WASTE-WATER TREATMENT WETLANDS IN COLD CLIMATES

The best prospects for successful wetland treatment should be in the w armer regions of the world, but studies in North America and Scandinav ia show that wetland treatment may be feasible also in cooler regions. A review shows that the number of wetlands of different types (free w ater surface, FWS; horizontal and vertical subsurface flow, SSF), trea ting different kinds of wastewater, is steadily increasing in most par ts of the cold temperate regions of the world. The major wetland engin eering concerns in cold climates, which are discussed in this paper, a re related to: (1) ice formation, and its implications for hydraulic p erformance; (2) hydrology and hydraulic issues besides ice formation; and (3) the thermal consequences for biologically or microbiologically mediated treatment processes. Energy- and water-balance calculations, as well as thermal modeling, are useful tools for successful design a nd operation of treatment wetlands, but the shortage of data makes it necessary to adopt a conservative approach. The treatment processes of ten appear less temperature sensitive in full-scale wetlands as compar ed to laboratory incubations. Several possible explanations are discus sed in the paper: (1) sedimentation playing a significant role, (2) ov erdimensioning in relation to some constituents, (3) seasonal adsorpti on (cation exchange) of ammonium, and (4) temperature adaptation of th e microbial community. Experience shows that cold climate wetlands can meet effluent criteria for the most important treatment parameters. T o gain wide acceptance, however, we need to become more specific about design and construction, and also about operation, maintenance and co st-effectiveness. These goals require detailed knowledge about process es in full-scale wetlands, including long-term changes and response to maintenance. (C) 1997 IAWQ.