مروری بر تصفیه شیرابه محل دفن زباله در تالاب‌های مصنوعی

Abstract
Introduction
Landfill leachate is a complex refractory wastewater which consists of extensive level of organic compounds, ammonia, and heavy metals. Contamination of water by landfill leachate has become a serious environmental concern worldwide due to its adverse impact on human health, aquatic organisms, and agricultural crop production. Basically, the pollutants found in landfill leachate include heavy metals, extraneous organic compounds (XOCs), organic compounds, and inorganic compounds. on the other hand, Advanced physico-chemical and biological leachate treatment technologies require continuous budget and energy supply and sufficient technical capabilities for the operation and maintenance of the equipments. Therefore, high-tech solutions are not sustainable for many landfill sites, particularly in developing countries. In recent years, constructed wetland (CW) has received promising attention in the treatment of landfill leachate, because of its costeffective and eco-friendly nature and simplicity in operation, in addition to higher treatment efficiency. Besides their small ecological footprint, CW systems possess similar aesthetic value as natural wetland systems. Constructed wetland systems consist of different media types, and typically, same species of emergent plants. The inherent features of wetland plant species such as extremely high transpiration rates, frost resistance, disease resistance, and tolerance to high heavy metal(loid) concentrations make them successful for remediation purposes. Additional characteristics including ease of rooting, fast establishment, quick growth, extensive rates of photosynthesis, and elevated usage of water make them successful in CW systems. Further, the clear advantage of using vegetation species in a CW is provision of supporting media for biological activities. Moreover, ecological advantages including carbon sequestration, erosion control, pollution prevention, and enhancing landscape appearance are some of the prevailing benefits of wetland plant species. Constructed wetland system is mainly classified into free water surface system (FWS) and subsurface flow system (SSF). In FWS, oxygen is prevalently introduced into the wetland via algal photosynthesis and atmospheric diffusion. In SSF, leachate flows underneath and through the plant rooting media, and subsequently leachate level is maintained below the tip of the substratum. In terms of fewer issues arising from odors, disease-related vectors, and public exposure, SSF is highly recommended for landfill leachate treatment. SSF is of two types, horizontal and vertical. In the horizontal flow systems (HSSF), the leachate is fed into the inlet and continues its way under the surface of the bed in a more or less horizontal path until it reaches the outlet zone. Conversely, in the vertical flow systems (VSSF), landfill leachate is fed on the whole surface area through distribution system and passes the filter in a more or less vertical path. In VSSF, greater oxygen transport is involved compared to the HSSF. As a result, VSSF is more efficient for removing ammoniacal nitrogen (NH3-N) and organic matter from landfill leachate. The efficiency of landfill leachate remediation achieved by CW depends upon different factors including the type of media used (sand, gravel, clay, or silt), availability of microorganisms, and selectivity of plants (monoculture or mixed beds). The purpose of this study is to investigate the use and efficiency of different types of constructed wetlands for the treatment of landfill leachate, the selection of suitable plant species, the mechanism of pollutant removal, and finally a summary of the performance of the constructed wetland system in the treatment of various leachate pollutants in laboratory and field studies.

Methodology
To carry out this study, keywords such as " Constructed wetlands", "landfill leachate pollutant " and "Landfill leachate treatment" were searched in the Web of Science, Google Scholar, ScienceDirect and SID databases. For these keywords, 164 articles were found from 1987 to 2023. After the screening, quality review and removal of repetitive and unrelated articles, 107 relevant articles were used. The main criterion for the selection of articles was the effectiveness of various types of constructed wetlands for the treatment of landfill leachate, the selection of appropriate plant species, and the mechanism and efficiency of pollutant removal. The quality of the articles was evaluated through the SJR index, the citation, the Impact Factor and the SNIP index.
Results
A review of the researches shows that Constructed wetlands for landfill leachate, on average, showed a removal efficiency of 60–80% for BOD5, with FWS and VSSF showing the highest removal. A closer look at the BOD5 range showed that FWS had been used only for very low inlet concentrations. For studies with higher BOD5, mostly hybrid CW have been used. COD removal efficiency, on the other hand, covered a wider range between 20 and 60%. While hybrid and VSSF CW showed similar COD removal, it should be noted that HSSF have been used for leachates with COD values in higher range compared to VSSF and hybrid CW. BOD5/COD ratio, while reduced in the effluent compared to influent, was almost predominantly similar in different types of CW. This can also be due to the lack of reported data in the literature, as many of the studies only report BOD5 or COD as an indicator of organic matter; therefore, the BOD5/COD ratio were not always available. CWs showed 60–75% removal efficiency in removing Ammonia-N, with hybrid CW being the most successful. It should be noted that hybrid CW have been used for leachates with the greater NH3 concentration as well. As expected, the greatest extent of nitrification was observed in VSSF and hybrid CW, while VSSF CW were most successful in removing TN. In terms of TSS and TP, while overall removal ranges of 50–65% and 55–80% for TSS and TP, respectively, are observed for all CW, HSSF and hybrid CWs proved to be the most successful in removing TSS and TP. The number of studies reporting heavy metal removal using CW was found to be small, making it harder to draw firm conclusions. VSSF CW appear to have been the most successful CW type in removing different types of heavy metals. Heavy metals removals have been reported in the range of 15–95%, with Phragmites sp. plants proving to be the most successful species in removing metals.
Conclusion
In the last couple of decades, an on-site treatment of landfill leachate with the help of CW is widely practiced in numerous nations in the world. It is obvious that CW could be the ideal technology for landfill remediation due to its cost-effective and eco-friendly nature. Additionally, establishing vegetation in landfill sites will facilitate erosion and hydraulic control by reducing infiltration of rainfall. It appears that the degree of success in terms of contaminant removal efficiency by CW systems varies depending upon the plant species selected, availability of microbial community, climatic conditions, physico-chemical properties of soil, and CW configuration. Also, treating landfill leachate using CW demands careful attention and design specifications must to be investigated case by case. To enhance the performance of biodegradable organic compound removal from landfill leachate, pre-CW strategies such as using aeration and sedimentation are needed, but their quantifiable removal performance enhancement and economic assessment should be investigated further. Constructed wetland can also be modified by biochar/zeolite/adsorbent addition, and combined with other methods, such as double-chamber anaerobic reactor and microbial fuel cell-coupled constructed wetland in order to increase the removal efficiency of pollutants.