Backyard Aquaponics
A place to discuss aquaponics

Intensive
The most popular growout systems for Murray cod in Victoria are intensive recirculating tank systems (RAS). Murray cod have proven to be very tolerant of high stocking densities (80-150 kg/m³) but oxygen injection is required with very high stocking levels. Murray cod display efficient food conversion (<1.5:1) in these systems with medium-fast growth rates (2g to 500-1000g in 12 months). Survival rates are >80%, but this is dependent on the management of water quality and good fish husbandry techniques. Smaller recirculation systems have stocking rates of 30-40 kg/m³, FCRs of 1.5-2.0 and fish reach market size in around 12-18 months. However, lower stocking densities generally reduce the risk of system failure, as there is a lower load on the biological filter.

The effect of stocking density upon the hydrodynamics of a circular tank, configured in a recirculation system, was investigated. Red drums Sciaenops ocellatus of approximately 140 g wet weight, were stocked at five rates varying from 0 to 12 kg m− 3. The impact of the presence of fish upon tank hydrodynamics was established using in-tank-based Rhodamine WT fluorometry at a flow rate of 0.23 l s− 1 (tank exchange rate of 1.9 h− 1). With increasing numbers of animals, curvilinear relationships were observed for dispersion coefficients and tank mixing times. Stocking densities of 3, 6, 9 and 12 kg m− 3 resulted in a 0.2-, 0.5-, 2.4-, and 3.2-fold decrease in mixing time relative to that observed for empty tanks

I operate several freshwater RAS systems in South Australia and have seen many others within the state. These systems run at moderate stocking densities (50-100 kg /cubic meter). All of these systems are very turbid despite continual filtration though 50 micron rotating drum filters and sumps which process the volume of the systems at least once per hour.

I have come to the conclusion that the turbidity is mainly due to a bacterial colonization of the water column. I think this because the turbidity acts in the same manner as high density aglae cultures ie. turbidity increases rapidly and lingers, followed by sudden death and rapid clarification of the system. This seems to happen independently to levels of nitrogeonous wastes.

I have tried to combat this problem using small doses of formalin and chloramine-t which work with varying success. These chemicals also adversly affect biological filtration, thus i would like to find a better way to combat this problem. i have also tried many probiotics / beneficial bacteria, none of which work.

If anyone knows what im talking about and hase a good solution i would love to hear it.
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Yes, i agree ozone does indeed help the problem, especially when used in conjunction with with protein skimmers.

We have attempted this on a small scale, however we found that we needed a large, expensive ozone generator and we had to inject pure oxygen into the machine to icrease it's ozone gerneration capacity, furthermore the cost of the stainless steel reaction chambers was high as large units were necessary to achieve the contact period necessary to 'nuke' the bugs. Also the necessity for protein skimmers, ozone destructor units and ORP meters to regulate its operation made it very costly and even then we were not entirely happy with its performance. I dread to think what the cost may be to set up a system to treat a large system with high stocking densities.

We have also tried passing the system through a 10 micron filter once per hour with no real improvement.

The concentration of DO present in water is affected by various factors, including altitude above sea level, water temperature and consumption rate, an increase in any of these factors reduces the amount of oxygen present in the water. A system that is designed to support a certain density of fish at sea level may not function as well if the water temperature, altitude or stocking density is increased. Furthermore, different species have different sensitivities to the DO concentration for example salmonids typically require minimum DO levels of around 6.0mg/L and tilapia require a minimum of 5mg/L. Now, the oxygen holding capacity of water at 16C (trout) is much higher than the oxygen holding capacity of water at 28C (tilapia), assuming the same altitude. Furthermore, tilapia are commonly stocked at higher densities than trout, increasing the demand for oxygen in a tilapia production facility. As roughly 250g of oxygen is consumed for every kilogram of feed eaten by the fish, the system design needs to take all these factors into consideration to ensure sufficient oxygen is available for the optimal growth and health of the fish being cultured.

At low to medium stocking densities water exchange or aeration are adequate for maintaining acceptable DO levels, and these methods are typically employed in raceways, and cages and earth ponds. Within a RAS the temperature of the water is controlled (usually heated) to optimal levels to promote rapid growth of the stock. Large water changes, such as are required to maintain DO levels, are not practical as this means the replacement water must be heated (or cooled) to the appropriate temperature, and this is an expensive process. Aeration is therefore used to maintain oxygen levels within RASs stocked at low to medium levels (<25 - 50kg/m3 - species dependant). Once densities of fish exceed the level where aeration is adequate to maintain DO levels, gaseous or liquid oxygen is added to the system to maintain appropriate DO levels within the water.

One of the key problems in RAS relates to the load of suspended solids and in particular to
very fine particles. The presence and accumulation of particulate wastes in RAS (faeces,
uneaten feed, and bacteria flocs) will impact negatively the water quality by affecting the
performance efficiency of the water treatment units. High suspended solids load has many
disadvantages:
o Particulate matter consumes oxygen during biological degradation which will
decrease the availability of oxygen for fish in culture (Rosenthal, 1997; Davidson
and Summerfelt, 2005).
o The brake down of organic wastes will increase the TAN concentration in the water
affecting nitrification (Liao and Mayo, 1974; Spotte, 1979; Davidson and
Summerfelt, 2005; Chen et al., 2006). Small quantities of unionized ammonia can
be toxic for epithelial tissues and disturb the elimination of protein metabolites
across gills (Peters et al., 1984).
o Solids support the growth of heterotrophic bacteria which can outgrow and compete
with nitrifyers. The nitrification process is strongly inhibited by heterotrophic
processes when high amounts of organic carbon are present (Zhu and Chen, 2001).
o Suspended solids offer an ideal temporary substrate for facultative pathogens while
they try to find a final host. Bullock et al. (1994) inferred that suspended solids may
be involved in bacteria gill disease (BGD) outbreak. Noble and Summerfelt (1996)
described that beside opportunistic microorganisms, non-infectious problems
prevail as high levels of suspended solids have caused mortalities in RAS.
o Particles can potentially clog biofilters and reduce their efficiency (Chen et al.,
1993; Rosenthal, 1997).
o Excessive solid loads can cause plugging within aeration columns, screens, and
spray nozzles orifices, which could ultimately result in system failure (Davidson
and Summerfelt, 2005).
o The organic C/N ratios in the water will negatively affect the efficiency of nitrifyers
(Rosenthal, 1997; Ebeling et al., 2006).
o The accumulation of solids can create anoxic conditions favourable for bacteria
responsible for the production of geosmin and 2-methylisoborneol causing offflavours
in cultured fish (Tucker and Martin, 1991).
o Gill tissue can be damaged by particles (Rosenthal, 1997) during feeding, drinking,
and breathing. Bullock et al. (1994) suspected that small suspended solids could
irritate gill tissue and provide an injured surface for attachment of any bacteria
(BGD) present in the water. Peters et al. (1984) found out that fin and gill lesions in
rainbow trout were induced partly by the accumulation of excretory and
decomposition products. Madetoja et al. (2000) showed that skin and mucus
abrasion dramatically enhanced the invasion of pathogenic agents into the fish.
o Fish vision can be affected by high suspended solids load, disturbing the recognition
of feed.
The proper management of suspended solids is one of the key factors determining the
successful operation of recirculating systems because of the elucidated potential impacts.
The design of a RAS to achieve the desired solid elimination has to take the following
aspects into consideration:
o The more quickly solids are removed from the water the less time they have to
break down and consequently less oxygen will be consumed by attached bacteria
(Bullock et al., 1994, 1997; Rosenthal, 1997; Davidson and Summerfelt, 2005).
Long residence times for particles in the system will affect their size due to share
forces and microbial degradation. Substances are leaching faster from smaller
particles than from big ones. Small particles, however, are more difficult to remove
from the culture water because of size and the proximity to water density.
o The methods to remove solids (sedimentation, filtration, and/or flotation) have to be
able to eliminate particles over a wide range of sizes. Normally a combination of
removal techniques are needed (Waller et al., 2003a; Orellana et al., 2005) specially
for the elimination of fine solids fraction (<20 μm) that do not settle in conventional
treatment processes such as gravity settling and microscreen, and accumulate in the
culture medium over time (Chen et al., 1993; Chen et al., 1994; Langer et al., 1996;
Rosenthal, 1997; Waller, 2001; Viadero and Noblet, 2002; Orellana et al., 2005).
o The size of fish and the water flow rates seem to be two closely related factors that
determine the characteristic of solid waste and because fish size and feed size are
known, these characteristics can largely be predicted. Small fish produce small
particles and need high quantities of feed per unit weight in order to satisfy their
energy requirements, while big fish produce larger particles and need less feed per
unit of biomass, compensated for by a relatively lower growth rate (Franco-Nava et
al., 2004). The amount, characteristics, and size of solids indirectly determine the
choice of methods for efficient removal.
o High stocking densities are often aimed for (depending on the species) to boost the
profitability of a RAS. High stocking densities allow more fish biomass to be
produced per unit of culture. However, increasing stocking densities require a better
management of solid removal and highly reliable removal techniques. Solid loads
will increase rapidly. Waste removal from the system has to be efficient and
becomes costly if mechanical means are no longer sufficient.

These systems run at moderate stocking densities (50-100 kg /cubic meter).

In the other thread ... you introduced the postulation that pumped water flow alone was sufficient for aeration....

I have been pointing out that what you suggest is contrary to most accepted methodology....

If I'm doing continuous flow, do I need a sump tank?

As long as the pump keeps running, the water level in the fish tank remains steady.