The purpose of filtration in an outdoor pool is:
- to trap the particles through mechanical filtration;
- To favor the transformation of the organic matter in the assimilable mineral matter by plants and which is nontoxic for the fish, in the form of ions nitrates (NO3-). This transformation requires the presence of 2 types of nitrifying bacteria ( Nitrosomonas and Nitrobacter ) and oxygen which can be provided by the oxygenating plants, by setting water in motion (water jet, waterfall) or by a diffuser of air inside the filter.
Here’s how to make a pond and aquarium filter simple and effective.
Focus on pond filters
The different types of filters
It is possible to make vertical or horizontal filters for an outdoor pool :
- Verticals are the easiest to make, from a plastic bin (see details below).
- The horizontals are conceived from one or more rectangular electric boxes put in series and connected by an overflow. They have the advantage of being flexible. A box of 45 × 45 × 40 cm can filter a volume of 3.5 m³ with fish or 7 m³ without fish.
The filtration circuit
The filtration circuit of a pond does not have a bottom drain. The pump dipped into the pond is connected to the filter placed above the bank, which then discharges the water into the basin by gravity.
Example of a multi-compartment horizontal filter
The idea is to offer a minimum of 3 compartments:
- A settling tank allows the deposit of sludge and large waste at the bottom (some models replace this compartment by brushes or by a vortex spiraling up the water, causing the sedimentation of large particles). Provide a drain valve to facilitate the cleaning of this compartment.
- The overflow of this first filter then passes through 3 or 4 compartments containing granulates of finer and finer granulometry (20 to 4 mm) made of pozzolana, clay balls, quartzites, basalts, silicas or other rocks up to fine sands (2 to 0.8 mm), or synthetic foams of increasingly fine porosity. Nitrobacter and Nitrosomonas bacteria, responsible for biological filtration, need to be reintroduced into these filters each spring if the filtration has been interrupted during the winter.
- A third compartment is used to store the filtered water before it is discharged into the pond to carry out control or simply empty the filter.
Many materials enlarge the attachment surface of bacteria and increase the oxygenation of the water, making biological filtration more efficient, such as synthetic foams (Japanese carpets), bio-balls (floating and creating motion) clay or pozzolana. The addition of an aquarium air diffuser in this compartment can also be considered.
Note: zeolite is clay that absorbs ammonium ions and toxic heavy metals and releases sodium, calcium, potassium, and magnesium ions in exchange. You can regenerate the zeolite by immersing it in salted water (100 g of salt / l of water for 24 hours). Activated carbon also fixes ammonium, which reduces the role of nitrifying bacteria. Phosphate absorbers can also be added if the amount of organic waste is important (koi pond).
Good to know: the maturation of biological filtration requires 4 to 6 weeks. Also, it is gradually that you introduce your fish after the creation of a pond or you will resume feeding in the spring to limit organic pollution. The speed of passage of water in the filter acts on its efficiency.
Make a vertical pond filter
With a bell drill, drill a plastic bin with 2-hole cover:
- one at the bottom for the water supply, 32 mm;
- the second in the upper part of the trash, with a diameter a little larger for the output.
Note: the size of the bin, the position of the filter, the size of the inlet and outlet holes are to be modulated according to the power of the pump, the amount of water to be filtered, its degree of fouling …
Good to know: you can also place the water inlet at the top and insert a T-tube pierced with small holes at its base which will allow the rise of the water and its output at 3/4 of its height as on the scheme.
- Attach a wall-pass to these holes and connect the pipes that allow the water to enter and return.
- Glue and silicone the pipes to seal them.
- Fill the bin with a layer of lava rock or clay balls.
- Overlay a coarse cell foam and a thin cell foam about 5 cm thick in the direction of water flow.
- Introduce nitrifying bacteria sold in pet stores.
- Place the filter above the water level, on the bank, and possibly connect it to a waterfall.
- Run the pump, checking that the flow is correct.
- You can add a UV lamp to the filter to limit micro-algae.
Good to know: Filtration can be interrupted when the fish have stopped feeding or when the water drops to 8 or 10 ° C. Some filters, however, allow a slower circulation that you can continue to operate during the winter. If not, empty them of their contents, clean them thoroughly, disinfect them, and leave them in place. In spring, replace the starter bacteria provided for this purpose with zeolite, if necessary.
Make a swirl filter
A vortex filter can be installed upstream to facilitate settling of larger particles:
- Get a 25-liter drum with a lid.
- Drill a drain hole at the bottom and insert a drain cock.
- Drill a 32 mm hole at mid-height and add a wall pass.
- Forcibly insert a piece of 32 mm pipe bent inside to direct the jet towards the wall of the barrel.
- Glue and silicone the pipe to the wall-pass.
- Drill an exit hole 35 mm in diameter at 3/4 of the height and add a wall pass.
- Insert an angled pipe, facing the top of the barrel so that it collects purified surface water.
- Direct the output of this filter to the filter made in step 1.