How to construct a vegetable pond filter
This is how a vegetable filter could look when finished and planted. It shows the basic principles of a vegetable or mini reed-bed in action
Build your own vegetable pond filter
If you want to incorporate a ‘veggy’ filter into your present system and your pond is bigger than 2 cubic metres then you could try building one.
If you had an old filter tank, it may cost you nothing, or at least just the cost of a few bags of inert gravel and some plants. An old Blagdon style filter tank. These came in a variety of sizes; for a vegetable filter, the larger the better. For starting from scratch, notice the inlet and outlet fittings and also how the interior divisions are put together.
The parameters you are working with are to try to circulate half the volume of the pond water through the system every hour.
Also, in order to give the water a thorough treatment, the surface area of the system wants to be approximately one third that of the pond.
This is an ideal that is out of the question for many people and since a min-reed bed system is very often an add-on to a biological filter system, it may often be the case that not only is ‘anything is better than nothing, but it proves to be quite adequate’.
Options Small to medium ponds
Any large container or series of containers that will hold water and to which you can fit outlet pipes to will do. A large old filter of the basic Blagdon type where water feeds in a the top of the tank, goes down through some filter brushes and up through filter medium to flow out back into the pond would be perfect. However any waterproof container can be adapted.
Starting from Scratch
A larger system could also be devised from a series of filter boxes or domestic loft header-tanks.
Alternatively, you build filter chambers out of concrete blocks laid on a 10cm concrete slab, rendered on the inside with fibre-reinforced cement and sealed with a tanking slurry like Vandex. These would be simply a tank, or even a series of tanks built one above the other, with the water pumped up from the pond, flowing from top to bottom.
At its simplest, the supply of water comes in from a series of pipes distributed along the back edge of the top tank, and would trickle down though a planting of reed in inert gravel (15-30mm). I would prefer the ‘trickle up’ solution to the ‘trickle down’ method, but the former method would require considerably more engineering, by having to pipe the water from a collection or settlement chamber, into and below the planted chamber in order for it to rise up and overflow into the pond or the next filter bed.
WHEN CAN I EXPECT RESULTS?
The system will start mechanically filtering straight away, but allow at least a month in the growing season for all the plants and the micro-flora to get established. Even when there is very little plant growth in the winter, there should be a certain amount of biological filtration from the bacteria in the gravel. Switch it off in very cold weather; you will just have to hope that everything holds its breath. The algae do tend to find a ‘window of opportunity’ briefly in the spring since they can grow profusely at much lower temperatures than many of the ‘higher plants’, but once summer is in full swing the algae haze is soon dealt with.
COST OF POND FILTER LIKE THIS?
It is worth experimenting with what you might have lying around. If you already have a multi-chamber biological filtration system, it may be just enough to plant up the last chamber in the series of tanks. If you only have a submersible pump supplying water to a waterfall, then the main expense is over.
Incorporating a vegetable filter or mini-reed bed at the top can just mean the expense of a header tank, a sheet of HDP or ABS plastic and two cheap plumbers fittings – maximum £30. As for the plants they are the cheapest. If you live near rhymes, dykes or waterways that are constantly being dredged on a rotational basis, then common reed, scraped out by the ton, can be rescued by the handful. Gravel is generally less than £2 a bag if you choose the cheapest inert ‘cracker grit’.
TOOLS TO BUILD POND FILTER
THE NUMBER ONE TIP FOR GETTING IT RIGHT
Let them at it ... the nutrients that is! Which means above all, get the right plants for the job - the common reed. A decent quantity planted 15cm apart, being fed water from the pond 24 hours a day, 7days a week.
Just cut the plants back at the end of the year (more if it is Cress or such like). If there is a drain plug accessible on the tank, backwash on an annual basis. Otherwise dig out, divide the plants and replace the gravel every 4 to 7 years.
Installing the Oase Filtofall.
The tank is placed in position and the shape is cut around the shape and a hole is dug to a depth at which the container protrudes about 10cm from the ground. A little sand is put in the excavation to help with the final levelling of the unit. The unit is set in level with a spirit level.
With the unit in place, the protruding edge is disguised with rocks and soil. Aquatic baskets can be used in your filter container, or the whole thing can be filled with rough gravel on top of the base plate. This Oase Filto-Fall has a custom made basket that is placed in the tank.
The Rough gravel, 32-25mm, is poured in up to about 10cm below the top edge and levelled out.
On top of this, smaller gravel (or in my case an expanded clay filter medium) is poured in on top level with the top edge in the Oase Filto-Fall but below the outlet on you homemade version. Norfolk Reed (Phragmites australis) is divided up and planted in the top medium.
Switch on and keep running 24/7.
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