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3 Key Design Factors for Bog Filtration

Pond builders agree on requirements for effective bog construction.

Pond designers and builders agree that the goal of bog filtration, as with any filtration system, is clean, healthy water. Whether including it as the primary or supplemental filtration system on a new or remodeled pond, they said three key design factors remain the same.

pond bog filtration
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1. Make the surface area proportionate to the pond.

In this case, surface area refers to the rock, gravel and plant roots needed for medial bacteria to grow, trap and break down organic wasted and convert it into plant food. Most experts said they use a 10 percent to 20 percent ratio. For example, 20 percent of a 10-foot-by-10-foot (100 square feet) pond would yield a 4-foot-by-5-foot bog about 12 inches deep.

Builders determine the big depth and size by the pond’s filtration needs. Some experts, including Jay Bearfield, owner of Liquid Landscape Designs in Carlisle, Mass., oversize the bog to about one-third to one-half the size of the pond. "I find over-sizing the system allows for the future stocking rates of fish and flora without the concern of a system being overly taxed, although it still can happen," he said.

Sizing the bog proportionate to the pond proves crucial because the bog must be able to timely and efficiently turn over the entire pond volume to maintain clean, healthy water. The turnover rate of once per hour is minimum. Pond builders consider two to three times per hour optimum, depend­ing on the size of the pond.

"If a pond can be turned over two to three times per hour it's going to be a very healthy, oxygen­rich environment for plants, fish and all aquatic life," said Dave Jones, owner of The Pond Professional in Woodstock, Ga. "If water passes through too quickly, then the bacteria don't have a chance to bio-remediate the organics, which can come back into the pond. If the water passes through too slowly you run the risk of stagnation pockets."

2 & 3. Proper pump size and plumbing facilitate turnover.

Pump size varies according to the volume of pond water turned over one to three times per hour, said Peter Stopen, with Pondview Landscaping Inc. in Brick, N.J. The plumbing acts as the conduit system that transports and diffuses water into the bog system. The diffuser is the slotted pipe on the bottom of the bog from where the water flows up. Connected to the diffuser is a clean-out pipe rising to the top of the bog, Stopen said.

Builders can custom-build the diffusing system and clean-out vault to fit the bog space, or they can use prefabricated components. Regardless of preference, Bearfield said the system should diffuse the water evenly throughout the bog to use the greatest surface area of the media without developing stagnation pockets. It also should create a low point in the system plumbing with a catch that handles the settlement of heavier organic materials that become suspended in the water, he said. Easy clean-out and maintenance increases the pond and bog life spans and reduces the need for dredging.

The water flows up through layers of rock or gravel, but plant roots also are part of the filter system. "For a pond in full sun to have the nutrient load handled to maintain the sometimes difficult algae balance, placement and exposure are important factors," Bearfield said. Because more plants are available for full sun exposure than shade, he said, bog placement in a full shade environment might not allow for a correct plant load to harness the nutrients before algae can.

When customers want a water feature that incorporates bog filtration, size enters into the equation because the bog has a footprint. The customer does not necessarily need a large yard, however, to accommodate a pond and bog.

Chuck Beer, owner of The Water Shed in Vancouver, Wash., features a bog system using two whiskey barrel halves and a small fish tank at his shop. Water from a small fish tank housing baby koi is pumped to a whiskey barrel half located on a terrace above the tank.

The barrel holds plastic liner and PVC piping. Beer laid the PVC piping in a grid, covered it with pea gravel and planted Creeping Jenny.

Water spills from a spout in the lip of the whiskey barrel half down to the second half of the whiskey barrel on a lower terrace. Then it spills over that barrel back down into the small fish tank.

Beer located the setup just off the shop entrance. He said it is small enough to fit on a patio.

Despite the small size, Beer's whisky barrel setup still incorporates all three key design factors of surface area proportionate to the pond (fish tank), proper pump size and plumbing. There is no set size for a bog if used as a standalone feature, Stopen said.

Experts agree that common mistakes made in bog filtration construction include improper sizing of the bog and pump, improper installation and omission of key components. Stopen added that not pruning the dead from bog plants often is overlooked and adds excess nutrients into the water.

"You have unhappy customers when things aren't sized properly," Jones said. "It boils down to understanding the basics of pond design that meet or exceed the customer's goals."

Builders need think of the pond, bog filtration and all of its parts as a whole unit of integrated systems that must work together to create a clean, healthy environment, Jones said. If one piece is missing or incorrect, then the whole system is out of balance.

"We want a pond to be healthier 5 years after it's built than the first year it was built," Jones said. "Too many people without the knowledge put in ponds that are designed to fail. You must design it so that it can be healthy and function properly."

By Ramona D. Marek