Plants have many well-documented influences in treatment wetlands, but differences in individual species' effects on year-round and seasonal performance are poorly understood. In this study, we evaluated plant effects on seasonal patterns of organic carbon removal (measured as COD) and sulfate concentration (used as an indicator of rootzone oxidation) in replicated, batch-loaded, greenhouse microcosms simulating subsurface treatment wetlands. Microcosms were planted with monocultures of 19 plant species or left unplanted as controls, dosed every 20 days with synthetic secondary wastewater, and operated over 20 months at temperatures from 4 to 24 • C. Study-long COD removal averaged 70% for controls and 70-97% for individual species. Most species enhanced COD removal significantly and the benefits of plants were greatest at 4-8 • C because COD removal decreased at low temperatures in controls but displayed limited seasonal variation in planted microcosms. Removal was significantly better at 24 • C than 4 • C with two species (Panicum virgatum and Leymus cinereus), significantly poorer with two species (Carex utriculata and Phalaris arundinacea), and did not differ with 15 species. Only one species showed a significant positive correlation between temperature and COD removal (Iris missouriensis, r = 0.67), while two species showed significant negative correlations (better when colder: Carex nebrascensis, r = −0.67; C. utriculata, r = −0.93). High COD removal throughout the study was strongly associated with high SO 4 concentrations at low temperatures, suggesting that plant performance is related to rootzone oxidation and species' abilities to promote aerobic over anaerobic microbial processes, particularly in winter. Results indicate that improved year-round and cold-season COD removal is common across diverse wetland plant species and novel species can be as good or better than those typically used. Better performing species were largely in the sedge and rush families (Cyperaceae and Juncaceae), while poorer performing species were largely in the grass family (Poaceae).

Treatment wetlands (TWs) are widely used for treating domestic, agricultural, and industrial wastewater, stormwater runoff, and acid mine drainage; natural wetlands are also exposed to these pollutant sources. Currently, few plant species are used in the majority of TWs, and these are often non-native and/or weedy. We are working to identify native species for year-round use in cold-region TWs, particularly the Rocky Mountain region, and explore the basis of differences in performance. In studies presented here, we evaluated chemical oxygen demand (COD) removal from simulated wastewater in microcosms planted with monocultures of 19 species. Experiments were conducted over one year at seasonal temperatures of 4-24°C. With some species and in unplanted controls, COD removal declined at cold temperatures during dormancy, as expected with normal temperature dependence of microbial processes. However, COD removal was constant across seasons with the majority of species. Average COD removal exceeded 90% for Carex aquatilis, C. bebbii, C. praegracilis, C. utriculata, Schoenoplectus acutus, Juncus arcticus, J. torreyi, and Deschampsia cespitosa; of these, only S. acutus is widely used. In contrast, the widely used (and frequently invasive) species T. latifolia, P. australis, and P. arundinacea were somewhat less effective, with average COD removals of 84%, 74%, and 83%, respectively. Redox, sulfate, and root oxygen loss measurements suggest that plant-mediated oxygen transfer may explain the ability of some species to offset the effect of temperature on microbial processes and maintain high COD removal in all seasons. Results indicate that many non-weedy, regionally native species may be candidates for use in TWs or for rehabilitation of natural wetlands exposed to certain pollutants. In addition to the species we studied, other Obligate Wetland and Facultative Wetland species of the Cyperaceae and Juncaceae merit investigation.

Phase A of the Psyttalia WwTP, providing preliminary and primary treatment for the wastewater of the 3.3 million inhabitants and the industries of the Athens greater area was commissioned in mid 1994. Treated effluent is discharged at 65 m depth in the Saronikos Gulf, via a long twin outfall. A new EUR 140 million contract was signed in December 1998 for the expansion of the treatment plant (Phase B), to include biological treatment (carbon removal, nitrification, partial denitrification) of the wastewater as well as treatment of the sludge produced (thickening, anaerobic digestion, dewatering). Effluent quality shall conform with the EU urban wastewater treatment Directive (91/271/EEC). It is expected that the new Phase B works shall be commissioned in early 2002. Several options for the utilisation and disposal of the produced sludge are currently evaluated, including thermal drying and use in cement factories and power plants, co-composting with municipal refuse, etc.