POTENTIAL USE OF CONSTRUCTED WETLANDS FOR WASTE-WATER TREATMENT IN NORTHERN ENVIRONMENTS

Constructed wetlands consist of soil filled beds with aquatic plants. Wastewater is treated when flowing through these beds. It has been que stioned if constructed wetlands will be able to operate when subjected to cold conditions in sub arctic regions. Experience from Norway indi cates that significant biological activity occurs at temperatures betw een 0 and 5-degrees-C, and that high removal rates of nutrients and or ganic matter are achieved in ponds and soil amended with wastewater at these temperatures. Results from using constructed wetlands in Denmar k, Sweden and North America show that winter performance is not signif icantly reduced as compared to other seasons, but in order to obtain h igh removal of organic matter and nitrogen in cold climates aerobic pr etreatment is probably a prerequisite. Cold climates may also require careful installation of larger and deeper systems with a longer detent ion time. Results of 15 months operation of a Norwegian multi-stage co nstructed wetland pilot plant optimised for nutrient removal, show 55% nitrogen and 98% phosphorus removal. The large phosphorus removal is obtained by using sand with a high content of iron oxides and a fabric ated porous medium that has a high phosphorus adsorption capacity. It remains to be seen if long term cost efficient phosphorus removal can be obtained in constructed wetlands. The results indicate that properl y designed constructed wetlands can operate satisfactorily in a cold c limate. When adequate design criteria are developed several possible a pplications exist for these simple low maintenance systems as main tre atment system, or in conjunction with other treatment methods.