In biofloc systems, a major factor that controls ammonia
concentration is the C:N ratio of feed and other inputs.
A feed with a 30 to 35 percent protein concentration has
a relatively low C:N ratio, about 9 to 10:1. Increasing the
C:N ratio of inputs to 12 to 15:1 favors the heterotrophic
pathway for ammonia control. The low C:N ratio of feed
can be augmented by adding supplemental materials with
6
high C:N ratio. Or, the input C:N ratio can be increased
by reducing feed protein content. Ammonia uptake by
heterotrophic bacteria occurs rapidly after carbohydrate
supplementation. Ammonia control through the heterotrophic
pathway is often more stable and reliable than algal
uptake or nitrification.
Many practical and processed materials have been
used as carbon sources in biofloc systems, including grain
pellets, molasses, sugar cane bagasse, and chopped hay,
among others. Carbohydrate materials should be low-cost
and convenient. Organic matter that breaks down easily
and quickly is best. Heterotrophic bacteria in biofloc systems
can act on simple organic matter rapidly, within minutes
to hours. Simple carbohydrates such as sugar (sucrose
or dextrose) or starch will have the quickest effect. The best
carbon source to add during system start-up, when the
most rapid response is needed, is simple sugar.
To promote exclusive control of ammonia concentration
by the heterotrophic pathway, carbohydrate additions
must be made in accordance with feeding rate. For every 1
kg of 30 to 38 percent protein feed added, add 0.5 to 1 kg of
a carbohydrate source such as sugar. More carbohydrate is
needed at the higher protein level. It is clear that relatively
large quantities of carbohydrate must be added to control
ammonia concentration this way. Less carbohydrate can
be added if other ammonia removal pathways are operating
simultaneously in a biofloc system.
There are several drawbacks to continually adding
organic carbon to control ammonia. This pathway
encourages the production of bacterial solids, which accumulate.
If not controlled, solids concentration may reach
levels that cause gill clogging. More oxygen will be needed
to support the respiratory demands of a greater bacterial
load, and additional energy is needed to keep solids
in suspension. High rates of water respiration (oxygen
consumption) reduce response time in the event of system
failure. Capacity must be added to remove, treat, and
dispose of accumulated solids.
Ongoing carbon supplementation is required to control
ammonia with this approach. In order to stop carbon
supplementation, a system must be “weaned.” Stopping
the supplemental carbon abruptly before the nitrification
pathway is sufficiently developed will lead to water quality
instability and potentially detrimental spikes of ammonia
and/or nitrite. Once carbon supplementation ceases,
superintensive biofloc systems naturally tend toward the
nitrification pathway of ammonia control.