Aquaculture

The Importance of Aquaculture

All living beings require oxygen for their existence. Creatures of land take this vital element from air whereas water creatures take it from the water source they live in. The natural biological and physical factors constantly maintain balance by replenishing the loss of oxygen thru natural means.

The dissolved oxygen values in water depend upon the interaction of biological and physical factors. The biological factors are the quantum of fish or prawn, the amount of organic load in the form of dissolved and particulate organic material, aerobic bacterial populations and primary producers like phytoplankton and macrophytes. The physical factors are temperature, wind velocity and /or turbulence, salinity, and a balance with other gases like carbon dioxide and ammonia. While freshwater has a higher value of dissolved oxygen, in salt water it is less. As the salinity and temperature increase, the solubility of oxygen in water is affected.

The concentration of dissolved oxygen follows a natural sequence in a 24-hour period. As the sun rises, the production of oxygen increases through photosynthesis. And as the sun sets, the dissolved oxygen levels fall due to respiration by plant and animal life, reaching minimum level towards the early hours of the morning.

In aquaculture management, additional inputs in the form of feed and aeration facilitate the aquaculturist to increase the oxygen carrying capacity in water, leading to realization of higher yields per unit area. Thus, properly sized aeration systems have assumed great importance in the context of semi-intensive and intensive systems of aquaculture.

The table below gives solubility of oxygen in fresh water, at sea level, for different water temperatures.

Temp oC

Oxygen Solubility (ppm)

0o C

14.6

5o C

12.8

10o C

11.3

15o C

10.1

20o C

9.1

25o C

8.2

30o C

7.5

35o C

6.9

40o C

6.4

As shown in the above table, oxygen solubility drops with an increase in temperature. Similarly, with an increase in salinity, there is a corresponding drop in oxygen solubility. For 35-ppt sea water the solubility values of the chart above would reduce by about 15%.

Aeration Techniques

Aeration can be accomplished by mechanical aerators or underwater air diffusers. Mechanical aerators agitate the water to produce liquid to air contact, while underwater diffusers introduce air bubbles from a depth to achieve oxygen transfer and mixing.

There are a wide variety of surface aerators such as paddle wheels, pumps which spray water into the air, and several other devices. The one aspect in common with all these systems is that they all expend a great deal of kinetic energy in throwing large quantities of water into the air. Obviously if the systems are expending energy in this task, the energy is not being directly used to aerate or mix the water in the fish culture system, making them power inefficient.

Surface agitators often look very impressive, but their influence over the oxygen levels in the aquaculture system is rather localized to the area surrounding the equipment. This factor becomes very apparent in ponds with a water depth of more than 1-metre. When oxygen levels are measured at depth or in the sediment, very low levels of oxygen are recorded. The low dissolved oxygen levels may lead to anaerobic sediment conditions and deterioration in water quality.

Bubble type aeration systems are replacing many mechanical aerators because of their low maintenance, high reliability, safety, flexibility and higher oxygen transfer efficiency. They are better at removal of waste gases such as ammonia and carbon dioxide. In this arrangement atmospheric air is pushed through water using diffusers, which causes the air to bubble and rise to the surface of the water – and in the process, the oxygen transfer takes place.

Diffusers are designed to deliver either coarse (approximately 4-6-mm), medium (approximately 2-3-mm), or fine (approximately 1-mm) air bubbles. Coarse-bubble systems require the lowest air pressure and are very resistant to clogging, but are about one third as efficient as medium bubble systems in transferring oxygen to the water. The medium bubble diffuser requires only slightly higher air pressure, but it’s has superior oxygen transfer. The fine-bubble diffuser’s superior oxygen transfer usually does not compensate for its higher-pressure requirement and much more frequency clogging. Fine bubble diffusers, therefore, are typically chosen for pure oxygen or ozone systems where pressure requirements are usually less important than transfer efficiency. Overall, however, medium-bubble diffusers are the most popular among aquaculturists.

Conventional Diffuser Techniques

Diffuser Type

Applications

Specifications

Drilled PVC-type         (Coarse bubble aeration) Primarily for agitation, de-icing, prevention of Sludge  Settling Low oxygen transfer rates.

Flow (m3/hr) = 5nd2

n = no. of holes                

d = dia of holes in cm.(Ave. Velocity: 7.8-m/sec)

Medium Pore Diffuser using stones made of glass bound silica General aeration in hatcheries Medium Diffuser transfer rates.

Approx. bubble size 1-3- mm.

Flow Rate: 01–0.15-m3/hr of 25-mm length

Fine bubble Diffusers – Rubber membrane type Pure gas applications. For high stocking density areas like in storage/ transport tanks, emergency aeration High Oxygen transfer efficiency.

Approx. Bubble size 0.5-mm

Flow rate = 0.2–0.4-m3/hr per cmof membrane area.

Conventionally, in hatcheries where multiple tanks need to be continuously aerated, the use of a centralized air blower with a distribution pipe grid is the general accepted practice. Along the distribution grid, flexible rubber pipes are connected that have ceramic diffusers/stones on the ends, immersed in tanks, through which the air is diffused into the tank. This arrangement offers a host of advantages such as centralized air supply, low installation cost, low maintenance, high flexibility, efficient oxygen transfer and easy operations. Since the intended role is very critical, high reliability and performance is of major concern. As these air blowers have to run continuously, reliability and energy efficiency is important.

TMCFS Aqua Series Air Blowers are especially developed for Aquaculture applications and offer 100% Oil Free Air supply and are rated for continuous duty applications. Unlike conventional air blowers, the Aqua Series Blower has modified end plates, which are physically isolated from the lubrication chambers to ensure 100% oil free air even under worst case of oil seal failure – providing total protection to the hatchery.

 

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