Biofilters for Freshwater Crayfish Purging Systems
While this information sheet explains the operation of biofilters for purging systems, a large supply of good quality water which allows you to either run a flow through system or a regular dump and refill every week (bore water is often ideal for this) would be preferable to running a complex and often expensive biofilter.
Recirculating water systems are usually used when there is no other alternative, usually when there is limited amounts of water available, or for improved efficiency if large volumes of water are required to be heated or cooled.
A recirculating or water reuse system is a series of water treatment units that combine to create a self-contained marine or freshwater environment. The system usually contains apparatus in the following sequence:
- purging tank for animals
- settlement tank or solids filter
- protein skimmer/foam fractionator
- biofilter
- reservoir
- pH buffering unit
- pump(s), aerators and, if required, heating and cooling units.
More than one of these functions may be performed in (or by) the same unit. For example, the biofilter may be housed in the reservoir, with the substrate acting as the pH buffer (limestone or shellgrit). The most complex of these components is the biofilter, since it is essentially a collection of living organisms (bacteria), growing on small particles or substrates which give a very large surface area. Therefore, the biofilter has requirements that must be met if the bacteria are to survive and function. Any deviation from these requirements may impair the performance of the bacteria or, if severe, may result in the collapse of the biofilter, through death of the bacteria.
The bacteria in the biofilter live on the nitrogenous wastes from the crayfish (or fish) in the stock tank. The bacteria oxidise toxic ammonia to less toxic nitrite and then to very much less toxic nitrate (nitrification). Then heterotrophic and autotrophic bacteria convert nitrate to nitrous oxide or nitrogen gas (denitrification). Because the bacteria are living organisms, they require large amounts of oxygen. It actually takes 4 kilograms of oxygen to oxidise 1 kg of ammonia. Biofilters range from an aquarium undergravel filter (figure 1) through to up-flow filters (figure 2), trickling down-flow filters (figure 3) and rotating bio-drums (figure 4).
Figure 1
Figure 2
Figure 3
Figure 4
The major requirements of the bacteria are:
- water flow; maximum of 1 litre (1L) of water flow per second (s) per square metre(m2), (or 1L/s/m2) of filter medium;
- nitrogenous nutrient (1mg NH4- N/day/m2 of filter medium at 20°C);
- a slightly alkaline pH (around 8 is preferred);
- oxygen (as high as possible, >70% saturation would be preferred);
- a substrate for the bacteria to grow upon.
Since the biofilter is ‘alive', the recirculating system cannot be turned on and off at will; the biofilter environment must remain stable. The bacterial population take a long time to establish; over what is called the startup period. This startup period is needed again each time after the system has been unloaded, or turned off. Depending upon nutrients, water flow rate and temperature, a biofilter typically takes 40-60 days to establish the required bacterial population.
Startup is usually performed with the biofilter isolated from the other reuse system components. An older method of startup is to gradually stock and feed with a hardy fish species over a period of 3-4 weeks. Alternatively a new filter may be ‘seeded' with substrate from an established filtration system, which contains large populations of bacteria. A newer, and now preferred, method is to feed the filter with chemical ammonia and nitrite which has been shown to reduce startup time by up to a quarter of that for the old methods.
When the system is operating, it is recommended that the biomass (total weight) of crayfish in the purging tank be changed by no more than 3% over a three day period, although supplementary feeding of the bacteria with chemical ammonia can be used to compensate for larger stock changes. Purging systems, therefore, present some unique problems in operating biofilters, since stock is continuously being received and despatched. As well, any group of purging crayfish have a declining ammonia output; over time to almost zero output, once fully purged (purging of the guts of the crayfish takes approximately two-three days).
It is common practice and recommended that the system also have some continuous water exchange (usually around 3 - 10% of the total water volume per day) to remove metabolites and nitrate (the final product of denitrification) which while low in toxicity may encourage undesirable and sometimes harmful bacterial and algal blooms.
At some standard temperatures (10°C, 20°C and 30°C) for biofilter media of crushed gravel 2.5cm diameter [200m2/m3] or bio-balls [403m2/m3], the following surface area and volume of substrate and maximum water flow through the substrate are required per kilogram of crayfish. The values, used in the example below, assume that crayfish produce similar quantities of ammonia to gold fish (approx. 0.25g/Kg/day) although the quantity is often taken to be double this amount to allow for heterotrophic bacteria. So, using a rate of 0.5g/Kg of body weight for ammonia excretion, 1kg of crayfish require, the amount of biofilter substrate shown in Table 1.
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| TABLE 1: AMOUNT OF BIOFILTER SUBSTRATE AND WATER FLOW REQUIRED PER KILOGRAM OF CRAYFISH. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| BIOFILTER SUBSTRATE | ||||||||||
SURFACE AREA (m2) |
VOLUME (m3) |
MAXIMUM FLOW (L/s) |
||||||||
TEMP. (OC) |
(1) |
(2) |
(1) |
(2) |
(1) |
(2) | ||||
10 |
2.0 |
2.0 |
10.0 |
5.0 |
2.0 |
2.0 |
||||
20 |
1.0 |
1.0 |
4.8 |
2.4 |
1.0 |
1.0 |
||||
30 |
0.6 |
0.6 |
3.2 |
1.6 |
0.6 |
0.6 |
||||
| 1 = Crushed Gravel; 2 = Bio-balls | ||||||||||
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The water flow is given as a maximum rate, since the faster the flow the less is the time available for the bacteria to remove the ammonia. Similarly, the slower the flow rate the more time the bacteria have to remove the ammonia. So management of a biofilter requires some balancing to obtain the optimum performance from the system.
Once the size of the filter is known, a bacterial population is required. Using the chemical addition method mentioned earlier a startup over a period of 20 days is recommended. While this method is designed to ensure that a suitable bacterial population is present, the filter will still not necessarily have a bacterial population sufficient to cope with the anticipated loadings, so rates should then be increased and monitored with ammonia and nitrite tests to check filter performance.
The initial rates of addition of nitrogenous chemicals, are 46mg NH4Cl- (ammonium chloride) per litre of water per day, and 73mg NaNO2 (sodium nitrite) per litre of water per day; these chemicals should be added over a 24hr period, ie. dissolved in water and trickled into the system (a constant head siphon is ideal for this purpose). Within a few days of starting up a biofilter, it is beneficial (particularly in sterile waters) to give a kick start, ie. seed the biofilter with bacteria by adding a small quantity of substrate from an operating biofilter.
The biofilter can now be artificially fed until the appropriate loading is reached. The rate of feeding is 0.25g of ammonia for every kilogram of crayfish that will be purged in the system (ie. if the system usually contains 1kg of crayfish but it only contains 0.5kg then it is fed with 0.125g of ammonia as NH4Cl). Also this method can be used throughout the later purging use of this system to compensate for periods of low stocking. If purged crayfish are held for an extended period, after a few days they produce an insignificant quantity of ammonia then the chemical ammonia addition should be increased to compensate, added as a constant drip over a 24hr period.
Ammonia and nitrite levels should continue to be monitored so that any problems (ie. a biofilter crash) can hopefully be noticed at an early stage and the appropriate remedy applied (eg ; flushing with clean water, reducing ammonia input, removing stock).
The biofilter should be designed at the start to cope with all the demands and conditions that will be expected of it, although this can often be difficult. The main point to remember is that even though knowledge is expanding on biofilter operations, it is still more of an art form than a science and consequently there is no substitute for experience when operating these systems.
References
Manthe D.P. and Malone R.F., (1987). Chemical Addition for Accelerated Biological Filter Acclimation in Closed Blue Crab Shedding Systems. Aquacultural Engineering (6) pp 227-236.
Hart, P and O'Sullivan, D. (Eds) Recirculating systems: Design, Construction and Management. Key Centre for Aquaculture Workshop Series. University of Tasmania.
Wheaton. F.W. (1077). Aquacultural Engineering. Wiley-Interscience, New York, USA
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