed flow with mechanical aerators vary from 20~50 HP/ 1000 m3.

Especially, it is important that the mixing power requirement be checked on design aerated lagoons for the treatment of domestic wastewater. The following is a example for aerated lagoons:

Determine the surface aerator power requirement:

 1. Assuming the following conditions:

     a. Oxygen requirement: 550 kg O2/ day
     b. Lagoon volume: 15000 m3
     c. The aerators are rated 1.1 kg O2/HP.hr
         We ignore the correction factor.

2. The amount of oxygen transferred per aerator per day is: 1.1 kg O2/ HP.hr X 24 hr =
    26.4 kg O2/HP.day

   HP= (550 kgO2/day) / (26.4 kg O2/HP.day) = 20.8

3. Check the power requirement for completely mixing:

   Assuming the completely mixed-flow, the power requirement is 20 HP/ 1000 m3.
   a. Power required: (20 HP/ 1000 m3) X ( 15000 m3) = 300 HP ( Total power for surface
                                aerators)

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Generally,  for designing to treat domestic wastewater others criteria:
   1. the power needed to meet the mixing requirement
   2. mean cell-residence time 3~10 days
   3. the ratio of oxygen required to BOD5 removed is 0.7 ~ 1.4

Oxygen is supplied by means of surface aerators or diffused air units. 
When we do process design of aerated lagoons, we shall consider the following factors:
    1. BOD removal
    2. Effluent characteristics
    3. Oxygen requirements
    4. Temperature effects

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Note: hp X 0.7457 = kw
          ft X 0.3048 = meter
          ft3 = 0.027 m3

                                                                             -------- 02/24/2001 

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