Biofloc and RAS Culture Systems

Historically, land based aquaculture involved creating a mini-lake or a mini-ocean using soil and liners.  The system depended on pumping large amounts of new, clean water through the ponds to rinse out shrimp or fish waste.  But if the incoming water became polluted, either by other farmers or other water users, the crop was at risk.  Further, the discharge of nutrient rich water was increasingly viewed as irresponsible and unsustainable.

Over the years, engineering improvements resulted in equipment that allowed for aquaculture wastewater to be filtered, treated and re-used over and over again. The combined use of filters, settling tanks, sterilizers and ozonators form the basis of a Recirculating Aquaculture System (RAS).  A good filter system, combined with strong aeration or pure oxygen injection, can sometimes allow very high density culture if the target species is compatible for that.

The drawback of RAS culture is that it often involves expensive equipment and highly trained managers to keep it operating.  The equipment, such as the settling tanks and biofilters, often take up significant amounts of space, either in or adjacent to the culture tanks.

Biofloc with Shrimp

The main goal of culture filtration is the removal of nitrogen waste, usually in the form of toxic ammonia.  The main source of ammonia is protein  digestion, either by the culture animals directly, or the microorganisms feeding on the fish or shrimp waste.  Ammonia itself can be used as a food source by autotrophic bacteria (Nitrosomonas and Nitrobacter) and algae.  There is also a wide range of heterotrophic bacteria that can use ammonia or other nitrogen compounds if they also have access to an easily digestible, high energy source, like sugar.

In the past few years management technologies have been developed that encourage the growth of both autotrophic and heterotrophic bacteria, sometimes with microalgae, right in the culture tank by adding sugar.  The bundles of algae/bacteria that grow under these conditions are called biofloc.  For some kinds of animals, like marine shrimp, biofloc is an excellent food source.  In fact, many studies show that on most shrimp farms, only 1/3 of the feed's nutritional value is digested and absorbed on the first eating.  A little more is absorbed as the shrimp eat their own feces.  But more than half of the food value either settles into the pond mud, or gets rinsed out or gets filtered out in the RAS systems.  Either way, dealing with the waste is often an expensive and time consuming problem.

In marine shrimp biofloc systems, if properly managed, nearly all the shrimp waste is converted into edible food for the shrimp, which is always available.  The shrimp grow faster, with better feed conversion rates, little water or waste discharge and limited filter systems.  The key to the success is strong aeration and water movement, which keeps the shrimp waste, feed and biofloc suspended in the water column.  It also requires skilled management.  Too much sugar leads to an overgrowth of bacteria and oxygen levels drop.  Not enough sugar can lead to rising ammonia levels and an accumulation of shrimp waste.  Add sunlight and algae into the mix, and matters can be quite complicated.

Miami Aqua-culture, Inc. offers consulting and training services as well as on going management assistance through the internet and video conferencing.  Let us show you how we can help!

Biofloc Shrimp Tank Design

High density shrimp culture using biofloc technology can be applied to a lot of designs.  But some work better than others.  In the traditional style semi-intensive outdoor ponds, shrimp densities are about 20 per square meter (20/m2).  In the clear water RAS tank or raceways, the density is usually 100-200 shrimp/m2.  In the biofloc tanks the density can approach 500 shrimp/m2, with some going higher than that.

The key design element is strong aeration and water movement which keeps shrimp waste suspended in the water column.  This is easier said than done.  Some large, older shrimp ponds have been retrofitted with strong aeration and mixing with improved production results.  But they often still have pockets of heavy sludge accumulation due to their sheer size and shape.  These pockets of sludge buildup are now viewed as prime breeding grounds for disease organisms such as EMS.

The best success so far seems to be oval shaped tanks, or rectangular raceways with center dividers creating a circular current.  The tanks are 100-400 feet long and 12-30 feet wide with a water depth of 3-4 feet.  The picture below is from Florida Organic Aquaculture, one of the largest shrimp farms in Florida.

Another Florida client of ours produces shrimp and fish year round in biofloc raceways as shown below.  Locating in Florida makes it easier to achieve the 85F water temperature in which these shrimp and tilapia thrive. 

But even in Florida we benefit from a greenhouse design in winter.  We also must compete with excellent quality shrimp caught from the wild all year.  For these marketing reasons, biofloc shrimp farms in Iowa, North Dakota, Minnesota, Wisconsin, Michigan plus even Canada and northern Europe are showing great potential.

PL-101, 21.35 gm.  No picture.

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