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Treatment wetlands can remove nutrients from inflow sources through biogeochemical processes. Plant composition and temperature play important roles in the nutrient removal efficiency of these wetlands, but the interactions between these variables are not well understood. We investigated the seasonal efficiency of wetland macrophytes to reduce soil leachate concentrations of total nitrogen and total phosphorus in experimental microcosms. Each microcosm contained one of six vegetation treatments: unplanted, planted with one of four species (Carex lacustris, Scirpus validus, Phalaris arundinacea and Typha latifolid) in monoculture or planted with an equal abundance of all four species. Microcosms were also subjected to two temperature treatments: insulated microcosms and microcosms exposed to environmental conditions. A constant nutrient solution containing 56 mg/l N and 31 mg/l P was added to all microcosms three times a week. Water samples were analyzed monthly for total dissolved nitrogen and total dissolved phosphorous. Microcosms exhibited a typical pattern of seasonal nutrient removal with higher removal rates in the growing season and lower rates in the winter months. In general, planted microcosms outperformed unplanted microcosms. Among the plant treatments, Carex lacustris was the least efficient. The four remaining plant treatments removed an equivalent amount of nutrients. Insulated microcosms were more efficient in the winter and early spring months. Although a seasonal pattern of nutrient removal was observed, this variation can be minimized through planting and insulation of wetlands.

Quarterly water quality data from 1998 to 2003 for eight single-family domestic systems serving 2-7 people in Ohio, USA, were studied to determine the cell-to-cell and system wide pathogen reduction efficiency and effectiveness of these systems in meeting compliance standards. Two-cell domestic wastewater treatment systems displayed significant variability in their cell-to-cell performance that directly impacted the overall ability of systems to meet effluent compliance standards. Fecal coliform was effectively reduced (approximately 99%) in these systems while two-thirds of the input biochemical oxygen demand was mitigated in each of the cells of these systems. Fecal coliform and biochemical oxygen demand were typically reduced below 2000 counts per 100 ml and 15 mg/l (respectively) before discharge to surface waters. Total suspended solids were reduced by approximately 80% overall with cell one retaining the majority of the solids (approximately 70%). These systems discharged more than 18 mg/l of suspended solids in less than 5% of the samples thus displaying a very high compliance rate. Ammonia and total phosphorus were less effectively treated (approximately 30-40% reductions in each cell) and exceeded standards (1.5 mg/l) more frequently. Analyses based on the number of occupants indicated that the two-cell design used here was most effective for smaller occupancy systems. More study is required to determine the value of this design for large occupancy systems. In the future, wetlands should be evaluated based on the total loads delivered to the watershed rather than by effluent concentrations.

Four wetland plant species (Scirpus validus, Carex lacustris, Phalaris arundinacea, and Typha latifolia) were grown in monoculture and as a four-species mixture to compare effectiveness of nutrient removal in controlled 18.93-l outdoor subsurface treatment wetland microcosms. A nutrient treatment that mimicked single-resident domestic effluent consisted of two levels of nitrogen (N) and phosphorus (P) [low (56 mg/l N and 31 mg/l P) and high (112 mg/l N and 62 mg/l P)] of nutrient solution applied three times weekly. The plants were established and maintained for one year before the nutrient treatment and monthly water sampling commenced; water sampling began July 31, 2001 and ended October 23, 2001. We tested four hypotheses: (1) vegetated microcosms are more effective at reducing concentrations of total N and total P from soil leachate than unvegetated, (2) there is a differential species effect on the potential to reduce N and P, (3) plant mixtures are more effective than monocultures at reducing N and P, and (4) the microcosms will be least effective at reducing N and P concentrations in October compared to August. We found support for hypotheses 1, 2, and 4, but our results are inconclusive for the third hypothesis. Total N and total P in the soil leachate were significantly higher from unvegetated microcosms compared to vegetated. S. validus was most effective and P. arundinacea was generally least effective at reducing N and P in monocultures, with treatment capabilities similar to unvegetated microcosms. The four-species mixture was generally highly effective at nutrient removal, however the results were not significantly different from the monocultures. At the end of the growing season (October) treatment efficiency was significantly less than earlier months, especially for the unvegetated treatment., (Copyright 2003 Elsevier Ltd.)