Among various water quality parameters for a successful aquaculture practice, Dissolved oxygen content

in the water is one of the most important parameters, as the oxygen is a vital for all the organisms living in the

water and having an aerobic type of respiration. The purpose of aeration is to increase the concentration of

oxygen in the water. In scientific aquaculture practices, this is more critical because, often the rate of consumption

of oxygen is much higher than the natural rate of replenishment of oxygen in the water through diffusion from

atmosphere and photosynthesis of aquatic plants. Oxygen is one environmental parameter that exerts a

tremendous effect on growth and production through its direct effect on feed consumption and metabolism

and its indirect effect on environmental conditions. Oxygen affects the solubility and availability of many nutrients.

Low levels of dissolved oxygen can cause changes in oxidation state of substances from the oxidized to the

reduced form. Lack of dissolved oxygen can be directly harmful to culture organisms or cause a substantial

increase in the level of toxic metabolites. It is therefore important to continuously maintain dissolved oxygen at

optimum levels of above 3.5 ppm

Ignatius, Boby

CMFRI Digital Repository

85Winter School on Technological Advances in Mariculturefor Production Enhancement and SustainabilityAerationAmong various water quality parameters for a successful aquaculture practice, Dissolved oxygen contentin the water is one of the most important parameters, as the oxygen is a vital for all the organisms living in thewater and having an aerobic type of respiration. The purpose of aeration is to increase the concentration ofoxygen in the water. In scientific aquaculture practices, this is more critical because, often the rate of consumptionof oxygen is much higher than the natural rate of replenishment of oxygen in the water through diffusion fromatmosphere and photosynthesis of aquatic plants. Oxygen is one environmental parameter that exerts atremendous effect on growth and production through its direct effect on feed consumption and metabolismand its indirect effect on environmental conditions. Oxygen affects the solubility and availability of many nutrients.Low levels of dissolved oxygen can cause changes in oxidation state of substances from the oxidized to thereduced form. Lack of dissolved oxygen can be directly harmful to culture organisms or cause a substantialincrease in the level of toxic metabolites. It is therefore important to continuously maintain dissolved oxygen atoptimum levels of above 3.5 ppm.Dissolved oxygen is measured either in mg per litre (mgl-1) or parts per million (ppm) with 0 ppm representingtotal oxygen depletion and 15 ppm representing the maximum or saturation. The solubility of oxygen is influencedby several factors. The solubility of oxygen decreases as the temperature increases; decreases exponentiallywith increase in salinity; decreases with lower atmospheric pressure and higher humidity and increases withdepth. Water temperature is the main limiting factor in aquaculture farms. Oxygen from atmosphere entersthe water naturally through air/water interface. This process is enhanced by wind action which creates a mildturbulence at the water surface.Need for artificial aeration in culture systemz Rise in atmospheric temperature causes an increase in the rate of biological degradation of organic matterand subsequent depletion of oxygen concentration in water.z Prolonged cloudy conditions causes reduction in the photosynthetic activity by green plants in ponds resultsin reduction in oxygen concentration.Aeration, Filtration and Disinfectionin MaricultureBoby IgnatiusCentral Marine Fisheries Research InstitutePost Box No. 1603, Ernakulam North P. O., Cochin, Kerala, Indiaboby_cmfri@yahoo.com86Course Manualz Increased stocking rate of animals in semi intensive and intensive farming practices requires greater amountof oxygen for respiration for all aquatic organisms results in depletion of oxygen concentration in water.z Aeration in ponds helps in mixing and circulation of pond water. Mixing and circulation is more critical inscientific farming which also helps in feed distribution and waste disposal.AeratorsAerators are mechanical devices which increases the dissolved oxygen content of the water. Aeratorsutilizes the energy input to increases the surface area of water available for oxygen transfer andmix water withoxygen to ensure the liquid medium with of oxygen concentration is brought in contact with oxygen or air.Oxygen Transfer ProcessThree steps are involved in the transfer process of oxygen into watera. Transfer of oxygen in the gas to gas liquid interfaceb. Transfer across the gas liquid interfacec. Transfer of oxygen away from the interface into the liquidTypes of aeratorsDifferent types of aerators used in aquaculture are1. Gravity aerator: In gravity aerators, the water falls under gravity and air is mixed into it from the surroundingatmosphere. They can man made or natural.2. Surface aerator: this type of aerators are commonly used on ponds and  can also be used in large tanks,cages etc. They are used to break up or agitate the surface of water so that oxygen transfer takes place.There are different designs of surface type aerators available. Paddle wheel aerators and spray type surfaceaerators are commercial type surface aerators.3. Diffuser aerator: Diffuser aerators inject air or oxygen into a body of water in the form of bubbles. Oxygenis transferred from the bubbles to the water by diffusion across the liquid film.4. Turbine aerator: Turbine aerator consists of a propeller submerged in the water to be aerated. Circulationof water by Rotation of propeller causes greater aeration to occur at the surface. The main disadvantage ofusing this type of aerator is that propellers can cause damage to the fishes.Aeration in aquaculture pondsSteps to maintain optimum levels of Dissolved oxygen would be to support  major factors that increaseDO and put into check the factors that decrease DO. Photosynthesis plays a major role in oxygen production;respiration of all living organisms in the pond is the major factor involved in oxygen consumption. Oxygenconcentration in pond water exhibits a diurnal pattern, with the maximum occurring during the peak ofphotosynthesis in the afternoon and the minimum occurring at dawn due to night time respiration. The magnitudeof DO fluctuation is small and occurs around the level of saturated DO when plankton density is low andincreases asplankton density increases. Supplemental aeration is generally provided during night time whenDO increases to levels below 4.0 ppm. Photosynthesis of phytoplankton is the major contributor of DO duringthe day and diffusion accounts for increases when DO is below saturation at night. Diffusion at night can be87Winter School on Technological Advances in Mariculturefor Production Enhancement and Sustainabilitytremendously facilitated with the use of aerators, which exposes more water surface to equilibrate withatmospheric oxygen. Through reverse diffusion, anaerator operated during the day will tend to removesupersaturated DO. The net effectis a milder diurnal fluctuations of DO similar to the conditions of lowphytoplankton density. Such conditions are favorable for semi-intensive culture of prawn and shrimp.Photosynthetic oxygen production is also significantly reduced when  large scale depletion  of plankton populationoccurs. Under these conditions, flushing out decaying plankton, providing for additional aerators and a eratingfor additional hours may be necessary to maintain DO at optimum levels. When plankton density is high, thepenetration of sunlight through water gets reduced, thereby reducing photosynthetic oxygen production in thebottom ofthe water column. High plankton density often results from high nutrient loads and other theseconditions, large quantities of feed and faecal wastes are found on the pond bottom. This causes an increase inbacterial population and metabolic activity in the bottom sediments leading to high oxygen demand in thebottom sediment. Limited light penetration and increased DO consumption in the bottom may cause significantlylower DO compared to the top layer of the water column. If this causes DO to deplete to lower than criticallevels, disastrous effects on the bottom living organisms may happen. Limited light penetration (low secchi discreading) can also cause differencesin the temperature of the top and bottom layer. Temperature stratificationusually occurs during calm and warm afternoons.FiltrationRemoval of particles from a water flow in important in aquaculture. Suspended solids, dissolved solids andorganic matter were removed from water by filtration of water through suitable media.Types of filtration1. Mechanical filtrationIn aquaculture, mechanical filtration is used primarily for the separation of solids and liquids. A mechanicalfilter is a filter that is set into the water flow to collect the particles and larger objects and allow water to passthrough. Mechanical filters uses differences in particle size of the solution (or mixture) components to extractone part form the other. The simplest type comprises a static screen, a grating or perforated plate. They areusually simple in operation and relatively easy to maintain.In sand filters, water is allowed to flow through a layer of sand with particles of varying sizes and depth.The layer is not dense, but contains a number of channels and holes created between the particles that constitutefilter medium. When water is passes through the filter medium, particles larger than a certain size will betrapped in the medium.Various types of mechanical filters are (i) stationary screen (ii) rotary screen ( iii) vibratory screen2. Gravitational filtrationGravitational filtration utilizes the force of gravity to separate particles from fluid. Density difference of thesuspended particles and water is used in this type of filtration. A simple example of gravity filtration issedimentation. Sedimentation is a process of allowing particulate materials having density greater than that ofsuspending liquid to settle out under gravitational forces. The settling process of the suspended particles can beincreased by aggregating the suspended particles by addition of certain chemicals (coagulation) or by addingchemicals to produce insoluble compounds with suspended particles (precipitation).88Course Manual3. Biological filtrationConcentration of ammonia in culture water is reduced by biological filtration process. Biological filters aredevices to culture microorganism that will perform the given task of reducing the ammonia concentrationwhen water with high ammonia level floes through them. In water both Ammonia (NH3) and ammonium ion(NH4+) are present and their sum is known as Total Ammonia Nitrogen (TAN) and their proportions vary withpH.  Ammonia (NH3) is toxic to fish and their presence in water is important in aquaculture practices.Biological filters (biofilters) are used to maintain water quality in recirculating or closed aquaculture systems.Biofilters are also used to improve water quality before water is discharged from a facility. Biological filers areformed as a component of the main filtration system which ensures water quality in an aquaculture farm.However it is very important in recirculating aquaculture or aquarium system.In biological filters, bacteria are used to convert ammonia in various steps. (i) Conversion of ammonium toNitrite (ii) conversion of Nitrite to Nitrate and (iii) Conversion of nitrate to molecular nitrogen. The first twosteps, known as nitrification, are performed by specific bacteria which oxidize ammonia. The autotrophicbacteria, Nitrosomonas bacteria utilize ammonia as a food source and produce nitrite. This nitrite is furtherconverted to nitrate by Nitrobacter. These bacteria grow and colonize on the filter medium of biological filter.Both nitrifying bacteria will grow and colonize the biofilter as long as there is food available. The efficiency ofthe nitrification process depends on the optimum growth of bacteria on the biofilter medium. One of the mainfactors affecting bacterial growth is the amount ammonia in the water. Other factors regulate are temperature,oxygen concentration, pH, salinity, organic substances and toxic substances.DisinfectionWater drawn from coastal waters, estuaries and rivers used for various aquaculture activities often formsan efficient means for the introduction and spread of infectious diseases in the system.  So it is very essential tohave a pathogen-free water source for success in aquaculture. For aquaculture, the supply water to the farm orhatchery is disinfected by various methods. Disinfection of wastewater before discharging is necessary to avoidthe pathogen contamination in the environment. Disinfection can be described as the reduction of microorganismssuch as bacteria, viruses, fungi and parasites to a desired concentration. The aim of disinfection of water in fishfarming is toreduce the risk of transfer of infectious dieses from water to the fishto an acceptable level.There are several methods for disinfecting water. Disinfectants can be grouped as chemical and non chemicalagents. A four group classification for disinfectants is (i) chemical agents (ii) physical agents (iii) mechanicalagents and (iv) radiation. Even though various methods can be used for disinfecting water, the quality of thewater to be disinfected is of major importance. Pure inlet water is much simpler to disinfect than the outletwater because latter contains more particles. Turbid water and water with a high content of organic substancessuch as reuse water are also more difficult to disinfect and therefore not commonly disinfected. For disinfectionof water supplies to aquaculture facilities, UV light and ozone are commonly used. When starting disinfection,one must be aware of the production of disinfection bye products which might be harmful for fishes andhumans. Disinfection can be performed in different situations in aquaculture. Water equipment, buildings andeffluents can all be disinfected.89Winter School on Technological Advances in Mariculturefor Production Enhancement and SustainabilityUltraviolet LightUV light is electromagnetic radiation with a wavelength of 1-40nm located at lower end of the visiblespectrum and beyond. The ability of UV light to inactive and destroy microorganisms varies with both wavelengthand the microorganisms to be inactivated. The most effective wave length for disinfection is 250 – 270nm. UVlight damages the genetic materials in the microorganisms which results in their inactivation and death. Theefficiency of UV light depends on various factors like lamp intensity, age of the lamp, cleanliness of the lampsurface, distance between the lamp and organisms to be inactivated, duration of UV exposure and purity ofwater. UV lamps need to be replaced regularly, atleast once in a year. UV light transmission through waterdepends on the turbidity of water, lower in turbid water.UV lights can be placed in the water flow which is the usual method or above water flow. The doserequired to kill pathogenic microorganisms depends on the organism. Most of the common bacteria requires alower dose while viruses, which are more difficult to disinfect, needs stronger exposure intensity and duration.OzoneOzone is a very strong oxidizing agent, highly toxic to all forms of life. Ozone, is a colourless gas which isvery unstable and will quickly be broken down to O2. Ozone inactivates the microorganisms by damaging cellmembranes and nucleic acids, breaking long chain molecules into simpler forms.  Another advantageous effectof using Ozone in aquaculture systems is that by oxidation it reduces the amount of NH3, NO2, biologicaloxygen demand. When using ozone as a disinfectant, it is recommended that particles be removed from waterbefore the ozone is added, otherwise much of ozone will be used to oxidize the particles. When adding ozoneto water, special injection system has to be used to ensure good gas water mixing. The ozone needs to givensufficient time to function and oxidize microorganisms.  For effective inactivation of pathogens, ozone can beapplied in a high dose for a shorter duration and vice versa. Overdosing must be avoided because this may killthe fish. Most pathogens are killed by an ozone dose of 0.1-1.0mg/L and contact time of 1-10 min, but thisvaries with the organisms.The water quality parameters like concentration of dissolved organics, particular organics, inorganic ions,pH and temperature will have large impact on the residual ozone concentration after a given time. Because ofthese variations, it is important to add enough ozone to obtain a satisfactory residual concentration to achievedisinfection.One of the major problems with ozone disinfection is that it is highly toxic to fishes and humans. Ozone istoxic to fishes even at lower concentrations as it oxidizes gill tissue of fishes. Therefore after disinfection, anyresidual ozone present in water should be removed or destroyed. An adequate retention time ensures thatmost of the ozone has reacted and the product is mainly oxygen gas. Being a very strong oxidizing agent, it willoxidize all materials which comes in contact. Ozone will destroy most of the plastic for some extent. Ozonewill oxidize metals causing significant corrosion problems.ChlorineChlorine is a very effective disinfectant for water and the most common method used for disinfection ofwater. It is normally obtained by adding liquid sodium hypochlorite to water, or solid calcium hypochloritemixed into the water or pure chlorine gas. All these compounds are strong oxidizing agents and have the abilityto break down organic molecules.90Course ManualFor effective disinfection using chlorine, specific contact time is required which include time for dissociationin water, time for diffusion through cell wall and time to inactivate selected enzymes. Presence of residualchlorine after disinfection is critical for fishes and overdoing must be avoided.  Water containing chlorine is verytoxic to fishes. When disinfecting tanks or other equipments with chlorine, it is important that sufficient cleanwater is used to wash away the chlorine residues produced.  There for when using chlorine as a disinfectant, amethod for dechlorination must be included.

Aeration, Filtration and Disinfection

in Mariculture

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Aeration, Filtration and Disinfection in Mariculture

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