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Three units of free water surface (FWS) constructed wetlands treating domestic wastewater under tropical conditions were examined in terms of water quality and biomass characteristics. One unit (L2) was planted with Scirpus grossus, one with Typha angustifolia (L3), and the unplanted third (L1) served as control. Influent and effluent quality parameters: biological oxygen demand (BOD(5)), nitrate (NO(3)(-)-N), ammonium (NH(4)(+)-N), phosphorus (P), total suspended solids (TSS) and fecal coliforms were regularly measured. The average BOD(5) reductions were 37.0%, 58.5%, and 53.8% for units L1, L2, and L3, respectively. The planted units removed pollutants more effectively although there was no significant difference between the Scirpus grossus and Typha angustifolia units. Plant growth was monitored in marked quadrats by measuring shoot height and other growth parameters. The above-ground organs in L2 and L3 was harvested whenever the shoots reached maximum shoot height and formed flowers. Scirpus grossus had sustainable above-ground biomass production but Typha angustifolia could not sustain repeated harvestings with the above-ground biomass production declining significantly following four consecutive harvests.

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Improvement of primary effluent quality by using an integrated system of emergent plants (Scirpus grossus in the leading subsurface flow arrangement) and submergent plants (Hydrilla verticillata in a subsequent channel) was investigated. The primary effluent was drawn from a septic tank treating domestic sewage from a student dormitory at the University of Peradeniya, Sri Lanka. Influent and effluent samples were collected once every 2 weeks from May 2004 through July 2005 and analyzed to determine water quality parameters. Both the emergent and submergent plants were harvested at predetermined intervals. The results suggested that harvesting prolonged the usefulness of the system and the generation of a renewable biomass with potential economic value. The mean overall pollutant removal efficiencies of the integrated emergent and submergent plant system were biological oxygen demand (BOD5), 65.7%; chemical oxygen demand (COD), 40.8%; ammonium (NH4+-N), 74.8%; nitrate (NO3--N), 38.8%; phosphate (PO43-), 61.2%; total suspended solids (TSS), 65.8%; and fecal coliforms, 94.8%. The submergent plant subsystem improved removal of nutrients that survived the emergent subsystem operated at low hydraulic retention times. The significant improvement in effluent quality following treatment by the submergent plant system indicates the value of incorporating such plants in wetland systems.

Abstract:Arsenite, As(III) is the most soluble form of arsenic species. Arsenic removal efficiency by laterite (commonly found in Sri Lanka) was examined as a function of pH, initial arsenite concentration, laterite dosage, contact time and mixing rate. More than 90% arsenite removal could be achieved within 5 minutes when pH is around 10. By treating the water at this pH range, the current USEPA standard for arsenic in drinking water (10 ppb) can be maintained when the arsenite / laterite ratio is less than 10(g/g). Results of the study showed that naturally available laterite in Sri Lanka can be used as an effective adsorbent to treat arsenic contaminated water.