Bog filters, sometimes called veggie or vegetable filters, remove organic and inorganic waste from pond water. A pump forces water through PVC pipe buried in pea gravel. The pea gravel traps the waste and the bog plants planted in the gravel use the waste as food, removing it from the filter. Bog plants are marginal plants that enjoy wet roots but don't like being submerged in water. Cattails, ribbon grass and sweetflag are all examples of bog plants. Bog filters also provide habitat for frogs and toads and food for birds. While somewhat challenging to build, an established bog filter is a beautiful site.
Dig a hole for your bog filter. It should be 12 inches deep (or deep enough that the bottom of the bog filter is 1 to 2 inches below the surface of the pond) and as wide as your pond. Make it long enough to accommodate the size filter your pond requires. Your bog filter should be 10 per cent of the size of the surface area of the pond. If your pond is heavily stocked, make your filter at least 20 per cent of the size of the surface area of the pond. Always make your bog filter larger than the size you think you will need.
Install the plastic lining according to manufacturer's directions. Use decorative stones to secure and hide the edges of the lining.
Drill 1/4-inch holes every 6 to 8 inches on one side of the pipe. This is the distribution pipe for the bog filter.
Center the distribution pipe in the middle of the bog filter and lay it so it runs the length of the filter. Adjust the pipe so the drilled holes are at a 45-degree angle to the bottom of the bog filter. One distribution pipe can handle a 5-foot wide bog filter. If you have a wider bog filter, use one distribution pipe for every 5 feet and space them 3 to 5 feet apart.
Attach the piece of PVC pipe with the female adaptor to the distribution pipe using PVC pipe glue or any waterproof glue. The female adaptor should be on the pond side of the drystone wall and should protrude 1 to 2 inches into the pond.
Glue the end cap onto the PVC distribution pipe. The cap forces water to flow out of the holes rather than the end.
Build a drystone wall with the concrete blocks or brick to separate the bog filter from the pond. Use brick or small pieces of rock to build an opening for the PVC pipe and female adaptor. Don't make the drystone wall watertight as the cleaned water from the bog filter will recirculate back into the pond. Cap the top of the drystone wall with decorative rocks. The drystone wall should be several inches higher than the surface of the pond.
Place the pond pump on the opposite side of the pond from the bog filter.
Attach the pond pump to the female adaptor using the flex hose.
Hose down the pea gravel washing as much dust and dirt from the gravel as possible.
Fill the bog filter halfway with the 3/4-inch pea gravel.
Remove your bog plants from their pots and arrange them in the filter.
Fill the bog filter with the rest of the 3/4-inch pea gravel. Be careful not to damage your bog plants. The gravel should be 1/4 to 1/2 inch higher than the level of the water in the filter. You may have to wait several days and add more gravel to get the proper height.
Paint the concrete blocks or bricks with black or brown latex paint to make them less noticeable. Scatter the same decorative rocks you used to secure the plastic lining and cap the drystone wall throughout the bog filter to tie everything together. You can partially bury the stones in the pea gravel or simply place them on top of the gravel. Keep the surface of your bog filter clear of dead leaves, branches and other large organic debris. If you don't like the look of the drystone wall,plant reeds or cattails in front of it to hide the concrete. Be sure you don't block the PVC pipe.
Don't wash the soil from the roots of your bog plants; the roots may be damaged if you do. Keep the holes on your distribution pipe at a 45-degree angle. This angle allows water to leave the holes but keeps the holes from becoming clogged with debris.