Water Quality: The Importance of Waste Products & Waste Removal In Intensive Silver Perch Culture.



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The main sources of waste in Intensive Silver Perch Farming and other recirculating culture systems are those of waste metabolites and uneaten feed. Wastes include faeces, urine, total ammonia, mucus, dead fish and scales.

Basically there are two requirements of filtration needed to sustain Intensive Silver Perch Farming and other high intensity aquatic culture environments. These requirements cover;

1: removal of solid wastes

2: removal of dissolved wastes

The nature of these wastes is outline below and the various types of equipment available for filtration is outlined in part (b).

Nitrogen Gas
Nitrogen gas is not normally considered to be of any consequence to the aquatic environment nor is it strictly speaking a waste product. However it can be of considerable concern in high intensity systems as the gas can become super saturated as a result of high, water pumping pressures and pumping system leaks acting as venturi inlets on the suction side. The super saturated nitrogen causes “Gas Bubble Disease” which reduces respiratory circulation with the formation of bubbles leading to asphyxiation. Nitrogen gas is essentially not a waste product but can influence production, in a similar way, especially if it occurs at low levels and goes undetected, adding to an undefined environmental stress. A situation that can influence production schedules.

Total Ammonia (NH3 + NH4)

Ammonia is highly toxic to all fish species and represents the major dissolved waste metabolite. It is secreted predominantly across the gills and as a minor component of the urine excretion. Ammonia exists in water in two forms (NH3 unionised ammonia and NH4 ionised ammonium). (Forteath 1990) The most toxic of the two being NH3.


The two forms of ammonia exist in an equilibrium relationship with the pH so as the pH changes the relationship between NH3 and NH4 changes. (temperature also effects pH but not significantly in a recirculating system) That is to say the lower the pH the lower the concentration of highly toxic NH3 and the higher the concentration of the less toxic NH4.

Aquaculturists can use this fact to reduce toxic concentrations or limit the toxic level of ammonia by controlling pH to slightly less than seven. Any background levels of NH3 should raise concerns as it indicates increases in metabolic wastes. However an acceptable level of 0.025mg/l is generally referenced for NH3 and 0.5mg/l for Total Ammonia.

ammonia toxicity diagram

ammonium -------------------------- ammonia

Low toxicity------------pH 7----------- High toxicity

(From Forteath 1990)

The above diagram represents a method to relieve the dangers on an "over cooked" pond. In intensive silver perch farming or in normal pond fish culture, the situation will occur where the pond is over stimulated from over feeding or over fertilising. Algal levels increase causing pH to rise where ammonia levels can easily become critical.

Cautiously reducing pH can save the crop. Addition of aluminium sulphate as a dissolved liquid spray works quickly but remember that sudden environmental changes create dangerous stress levels also. This process creates a flocculate in turbid ponds which would cause gill damage and stress. Using filtered water exchange can also dilute the toxicity.

Any background levels of ammonia can be considered a culture stress and should be removed. Monitoring recirculating systems for ammonia should be done every day and in a relationship to feeding so that levels can be measured after feeding. The highest concentration usually occurs about 12 hours after feeding.


Please refer to our

Remote Sensing Information



Where the feeding frequency is high the levels of total ammonia could tend to be constant although high feeding frequency would favour a more stable nitrifying bacterial population in the biological filtration system. This effect should result in consistent reduction of ammonia and reduce the cyclic nature of the nitrifying bacteria.


Organic Nitrogen
Organic nitrogen (Org-N) refers to the amount of nitrogen in organic material. In the recirculating system organic nitrogen can become dissolved or suspended in the water column. Organic nitrogen build up comes form plant matter faeces and uneaten feed and is acted upon by heterotrophic bacteria, which increases the toxic metabolites of ammonia, nitrite and nitrate circulating in the culture medium.

A: Nitrite (NO2)
Ammonium NH4 is converted to Nitrite (NO2) by the group of autotrophic bacteria known as Nitrosomonas spp (species). Nitrite is toxic as it prevents fish blood from carrying oxygen (Forteath 1990). “In fact nitrite oxidises the iron in haemoglobin to methaemoglobin which lacks the ability to bind oxygen”. Nitrite poisoning is more a problem in freshwater as saltwater species can tolerate nitrite levels 50 to 100 times greater. In freshwater the maximum safe level of nitrite is 0.5mg/l while in saltwater the level is 25mg/l.

B: Nitrate (NO3)
Nitrite is further reduced to nitrate by the biological activity of the nitrifying bacteria known as Nitrobacter spp. “ Technically speaking nitrate is formed by the complete oxidation of ammonia” (Forteath 1990). Nitrate is considered “basically harmless” (Piper 1983) up to concentrations exceeding 400ppm however Forteath mentions some fish species can be affected in recirculating systems (yellow perch and eels) at concentrations exceeding 23mg/l. However he goes on to mention crustaceans and molluscs can tolerate levels much greater than fish. For example, Penaeus monodon survives and grows at concentrations of 200 mg NO3/l and molluscs have survived concentrations of 4,482mg/l NO3. However any indication of a build up of nitrogenous waste products in a recirculating system is cause for alarm and reduction procedures (nutrient stripping or water exchange).


Carbon Dioxide
Carbon dioxide is a product of aerobic respiration and can be toxic and harmful to fish at concentrations exceeding 20ppm (Piper 1983). Water in recirculation systems should not exceed a maximum of 5 ppm CO2 but even this level can be harmful if the O2 falls below 3ppm. Carbon dioxide is easily removed by sufficient agitation of the medium and should not normally be a problem in recirculating systems. The soluble carbon is dissipated easily with devices such as packed columns and foam fractionators (Timmons 2000).


Uneaten Feed/Dead Fish/Scales/Faeces
These are all items generally covered under particulate matter or the major items of solid wastes. They represent the major problem of clogging in the mechanical and the biofiltration systems and for that reason are the first filtration items to be addressed in the filtration system. This is achieved by the mechanical filtration components. Decaying matter from metabolic function needs to be removed as it quickly leads to heterotrophic bacteria build-up, anaerobic pockets (Timmons 2000) and reduced water quality.

Uneaten feed enters the system occasionally as the result of over feeding or as the result of a stressful event when fish suddenly go off the diet. Recirculating systems should be designed to manage this problem and removal of any physical substances, including feed, scales and carcases would make up a major component of the first stage of the filtering system. Recirculating system designs should be continuously self-cleaning as uneaten feed and other debris quickly reduces water quality.


Suspended and Dissolved Solids
“Fine solids in water leave tangible residues when the water is filtered (suspended solids) or evaporate to dryness (dissolved solids). Suspended solids make the culture cloudy or opaque and include chemical precipitates, flocculated organic matter, living and dead planktonic organisms and inorganic particles (Piper 1983). Turbidity is the term used to describe the presence of suspended solids and in intensive culture is often a large component of the feeding process. For example, some high energy feeds used in tank culture contain high levels of fish oil which can easily stain the water “increase turbidity” and give off a fish oil smell. This situation has led to stress and mortality in an intensive barramundi (Lates calcarifer) system where water exchange was kept to an absolute minimum so as not to reduce water temperature. (Personal Observation Infinity Fish Farm Sydney).

Timmons discusses the inverse relationship of ammonia filtration to the level of dissolved organics as it has been reported that ammonia removal was proportionally inhibited by the concentration of the organic loading which underlines the acute necessity for the control and removal of organics, both dissolved and suspended.

Notice the turbity. This factor reduces productivity. The authors  Farm 1990


Stage 1 (1990)

silver perch farming


Stage 2 (1998)



Designing Settlement and Effluent Ponds

Essentially settlement ponds have to be designed to handle the loading for the worst-case effluent for the particular situation. Huguenin suggest that typical sedimentation pond depth is between 1.2 and 1.8 meters and the length should be in the range of between 4:1 and 8:1. So in the above part (a) the sediment pond would be approximately 6 meters by 50 meters to achieve an adequate technical result.

Basically sedimentation involves the reduction of solids in suspension by reduction of velocity in comparison to the farm discharge velocity. The pond or ponds construction will be based on this rate. Huguenin states that with the settling velocity (Vc) equal to D x Vh / length, complete particle removal will result (where D is Depth, L is Length and Vh is horizontal velocity).


Which ponds are sedimentation ponds? Fram production here was 5 tonnes per magalitre with 6 x 1megalitre production ponds

Taree Intensive Silver Perch Farming (1989)

For example the sedimentation ponds design, at the Taree Intensive Silver Perch Farming operation (photograph), were such that the water flow of 1000 litres per minute or 60m3 per hour from a 100mm pipe was reduced across a 5 meter width by 15 meter length by 1 meter deep pond (see photo). This was the first intensive silver perch farming venture in Australia. It was based on over 2 years of private research and pilot scale development and the following standards are used to calculate sedimentation requirements for fish farm licence approval in NSW.

Vh =Q/W x D

Vh = 0.0166/5 x 1
Vh = 0.0166m3/sec / 5
Vh= 0.0033m/sec or 3.3litres per second.

Vc = Q/L x W
Vc= 0.01666 / 5 x 15
Vc= 0.01666 / 75
Vc= 0.0002m/sec
Vc= 17.28m/day


Huguenin mentions that typical Vc (overflow) rates for warm water fish were 160-175 m/day so the above sedimentation was considerably over designed or could handle far greater loads.

Further to the above, vertical plastic curtains were used in the sedimentation ponds to cause the effluent water to have to rise and fall, firstly, under the first curtain and then over the next and the back under the next etc. Thus extending the ability for sediment reduction. The second sedimentation pond which fed into the duel biological filters used a similar curtain system but also utilised water hyacinth plants to absorb dissolved nutrients.

These plants also tended to have a secondary function, as their root system was very efficient at binding clay sediments and removing them from the water flow as well, which helped protect the biofilter from clogging.

The sludge accumulation in these ponds was always of a black oily and smelly consistency. However the obvious anaerobic nature of the sediment never appeared to be a problem for the system. The sludge was removed by use of a submersible sludge pump using a clear plastic hose so as the water became clear the pump could be moved along the bottom in the collection sump, which ran through the middle of the pond. The sludge was pumped to evaporation paddocks on the property.

Prior to the construction of any fish culture system all the associated flows rates and biological functions should be checked mathematically. One of these tests would be the calculations on the expected sediment loading/ effluent out, prior to construction to determine Vh and Vs values and consequent sedimentation pond dimensions for the relevant discharge effluent expected.

The design of the sedimentation pond should include practical means for allowing control of inlet and outflow turbulence flow and short-circuiting flow as well as removal of sludge as required (Huguenin). In the pursuit of high quality effluent water a double sedimentation system may have advantages. The use of collection sumps with drain off valves where high effluent deposits can be removed to an evaporation area has advantages in sustaining the function of the sedimentation pond while effectively reducing the BOD on the pond as sludge can be removed daily if required.

The preferred method of sludge management in NSW silver perch culture is for sludge to be pumped from a pond sump to evaporation. A system could be utilised by a second crop such as grapes and stone fruit. Both of these methods have undergone preliminary trials in the Hunter Valley in conjunction with silver perch Bidyanus bidyanus culture but with no confirmed advantage. However, from personal work with pond sludge the results achieved with crop culture can be very promising as one can repeatedly cause “grass to grow on bare gravel” with just one application.

In the situation of limited space for sedimentation ponds, today there are positive alternatives. Huguenin discusses the “ongoing dramatic change in the sophistication and availability of pre-packaged (modular and pre-built) wastewater treatment plants and systems for modest flow rates well within our area of interest”. And it is this type of equipment that could be utilised to replace sedimentation ponds where there is insufficient land on the aquaculture site and where the effluent is returned to a waterway.



Silver Perch Farming Permit Requirements

no effluent to reach any natural waterway
effluent pond (>2 x the volume of largest culture unit) purpose-built and constructed of clay; isolated from run-off


effluent reused/irrigated, with sufficient land available for irrigation

Note: In personal communication with a NSW Fisheries Representative, Steve Boyd, the situation in pond culture is that the sedimentation pond of twice the size of the largest growout pond is an essential requirement and alternatives such as sedimentation devices and swirl separators are not considered alternatives to permit regulations. (2004)


However the efficient removal of waste solids has some significance as the waste sludge could be a potential marketable product and its efficient collection would certainly help that process. Such passive devices should be considered irrespective as they improve filtration efficiency.

Such devices as the “swirl concentrator” or swirl separator use a flow principal that removes solids continuously with very little maintenance requirements. They concentrate and remove sludge at water rates as low as 1-5% and have been used in comparison trials with traditional sedimentation ponds used on trout farms.

For example, with a flow rate of 500 litres per second the traditional settling pond would average a surface area of 500-750m2. A swirl concentrator would achieve similar results with a device 9 meters in diameter and with a surface area of 64m2.

Mechanical filtration devices, as mentioned earlier, can be used to effectively remove particulate matter from the effluent water and there are velocity reduction devices such as the “Tiled Plate Interceptor “design (source Muir 1982) or the lamella separator (Cripps 1994) which act as 3 dimensional sedimentation devices. Ironically the action of a biological filter is also a very good mechanical filter and is the main reason for the “correct steps” approach in placement of filtration components in series in a recirculating system. A simplistic statement but I have seen them built out of sequence.


silver perch









Additional Note

The product, below, was sold in May 2004. Notice the "paint brush tails", the distorted bodies and the "axe head" deformity in the heads. This is not the way to make money from silver perch farming. Interestingly the whole consignment appeared to be deformed.

Product like this is disappointing and the marketing damage created is irrevocable.

Silver Perch Deformities Damage Market Potential










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Aquaculture Farm Environment. Web Address;

Boyd, C. E., Water Quality Management For Pond Fish Culture. 1982. Developments in Aquaculture and Fisheries Science. Volume 9. Elsevier Pub.

Cripps, S. J.,1994. Minimising outputs: Treatment. Division of Sanitary Engineering, (Aquaculture Engineering) Lulea University of Technology. S-971 87 Lulea, Sweden.

Forteath, N. A handbook on recirculating systems for aquatic organism 1990 Fishing Industry Training Board of Tasmania

FRDC, Research Grant 94/132. The use of oysters as natural filters of aquaculture effluent. Web Site, http://www.

Huguenin, J.E. & Colt, J. Design & Operating Guide For Aquaculture Seawater Systems Second Edition 2002 Elsevier ISBN 0-444-50577-6

Piper, R.G., et al. Fish Hatchery Management 1982 ISBN 0-913235-03-2

Study Guide SQQ 633 Engineering Aspects of Aquaculture

Timmons, M. B. & Losordo, T.M. Edit. 2000. Aquaculture Water Reuse Systems:
Engineering Design & Management. Elsevier Pub. ISBN 0-444-89585-x