Wetland construction seminar REPORT


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Wetland construction seminar REPORT

  1. 1. Wetland construction 1. INTRODUCTION 1.1 GENERAL Globally, most of the developing countries are geographically located in those parts of the world that are or will face water shortages in the near future. Moreover, the existing water sources are contaminated because untreated sewage and industrial wastewater is discharged into surface waters resulting in impairment of water quality. The treatment of wastewater using Constructed Wetland (CW) is one of the suitable treatment systems, used in many parts of the world. Wetlands are defined as land where the water surface is near the ground surface long enough each year to maintain saturated soil conditions, along with the related vegetation. Marshes, bogs, and swamps are all examples of naturally occurring wetlands. A “constructed wetland” is defined as a wetland specifically constructed for the purpose of pollution control and waste management, at a location other than existing natural wetlands. Wetlands can be used for primary, secondary, and tertiary treatments of domestic wastewater, storm wastewater, combined sewer overflows (CSF), overland runoff, and industrial wastewater such as landfill leachate and petrochemical industries wastewater. The most common systems are designed with horizontal subsurface flow (HF CWs) but vertical flow (VF CWs) systems are getting more popular at present. The most commonly used species are robust species of emergent plants, such as the common reed, cattail and bulrush. In this seminar paper the inlet and outlet wastewater physico-chemical parameters were analysed during the retention period. The parameters studied were pH, BOD, COD, DO, Total Suspended Solids, Total Dissolved Solids, Nitrogen and Phosphorus. The percentage removal of the parameters were analysed and studied until the percent removal rate gets stabilized. 1.2 ADVANTAGES OF CONSTRUCTED WETLANDS  Wetlands can be less expensive to build than other treatment options  Utilization of natural processes,  Simple construction (can be constructed with local materials),  Simple operation and maintenance,  cost effectiveness (low construction and operation costs), Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 1
  2. 2. Wetland construction  Process stability.  Low energy demand.  Low environmental impact 1.3 LIMITATIONS OF CONSTRUCTED WETLANDS  large area requirement  wetland treatment may be economical relative to other options only where land is available and affordable.  Design criteria have yet to be developed for different types of wastewater and climates. 1.4 NATURAL WETLANDS VS. CONSTRUCTED WETLANDS A natural wetland is an area of ground that is saturated with water, at least periodically. Plants that grow in wetlands, which are often called wetland plants or saprophyte, have to be capable of adapting to the growth in saturated soil. Constructed wetlands, in contrast to natural wetlands, are man-made systems or engineered wetlands that are designed, built and operated to emulate functions of natural wetlands for human desires and needs. Engineered to control substrate, vegetation, hydrology and configuration. It is created from a non-wetland ecosystem or a former terrestrial environment, mainly for the purpose of contaminant or pollutant removal from wastewater (Hammer).These constructed wastewater treatments may include swamps and marshes. Most of the constructed wetland systems are marshes. Marshes are shallow water regions dominated by emergent herbaceous vegetation including cattails, bulrushes, and reeds. Fig 1.1 A natural wetland: Tasek Bera, a freshwater swamp Fig 1.2 A constructed wetland: Putrajaya system Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Wetlands Page 2
  3. 3. Wetland construction 2. LITERATURE REVIEW ATIF MUSTAFA(2013) conducted treatment performance of a pilot-scale constructed wetland (CW) commissioned in in Karachi, NED University of Engineering & Technology, was evaluated for removal efficiency of biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), ammonia-nitrogen (NH4-N), ortho-phosphate (PO4-P), total coliforms (TC) and faecal coliforms (FC) from pretreated domestic wastewater. Monitoring of wetland influent and effluent was carried out for a period of 8 months. NED wastewater treatment plant (WWTP) treats wastewater from campus and staff colony. The wastewater contains domestic sewage and low flows from laboratories of various university departments. The constructed wetland is planted with common wetland plant (Phragmites karka). The key features of this CW are horizontal surface flow. Treatment effectiveness was evaluated which indicated good mean removal efficiencies; BOD (50%), COD (44%), TSS (78%), NH4-N (49%), PO4-P (52%), TC (93%) and FC (98%). YADAV and JADHAV (2011) construct wetland unit combined with surface flow and planted with Eichhornia crassipes was built near Technology Department, Shivaji University, Kolhapur ( Latitude 160 40’ N, Longitude 740 15’ S). Maharashtra situated in Western part of India. The campus wastewater was let into the constructed wetland intermittently over 30 days.The study was performed in two sets A and B which were run in the months of December and January respectively. The parameters analysed for the study were pH, Dissolved Oxygen, Biochemical Oxygen Demand, Chemical Oxygen Demand, Total Suspended Solids, Total Dissolved Solids, Nitrogen and Phosphorus. Only quality of wastewater was analysed during the study period of 2 months i.e. December and January. The sampling took place daily at both inlet and outlet of constructed wetland system. Treatment effectiveness was evaluated which indicated good mean removal efficiencies; BOD (95%), COD (97%), TSS (82%), NH4-N (43%), PO4-P (49%). Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 3
  4. 4. Wetland construction 3. WETLAND CONSTRUCTION Wetlands, either constructed or natural, offer a cheaper and low-cost alternative technology for wastewater treatment. A constructed wetland system that is specifically engineered for water quality improvement as a primary purpose is termed as a ‘Constructed Wetland Treatment System’ (CWTS). In the past, many such systems were constructed to treat low volumes of wastewater loaded with easily degradable organic matter for isolated populations in urban areas. However, widespread demand for improved receiving water quality, and water reclamation and reuse is currently the driving force for the implementation of CWTS all over the world. The ability of wetlands to transform and store organic matter and nutrients has resulted in a widespread use of wetland for wastewater treatment worldwide. Recent concerns over wetland losses have generated a need for the creation of wetlands, which are intended to emulate the functions and values of natural wetlands that have been destroyed. Natural characteristics are applied to CWTS with emergent macrophyte stands that duplicate the physical, chemical and biological processes of natural wetland systems. The number of CWTS in use has very much increased in the past few years. The use of constructed wetlands in the United States, New Zealand and Australia is gaining rapid interest. Most of these systems cater for tertiary treatment from towns and cities. They are larger in size, usually using surface-flow system to remove low concentration of nutrient (N and P) and suspended solids. However, in European countries, these constructed wetland treatment systems are usually used to provide secondary treatment of domestic sewage for village populations. These constructed wetland systems have been seen as an economically attractive, energy-efficient way of providing high standards of wastewater treatment. Typically, wetlands are constructed for one or more of four primary purposes: creation of habitat to compensate for natural wetlands converted for agriculture and urban development, water quality improvement, flood control, and production of food and fiber (constructed aquaculture wetlands). Constructed wetlands are based upon the symbiotic relationship between the micro organisms and pollutants in the wastewater. These systems have potential to treat variety of wastewater by removing organics, suspended solids, pathogens, nutrients and heavy Metals. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 4
  5. 5. Wetland construction A constructed wetland is a shallow basin filled with some sort of filter material (substrate), usually sand or gravel, and planted with vegetation tolerant of saturated conditions. Wastewater is introduced into the basin and flows over the surface or through the substrate, and is discharged out of the basin through a structure which controls the depth of the wastewater in the wetland. A constructed wetland comprises of the following five major components: • Basin • Substrate • Vegetation • Liner • Inlet/Outlet arrangement system Fig 3.1 Components of constructed wetland(UN-HABITAT, 2008.) Basin Systems have been designed with bed slopes of as much 8 percent to achieve the hydraulic gradient. Newer systems have used a flat bottom or slight slope and have employed an adjustable outlet to achieve the hydraulic gradient. Subsrate Many different media sizes have been tried for the bed, but gravel less than 4 cm diameter seems to work best. Larger diameters increase the flow rate, but result in turbulent flow. Smaller media gives a reduced hydraulic conductivity, but has the advantage of more surface Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 5
  6. 6. Wetland construction area for microbial activity and adsorption. Soil is sometimes used to remove certain materials due to the ability of reactive clays to adsorb heavy metals, phosphates, etc.. Vegetation Three types of plants are normally used  Cattails, which are a favorite food of muskrats and nutria.  Bulrush is also high on the mammals food list, but they should not be attracted to the wetland if the water surface is kept below the media.  Reeds are used most often in Europe because they are not a food source for animals. However, they are not allowed in some areas due to their tendency to spread and push out native vegetation. The type used will also depend on the local climate and the substances to be removed. In some instances decorative plants are used, but results show them to be less effective and require more maintenance. Control of the water level can be used to increase root penetration and control weeds. Liner The liner goes under the entire system and can be a manufactured liner or clay. This prevents the wastewater from infiltrating into the ground before it is treated. A berm around the system prevents runoff from entering the system. Inlet The inlet can be a manifold pipe arrangement, an open trench perpendicular to the flow, or weir box. The manifold arrangement can be a pipe with several valve outlets or a simple perforated pipe. Coarse gravel allows rapid infiltration of the water. The inlet purpose is to spread the wastewater evenly across the treatment bed for effective treatment Outlet The outlet structures used are similar to the inlet structures. One preferred addition is making the outlet adjustable to allow the control of water level. The level could be lowered when a large amount of rainfall is expected or raised for maximum cross-sectional use of the media. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 6
  7. 7. Wetland construction 3.1 TYPES OF CONSTRUCTED WETLAND CW could be classified according to the various parameters but the two most important criteria are water flow regime (surface and sub-surface) and the type of macrophytic growth. Different types of constructed wetlands may be combined with each other (so called hybrid or combined systems) in order to exploit the specific advantages of the different systems. The quality of the final effluent from the systems improves with the complexity of the facility. Emergent macrophytes based systems can be constructed with surface flow, subsurface horizontal flow and vertical flow (fig 3.2 ) Fig 3.2 Classification of constructed wetlands for wastewater treatment ( Vymazal and Kropfelova, 2008) 3.1.1 SURFACE FLOW SYSTEMS A typical surface flow (SF) or free water surface constructed wetland (FWS CW) with emergent macrophytes (usually covering more than 50 percent), is a shallow sealed basin or sequence of basins, containing 20-30 cm of rooting soil, with a water depth of 20-40 cm. As the name suggests, the waste water flows above the ground exposed to the atmosphere. Inflow water Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 7
  8. 8. Wetland construction containing particulate and dissolved pollutants slows and spreads through a large area of shallow water and emergent vegetation. FWS CW typically have aerated zones, especially near the water surface because of atmospheric diffusion, and anoxic and anaerobic zones in and near the sediments. In heavily loaded FWS wetlands, the anoxic zone can move quite close to the water surface. Long retention times and an extensive surface area in contact with the flowing water provide for effective removal of particulate and organic matter. The sediment, plant biomass and plant litter surfaces are also where most of the microbial activity affecting water quality occurs, including oxidation of organic matter and transformation of nutrients. Biomass decay provides a carbon source for denitrification, but the same decay competes with nitrification for oxygen supply. Low winter temperatures enhance oxygen solubility in water, but slow microbial activity. Fig 3.3 Typical configuration of a surface flow wetland system (Kadlec and Knight,1996) 3.1.2 SUB-SURFACE SYSTEMS These systems have the water level designed to remain below the top of the substrate. They have the added component of emergent plants with extensive root systems within the media. CW with sub-surface flow may be classified according to the direction of flow; horizontal and vertical (Fig.3.2) on one hand, and into soil, sand and gravel based wetlands on the other hand. The substratum provides the support and attachment surface for microorganisms Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 8
  9. 9. Wetland construction able to anaerobically (and/or anoxically if nitrate is present) reduce the organic pollutants into CO2, CH3, H2S etc. The substratum also acts as a simple filter for the retention of influent suspended solids and generated microbial solids, which are then themselves degraded and stabilized over an extended period within the bed, such that outflow suspended solids levels are generally limited. The provision of a suitably permeable substrate in relation to the hydraulic loading to obviate surface ponding tends to be the most expensive component of the subsurface flow systems, and catered for. Sub-surface systems are also referred to as planted filters, reed beds, root zone method, gravel bed hydroponics filters, vegetated submerged beds or artificial wetlands. Fig 3.4 Typical configuration of a sub-surface flow system(Kadlec and Knight,1996) HORIZONTAL FLOW SYSTEMS (HFS) It is called horizontal flow because the wastewater is fed in at the inlet and flows slowly through the porous medium under the surface of the bed in a more or less horizontal path until it reaches the outlet zone where it is collected before leaving via level control arrangement at the outlet (fig 3.6). During this passage the wastewater will come into contact with a network of aerobic, anoxic and anaerobic zones. The aerobic zones occur around roots and rhizomes that leak oxygen into the substrate. CW with horizontal subsurface flow requires much larger vegetated bed area in order to effectively eliminate phosphorous and nitrogen. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 9
  10. 10. Wetland construction HFS are very efficient in removing organic matter and suspended solids. BOD removal rates range between 65 percent and 90 percent, with average BOD effluent concentrations below 30-70 mg/l. Typical effluent TSS levels are below 10–40 mg/l and correspond to 70–95percent removal rates. Pathogen removal amounting to 99 percent or more (2–3 log) total coliforms has been reported by Crites and Tchobanoglous . Tropical and subtropical climates hold the greatest potential for the use of HFS. Cold climates tend to show problems with both icing and thawing. Water stress of plants in a HFS is an important issue to be considered especially in households systems during periods without inflow (e.g. during holidays). Fig 3.5 Horizontal flow constructed wetland Fig3.6 Schematic cross-section of horizontal flow constructed wetland (Morel and Diener ;2006) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 10
  11. 11. Wetland construction VERTICAL FLOW SYSTEMS (VFS) VFS are shallow excavations or above-ground constructions with an impermeable liner, either synthetic or clay. They are characterized by intermittent (discontinuous) loading and resting periods where the waste water percolates vertically through the substrate. Intermittent and batch loading enhances oxygen transfer and thus nitrification. The main purpose of plant presence in VFS is to help maintain the hydraulic conductivity of the bed. VFS have a typical depth of 0.8–1.2m. For small systems (i.e. single households) receiving septic tank effluents, Cooper (1999) proposes the use of two vertical –flow beds in series. Removal efficiencies in terms of BOD, COD, ammonia-N and pathogens of VFS are generally higher than comparable HFS. However removal of suspended solids is somewhat lower than in HFS. Average removal efficiencies are typically within a range of 75-95percent and 65-85 percent in terms of BOD and TSS respectively. Pathogen removal in terms of total coliforms are typically within a range of 2-3 log and can be as high as 5 log as seen in Nepal. This could be resolved by recycling of the effluent into the pretreatment unit, e.g. septic or Imhoff tank (Arias and Brix, 2006). One of the major threats of good performance of VFS is clogging of the filtration substrate. Therefore, it is important to properly select the filtration material, hydraulic loading rate and distribute the water evenly across the bed surface in order to avoid overloading of certain parts of the surface. Given their reliance on a well functioning pressure distribution, they are more adapted to locations where natural gradients can be used, thus enabling the filter by gravity. Since flat areas require the use of pumps, they are thus dependent on a reliable power supply and frequent maintenance. VFS could be further categorized into down-flow and up-flow depending on whether the wastewater is fed onto the surface or to the bottom of the wetland. VFS are primarily used to treat domestic or municipal sewage. The system has also been successfully applied to municipal, industrial. (Explosives, food processing, airport de-icing water, acid mine drainage) as well as agricultural (aquaculture, swine feedlot) wastewaters (Behrends et al,1996) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 11
  12. 12. Wetland construction Fig 3.7 Schematic cross-section of vertical flow constructed wetland( Morel and Diener,2006) Fig 3.8 Vertical flow constructed wetland Downflow Vertical flow constructed wetlands (VF CW) comprise a flat bed of graded gravel topped with sand planted with macrophytes. The size fraction of gravel is larger in the bottom layer (e.g. 30-60 mm) and smaller in the top layer (e.g., 6 mm). VFCW are fed intermittently with a large batch thus flooding the surface. Wastewater then gradually percolates down through the bed and is collected by a drainage network at the base. The bed drains completely free and it allows air to refill the bed. This kind of dosing leads to good oxygen transfer and hence the ability to nitrify. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 12
  13. 13. Wetland construction Fig 3.9 Typical arrangement of a downflow vertical-flow constructed wetland ( Cooper et al.,1999) Upflow In vertical-up flow CW the wastewater is fed on the bottom of the filter bed. The water percolates upward and then it is collected either near the surface or on the surface of the wetland bed. These systems have commonly been used in Brazil. The beds are filled with crushed rock on the bottom, the next layer is coarse gravel and the top layer is soil planted with Rice (Oryza sativa). This treatment system is called in Brazil ―filtering soil‖ (Fig 3.10). However, outside Brazil, the layer of soil is usually not used and beds are filled with gravel and usually planted with common species such as Phragmites australis. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 13
  14. 14. Wetland construction Fig 3.10 Schematic representation of a constructed wetland with vertical up-flow ( Vymazal.2001) Tidal flow Tidal flow systems are a new form of. Tidal flow systems were developed to try to overcome some of the problems that were seen in the early forms of VFS related to clogging of the surface. Upflow systems have been used for about 20 years but they suffer from the problem that distribution is below the surface and hence hidden from the observers. In tidal flow systems at the start of the treatment cycle the wastewater is fed to the bottom of the bed into the aeration pipes. It then percolates upwards until the surface is flooded. When the surface is completely flooded the pump is then shut off, the wastewater is then held in the bed in contact with the micro-organisms growing on the media. A set time later the wastewater is drained downwards and after the water has drained from the bed the treatment cycle is complete and air diffuses into the voids in the bed. HYBRID SYSTEMS Various types of CW may be combined in order to achieve higher treatment effect, especially for nitrogen. In these systems, the advantages of the HSF and VF systems can be combined to complement each other. Therefore, there has been a growing interest in hybrid systems (also sometimes called combined systems). Hybrid systems used to comprise most frequently VFS and HFS arranged in a staged manner (fig 3.11); however, all types of CW could be combined. In hybrid systems, the advantages of various systems can be combined to Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 14
  15. 15. Wetland construction complement each other. It is possible to produce an effluent low in BOD, which is fully-nitrified and partly denitrified and hence has much lower total-N concentrations. The design consists of two stages of several parallel VF beds (―filtration beds‖) followed by 2 or 3 HF beds (―elimination beds‖) in series. The results indicate very good removal for organics (BOD5 and COD) and TSS while removal of nitrogen is enhanced with no nitrate increase at the outflow. Fig 3.11 Schematic arrangement of the HF-VF hybrid system according to Brix and Johansen. (Vymazal.2001) 3.2 WETLAND PLANTS The role of wetland vegetation as an essential component of CW is well established. Emergent plants contribute both directly and indirectly to the treatment processes. In spite of the fact that the most important removal processes in CW are based on microbial processes, the macrophytes posses several functions in relation to the water treatment. They influence treatment process in CW by their physical presence and metabolism. In general, the most significant functions of wetland plants (emergents) in relation to water purification are the physical effects brought by the presence of the plants. The plants provide a huge surface area for attachment and growth of microbes. The physical components of the plants stabilise the surface of the beds, slow down the water flow thus assist in sediment. Plants also provide microorganisms with a source of Carbon. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 15
  16. 16. Wetland construction However, not all wetland species are suitable for wastewater treatment since plants for CW must be able to tolerate a combination of continuous flooding and exposure to wastewater or storm water containing relatively high amounts of pollutants. A portion of the nutrients is retained in the undecomposed fraction of the plant litter and accumulates in the soils. Plants oxygenate the root zone by release of oxygen from their roots, and provide aerobic microorganisms a habitat within the reduced soil. Plants have additional site-specific values by providing habitat for wildlife and making wastewater treatment systems aesthetically pleasing. Wetland species of all growth forms have been used in treatment wetlands. However, the most commonly used species are robust species of emergent plants, such as the common reed, cattail and bulrush. The larger aquatic plants growing in wetlands are usually called macrophytes. Three most important criteria form the basis for the plant choice:  Availability in climate zone Pollutant removal capacity (treatment efficiency) and tolerance ranges Plant productivity and biomass utilization options 3.2.1 PLANTS IN WETLANDS The larger aquatic plants growing in wetlands are usually called macrophytes. The term includes aquatic vascular plants (angiosperms and ferns), aquatic mosses, and some larger algae that have tissues that are easily visible. Although ferns like Salvinia and Azolla and large algae like Cladophora are widespread in wetlands, it is usually the flowering plants (i.e. angiosperms) that dominate. Macrophytes, like all other photoautotrophic organisms, use the solar energy to assimilate inorganic carbon from the atmosphere to produce organic matter, which subsequently provides the energy source for heterotrophs (animals, bacteria and fungi). As a result of the ample light, water and nutrient supply in wetlands, the primary productivity of ecosystems dominated by wetland plants are among the highest recorded in the world. Associated with this high productivity is usually a high heterotrophic activity, i.e. a high capacity to decompose and transform organic matters and other substances. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 16
  17. 17. Wetland construction The presence or absence of aquatic macrophytes is one of the characteristics used to define wetlands, and as such macrophytes are an indispensable component of these ecosystems. In spite of the fact that the most important removal processes in constructed treatment wetlands are based on physical and microbial processes, the macrophytes possess several functions in relation to the water treatment. Vegetation and its litter are necessary for successful performance of constructed wetlands and contribute aesthetically to the appearance. The vegetation to be planted in constructed wetlands should fulfill the following criteria: • Application of locally dominating macrophyte species; • Deep root penetration, strong rhizomes and massive fibrous root; • Considerable biomass or stem densities to achieve maximum translocation of water and assimilation of nutrients; • Maximum surface area for microbial populations; • Efficient oxygen transport into root zone to facilitate oxidation of reduced toxic metals and support a large rhizosphere. Two species, Phragmites sp. and Typha sp., widely used vegetation in constructed wetlands. Phragmites karka and P. australis (Common Reed) is one of the most productive, wide spread and variable wetland species in the world. Due to its climatic tolerance and rapid growth, it is the predominant species used in constructed wetland. Fig 3.12 Phragmites kark (common reed) fig 3.13 Cattail-typha angustifolia Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 17
  18. 18. Wetland construction 3.2.2 SELECTION OF WETLAND PLANTS Floating and submerged plants are used in an aquatic plant treatment system. A range of aquatic plants have shown their ability to assist in the breakdown of wastewater. The Water Hyacinth (Eichhornia crassipes), and Duckweed (Lemna) are common floating aquatic plants which have shown their ability to reduce concentrations of BOD, TSS and Total Phosphorus and Total Nitrogen. However prolonged presence of Eichhornia crassipes and Lemna can lead to deterioration of the water quality unless these plants are manually removed on a regular basis. These floating plants will produce a massive mat that will obstruct light penetration to the lower layer of the water column that will affect the survival of living water organisms. This system is colonised rapidly with one or only a few initial individuals. The system needs to be closely monitored to prevent attack from these nuisance species. Loss of plant cover will impair the treatment effectiveness. Maintenance cost of a floating plant system is high. Plant biomass should be regularly harvested to ensure significant nutrient removal. Plant growth also needs to be maintained at an optimum rate to maintain treatment efficiency. Fig 3.14 The Water Hyacinth Eichhornia crassipes The Common Reed (Phragmites spp.) and Cattail (Typha spp.) are good examples of emergent species used in constructed wetland treatment systems. Plant selection is quite similar for SF and SSF constructed wetlands. Emergent wetland plants grow best in both systems. These emergent plants play a vital role in the removal and retention of nutrients in a constructed wetland. Although emergent macrophytes are less efficient at lowering Nitrogen and Phosphorus contents by direct uptake due to their lower growth rates (compared to floating and submerged Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 18
  19. 19. Wetland construction plants), their ability to uptake Nitrogen and Phosphorus from sediment sources through rhizomes is higher than from the water. Fig 3.15 The Common Reed Phragmites karka 3.2.3 EXAMPLES OF WETLAND PLANTS There is a variety of marsh vegetation that is suitable for planting in a CWTS (see Table 3.1 ). These marsh species could be divided into deep and shallow marshes. Table 3.1: List of emergent wetland plants used in constructed wetland treatment systems (Lim et al.1998) Planting zones Common name Scientific name Marsh and deep marsh (0.3-1.0m) Common Reed Phragmites karka Spike rush Eleocharis dulcis Greater Club Rush Scirpus grossus Bog Bulrush Scirpus mucronatus Tube Sedge Lepironia articulate Fan Grass Phylidrium lanuginosum Cattail Typha angustifolia Golden Beak Sedge Rhynchospora corbosa Spike rush Eleocharis variegta Sumatran Scieria Scieria Sumatran Globular Fimbristylis Fimbristylis globulsa Knot Grass Polygonum barbatum Asiatic Pipewort Erioucauio longifolium Shallow marsh(0-0.3m) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 19
  20. 20. Wetland construction 3.2.4 ROLE OF PLANTS The main role of the aquatic plants (please be sure to select only aquatic plants because of the always water saturated environment that is a fundamental aspect in constructed wetlands) is to act as catalyzers in the purification process. This process, as seen before, is a combination of microbiological, chemical and physical processes. The plants haven’t a significant action as direct removal (for some substances, like N and P or organic matter, we can talk of a contribution in the order of 10-20% during the vegetative season); they offer instead a very efficient support for the growth of aerobic bacteria colonies on their rhizomes. Air is pumped towards the root zone by several mechanisms, like convection. Another important plant’s function is the maintenance and continuous re-establishment of the hydraulic conductivity inside the beds. Amongst all macrophytes, Phragmites australis or communis is the most used worldwide for its optimal performances, for its ability in developing deep roots (0.5-0.7 m), for its resistance to aggressive wastewaters and to diseases. Table 3.2 Role of plants (Brix, 1997) Macrophyte property Role in treatment process  Light attenuation – reduced growth of phytoplankton  Reduced wind velocity – reduced risk resuspension  Aerial plant tissue Aesthetic pleasing appearance of system  Storage of nutrients  Filtering effect-filter out large debris  Plant tissue in water Reduced current velocity –Increase rate of sedimentation, reduced risk of suspension  Provide surface area for attached biofilms  Excretion of photosynthetic oxygen –Increases aerobic degradation  Uptake of nutrients Roots and rhizomes in  Stabilizing the sediment surface-less erosion the sediment  Prevents the medium from clogging in VFS  Release of oxygen increase degradation(nitrification)  Uptake of nutrients Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 20
  21. 21. Wetland construction 4. REMOVAL MECHANISMS A constructed wetland is a complex assemblage of wastewater, substrate, vegetation and an array of microorganisms (most importantly bacteria). Vegetation plays a vital role in the wetlands as they provide surfaces and a suitable environment for microbial growth and fi ltration. Pollutants are removed within the wetlands by several complex physical(sedimentation,filtration,adsorption and volatisation) chemical(precipitation, adsorption hydrolysis, oxidation/reduction) and biological (bacterial metabolism,plant metabolism, plant absorption,natural die-off processes as depicted in Fig 4.1 Fig 4.1 The pollutant removal mechanisms in constructed wetland (modified from Wetlands International, 2003) Settleable and suspended solids that are not removed in the primary treatment are eff ectively removed in the wetland by filtration and sedimentation. Particles settle into stagnant micro pockets or are strained by flow constrictions. Attached and suspended microbial growth is responsible for the removal of soluble organic compounds, which are degraded biologically both aerobically (in presence of dissolved oxygen) as well as anaerobically (in absence of dissolved oxygen). The oxygen required for aerobic degradation is supplied directly from the atmosphere Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 21
  22. 22. Wetland construction by diffusion or oxygen leakage from the vegetation roots into the rhizosphere, however, the oxygen transfer from the roots is negligible (Figure 4.1). Table 4.1 the pollutant removal mechanism in constructed wetland Waste water constituents Removal mechanism Nitrogen filtration  Aerobic microbial degradation Anaerobic microbial degradation  Matrix sorption  Phosphorus Sedimentation  Soluble organics   Suspended solids Plant uptake  Ammonification followed by microbial nitrification   Matrix adsorption  Ammonia volatilazation( mostly in SF system)  Adsorption and cation exchange  Complexation  Precipitation  Plant uptake  Microbial oxidation/ reduction  Sedimentation  Filtration  Natural die-off  Pathogens Plant uptake  Metals Denitrification Predation Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 22
  23. 23. Wetland construction Fig 4.2 Oxygen transfer from roots(modified from Wetlands International, 2003) 4.1 NITROGEN REMOVAL MECHANISMS There are sufficient studies to indicate some roles being played by wetland plants in Nitrogen removal but the significance of plant uptake vis-à-vis nitrification/denitrification is still being questioned. Nitrogen (N) can exist in various forms, namely Ammoniacal Nitrogen (NH3 and NH4+), organic Nitrogen and oxidised Nitrogen (NO2- and NO3-). The removal of Nitrogen is achieved through nitrification/denitrification, volatilisation of Ammonia (NH3) storage in detritus and sediment, and uptake by wetland plants and storage in plant biomass (Brix, 1993). A majority of Nitrogen removal occurs through either plant uptake or denitrification. Nitrogen uptake is significant if plants are harvested and biomass is removed from the system. At the root-soil interface, atmospheric oxygen diffuses into the rhizosphere through the leaves, stems, rhizomes and roots of the wetland plants thus creating an aerobic layer similar to those that exists in the media-water or media-air interface. Nitrogen transformation takes place in the oxidised and reduced layers of media, the root-media interface and the below ground portion of the emergent plants. Ammonification takes place where Organic N is mineralised to NH4+-N in both oxidised and reduced layers. The oxidised layer and the submerged portions of plants are important sites for nitrification in which Ammoniacal Nitrogen (AN) is converted to nitrite N (NO2-N) by the Nitrosomonas bacteria and eventually to nitrate N (NO3-N) by the Nitrobacter Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 23
  24. 24. Wetland construction bacteria which is either taken up by the plants or diffuses into the reduced zone where it is converted to N2 and N2O by the denitrification process. Denitrification is the permanent removal of Nitrogen from the system; however the process is limited by a number of factors, such as temperature, pH, redox potential, carbon availability and nitrate availability. The annual denitrification rate of a surface-flow wetland could be determined using a Nitrogen mass-balance approach, accounting for measured influx and efflux of Nitrogen, measured uptake of Nitrogen by plants, and sediment, and estimated NH3 volatilisation. The extent of Nitrogen removal depends on the design of the system and the form and amount of Nitrogen present in the wastewater. If influent Nitrogen content is low, wetland plants will compete directly with nitrifying and denitrifying bacteria for NH4+ and NO3-, while in high Nitrogen content, particularly Ammonia, this will stimulate nitrifying and denitrifying activity. Figure 4.3: Nitrogen transformations in a constructed wetland treatment system (Cooper et al.1996) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 24
  25. 25. Wetland construction 4.2 PHOSPHORUS REMOVAL MECHANISMS Phosphorus is present in wastewaters as Orthophosphate, Dehydrated Orthophosphate (Polyphosphate) and Organic Phosphorus. The conversion of most Phosphorus to the Orthophosphate forms (H2PO4-, Po42-, PO43-) is caused by biological oxidation. Most of the Phosphorus component may fix within the soil media. Phosphate removal is achieved by physical-chemical processes, by adsorption, complexation and precipitation reactions involving Calcium (Ca), Iron (Fe) and Aluminium (Al). The capacity of wetland systems to absorb Phosphorus is positively correlated with the sediment concentration of extractable Amorphous Aluminum and Iron (Fe). Although plant uptake may be substantial, the sorption of Phosphorus (Orthophosphate P) by anaerobic reducing sediments appears to be the most important process. The removal of Phosphorus is more dependent on biomass uptake in constructed wetland systems with subsequent harvesting. 4.3 BOD5 REMOVAL The physical removal of BOD5 is believed to occur rapidly through settling and entrapment of particulate matter in the void spaces in the gravel or rock media. Soluble BOD5 is removed by the microbial growth on the media surfaces and attached to the plant roots and rhizomes penetrating the bed. Some oxygen is believed to be available at microsites on the surfaces of the plant roots, but the remainder of the bed can be expected to be anaerobic. 4.4 PLANT UPTAKE Nitrogen will be taken up by macrophytes in a mineralised state and incorporated it into plant biomass. Accumulated Nitrogen is released into the system during a die-back period. Plant uptake is not a measure of net removal. This is because dead plant biomass will decompose to detritus and litter in the life cycle, and some of this Nitrogen will leach and be released into the sediment. Johnston (1991) shows only 26-55% of annual N and P uptake is retained in aboveground tissue, the balance is lost to leaching and litter fall. 4.5 METALS Metals such as Zinc and Copper occur in soluble or particulate associated forms and the distribution in these forms are determined by physico-chemical processes such as adsorption, precipitation, complexation, sedimentation, erosion and diffusion. Metals accumulate in a bed Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 25
  26. 26. Wetland construction matrix through adsorption and complexation with organic material. Metals are also reduced through direct uptake by wetland plants. However over-accumulation may kill the plants. 4.6 PATHOGENS Pathogens are removed mainly by sedimentation, filtration and absorption by biomass and by natural die-off and predation. 6.7 OTHER POLLUTANT REMOVAL MECHANISMS Evapotranspiration is one of the mechanisms for pollutant removal. Atmospheric water losses from a wetland that occurs from the water and soil is termed as evaporation and from emergent portions of plants is termed as transpiration. The combination of both processes is termed as evapotranspiration. Daily transpiration is positively related to mineral adsorption and daily transpiration could be used as an index of the water purification capability of plants. Precipitation and evapotranspiration influence the water flow through a wetland system. Evapotranspiration slows water flow and increases contact times, whereas rainfall, which has the opposite effect, will cause dilution and increased flow. Precipitation and evaporation are likely to have minimal effects on constructed wetlands in most areas. If the wetland type is primarily shallow open water, precipitation/evaporation ratios fairly approximate water balances. However, in large, dense stands of tall plants, transpiration losses from photosynthetically active plants become significant. Fig 4.4 Experimental studies continue to be carried out in Florida and many other parts of the country as well as overseas to evaluate the performance of a variety of constructed wetlands systems.( United States EPA(2003) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 26
  27. 27. Wetland construction 5. CASE STUDY’S 5.1 CASE STUDY 1 5.1.1 EXPERIMENTAL SET UP The constructed wetland (CW) treatment system is situated in Karachi, NED University of Engineering & Technology, at a longitude 25° 56‘8” N and latitude 67° 06’ 44” E. The maximum and minimum temperatures during the study period were 36° C and 20° C, respectively. NED wastewater treatment plant (WWTP) treats wastewater from campus and staff colony. The wastewater contains domestic sewage and low flows from laboratories of various university departments. The primary treatment consist of a settling tank, after that the wastewater is transferred to an aeration tank and then to a secondary settling tank. The main objective to construct the wetland at the WWTP site is to evaluate the performance of simple and low-cost wastewater treatment technology. The effluent entering the CW system (cell) comes from a WWTP which treats domestic and institutional wastewater. The key features of this CW are horizontal surface flow (HSF). The HSF CW was selected as this kind of system does not have the clogging problem. The constructed wetland is designed as a plug flow reactor and length to width ratio (L: W) is 4:1. The cell design consists of a rectangular bed, bordered with masonry work of 0.25 m wall and concrete based floor to protect seepage of wastewater. The cell is 0.30 m above and 0.30 m below the ground, so no external water enters into the cell from the natural ground surface. The system is designed for a flow of 1 m3/d. The design of this pilot-scale constructed wetland consists of a bed that is rectangular in shape and is planted with common wetland plant (Phragmites karka). Pre-treated wastewater is fed in the storage tank placed at the influent end of the wetland; the influent entering the CW is controlled manually by adjusting the valve attached to the inlet pipe. The flow between inlet and outlet is by gravity. PVC pipes at the inlet and outlet zone are slotted to distribute and collect the wastewater. This slotting method reduces short circuiting in the cell and the whole bed is utilized to treat the wastewater. The treated wastewater from the outlet zone is collected is a small ditch walled with masonry bricks. The inlet pipe has a diameter of 5 cm while outlet pipe diameter is Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 27
  28. 28. Wetland construction 10 cm (Fig.5.1). The soil used for wetland bed was first checked that no roots of any other plants are present. Then the cell was half filled with the soil in such a way that wastewater will flow towards the effluent end by making inclination of 5 degrees. The bed was filled with water to settle the soil and facilitate the growth of macrophytes. The main features of the constructed wetland at NED University are presented in Table 5.1 while Fig. 5.1 shows the layout of CW added to the treatment train. Fig 5.1 Pilot scale constructed wetland layout: (1) secondary wastewater effluent from wastewater treatment plant to storage tank, (2) control valve, (3) inlet pipe, (4) constructed wetland (5) outlet pipe, (6) water level control box. Table 5.1: Main features of the constructed wetland Parameter Unit Value Length Metre 6 Width Metre 1.5 Height Metre 0.6 Surface area Metre 9 Hydraulic retention time Days 4 Flow Cubic metre per day 1 vegetation Metre 4 plants Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 28
  29. 29. Wetland construction 5.1.2 OPERATIONS Young plants of Phragmites karka were collected from the campus surroundings in late May, 2010. The plants were transplanted the same day in the bed at a density of four plants per square meter. The cell was filled with fresh water so that the macrophytes adapt to the new environment. Later on, the cell of CW was loaded with settled domestic wastewater from the campus and staff colony. Sampling scheme was initiated after an acclimatisation period of 6 weeks. For wastewater analysis, grab samples were collected in plastic bottles previously washed and rinsed with distilled water. Samples were collected after every two weeks from the inlet and outlet of the CW. Liquid samples were tested for pH, total dissolved solids (TDS), total suspended solids (TSS), 5 days biochemical oxygen demand (BOD5) at 20°C, chemical oxygen demand (COD), ammonia nitrogen (NH4-N) ortho-phosphate (PO4-P), temperature, dissolved oxygen (DO), faecal coliforms (FC), total coliforms (TC) using American Public Health Association standards methods . All samples were analysed in the laboratory of environmental engineering department. 5.1.3 RESULT During the eight months monitoring period which started in the month of September, 2010 and continued till late April 2011, samples were collected from the CW system and analysed for each of the various physical, chemical and microbiological parameters. Table presents the average inlet and outlet concentrations of each monitored parameter. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 29
  30. 30. Wetland construction Table 5.2 : water quality variables mean ± (standard deviation) for the constructed wetland S. No Variables Unit n Inlet Outlet Reduction(%) 1 BOD mg/L 16 68.6±23.6 34.0±15.5 50 2 COD mg/L 16 122.9±50.7 68.3±20.7 44 3 TSS mg/L 16 201.4±93.2 45±26.3 78 4 Ammonia- mg/L 16 19.2±4.8 9.7±4.6 49 nitrogen 5 Ortho-phosphate mg/L 16 7.6±1.9 3.7±2.3 52 6 Total coliforms Counts/100mL 12 2.1x106 8x103 93 7 Faecal coliforms Counts/100mL 12 1.1x106 3x103 98 8 ph _ 16 7.8±0.7 7.9±0.4 _ 9 Dissolved mg/L 16 1.7±0.6 4.5±1.2 _ oxygen 5.2 CASE STUDY 2 5.2.1 EXPERIMENTAL SET UP Yadav S. B., Jadhav A. S(2011) construct wetland as combined with surface flow and planted with Eichhornia crassipes was built near Technology Department, Shivaji University, Kolhapur. ( Latitude 160 40’ N, Longitude 740 15’ S). Maharashtra situated in western part of India. The climate of this region is tropical with an average annual rainfall of 1025 mm. The mean minimum and maximum temperatures during the study period were 15° C and 34°C respectively. The integrated subsurface flow artificial wetland constructed with brickwork of size 3 m × 1.25m having a depth of 0.6 m. The tank is plastered in cement mortar on both sides. The bottom of the tank is made up of 1:4:8 concrete portions to make it watertight. The necessary slope is provided at the bottom of the tank for sludge removal. The constructed wetland cell is of 3.06 sq m area and length to width ratio 2.45:1.25. The depth of the bed is 0.6 m. The bed has regular rectangular shape and a necessary slope is provided at the bottom of the tank for sludge removal. The inlet and outlet chambers are made up of PVC pipe of 12.5 m in diameter with a Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 30
  31. 31. Wetland construction control valve. At the base of the wetland unit two pipes of 10 cm diameter having inward holes at every 0.9 m distance run parallel in longitudinal direction from inlet chamber to outlet chamber to facilitate the drainage. OPERATIONS The campus wastewater was let into the constructed wetland intermittently over 30 days. The plants were monitored for general appearance, growth and health. The length of the plant was found to be similar to that of wetland plants in natural wetlands. Any invasive plants like ordinary grass were uprooted and removed immediately. The plant density spread vigorously within 2 months. RESULTS The overall system treatment performance was high and stable during the observation period Table 5.3: Physico-chemical parameters and percent removal at inlet and outlet of Set A. Sl.No Parameter Inlet Outlet Average Range Average Range % Average removal 1 pH 7.1-7.4 6.3 7-7.3 7.2 - 2 DO 0 0 3.4-7.1 5.56 - 3 BOD 122-1 163.2 14-21 21.9 86.19% 4 COD 169.2-176.40 171.9 18-25 23.10 87.35% 5 TSS 135-142 167.1 20-59 41 76% 6 TDS 465-505 537 99-120 199.1 69.95% 7 TN 13.8-14.95 17.3 6.95-7.98 9.08 43.33% 3.14-3.9 4.4 1.6-2.01 1.99 45% 8 Total phosphorus Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 31
  32. 32. Wetland construction Table 5.4: Physico-chemical parameters and percent removal at inlet and outlet of Set Sl.No Parameter Inlet Outlet Average % Range Average Range Average removal 1 pH 7.2-7.8 7.6 7.1-7.6 7.3 - 2 DO 0-0.1 0 4.9-6.2 7.1 - 3 BOD 132.4-141.8 136.9 9-24 22.6 95.8% 4 COD 167.4-178.8 173.45 10-28 24.9 97% 5 TSS 136.80-139.6 138.54 14-55 32.45 82% 6 TDS 467.4-486 433 90-110 109.81 71% 7 TN 14.01-14.42 14.20 7.12-7.82 6.79 43.07% 8 Total phosphorus 3.45-3.66 3.25 1.42-1.92 1.5 49.03% Fig.2: Water Weed: Eichhornia crassipes; Fig.3: Filling the C.W. by sewage; Fig.4: Placing the Eichhornia crssipes in the C.W.; Fig.5: Growth of Eichhornia crssipes Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 32
  33. 33. Wetland construction 6. CONCLUSION It is noted that CWs are now being increasingly used for environmental pollution control. Recent inventories have indicated that there are more than 7000 CWs in Europe and North America with the number increasing in Central and South America, Australia and New Zealand as well as Asia and Africa. Most of them are local soil/gravel based systems with either horizontal or vertical flow planted mostly with common reeds (phragmites australis). Constructed wetlands were implemented in a wide range of applications, such as water quality improvement of polluted surface water bodies, wastewater on-site treatment and reuse in rural areas, campuses, recreational areas and green architectures, management of aquaculture water and wastewater, tertiary treatment, and miscellaneous applications. Water monitoring results obtained from several demonstrations show that CWs could achieve acceptable wastewater treatment performances in removing major pollutants, including suspended solids, organic matters, nutrients, and indicating microorganisms, from wastewater influent. The results indicate that if constructed wetlands are appropriately designed and operated, they could be used for secondary and tertiary wastewater treatment under local conditions, successfully. Hence constructed wetlands can be used in the treatment train to upgrade the existing malfunctioning wastewater treatment plants, especially in developing countries. During hydraulic retention study, it was found that the BOD, COD was best removed in planted wetland than unplanted wetland. It is because of the oxygen diffusion from roots of the plants and the nutrient uptake and insulation of the bed surface. It is also found that the increases in the detention period of the wastewater the removal rate also increases. Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 33
  34. 34. Wetland construction 7. REFERENCE 1. Borkar.R.P , Mahatme.P.S(2011),” Wastewater Treatment with Vertical Flow Constructed Wetland” , International journal of environmental sciences Vol 2(2), pp 590 -603 2. Asghar Ebrahimi, Ensiyeh Taheri(2013)” Efficiency of Constructed Wetland Vegetated with Cyperus alternifolius Applied for Municipal Wastewater Treatment “, Hindawi Publishing Corporation Journal of Environmental and Public Health, Vol. 2013,pp 1-5 3. Hafeznezami1; Jin-Lee Kim(2012)” Evaluating Removal Efficiency of Heavy Metalsh in Constructed Wetlands”,journal of environmental engineering © asce / april 2012 ,pp 475482. 4. Atif Mustafa( 2013),” Constructed Wetland for Wastewater Treatment and Reuse:A Case ”, International Journal of Environmental Science an Study of Developing Country Development, Vol. 4(1),pp 20-24 5. Yadav S. B., Jadhav A. S(2011),” Evaluation of Surface Flow Constructed Wetland System by Using Eichhornia crassipes for Wastewater Treatment in an Institutional Complex”, universal Journal of Environmental Research and Technology All Rights Reserved Euresian publications ,Vol.1,pp 435-441 6 Chavan B.L, and Dhulap V.P.(2013),” Developing a Pilot Scale Angular Horizontal Subsurface Flow Constructed Wetland for Treatment of Sewage through Phytoremediation with Colocasia esculenta”, International Research Journal of Environment Sciences Vol. 2(2), pp 6-14 7. Dr. joachim schrautzer, Ralph otterpohl(2009),” constructed wetlands: potential for their use in treatment of grey water in kenya’’, environmental management management natürlicher resourcen,pp 1-77 8. “The use of constructed wetlands for wastewater treatment”, Wetlands International Malaysia-Office(2003) 9. “ Constructed wetland manual”, United Nations Human Settlements Programme (UNHABITAT),( 2008) Dept. of Civil Engineering, M.E.S College of Engineering, Kuttippuram Page 34