Best Management Practices in Aeration

Pond aerators and water movers play a critical role in setting up a desirable culture environment for prawns in large ponds. They are primarily used to maintain adequate oxygen levels and gaseous exchange. They also keep the pond water column well mixed and prevent stratification so that the water quality is consistent throughout the pond. The circular flows that are generated by aerators will also concentrate slow-settling wastes (such as slit, algae and feces) towards the centre of the pond, creating a mound of sludge that will be visible when you eventually drain the pond at the harvest time. This mound should ideally be less than 40m in diameter (1200m) in a 1hectare pond. The faster currents around the periphery of the pond sweep the pond bottom and keep this feeding area free of excessive organic debris. Aerators can be installed in position with tall stakes hammered into the mud, or with long ropes to the bank. The layout of pond aerators should be designed to achieve the maximum flow of pond water with the minimal energy input. In square ponds, the use of at least four aerators, each positioned 15-18 from the sides at each corner, allows sufficient water supply to the aerator and minimizes bank scouring.

One of the most common mistakes in placing paddle wheel aerators too close to the banks or corners where it is perceived that currents are tracking past in the generally circular pattern that develops. Directing flow across the path of another aerators should be avoided because this creates eddies and deposits wastes in places other than in the centre. Positioning in odd- shaped ponds needs to be undertaken on an individual basis, possibly by trial and error.

Key Instructions for Better Aeration Management:

1. Use Aerators where stocking density exceeds 30,000/ha. The use of aerators in pond has a strong influence on the maximum yield that can be achieved from the pond. Dissolved oxygen (DO) concentration should be more than 4ppm. If concentration falls below 4ppm during night and early morning, use aerators. If oxygen levels deplete in the pond, shrimp will start swimming near the water surface especially during early morning.
2. Use 1hp aerator for every increase in 400 kg of shrimp biomass in the pond. For biomass less than 500kg in extensive ponds there is no need for aerators. Fix aerators t least 3m away from the dike. Maintain the aerators RPM 80 to 100. Aerator should be positioned correctly and operated efficiently to minimize pond dike erosion and suspension of pond bottom sediments.
3. The position and orientation (Clockwise direction) of the aerators should encourage maximum water flow within the pond. It should be adequate enough to concentrate waste in the centre of the pond. Aeration can be used while application of lime fermented juice into the water. Stop Aeration during feeding and chain dragging.

If for any reason of mismanagement or equipment failure, the dissolved oxygen levels fall below 3 ppm, first increase mechanical aeration, and as the last choice perform water exchange. Aeration is required usually after 30 days of culture and during late evening to early morning period. Regular aeration is better practice. In farms with lower stocking density, low dissolved oxygen is mainly the result of organic wastes at the pond bottom, especially from un-removed sludge, dead benthic algae and excessive feeding. In such conditions, aeration should be provided when shrimp start surfacing or the bottom soil quality is bad and water has more turbidity and dark colour.

4. Suggested operation of aerators in shrimp ponds:

Days after stocking Aeration Requirement Running Time 1 to 30

During cloudy days/ during rain/ when less bloom

30 to 60 During cloudy days/during rain 4 to 6 hrs at night every 2 to 3 days 60 to 90 As above Every night for 8 hrs 90 to harvest (For biomass or <1.5 ton/ha) As above with additional Aerators Every night for 8 to 12 hrs, 1to 2hrs before feeding 90 to harvest (For biomass or >1.5 ton/ha)

All day every day expect around feeding

5. Poor layout of aerators in the pond can lead to erosion of the pond walls or bottom and significantly increase the amount of sediment in the sludge mound by the end of the crop. This can decrease the life span of ponds and increase maintenance costs.
6. The oxygen transfer efficiencies of the aerators in kilograms of oxygen transferred per kilowatt-hour of power applied to aerator shafts is presented below for the basic types of aerators:

Type of Aerator

Average oxygen transfer efficiency (kgO2 /kWh) Paddle Wheels

2.13

Propeller- Aspirator- Pumps

1.58

Vertical pumps

1.28

Pump sprayers

1.28

Diffused air Systems

0.97

The Purchase prices of different types of aerators do not differ greatly per kilowatt of motor size. Therefore, paddle wheel aerators will transfer oxygen to pond water at a lower cost than other types of aerators. However, all types of aerators have been used successful in aquaculture.

Source : NaCSA