needs of the fish and the microorganisms in the water, good aeration and agitation is fundamental.
The placement of aerators, usually
paddle-wheels, within the pond
must be thought through to prevent the flocs from settling in an un
-agitated area of the pond. The design of the tank or pond must also
allow you to effectively drain the
excess heavy sludge which does
settle out on the bottom.
As the Biofloc starts to develop in
the system, the water colour starts
off by turning green as an algae
bloom is induced, and although
algae does play an important role in
a Biofloc system, it is heterotrophic
bacteria which you want to dominate the microbial community. Heterotrophic bacteria are usually limited by the supply of Carbon within
a pond, and works best off a C:N
ratio of 10-20:1. By adding a carbon source to the water, you effectively allow the heterotrophic bacteria to multiply and dominate the
system, which becomes characterised by the water turning brown. In
brief, heterotrophic bacteria uses
the excess nitrogen in the system
which is supplied by your fertilizer
at first, and then from the uneaten
feed and excreta from the fish. It
removes these potentially toxic nitrogen compounds from the water,
as a biofilter does in a normal Recirculating Aquaculture System (RAS).
Together with the carbon source,
the bacteria then incorporates the
nitrogen into microbial protein in
the form of the flocs, which filter
feeding fish such as Tilapia and
Shrimps are able to consume. The
flocs typically have a dry-weight
protein content of 30-45%.
So in effect, you enhance the use of
heterotrophic bacteria to control
your nitrogen levels and produce a
supplementary microbial food
source which can allow you to save
up to 40% of your feed costs by
simply recycling the nutrients. The
key is to provide a favourable environment for the heterotrophic bacteria through a well mixed, well
aerated water body with an additional carbon source and of temperature from 20-35 degrees Celsius.
Monitoring the key water parameters is an important aspect of a Biofloc system. Nitrate, nitrite, ammonia, turbidity and total suspended
solids (TSS) should be measured at
least once every 2 weeks with dissolved oxygen, temperature and pH
monitored daily. The level of the
biofloc can be measured as TSS and
through the use of an Imhoff cone
which is a glorified rain gauge with
a different scale on the side. After 1
A well mixed, well aerated system.
litre of water is given 20 minutes to
settle in the cone, the level of the
biofloc can be measured and
should be around 50ml/l for Tilapia.
The stocking density of a biofloc
pond is generally around 15-30kg
fish / m³ at harvest, and should be
worked back to calculate how many
fingerlings are needed to initially
stock your pond, given its size and
the size at which you plan to harvest the fish.
Biofloc Technology offers clear opportunities as a Tilapia industry will
begin to develop in South Africa.
Setup and running costs are low as
no external filtration, pumping of
water; better heat conservation as
well as savings on feed becomes a
reality. The low environmental impact further punts this green technology to play a role in providing a
good, cheap source of protein to
our hunger stricken continent.
Please feel free to contact me with
any questions or where I may be
able to assist you. I would recommend reading Biofloc Technology,
A Practical Guide Book by Yoram
Avnimelech to anyone interested.
Scott Day is an Aquaculture Masters
Student, Stellenbosch University.
A biofloc pond within a tunnel