Aeration

Summary

Aeration is a basic means of management and control in Biofloc Technology (BFT) ponds. A proper aeration system is vital to the success of these ponds. The Aeration is needed in order to supply oxygen, to mix the water and control bottom sludge accumulation.

Oxygen added to the system by algae via photosynthesis is important during daytime , however, it is of limited significance and reliability in super-intensive systems. In the long run, algae do contribute O2 to the pond, since they respire & add to the O2 consumption of the pond.

The proper utilization of aerators is an essential means to minimize the coverage of the pond bottom by anaerobic (without oxygen) sludge.

In addition to replenishing oxygen, aeration mixes pond water, prevents water stratification, and helps move sludge (Acuipaula shrimp farm, Panama).

Aeration is a very important means to achieve higher yields in ponds. In addition, aerators are introduced to ponds to achieve several related goals:

1. Supply oxygen to the animals to overcome oxygen limitations and thus enable higher stocking growth and yields.

2. Distribute the oxygen in the pond, (a) horizontally and (2) vertically.

3. Mix water and sediment-water interface

4. control sludge coverage, location and drainage.

It is important that aerators achieve all the goals mentioned above as much as possible, in addition to merely supplying oxygen.

Aerobic and Anaerobic Conditions in Ponds

Oxygen (O2) is essential component affecting shrimp life as a variety of biological and chemical processes in the pond. However, there is a continuum of oxygenation levels, from fully oxygenated systems down to highly reduce systems.

The degree to which bottom sludge becomes anaerobic can be measured as its oxidation/reduction potential or “redox.” Progressively lower redox values are associated with a specific series of chemical processes (Table 1).

(i.e. A water sample that contains oxygen at saturation or close to it will have a redox potential of about 500 mv, while in a system with absolutely no oxygen and a presence of highly reduced hydrogen sulfide will have a redox of about -100 mv.

A series of metabolites are form in the anoxic or anaerobic bottom water and sludge (Table 1.)

These compounds are generally odorous, toxic, and highly undesirable for shrimp production. Sulfides lend the black color to the sludge and are very toxic (as H2S), at levels less than 1 mg/l.

Metabolites in Sludge Bottom sludge is an organic-enriched, anaerobic, soft, and normally black layer that develops on the bottom of ponds. The accumulation of organic residues at pond bottoms creates high rates of consumption of the often limited amount of oxygen available. A series of metabolites are formed in the sludge under anaerobic condition (Table 1.). This is commonly due to limited supply of oxygen to the pond bottom and to the high oxygen demand induced by the accumulation of organic residues. Sludge accumulation increases with an increase in stocking density feeding rates. The accumulation of sludge and the development of anaerobic conditions in the sediment were shown to be factor limiting shrimp growth (Avnimelech and Zohar, 1986).

In addition, the effect of sludge of sludge on shrimp growth may be especially hazardous, since shrimp live and eat at the pond bottom.

Hopkins et al. (1994) grew shrimp at high density in zero water exchange lined ponds using three treatments: (a) control, no sludge treatment, (b) sludge removed periodically, and (c)sludge mixed (Re-Suspended) periodically. Shrimp in treatment (a) hardly survived (survival = 0.2%) as compared to reasonable survival and growth in the other two treatments.

Effect of sludge on survival and growth of shrimp in zero water exchange pond

Sludge Treatment Remain Remove Re-SuspendSurvival (%) 0.2 32.8 54.1Production (Kg/Ha) 18 2406 3474

*Adapted from Hopkins et al., 1994.

In another experiment (Avnimelech, not published), shrimp were stocked at high density and grown in concrete ponds. Feed consumption was monitored for a week before and after scraping and cleaning of the sludge accumulated on the pond bottoms (Table2). Shrimp activity was limited when sludge was present, but improved greatly following sludge removal. The environmental stress induced by the sludge can also limit growth and induce disease outbreaks.

Aerator Placement And Sludge Deposition

A 1.2-ha, plastic-lined pond equipped with 14, 2-hp, long-arm paddlewheel aerators, about 25% ofthe pond was almost stagnant (Figure1a).

Limited water flow rate was associated with sludge accumulation (Figure 1b). Sludge of differentdepths covered about 40% of the pond’s area.

The long-arm paddlewheel aerators traditionally placed perpendicular to the pond levees were turned about 30° toward the center of the pond. A different water flow pattern resulted, and the stagnant fraction of the pond was reduced from 25% to 18%.

Another study by Delgado et al. (2001) used a relatively small (0.25 ha) pond with a sandy bottom andthree paddlewheel aerators (5 hp total power) placed radially. This study showed radial flow of water, and a central stagnant region with a flow rate of less than 2 cm/sec, which covered about 25% of the pond area (Figure 2). Early-morning oxygen concentrations in this region were very low (0-1 mg/l). Sludge accumulation paralleled the water velocity pattern (Figure 3). Sludge deposition was generally less than 1-2 kg/m2 and about 4-8 kg/m2 in the outer shell, in the center. Organic carbon accumulation in the pond bottom was less than 100 g carbon/m in the outer region, and 300-600 g carbon/m2 in the central region.

Means to Control Inorganic Nitrogen Accumulation

1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1. Nitrification is the process by which ammonia is converted to nitrites (NO2

-) and then nitrates (NO3

-). This process naturally occurs in the environment, where it is carried out by specialized bacteria. Ammonia is produced by the breakdown of organic sources of nitrogen.

2. Newly stocked ponds start by having small population of nitrifying bacteria, both ammonium and nitrite oxidizers. Ammonium oxidizing starts slowly when Total Ammonium Nitrogen (TAN) concentration is built up and nitrite oxidation starts later, when nitrites builds up.

Therefore, in Fig. 1 we see the sequence of events:

(a.) Ammonia concentration rises up to a maximal value about 2-3 weeks after stocking.

(b.) And, it declines with a parallel rise of nitrite.

(c.) After about 4-6 weeks both ammonium and nitrite concentrations are lowered, replaced by increasing concentrations of nitrate.

The slow growth of nitrifying bacteria is of utmost importance. For instance, when a pond is stock and fed, processes leading to NH4

+ build up in the water start. When NH4

+ conc- rises, NH4+ oxidizing bacteria

population starts to develop. However, it takes about 2 weeks before there are enough bacteria to oxidize the evolved NH4

+ , which would lower TAN levels.

Then, NO2- oxidizing bacteria start to develop. Again, before a sufficient amout of NO2

- oxidizers is developed, NO2

- conc- in the water rises, over about 4 weeks, until there are enough bacteria and both NH4

+ and NO2- conc- are controlled.

This period is critical to the stock shrimp. Inoculating ( i.e. Probiotics/Nitrite salt (NaNO2-) the pond

shortens this dangerous period.

Nitrogen Cycle in Biofloc Ponds