Constructed wetland is an economical, efficient and environment friendly technology for the treatment of wastewater. The aquatic plants and microorganisms in constructed wetlands play a key role in the treatment of wastewater. Plants add oxygen during the process of photosynthesis or by direct transport from the atmosphere through their stems and roots to the rhizosphere of constructed wetlands. Moreover, macrophytes provide optimum conditions and surface area for the attachment of microorganisms to perform their activities. The oxygen released in rhizosphere of constructed wetlands has significant impacts on nutrient, gas exchange chemistry, and contaminants removal. In this review, we have compiled the published data to explore the mechanism of oxygen release by the roots of macrophytes in constructed wetlands, rhizospheric oxygen measurement techniques, role of climatic factors, and ability of different macrophytes in oxygenation of rhizosphere in constructed wetlands. Numerous studies have been carried out on enhancing the efficiency of constructed wetlands by modifying design, macrophyte species, and size of wetlands. However, the part of combination of climatic factors, microorganisms, and plant species in augmenting oxygen release in the rhizosphere is not explored so far. Therefore, in this review, we tried for the first time to compile all the parameters effecting oxygen released in the rhizosphere of macrophytes in constructed wetlands that are using worldwide for the treatment of wastewater. Moreover, this review article will provide vital information to all researchers who are working on wastewater treatment by using constructed wetlands.
To illuminate future uncertainties and inform strategic plans, this chapter presents three qualitative scenarios for the status of the HKH through 2080. The scenarios (Box 4.1) emerged from a participatory visioning exercise for scenario development (also see Sect. 4.1.2) conducted by the chapter team and HIMAP secretariat between January and September 2016. Over six successive workshops, decision makers and scientists representing HKH countries determined what would constitute a "prosperous" HKH scenario for 2080-along with its less desirable alternatives, business as usual and downhill (Fig. 4.1).
The current study is designed to investigate the impact of plant growth promoting rhizobacteria (PGPR) on the growth of Typha latifolia and Phragmites australis and the consequent impact on oxygen concentration in their rhizospheres. Initially, uninoculated plants are tested for the optimum temperature and light intensity combination by exposing them to sixteen combinations. An automated monitoring system for temperature and light intensity is deployed at the experimentation site. The PGPR strains T2, T5, and T7 collected from National Agriculture Research Council, Pakistan, are used to inoculate the plants and analyzed for number of leaves, fresh plant biomass, total chlorophyll, and oxygen concentration in their rhizosphere.The results indicate that the strain T7 increased chlorophyll content, number of leaves, and fresh plant biomass in T. latifolia and P. australis. The oxygen concentration is also enhanced significantly in the rhizosphere of both plants. However, strains T2 and T5 are not observed to be influencing the growth of T. latifolia and P. australis. Moreover, the enhanced oxygen concentration in the plants' rhizosphere is observed to significantly reduce the time consumption for achieving over 90% chemical oxygen demand and biochemical oxygen demand removal efficiency. Hence, it is determined that the efficiency of constructed wetlands can be improved by augmenting the rhizospheric oxygen using PGPR to accelerate the plant development. The technology has practical implications and potential to be scaled up in future.
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