Bog Filtration
Mark Paulsen, Pond Place, Hoquiam, WA

A bog is an area, which consistently has a water level several inches below the "soil" surface. In nature, bog soils are usually very high in organic materials, and low in pH. Traditionally, using semi-permeable liners, ornamental bog gardens have imitated these conditions in order to grow plants (such as pitcher plants) which have adapted to these areas.  These are known as anaerobic bog gardens. For filtration purposes, bog gardens are constructed using waterproof pond liner (such as Firestone Pondguard 45 mil EPDM) and pea gravel. They may utilize a wide range of marginal pond plants, bog plants, and even many standard garden plants. Water is moved through the bog in a trickle fashion, making the bog more aerobic

These moving water bog gardens have two primary functions:

1) Beauty, The bog filter provides the perfect background to the pond, showcasing plants, while allowing more pond surface area to remain open. It also provides a natural looking transition from land to pond.

2) Filtration, Bog gardens are the ultimate pond filter for both water purity, clarity, and low maintenance. Unlike man made filters, bogs can completely process organic waste, including solid waste, and will even process inorganic waste, such as minerals.

Man made filters merely mechanically trap debris, and biologically detoxify waste. Waste by-products still build up in the water and create various problems, including a build-up of nitrates and phosphates, which promote algae growth. In a bog filter, gravel traps organic debris until it can be broken down by bacteria and used by the plants. All by-products are processed and used as food by the microbes and plants. Nature can balance itself, given the opportunity.

Bog Filter Construction:

1) The size of bog filter required will vary according to the organic load. For most purposes, a bog filter whose surface area is 30% of the surface area of the main pond will suffice, with up to 100% for heavy loads. (duck and geese ponds)

2) Excavate an area next to the main pond to a depth of at least 10". Build a retaining wall of concrete or concrete blocks to separate this area from the main pond, or mound soils up in the form of a burm.  The top of the wall should be 1-2" below the pond surface when filled. Remember to make the liner for the bog filter level, and to allow extra liner for the sides of the retaining wall. Use felt protection mat and/or scrap carpet to protect the liner from rough or sharp edges.

3) An alternate method is to build the bog filter as an extra-wide shelf, place the liner, and build the retaining wall on top of the liner, using rock if desired. Fill in any large cracks which would "leak out" gravel, using mortar or black expanding foam. Do not fill in cracks, which are shallower than 2" below pond surface.

4) Place liner in pond and bog filter, following vendor's liner installation instructions

5)  Install the pump in the main pond as far away from the bog filter as possible, using a pump which will circulate the pond volume at least once every three hours for pond under 4,000 gallons, and at least once every four hours for ponds over 4,000 gallons. Use over-sized pipe or tubing to maximize flow to the bog. Screen the pump with 1/8-inch mesh/ baskets in some fashion to prevent clogging. A full size foam pre-filter is unnecessary.

6) Use PVC pipe to build a distribution manifold the length of the bog. The pipe should be very over-sized (for example, if the pump has a 1" outlet, use a 2" pipe). Drill 1/4" holes about 3" apart, in a line about halfway between the side and bottom of the pipe. Cap the far end of the pipe. Lay it on the bottom of the bog as far from the main pond as possible. Bogs wider than three feet benefit from multiple lines. The outflow screens do two functions:
            A) Prevent pea gravel from being siphoned back into your pump.
            B) Diffuse the water into different areas of the bog.

7) Place decorative (usually flat) rock across the top of the retaining wall/allowing small cracks between and under rocks for water flow. Water will be pumped from the main pond into the bog filter, through the gravel, and flow back into the pond over the retaining wall through cracks between rocks.

8) Fill bog filter with pea gravel, mounding the gravel so it is about 3" above water level for most of the bog. Mix larger pieces of river rock into the top layer of gravel for a more natural look. Incorporate some of the rock around the pond and landscape in the bog filter to visually tie the landscaping together. Feel free to use some large pieces of wood, as well.

9) Plant the bog filter, using any moisture loving plants (now may be the time to experiment with Hydroponics tomatoes!). To a certain extent, your choices may be dictated by your climate, unless you are willing to sacrifice or repot tender species. Use tall, decorative marginals, such as Gunnera and Thalia, as focal points. Thalia, Iris pseudacorus. Iris versicolor, hostas, and day lilies are among those which thrive, and their fibrous roots provide excellent colonization sites for beneficial microbes in both summer and winter. Other tall marginals include cattails, pickerel, various aquatic and bog iris, lizard's tail, ribbon grass, various rushes, and horsetail. Use short ground-cover plants around the bog edge. Shade loving marginal plants, such as marsh marigold, aquatic forget-me-not, and yellow monkey flower can find homes in a bog filter. Other excellent fast growing ground covers to consider are creeping Jenny, Water Cress (edible!) variegated or green water celery (edible!), and aquatic mint (edible!). These fast growers will provide the bulk of the nutrient absorption, and will soften the transition between pond and land; in fact, they will probably grow out into the soil bordering the bog and fill in between the taller marginal plants. When planting the bog filter, wash soil away from the plant's root ball. Plant the entire root ball in the gravel. A small amount of soil will not harm the pond or filter, and will help the plant adjust to it's new home. To achieve a full effect, use up to one plant per square foot of bog filter surface, or as little as one plant per three square feet and let time do the rest. Fewer plants may mean reduced filtration until they fill in.

10) For faster cycling of nutrients, inoculate the pond with a pond bacterial culture. This is a good idea with a new pond, or if the pond accumulates organic debris quickly.

11) Little maintenance is required, other than harvesting excess plant material and some trimming of dead foliage in the fall. Trim and divide plants as needed. When plants are overcrowded, their growth slows, and they are not pulling as many nutrients from the water. Ground-cover plants grow especially quick.  Pulling plants out of gravel is quick and easy, as is trimming. Don't feel bad about harvesting them, as they make great compost and your garden is a better place for the nutrients than your pond. People and Koi will enjoy watercress, as a snack.  

12) With bacterial inoculation, sediment will build up very slowly, if at all. Every year or two, a partial cleaning of the gravel may be required, but probably only in areas where large amounts of inorganic debris are deposited (such as where wind blows dust in). The smaller the bog the more often it needs to have the deposits removed.

Note: Many installers use one pump to operate a waterfall/stream bed, and a 2nd pump to operate the bog filter. One pump may be used for both. A way to do this is to have the waterfall/stream bed empty into the bog filter or the bog empty into the waterfall.

There is a benefit in having a waterfall prior to the bog, Oxygen. The aerobic microbes, exposed to higher levels of dissolved Oxygen allow them to maintain high levels of population and activity. There can be many variations to these basic designs. Existing ponds may be retrofitted with a bog filter by making the bog slightly higher than the main pond, and connecting the two with a streambed; they don't even need to be that close to one another. Bog filters work incredibly well in promoting crystal clear water, exerting a high degree of control over algae, by removing the nutrients produced by the fish, birds and other creatures visiting your pond.
Savvy pond keepers use and enjoy bog for their beauty and simplicity.

Mark Paulsen, Pond Place, Hoquiam, WA                       Home