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The biofilter is the heart of every recirculating aquaculture system. Fish health, and therefore economic success, depend on correct operation of the biofilter. High ammonia and nitrite levels in fish tanks can be caused by several factors. One of these can be poorly designed or sub-optimal operation of the biofilter (too small, not mixed evenly, nitrate levels too high, pH too low, intoxication of the biofilter by salt or medical treatment, aeration too low or too high, etc.). The other main aspect of design failure is insufficient recirculation of the water. The biofilter can only degrade what it receives from the fish tank. If the recirculation rate is too low, even an over dimensioned biofilter will not lead to good water quality. To avoid this, follow the example in Chapter 2 to calculate the correct recirculation rate for your system.
In systems with low fish stocking density, a media growing bed can take over the role of both solids removal and biofiltration. If the solids load is too high, clogging and anaerobic areas can occur, which reduce the efficiency of biofiltration. Therefore, if the growing bed is to function as a biofilter, either a very low fish stocking or a separate solids removal device are recommended.
The most commonly used biofilter type in aquaponics and in RAS is the moving bed biofilter reactor (MBBR) (Figure 13, Table 6). The media of a moving bed filter consists of small (1-2 cm) plastic structures with high specific surface area (e.g. Kaldness k1). This filter media is kept in constant movement by aeration (e.g. through input of air through air-plates at the bottom of the biofilter tank). The constant movement of the media has a self-cleaning effect on the filter media and prevents extensive bacteria growth. For cleaning the moving bed filter should be disconnected from the RAS and then backwashed approximately once per week.
The carrier media supports microbial biofilm growth by providing a large surface area. Typically, MBBR are filled 40-60% with biocarriers, creating an absolute surface area of 300-600 m2/m3 bioreactor volume. Air movement creates shear forces on the biofilms and keeps growth and breakdown of the biofilm in equilibrium. If the biofilm on the carriers gets too thick, then aeration is too low, and if it is non-existent, then aeration is too high. A major advantage of MBBR is the degassing and aeration by air flow, which is not provided by fixed bed filters.
Fixed bed filters have fixed biofilter media. The fixed bed filter also works as a solids removal device as it has filtration capabilities to filter out leftover solids and organic compounds that have not been filtered out in the solids separation unit. If the organic loading is higher than the natural degradation on the surface, the filter cake can become clogged by particles and bacteria growth. The filter needs to be backwashed regularly and the backwash water treated separately (by sedimentation etc.). (Table 6).
Trickling filters are the last of the three common filter types and work by trickling water through a pile of biofilm carriers. The biggest benefit of the trickling filter is the high degassing effect through the high water to air surface caused through the trickling. The main disadvantage are the high pumping costs needed to bring the water to the required height. Since these carriers are not moved regularly like in a MBBR, the biofilm grows thicker on these carriers and reduces the nitrification rate. Trickling filters are very common in aquaponics, since they enable gas exchange (degassing of CO2 and aeration) in the one step. In addition, they only need water circulation and no additional aeration device like MBBR (e.g. a blower), which makes them a very easy to build system.
Figure 13: Two versions of suboptimal moving media biofilters: (left) biofilter containing too many biochips (photo R. Bolt); (right) biofilter with no aeration (photo: U. Strniša)
Table 6: Types of biofilters and their pros and cons in terms of system performance: moving bed biofilm reactor (MBBR), fixed bed filter and trickling filter
Biofilter type Basic construction Pros and cons Moving bed biofilm reactor (MBBR Nitrification ++ Filtration - Degassing + Fixed bed filter Nitrification + Filtration + Degassing - Trickling filter Nitrification +Filtration -Degassing ++ (if aerated)-
The fish tank(s), biofilter and grow bed(s) all need appropriate aeration. There are many ways to provide this, including using airlift pumps, water sprays, paddlewheels, rotors, blowers, and compressors. As with water pumping, water aeration needs to be reliable and energy-efficient. Aeration in smaller systems can be provided by using an energy-efficient and long-lasting air pump and food-grade vinyl tubing connected to airstones placed at or near the bottom of the tanks and grow beds. Air pumps are generally not large enough for aerating larger systems, which tend to use a regenerative blower or an oxygen generator.
In aquaponics, air pumps and air stones are used to force air into the water to provide plant roots and fish with oxygen. Air pumps are widely available in a range of sizes, from very small up to very large with a capacity to run from one to many airstones, each of which introduces hundreds of tiny bubbles of fresh, oxygen-rich air into the solution. While it is easier to push air out of an airstone that is in shallow water, you do not get as much oxygen into the water as you do if the airstone is deeper. When the airstone is deeper the large number of bubbles that come out are smaller because of the higher water pressure, which together have a greater surface area than fewer larger bubbles, and they have to travel further to the surface, with the surrounding water absorbing oxygen from the bubbles all the way to the top of the tank where they burst at the surface.
The basic oxygenation technologies are the U-pipe, oxygenation cone, and low head oxygenator (Figures 14-16, Table 7).
Table 7: Characteristics of different possibilities of high efficiency oxygen enrichment in RAS
U-PipeConeLHOPrinciplePressure increase due to water column, long contact path between water and gasPump overpressure. Widening cross section keeps bubbles in suspensionOverpressure by means of water column, large contact surface between water and gasPressure lossNoHigh(2-3 m, 0.2-0.3 bar)Medium(ca. 1m, 0.1 bar)EfficiencyHighHighMedium
One simple oxygenation technology to dissolve oxygen into the system water is the U-pipe (Figure 14). Oxygen is injected at the bottom of a 10-30 m deep pipe through which the system water flows. Due to the high hydraulic head, the high pressure leads to high dissolution of the oxygen into the water column. However, as this technique requires structures to be built deep into the ground, the method is often not implementable in practice.
Figure 14: U-pipe
An oxygenation cone (Figure 15) uses the same principle as a U-pipe. The difference is that the high hydraulic pressure is induced by a pump (which uses a lot of energy). This technology is especially suited to cover peaks in oxygen demand, and it has a high efficiency in terms of oxygen dissolution.
The low head oxygenator (LHO) (Figure 16) uses another method of oxygen enrichment. Water flows through a perforated plate and causes a high water to gas surface area in the mixing chamber below. LHOs operate very economically, although they cannot achieve oxygen concentrations as high as cones can.
Figure 16: Low head oxygenator
Figure 17 and Table 8 show different possibilities for low efficiency oxygen enrichment.
Figure 17: Different possibilities of low efficiency oxygen enrichment in aquaculture
Table 8: Characteristics of different possibilities for low efficiency oxygen enrichment in RAS
Fine-bubble oxygen entrainment or loadingCoarse-bubble oxygenCoarse-bubble compressed airApplicationMany fine bubbles that rise slowly and have a high surface to volume ratioHigh concentration gradient (because it is pure oxygen). Most of the time used for emergency oxygenationDoes not need pure oxygen but has a low efficiency because air contains only 21% oxygen. The rest is N2 etc. Can lead to oversaturation with N2Pressure loss1.5 barBeginning from 300 mbar + water columnBeginning from 300 mbar + water columnEfficiencyMedium (up to 20%);with high water column up to 100% at approx. 5-10 mLow (5%)Very low (1% of total volume)
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