What is filtration system ?

Filtration is used in aquaculture to keep perfect sanitary water conditions, keeping animals alive in a clean water. BVS uses this technic successfully since 5 years now. To be efficient, a circuit needs at minima, one pump, one media filter and one UV light. Increase technology in case of specificities.

 3 kinds of volumes:

SMALL VOLUME : 100  liters …up to 800 liters

MEDIUM VOLUME : 1000 liters ….up to 5m3

BIG VOLUMES: up to 5m3 .

System Components

System components includes equipment that is directly involved in the storage of thealive seafoods within the recirculation system. Major system components include the following.

Oxygen Generator or Source of Aeration

• Fish, lobster, shells require oxygen to survive. As fseafoods are usually stocked at high densities within the tanks simple aeration using mechanical aeration systems is often not sufficient. Oxygen can be added to the system via liquid oxygen and/or an oxygen generator, to maintain suitable oxygen levels at high stocking rates. Aeration pumps will provide the tanks with both oxygen and water circulation.

Mechanical Filtration

• Mechanical filtration removes suspended solids obtained from feces and un-eaten food. Removal of solids is important to ensure that pipes and equipment components do not become clogged with waste material. Decomposing waste matter left in the fish tanks will also consume available oxygen within the water column. There are many various types of mechanical filtration that will filter out different sized particles of waste matter. Some types include drum filtration, screen filtration, foam fractionation, settlement tanks, sand filters just to name a few. Mechanical filters require regular back-flushing to prevent the accumulation of sludge.
  

   

• Mechanical filtration small volumes    

  

• Mechanical filtration medium volumes    

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 Mechanical filtration big volumes 

• Biological Filtration

Fish, lobster, shells produce ammonia and nitrites as metabolic waste products which are toxic. These waste products therefore need to be converted into nitrates which are not harmful to the fish.

Nitrification where ammonia and nitrites are converted into nitrates by bacteria via oxidation.

Bio-filters consist of a medium with a large surface area upon which nitrifying bacteria will colonize after a few weeks. These bacteria will convert toxic ammonia and nitrites into non-toxic nitrates via oxidation. This process is known as nitrification. There are a number of different bio-filters on the market and some recirculation systems will often incorporate several into their design.

It will usually take a few weeks to a month before nitrifying bacteria colonize and the bio-filter becomes active. During this time stocking and feeding rates should be reduced.
 

. UV light

What is Ultraviolet?

Ultraviolet (UV) sterilization is used as a pretreatment and/or post treatment step to kill or inhibit growth of microorganisms, remove ozone, chlorine and trace organics and reduce total organic carbon (TOC). During UV sterilization, the water is exposed at a controlled rate to ultraviolet light waves. The bacteria absorb the UV radiation energy, which destroys or inactivates their DNA, thus preventing the bacteria from reproducing. UV systems may reduce 99% of bacteria in the water.

The Principle behind Recirculation

Recirculation systems occupy a very small area and allow the grower to stock fish at high densities and produce high yields per unit area. Recirculation systems are very intensive and therefore require a high level in management of stock, equipment and water quality. Thus it is important to have an understanding of the principles of recirculation systems if the system is to be managed effectively.

A recirculation system is essentially a closed system and involves fish tanks and filtration and water treatment systems. The fish are housed within tanks and the water is exchanged continuously to guarantee optimum growing conditions. Water is pumped into the tanks, through biological and mechanical filtration systems and then returned into the tanks. Not all water is 100% exchanged however as it is difficult to ensure that all waste products are converted or removed by the treatment process. Most culture systems recommend at least 5% to 10% water exchange rate per day depending on stocking and feeding rates.

Water Quality Management

As fish live and breathe within water it is important that optimum water quality conditions are maintained within the recirculation system especially when fish are stocked at high densities. The supply water carries oxygen to the fish and removes waste products such as feces, ammonium, carbon dioxide and uneaten food. These waste products are removed from the system and transformed into less harmful compounds or concentrations that do not effect the growth or health of the fish.

Water quality must be kept under optimum conditions to ensure the survival of both the culture fish species and the nitrifying bacteria inhabiting the bio-filter. Continuous records of water quality must be regularly taken to control for any changes that may occur. Water quality testing kits or probes are readily available from analytical supply stores however make sure that the equipment will measure within the required ranges and shop around as some can be quite expensive.

Temperature

• Maintaining temperature within the optimal range for growth of the selected culture species is vital. Fish grow more rapidly within this range and achieve improved food conversion ratios. Fish are also less stressed when held at their optimum temperatures and therefore become less prone to disease.Temperature is maintained using artificial heating or cooling as described previously. It is easily measured by using a thermometer.

Oxygen

• Dissolved oxygen is perhaps the most critical water quality variable and will depend on water temperatures, stocking and feeding rates and the effectiveness of the aeration installed within the recirculation system. Dissolved oxygen concentrations should be kept above 60% saturation (or around 5ppm) to ensure the survival and growth of the culture species.The activity of the bacteria within the bio-filter will also depend on the level of dissolved oxygen within the water column and usually become inefficient when oxygen levels fall below 2ppm.Declining oxygen levels can be caused by a number of factors such as high stocking rates that occur within the recirculation system and the decomposition of organic matter including feces and uneaten food. Low dissolved oxygen can be lethal to the aquaculture species. Some effects include stress, increased susceptibility to disease, poor feed conversion, poor growth and can cause mass mortalities in extreme cases.

  Signs of low dissolved oxygen can be detected when fish are observed rising to the surface and gulping air or gathering around the aeration device. There are also oxygen probes available that can effectively measure dissolved oxygen levels and therefore allowing the grower to detect and control dissolved oxygen before it reaches a critical level.

  Dissolved oxygen levels can be maintained by incorporating aeration devices into the recirculation system. Oxygen generators are also useful as they will supersaturate the water with oxygen before it enters the grow-out tanks.

pH Levels

• The pH is the measure of the hydrogen ion (H⁺) concentration in the water. The pH scale ranges from 0-14 with a pH of 7 being neutral. A pH below 7 is acidic and a pH of above 7 is basic. An optimal pH range is between 6.5 and 9 however this will alter slightly depending on the culture species.The pH within recirculating systems tends to decline due to the build up of carbon dioxide (C0₂) produced by the respiration of fish and bacteria within the bio-filter. Carbon dioxide will react with water to form carbonic acid and therefore push the pH levels downwards. This becomes critical when pH levels reach below 6.5 as the effectiveness of bacteria within the bio-filter becomes void.Sub-optimal pH also has a number of adverse affects on the culture species. It can cause stress, increase susceptibility to disease, low production levels and poor growth. Signs of sub-optimal pH include increase mucus on the gill surfaces of fish, damage to the eye lens, abnormal swimming behavior, fin fray, poor phytoplankton and zooplankton growth and can even cause death.

  pH can be maintained within the recirculating system by adding buffering agents such as sodium bicarbonate or calcium carbonate, and by aerating the water which reduces the buildup of carbon dioxide. Water should be checked for pH almost daily within a recirculating system by using pH probes or special testing kits that are available from water analysis suppliers.

Carbon Dioxide

• Carbon dioxide is produced by the respiration of fish and bacteria within the system. If carbon dioxide levels reach high levels it can cause respiratory problems as it will interfere with oxygen uptake. High carbon dioxide concentrations within the water column can also cause pH levels to decrease as mentioned previously.Carbon dioxide can accumulate within recirculation systems if it is not removed. This can be achieved by aeration.

Water Alkalinity and Hardness

• Alkalinity refers to amount of carbonates and bicarbonates in the water and water hardness refers to the concentration of calcium and magnesium. As calcium and magnesium bond with carbonates and bicarbonates, alkalinity and water hardness are closely interrelated and produce similar measured levels.Waters are often categorized according to degrees of hardness as follows:
  • 0 – 75 mg/I = soft
  • 75 – 150 mg/I = moderately hard
  • 150 – 300 mg/I = hard
  • over 300 mg/I = very hard

It is recommended that alkalinity and hardness levels are maintained above 50 mg/I which provides a good buffering (stabilizing) effect to pH swings that occur in ponds due to the respiration of fish and bacteria. However high water hardness can cause a build up of calcium deposits on pipes and fitting which can be difficult to remove.

Nitrogenous Wastes

• Ammonia is the main nitrogenous waste that is produced by fish via metabolism and is excreted across the gills as ammonia gas. Ammonia can also be produced from the decomposition of organic wastes resulting in the breakdown of decaying organic matter such as animals and uneaten food.Ammonia is present in two forms in water – as a gas NH₃ or as the ammonium ion (NH₄⁺). Ammonia is toxic to culture animals in the gaseous form and can cause gill irritation and respiratory problems.Ammonia within recirculation systems is broken down by the bacteria within the bio-filter via nitrification as mentioned previously. Ammonia levels will depend on the temperature of the water and its pH. For example at a higher temperature and pH, a greater number of ammonium ions are converted into ammonia gas thus causes an increase in toxic ammonia levels within the water.

  If ammonia levels become elevated it is important to check the effectiveness of the bio-filter within the recirculation system. Bio-filters can fail due to mechanical failure or biological failure caused by the inhibition of bacterial activity. Bacterial activity can be reduced due to toxicity from chemicals, natural aging, lack of oxygen, pH, etc. It is important to remember that bacteria are living organisms and require just as much care as the culture species. Bacteria survival is extremely important as it ensures the health, growth and survival of the culture species.

  If high levels of ammonia are present within the pond’s water, a number of measures can be taken. These include:

  • reduce or stop feeding
  • flush the tanks with fresh water
  • reduce the stocking density
  • increase aeration
  • in emergencies – reduce the pH level

The amount of ammonia present in the water can be calculated by recording the total ammonia¬nitrogen (TAN), pH and temperature (Table 1).

For example to obtain the concentration of NH₃: Water at pH 8.4, 28°C and 2mg/1 of TAN (sampled measurement) contains 15% NH₃ (from table). Therefore 2mg/1 x 15% / 100 = 0.3 mg/I of NH₃. Alternatively, ammonia testing kits and probes can be purchased from analytical supply stores.
Table 1: Percentage of TAN in the toxic unionized form NH₃ at different temperature and pH levels. Boyd (1982) “Water quality management for pond fish culture”.

Solids

• Solid wastes, or otherwise known as particulate organic matter often consists of feces or uneaten food. A build up of solid wastes within the system should be prevented as it can cause oxygen depletion and ammonia toxicity when it decomposes. Mechanical filtration and water exchange will remove the majority of organic matter from the system.Organic wastes are present in three main forms in the recirculation system:
  • Settling solids – accumulate on the bottom of the tank
  • Suspended solids – float in the water column and will not settle out of water
  • Fine and dissolved solids – float in the water column and can cause gill irritation and health damage to animal

Dissolved Ions

• Dissolved ion concentration should be checked before a water source is used as dissolved ions can be difficult to remove. This is particularly relevant when bore water is used as this source can sometimes be high in dissolved ion concentration which can effect the respiratory capacity of the animal.