Schematic diagram of a biofloc technology system.
Sustainability
2021, 13, 7255
3 of 15
1.1.1. Carbon–Nitrogen Ratio
In the aquatic environment, the carbon–nitrogen ratio (C/N) plays a vital role in
the immobilization of toxic inorganic N compounds into useful microbial biomass that
might act as a direct source of food for the reared aquatic species. Immobilization of
inorganic N occurs at a C/N ratio of organic matter above 10 and, hence, any alteration
in this ratio within the BFT system might result in a shift in microbial diversity, which
might further affect the water quality. For example, De Schryver et al. [
13
] observed
that a high C/N ratio favors the proliferation of heterotrophic bacteria, which leads to
significant changes in water quality and biofloc composition. As such, manipulation of
the C/N ratio can be achieved through modification of the carbohydrate content in the
feed or the addition of an external carbon source in the rearing water so that microbes
can assimilate waste ammonium for microbial biomass production. This will, in turn,
decrease the concentrations of ammonium/ammonia to less toxic levels, thus making
water exchange unnecessary [
14
]. Total suspended solids (TSS) is another important water
quality parameter whose concentration in aquatic ecosystems depends on the C/N ratio.
Xu et al. [
15
] observed that a high C/N ratio (15:1 and 18:1) rapidly increased the TSS
concentrations in water, which negatively affected the growth performance of L. vannamei.
Moreover, the authors anticipated that production costs would be reduced under the
C/N ratio of 12:1 compared to 15:1 and 18:1 due to reduced utilization of organic carbon,
saving approximately 20,000 L of molasses per hectare of shrimp production at the same
stocking density. Pérez-Fuentes et al. [
16
] also found that, under high-density cultivation
of O. niloticus in a BFT system, C/N ratios exceeding 15:1 promoted the production of
dissolved salts and settled biomass, which affected the growth performance of fish. The
authors recommended a C/N ratio of 10:1 as the optimum condition for the production of
O. niloticus reared under similar conditions. In another study, Silva et al. [
17
] also observed
poor water quality (high TSS, turbidity, alkalinity, and settleable solids) at a C/N ratio
of 20:1, which affected the growth performance of O. niloticus. Similar results have been
reported in Clarias gariepinus [
18
,
19
]. However, Yu et al. [
20
], Haghparast et al. [
21
], and
Wang et al. [
22
] reported better growth performance and immune stimulation in carp at
high C/N ratios (20:1 and/or 25:1) reared in BFT. The discrepancy in results could be
attributed to the difference in species and the source of organic carbon.
1.1.2. Source of Organic Carbon
Different carbon sources, such as molasses, glucose, cassava starch, cornmeal, wheat
flour, sorghum meal, sugar bagasse, sugar, rice bran, ground bread crumb, glycerol, and
anhydrous glucose, are used to enhance nutrient dynamics through an altered C/N ratio
as well as improving the production of crustaceans and certain finfish species [
20
,
23
–
29
].
The efficient establishment of flocs by different carbon sources mainly depends on their
carbon content and speed of degradation, hence indicating that certain carbon sources
are more efficient in promoting floc formation than others. Generally, simple sugars such
as molasses are degraded faster than complex sugars such as cassava starch, leading to
improved water quality, as indicated by lower concentrations of ammonia and a higher
growth rate of beneficial microbial biomass [
2
]. Molasses are the most widely used carbon
sources in BFT systems during larval, nursery, and grow-out phases due to their efficiency
in improving water quality for the sustainable production of aquatic species [
2
,
30
].
One of the most elegant flexibilities of BFT systems is the capability of reusing water-
containing flocs for the production of certain detritivorous species under intensive culti-
vation. This practice aims to prevent the discharge of nutrient-rich wastewater into the
environment, which might result in pollution. Liu et al. [
31
] conducted a 56-day experi-
ment to elucidate the effect of no carbohydrate addition applied to control water quality in
water-reusing BFT systems for tilapia (GIFT Oreochromis niloticus, 99.62
±
7.34 g). Results
indicated no significant difference in growth performance between fish culture in tanks
with or without carbohydrate (glucose) addition, hence indicating the feasibility of no