High Performance Constructed Wetlands for Cold Climates

Abstract

In 1991, the first subsurface flow constructed wetland for treatment of domestic wastewater was built in Norway. Today, this method is rapidly becoming a popular method for wastewater treatment in rural Norway. This is due to excellent performance even during winter and low maintenance. The systems can be constructed regardless of site conditions. The Norwegian concept for small constructed wetlands is based on the use of a septic tank followed by an aerobic vertical down-flow biofilter succeeded by a subsurface horizontal-flow constructed wetland. The aerobic biofilter, prior to the subsurface flow stage, is essential to remove BOD and achieve nitrification in a climate where the plants are dormant during the cold season. When designed according to present guidelines a consistent P-removal of > 90% can be expected for 15 years using natural iron or calcium rich sand or a new manufactured lightweight aggregate with P-sorption capacities, which exceeds most natural media. When the media is saturated with P it can be used as soil conditioner and P-fertilizer. Nitrogen removal in the range of 40–60% is achieved. Removal of indicator bacteria is high and < 1000 thermotolerant coliforms/100 ml is normally achieved.

Key Words:

• Constructed wetlands
• Cold climate
• Design
• Pre-treatment biofilters
• Treatment performance
• Seasonal dependence.

ACKNOWLEDGMENT

The research reported here was partly funded by European Commission under contract no. EVK1-CT 2000-00065 EU 5th FP RTD PRIMROSE).

Notes

*Graywater systems.

^a Based on arithmetic mean of concentrations, n = 5–50grab samples.

^b No. of persons (p) served in the parenthesis.

^c 7-day BOD is standard in Norway.

^d Termotolerant coliform bacteria, TCB per 100 ml.