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CHAPTER TWO LITERATURE REVIEW
Wetlands
Natural wetlands commonly represent the areas which are found between land and water bodies. They are intermediary areas linking the two surfaces and have been recognized as natural resource throughout the history of mankind. The submissive nature of wetlands arising from different factors makes it almost impossible to give one general definition. So that, they can be defined in several ways depending on those factors which include: personal perspective, existing water and plant condition, landscape position/geographic setting, and wetland diversity and function (Thomas and William, 2001; Scholz, 2006; Kadlec and Wallace, 2009). Although there are a variety of ways to define the wetland system, the most widely agreed definition was formulated by the International Union for the Conservation of Nature and Natural Resources (IUCN) in the Ramsar Convention, in 1980. According to the convention, wetlands are defined as: ‘areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters.’ Article 1.1 Wetlands are well known for giving magnificent services as biological filters to protect water resources of both surface and groundwater. Natural wetlands have acted as ecological buffer all the time to protect the environment. But conducting researches and the advancement of using wetland treatment technology for treating various wastewaters is a development started in the early 1950s, in Germany. In the United States, researches on wetlands began in the late 1960s and extended in scope during the 1970s. Following this, treating wastewater using wetlands emerged as an appropriate alternative technology globally (Thomas and William, 2001). Wetlands have distinctive characteristics which make them different from major ecosystems plainly known (Kadlec and Wallace, 2009). As nearly all forms of biological productivity highly depend on the amount of water available, water is the most important factor affecting the wetland environment and the associated forms of life (RCS, 2013). Ramsar Convention (2013) pointed out that:‘Wetlands are among the world’s most productive environments. They are cradles of biological diversity, providing the water and primary productivity upon which countless species of plants and animals depend for survival. They support high concentrations of birds, mammals, reptiles, amphibians, fish and invertebrate species. Wetlands are also known to be important storehouses of plant genetic material.’ (Ramsar convention, 2013: p.8). In view of this, wetlands are recognized for giving protection for the environment especially water bodies by removing pollutants in discharged wastewater ranging from rainfall runoff to strong wastewater such as community sewage over many years (Robert, 2004). They have been employed as convenient wastewater discharge sites for as long as sewage has been collected, at least 100 years in some locations (Kadlec and Wallace, 2009), so that they have been receiving increasing attention as effective alternatives for wastewater treatment (Charles and Ian, 2009). In this regard, bacterial metabolism and physical sedimentation are the two major processes acknowledged for the performance of wetlands in treating wastewater (Kadlec and Wallace, 2009; Zhai et al, 2016; Szklarek et al, 2018).Furthermore, immense and highly indispensable services for human well-being and poverty reduction are offered by wetland ecosystems (Kent, 2001; WRI, 2005; Vymazal, 2010). Some of the most important ecosystem services include: aquatic and wildlife habitat (Kent, 1994; Vymazal, 2010; Si et al, 2014), educational and scientific venues, flood flow alteration (Vymazal, 2010), groundwater recharge, elemental transformation, particle retention and sources of raw materials, recreation, and soil stabilization (Kent, 2001; WRI, 2005). In Ethiopia, it is estimated that the wetlands covered 2% of the country’s land surface (Afework, 2005). But much of these resources are exposed to exploitation and signs of wetland degradation have become out of control across the country. This is mainly due to the absence of clear policy frameworks except the scattered efforts made by different sectors. In the mean time, scholars have been contributing a lot to create awareness on the benefits and status of wetlands since the beginning of the 1990s. There have been different views among scholars regarding wetland policies and strategies; some arguing that the issue of wetlands has been addressed in the general framework of the existing policies and development strategies and others have stressed the need for a standalone wetland development policy. Currently, the Federal
Environment, Forest and Climate Change Commission (EFCCC) seems to have been working in this line for realizing the signing of the Ramsar Convention and approval of the draft wetland policy (Tadesse and Solomon, 2014).
Declaration
Abstract
Dedication
Acknowledgements
Table of contents
List of figures
List of tables
List of abbreviations
CHAPTER ONE INTRODUCTION
1.1 Study background
1.2 Statement of the Problem
1.3 Study justification/Rationale
1.4 Research Questions
1.5 Objective of the Study
1.5.1 General Objective
1.5.2 Specific objectives
1.6 Ethical Considerations
CHAPTER TWO LITERATURE REVIEW
2.1 Wetlands
2.2 Constructed Wetlands as an Attractive Technology for WWT; an Overview
2.3 Advantages of Constructed Wetlands
2.4 Types of Constructed Wetlands
2.4.1 Free Water Surface systems
2.4.2 Subsurface Flow (SSF) systems
2.5 Design Factors of Constructed Wetlands
2.5.1 Wetland Plants
2.5.2 Substrate
2.5.3 Retention Time
2.5.4 Water Depth
2.5.5 Seasons of the Year
2.6 Pollutant Removal Mechanisms in Constructed Wetlands
2.6.1 Physical Mechanisms
2.6.2 Biological Mechanisms
2.6.3 Chemical Mechanisms
2.7 Removal of Pollutants in Constructed Wetlands
2.7.1 Removal of Organic Compounds
2.7.2 Suspended Solids
2.7.3 Removal of Nutrients
2.7.4 Fecal Coliform Removal
2.8 Reaction Kinetics
2.9 Application of Constructed Wetlands
2.9.1 Application of Constructed Wetlands for Domestic Wastewater Treatment
2.9.2 Application of Constructed Wetlands for Storm Water Treatment
2.9.3 Constructed Wetlands for Industrial Wastewater Treatment
2.9.4 Application of Constructed Wetlands for Agro-Industrial Wastewater Treatment
2.9.5 Constructed Wetlands for Leachate Treatment
2.9.6 Constructed Wetlands for Acid Mine Drainage Treatment
2.9.7 Constructed Wetlands for Agricultural Runoff Treatment
2.9.8 Constructed Wetlands for Pesticide Treatment
2.10 Studies on Performance of Constructed Wetlands in Ethiopia
CHAPTER THREE MATERIALS AND METHODS
3.1 Description of the Study Area
3.2 Construction of the wetland systems
3.2.1 Determination of the size of wetland cells
3.2.2 Hydraulic retention time of the wetland systems
3.2.3 Layout and configuration of the pilot-scale constructed wetlands
3.2.4 Type of filter media used
3.2.5 Plant species used in the study and planting procedures
3.2.6 Sedimentation tank
3.2.7 Installation of inlet and outlet pipes
3.2.8 Lining of the wetland beds
3.3 Monitoring of performance (removal efficiency) of the constructed wetland systems
3.4 Sampling and laboratory analyses
3.5 Data analysis
3.6 Total cost of construction
3.7 Sources of meteorological data of the study area
CHAPTER FOUR RESULT AND DISCUSSION
4.1 Characteristics of the Domestic Wastewater
4.2 Temperature, pH, Electrical Conductivity and Dissolved Oxygen
4.3 Removal of Biochemical Oxygen Demand (BOD5)
4.4 Removal of Chemical Oxygen Demand (COD)
4.5 Removal of Total Suspended Solids (TSS)
4.6 Removal of Ammonium (NH4+)
4.7 Removal of Nitrate (NO3-)
4.8 Removal of Total Nitrogen (TN)
4.9 Removal of Phosphate (PO43-)
4.10 Removal of Total Phosphorous (TP)
4.11 Removal of Fecal Coliform (FC)
4.12 Kinetic parameters determination
4.13 Plant tissue nutrient (N and P) content
CHAPTER FIVE CONCLUSION AND RECOMMENDATION
5.1 Conclusion
5.2 Recommendations
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