Build your own vegetable filter

You want to incorporate a veggy filter into your present system and or your pool is bigger than 2 cubic metres. 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. You can see the old style header tank that Oase use for the Filto-Fall, many companies have used these for biological filtration and they come in a variety of sizes. For use as a vegetable filter, the larger the better. If you are starting from scratch with a basic header style tank and some plastic sheet and fittings, notice how the biological filter manufacturers fit their inlet and outlet fittings and also how the interior divisions are put together.

Your main aim is to try to circulate half the volume of the pool 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 pool. 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 just a little bit makes a big difference; just enough to tip the scales.

Options: small to medium pools

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 pool would be perfect. However any waterproof container can be adapted.

Starting from Scratch.

Take the waterproof container and cut out profile of the inside of the tank. Then using silicone or glue gun fix it into place so that a compartment 10cm wide at one end of the container. This will be the settlement chamber. There needs to be a 5cm gap between the bottom of the divider and the base of the tank.

Two 10cm wide strips of plastic are glued into place on the wall of the tank to support the divider.

A filter medium support plate is made from a sheet of plastic drilled with holes. This should have a curved profile on the corners of one end to fit the shape of the box and square at the other end to fit tightly against new divider.

The drilled filter-medium support plate should have angled strips of plastic 5cm wide glued to it. These will help support the plate that holds the filter medium above the base of filter tank.

The tank is drilled using a hole-borer with 40mm hole as an outlet at the away from the settlement chamber with the outside edge of the hole 50mm below the top edge of the tank.

A 25mm hole for the inlet hose tail is drilled in the side of the settlement chamber with the outside edge 25mm below the top edge of the tank.

Fit a flanged water pipe fitting to the outlet and a water butt style fitting to the inlet.

With the filter in position and the fittings and support plate in place fill the filter compartment with coarse inert gravel to 10cm short of the outlet. It is topped up with gravel to just below the outlet.

In this type of filter a basic tank is divided into a settlement chamber and a filtration chamber. One is divided from the other by a sheet of stiff plastic silicon glued into place. This is reinforced by other pieces of plastic forming divisions in the settlement chamber. These would normally be used to support the filter brushes if you want to use them.

The filtration chamber is filled to the outlet spout with a filter media. This is supported from the bottom by another sheet of plastic peppered with holes that allows the water moving in from the settlement chamber to pass through it up into the filter medium. This sheet of plastic, which could be a stiff mesh of some sort, is supported from the bottom by angled strips of plastic silicon glued into place.

The whole unit can be set in the ground, and thus disguised, blend in as part of the backdrop of plants around the pool. Alternatively it can be incorporated in the feed to a waterfall as I did with this Filtofall.

The tank is placed in position and the shape the tank is marked on the ground. Remove the tank and use the markings as a guide for digging a hole to a depth at which the container protrudes about 10cm from the ground.

The pipework for the water supply from the pump is set in the ground and leading to the inlet to the filter.

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. This Oase Filto-Fall sits at the top of a waterfall.

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 waterproofing 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 pool, 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 pool or the next filter bed.