Studies on mechanisms and efficiencies of wastewater treatment with constructed wetland,mainly from developed countries,conducted mainly in the past 2 decades are reviewed.The mechanisms of wastewater treatment by wetlands are extremely complicated,including a series of physical,chemical,and bio chemical processes,such as special and non special adsorption,exchange,sedimentation,assimilation,decomposition,volatilization,and so on.There are lots of components,e.g.plants,soils,and microorganisms,in wetlands making contributions to water quality improvement.Of all plants in constructed wetlands, Phragmites communis is used most widely and frequently.But wetlands covered by this single species are inferior to those covered by many species including it and other plants with reference to removal efficiency of nutrients and pollutants.In order to consummate theories about constructed wetlands for wastewater treatment,and to further improve removal efficiencies of pollutants and wetland ecosystem health,the following research fields are suggested to consolidate or further conduct in the future:① deeper and more systemic research on mechanisms of wastewater purification by wetlands;② studies on sustainable development of constructed wetland;③ experimentation on the suitability of local and indigenous wetland macrophytes for removal of nutrients and heavy metals from different types of wastewater;④ exploration for better constructing,maintaining,and using measures to maximize the wastewater treatment efficiency;⑤ controlling disease vectors and animal pests,and preventing them from jeopardizing human health and lives;and ⑥ ascertaining reasons producing noxious and odorous gases and then trying to control them effectively.
The role of forest litter as an acid-base buffering system was assessed by litter from plantation and natural forests in South China. Samples were either extracted with acid solutions or titrated with acid or base solutions. Litter was found to be a strong acid-base buffering system. Two legume species, Acacia mangium Willd and A. auriculaiformis A. Cunn, had very high litter pH values of around 6, which was 2 pH units higher than that of the soil where they grew. Litter of all other plantation species had litter pH of around 4, similar to that of the soil. Both legume species have high potential to neutralize soil acidity and the litter layer could act to shield soil against acid rain. The current stand of two legume species was estimated to be able to raise rain acidity by 0.1 to 0.4 pH units. Inorganic ions did not fully explain the pH pattern of different litter extracts, but high sodium and low nitrate partly accounted for the high pH of the two legume species. Some natural forest species had extremely low pH. As a whole, the litter of the natural climax forest was the driving force of soil acidification. Although plant residues are strong acid-base buffering system and able to adjust acidity of environment, only a few species can be expected to make soil more acid or alkaline through this mechanism since most species have litter pH values similar to those of soil where they grow.
