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

see part 1 of this pond filter article here

Build your own vegetable pond filter

Larger image to show how to build a vegetable pond filter ... click to read pond filter article

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

• Take a waterproof container (a domestic header-tank is perfect) and glue with silicone or hot glue gun a profile that will fit inside to make small compartment 10cm wide that will be the settlement chamber. There needs to be a 5cm gap between the bottom of the divider and the bottom of the tank.10cm wide strips of plastic glued into place between the profile and the inside wall of the container act as braces and supports.
• A filter medium support plate is made from a sheet of plastic drilled with holes.
  Glue 5cm wide strips of angled strips of plastic on the underside of the filter medium support sheet. These will help support the sheet from sagging when the weight of the filter medium is pressing down on it.
• The container itself is drilled with a ‘hole-borer’ attachment to an electric drill. A 40cm hole is made in the front to take the 40mm waste pipe fitting that will make up the outlet. Another 25mm at one end of the new small ‘settlement chamber’ will take the standard 25mm hosetail fitting that will be the inlet.
• With the filter in position and the fittings and support plate in place, the main filter chamber is filled with coarse inert (not limestone) gravel or even better Alfagrog to 10cm short of the outlet. This is topped of with smaller gravel to just below the outlet.
  The same procedure would be adequate for a DIY biological filter except that the medium ought to be an Alfagrog or hortag.

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 tools you would need if you were starting from ‘scratch’ with a basic plastic or fibreglass tank:
• A glue gun or a silicon glue and dispenser.
• A power drill with hole borer attachment for the outlet pipe, which should be a minimum of 40mm.
• You will also require a holeborer of the size for the inlet hole, which should be adequate for the hosetail, which in turn should fit the tubing from the pump.
• Timewise, even constructing from scratch, the large header tank method of building will take less than an hour to build and probably 2 hours to dig in and fit up.

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.

MAINTENANCE?

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.

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