6 September 1996

Pond system turns dirty water into clean habitat

Pollution control rules are only just beginning to bite in Canada. In this report on dairying in the Quebec province Jessica Buss details a novel system used to purify water

CIRCULATION wash water containing chemicals is being treated using a series of five ponds planted with cats-tails, bulrushes and wet meadow grasses on one Canadian dairy unit.

Mylene Daoust, Argus environmental consultant, Sainte-Foy, Quebec is coordinating this Env-ironment Canada funded project.

The system has been set up at Louis and Clement Turmels 80-cow unit at Sainte-Marie-de-Beauce. "This is an efficient way to treat what is left in the water but it would need a large area to treat all slurry," says Ms Daoust. Polluted dairy water used to be discharged into the farm ditch.

The system can cope with 0.5 cu m of liquid a day within the 0.7ha (1.7-acre) site. Filtered water from the final pond will overflow into the ditch.

"The first pond is a settlement tank that takes off a great proportion of the pollutants, but not enough," she says. From this the water goes into a second, wet meadow, pond. This comprises a dense crop of cat-tails and bulrushes that filter the water. The water level is a few cm deep but may be increased to 30cm (12in) deep.

"These plants accumulate some nitrogen and phosphates in their tissues," she says. "But their principal role is to transport oxygen into their stems and create an environment for aerobic and anaerobic organisms. These micro-organisms work to remove the pollutants using oxygen transported by the plant."

The third pond is smaller than the second but contains the same type of plants. These two ponds have been separated for the project to assess if the first area is sufficient for treatment or if a larger area is needed.

Water flows into each of the ponds through small holes in a supply pipe at one end of the pond. From the far end it then flows to the next pond.

The last pond contains a variety of plant species creating a wildlife habitat. It is up to 1.2m (3.9ft) deep to keep a free water surface in the centre, for plants will not grow in water this deep, she says. This pond overflows into the stream that once took the untreated water.

For practical on-farm use one pond comprising a sequence of plant species would be sufficient.

&#8226 Settlement pond.

&#8226 Water meadow with bulrushes and cats-tails.

&#8226 Pond with wet meadow grasses.

&#8226 Wildlife habitat pond.


The final deep pond contains a variety of plants and creates a wildlife habitat.