Study On Purification Of Domestic Wastewater By Constructed Wetlands And Screening Of Dominant Wetland Plant Species

Constructed wetlands are among the recently proved technologies for wastewater purification. Five plant species (Pennisetum purpureum Schumach., Phragmitas communis Trin., Canna indica L., Typha latifolia L, and Vetiveria zizanioides Nash.) were planted into constructed wetlands to treat domestic wastewater. The experiment design included two 8m X 3m soil- and gravel-based wetlands and sixteen 2m X 1m soil-based wetlands planted with above five species in monoculture (3 replicates in each species and one without vegetation). Comparisons of the wastewater treatment efficiency between 8mX3m soil- and gravel-based wetlands, and between the five plant species are studied in this paper. The results are as follows:1. The removal rate of CODcr> BODs in sub-surface constructed wetland is significantly higher than that in surface flow wetland. But the ability to remove nitrogen in sub-surface constructed wetland is lower than that in surface flow wetland. The ability to remove P in sub-surface wetland performed better with the operating of wetland bed.2. 2m X 1m wetlands provided significant wastewater treatment ability, but vegetation further improved many treatment efficiencies. After five days retention, the average of fifteen 2m X 1m planted wetlands showed a 80.58+18. 56% reduction in total nitrogen (TN), 76.54 + 23. 21% reduction in ammonia (NH4-N), 86.95 ?. 66% reduction in total phosphate (TP), and 91.56 ?. 05% reduction in soluble phosphate (SP).3. The ability to remove pollutants changes with season and plants' growth. P. purpureum significantly out-performed other plants in effluent quality improvementin May and June, while P. communis and C. indica performed best from August to December. But there is little significant difference in removal of organic matter between the wetlands of different plant species. In May and June, C. indica and P. communis are better than V. zizanioides and T. latifolia in removal of nitrogen, and T. latifolia and V. zizanioides performed better than C. indica and P. communis in removing phosphorus. From August to December, P. purpureum is better than V. zizanioides and T. latifolia in removing nitrogen and phosphorus.4. The ability to remove pollutants in summer and autumn, especially from June to September, is significantly superior to that in winter, for there is lower sewage loading rate, better plant growth and preference temperature making for microbic activity in this time.5. The root of the five plant species distributes mostly in upper 0-15cm of the soil, whereas little below 15cm soil. The root biomass changes with plant species: P. communis, C. indica, and P. purpureum (in May) had more root biomass than other species. There is a positive relationship between root biomass and treatment efficiency.6. The nutrient accumulation is higher in upper 0-15cm soil than in 15-30cm of the soil. There is a positive relationship between nutrient accumulation and sewage removal rate.7. The above- and below-ground biomass in the subsurface wetland is more than that in surface flow wetland. For biomass in monoculture, V. zizanioides, P. communis and P. purpureum had more above-ground biomass than others; T. latifolia, V. zizanioides and P. communis had more below-ground biomass than others.