Tài liệu Wetland - An introduction docx

WETLAND - AN INTRODUCTION … Le Anh Tuan Catholic University of Leuven, Belgium December, 2003 oOo I. WHAT IS WETLAND ? Wetland can be understood as an ecological system lie on a continuum between uplands, where excessive water is not a factor for plant growth, and deeply flooded lands, or aquatic systems, where flooding excludes rooted emergent vegetation. Figure 1 shows how to describe this wetland concept. Figure 1. Wetland in general meaning (Kadlec & Knight, 1996) More than 100 years before, in many cities, towns and villages around the whole world, natural wetlands have been used as convenient wastewater discharge sites as long as sewage has not been collected (Kadlec and Knight, 1996). There is over 6 percent of land surface of the world, or 8.6 million km2, is wetland (Bazilevich et al, 1971, Maltby & Turner, 1983). In natural wetlands, the 5-day Biochemical oxygen demand (BOD 5 ) from wastewater can be removed by the support of a large and diverse population of bacteria which grow on the submerged roots and stems of aquatic plants. These plants have also the ion exchange or adsorption capacity in wastewater. In addition, the wastewater solids will be accumulate in the wetland bed by their quiescent water conditions. Other aspects of wetlands are considered as determining factors on climatic stable and balance. Wetlands are residence places for many kinds of wildlife as birds, retiles, amphibians, and so on. Over the past 20 years, the application of constructed wetlands (CW) for domestic and municipal pollution control has been rediscovered and has gradually developed in many European countries and many parts of the world today. The most advantage of CW is their simple construction, low energy process requiring minimal operational cost. However, constructed wetland treatment may be economical relative to other options only where land is available and affordable. Their performance may be less consistent than in conventional treatment. Otherwise, the biological components are U p land Wetland A q uatic Arbitrar y Arbitrar y Hi g h water table Low water table Hi g h water Low water Seasonally saturated soils Seasonally flooded sensitive to toxic chemicals, such as ammonia and pesticides, flushes of pollutants or surges in water flow may temporarily reduce treatment effectiveness . The water and environmental scientist found that CW could be applied as a useful domestic wastewater treatment system for small village communities with populations of 50 – 1000 person equivalents. Wetland treatment systems use aquatic plant species and shallow, flooded or saturated soil conditions to provide various types of wastewater treatment. There are two basic types of CW treatment systems; i.e. constructed surface flow (SF) wetlands and constructed subsurface flow (SSF) wetlands. According to IWA (2000), SF wetlands are densely vegetated by a variety of plant species and typically have water depths less tan 0.4 m. Open water areas can be incorporated into a design to provide for the optimization of hydraulics and for wildlife habitat enhancement. SSF wetlands use a bed of soil or gravel as a substrate for the growth of rooted emergent wetland plants. The bed depth in SSF wetlands is typically between 0.6 and 1.0 m, and the bottom of the bed is sloped to minimize water flow overland. According to Melbourne Water, 2002, There are three main elements to wetland systems – pre-treatment, inlet zone and macrophyte or wetland zone. An Ephemeral Zone may also be required if the pre-treatment element is insufficient. Each has performance criteria that are described below and illustrated in Figure 2. Figure 2: Schematic representation of a constructed wetland system (Melbourne Water, 2002) During the period 1960 – 1980, Seidel and her co-workers at the Max Planck Institute in Germany has developed the origins of SSF wetland technology (Seidel, 1976; Kickuth, 1977). Numerous studies on water and wastewater treatment with wetlants plants have been published from 1955 through that lat 1970s. In mid-1985, the British Water Research Centre first proved the potential of horizontal flow through the reed bed treatment systems in improvement the quality of wastewater. Between 1985 and 1990, Weyerhaeuser Company began two separate pilot studies of SSF wetland systems to treat pulp and paper mill effluents. Treatment troughs were planted with cordgrass (Spartina cynosuroides), cattail (Typha latifolia) or common reed (Phragmites australis) (Thus, 1989, 1990 a,b, 1993); Since 1985 to now, hundreds of CW systems were built around the world, special in European countries (Austria, Belgium, Denmark, France, Germany, Sweden, Switzerland, the Netherlands, United Kingdom), North America, Australia and Asia (China and India). In September 1990, the International Conference on Constructed Wetlands was held in Cambridge for presenting the European Design and Operations Guidelines for Reed Bed Treatment Systems (Cooper & Findlater, 1990). SSF wetlands that use gravel substrates have been used extensively in the United States (Reed, 1992). From last 10 years to now, there many scientists in many different fields have a great and wide research for several solutions concerning with the wetland systems. Besides many progress measure achievements in hydrology, ecology, chemistry, bio-environment, natural resources management,… many mathematical and physical models for wastewater flow in constructed wetlands have been established. Gerald A. Moshiri, 1993, have listed major events notable first include the use of constructed wetlands for treating: • 1956 - livestock wastewaters - experimental; Seidel • 1975 - petroleum refinery wastewaters - operational; Litchfield • 1978 - textile mill wastewaters - operational; Kickuth • 1978 - acid mine drainage - experimental; Huntsman • 1979 - fish rearing pond discharge - operational; Hammer and Rogers • 1982 - acid mine drainage - operational; Pesavento • 1982 - reduction of lake eutrophication - experimental; Reddy • 1982 - urban stormwater runoff - operational; Silverman • 1983 - pulp/ paper mill wastewaters - experimental; Wolverton • 1985 - seafood processing wastewaters - experimental; Guida and Kugelman • 1988 - compost leachate - operational; Pauly • 1989 - sugar beet processing plant wastewaters; Anderson • 1989 - reduction of lake eutrophication - operational; Szilagyi • 1990 - harbor dredged materials - experimental; Pauly • 1991 - pulp/paper mill wastewaters - operational; Thut Concerning mathematical and physical models for describing treatment wetlands, we may get many published technical papers. Jerald L. Schnoor (1996) have published to introduce groundwater contaminant solute transport equations. Datta (2002) have considered convection-dispersion equations in a flowing stream and in a saturated porous solid. R.H. Kadlec (2002) have reported, in Elsevier Science Journal, his observations on effects of pollutant speciation in treatment wetlands design, including potential effects of distributions of detention times and first order removal rate constants. Lin, et al. (2003) have performed their hydraulic tracer tests in the Predo Wetlands, Riverside County, California, USA and evaluated by comparing the breakthrough curve (BTC) of Rhodamine WT® and bromide in the determination of hydraulic characteristics of constructed wetlands. II. CONSTRUCTED WETLAND DESIGN PROCEDURE II.1. Types of constructed wetland Constructed wetlands can be classified by their treatment functions. Figure 3 shows the types of constructed wetland. Figure 3. Types of constructed wetland Based on the ecological engineering of natural wetlands, the constructed free water surface (FWS) wetlands were build for wastewater treatment purposes mainly. They are considered as the mimic hydrological regime in small-scale shallow basins constructed of soil and aquatic plants with their water balance: water flows in and out over the soil surface and losses to evapo-transpiration and infiltration within the wetlands. Although not all wetland species are suitable for wetland treatment (R.H. Kadllec et al, 2000), but we can find the common emergent macrophytes such as: common reed (Phragmites australis), bulrushes (Scripus spp.), spikerush (Eleochris spp.), cattail (Typha spp.), …; floating plant species: water hyacinth (Eichhornia crassipes), duckweed (Lemma spp.), water spinach (Ipomoea aquatica), …; floating- leaved, bottom-rooted macrophytes such as: water lilies (Nymphaea spp.), lotus (Nelumbo spp.), cowlilies (Nuphar spp.), …; floating mats such as: cattail (Typha spp.), common reed (Phragmites australis), …; and submerged aquatic plants as: waterweed (Elodea spp.), water milfoil (Myriophyllum spp.), naiads (Najas spp.), … Depending on the characteristics of constructed soil, water and kind of conspicuous plants (macrophytes), we can distinguished following the figures 4 below: CONTRUCTED WETLAND Free water surface (FWS) treatment wetland FWS wetland with emergent macrophytes FWS wetland with free floating macrophytes FWS wetland with floating-leaved, bottom-root macrophytes FWS wetland with floating mats FWS wetland with submersed macrophytes Subsurface Flow (SSF) treatment wetland Horizontal-flow systems Vertical -flow systems Figure 4: Types of Free water surface treatment wetlands Outlet weir/pipe Inlet pipe Lined basin (b) FWS wetland with floating plants Outlet weir/pipe Inlet pipe Low-permeability soil (a) FWS wetland with emergent macrophytes Outlet weir/pipe Inlet pipe Lined basin (c) FWS wetland with rooted, floating leaf plants Outlet weir/pipe Inlet pipe Low-permeability soil (d) FWS wetland with floating emergent macrophyte mats Outlet weir/pipe Inlet pipe Low-permeability soil (e). FWS wetland with submerged macrophytes The SSF wetlands are designed as a basin or channel with a boundary to prevent seepage and a suitable depth bed of porous media that support the emergent plants. The wastewater will flow in a high level site of the wetland and then flow through the porous media of sand soil and plant rhizosphere. The wastewater is treated by the physical-chemical and biochemical complex processes of filtration, sorption and precipitation processes in the soil and by microbiological degradation. Finally, the treated wastewater flow out in the bed that remain below the top of the gravel/stone/rock media. The SSF systems have also several other names, such as: vegetated submerged bed, root zone method, microbial rock reed filter, and plant-rock filter systems. The SSF wetland types have several advantages if compared with the FWS wetland types. It is found that in the constructed SSF wetland, the available of wastewater treatment is better than the constructed FWS one. Wastewater flowing subsurface media may also avoid of the little risk of heavy odors, dark-color exposure and insect vectors effects. The area application for SSF wetland can be smaller than a FWS system with the same wastewater withdraw conditions. Constructed soil-and gravel based subsurface flow (SSF) wetlands are common used to treat mechanically pretreated municipal wastewaters in many places in the world. There are two types of constructed SSF wetlands, i.e. horizontal-flow systems and vertical-flow systems (Figure 5). As their names, this classification is based on the flow direction to the soil-and gravel layers. This technology is generally limited to systems with low flow rates and can be used with less than secondary pretreatment (Kadllec et al, 2000). In most of the systems in the United States, the flow path is horizontal, although some European systems use vertical flow paths (USDA - NRCS, EPA - Region III). The Mekong delta (MD), the most downstream part of the Mekong river, is draining an area of 600,000 km 2 along a course of 3,650 km through China, Miamar, Thailand, Laos, Cambodia before entering Vietnam. In topographical speaking, the MD is typified by its flatness, especially in the lower part, the average slope of land is about 1/100,000 (1 cm/km). In high flood period, haft of the Delta is partly submerged. The inundation has recorded that 2.5 m depth near the border of Cambodia - Vietnam to 0.7 m northwest of My Tho. Along the 600 km-coastal areas, the tide effects strongly the hydrological regime of the MD. Generally, there are two kinds of tidal variations: the semidiurnal tide (twice daily) in the East Sea and the diurnal tide (daily) in the Gulf of Thailand. These tides result in a complex movement to the rivers and canals and also an intrusion of saline water in the downstream part of the delta. These hydrological characteristics make the Delta become a wetland continually. In the Mekong River Delta, it seems fitting with the horizontal-flow systems more than vertical one due to its upper groundwater level is too high, nearly remains below the land surface a few decimeters. However, there is a great lack of wetland hydraulic research in Vietnam, especially in the Mekong Delta. From the general view, natural wetlands have been used as convenient floodwater and wastewater discharge sites for as long as drainage systems have not been collected. Figure 5: General arrangement for the constructed wetland with (a) a horizontal flow and (b) a vertical flow Inlet flow Outlet flow Outlet collector Plants Big stones Big stones Im p ermeable liner Medium (gravel, sand, crushed stones) Water level maintained (a) Longitudinal section of a constructed wetland with horizontal SSF (Vymazal, 1997, modified) 25 cm ~ 8 cm 6 mm-washed pea-gravel "shar p " sand ~ 15 cm ~ 10 cm ~ 15 cm 12 mm-round washed- g ravel 30-60 mm-round washed- g ravel Perforated pipe ( ~110 mm o.d. ) Solid pipe Feed dosed intermittently over whole surface 1% slope Large stones Network of agricultural drainage pipes LDPE liner Free-draining outlet (b) Typical arrangement of a vertical reed bed system (Cooper, 1996, modified) II.2. Design procedures In general speaking, the constructed SSF wetlands can be designed and built almost anywhere that the emergent plant species can be planted and grown up. In the winter freezing areas, the application of the constructed wetlands may be limited. When designing a constructed wetland, some considerations should be noted: • Site selection + Topography + Soil types and their permeability + Hydrological factors + Water and land use rights + Social, environmental and public health considerations • Treatment expectations + BOD 5 removal + Suspended solids (SS) removal + Nitrogen removal + Phosphorus removal + Heavy metals removal + Refractory organics removal + Bacteria and virus removal • Process variables + Design objectives + BOD 5 loading rates + Hydraulic loading rates + Water depth (in FWS systems) + Detention time • Pre-application treatment • Vegetation/ plants + Emergent plants + Submerged aquatic plants • Physical design factors + System configurations + Distribution system + Outlet structures + Vector control (in FWS wetland cases) + Vegetation harvesting • Costs and maintain + Study plan and permits + Land acquit + Construction + Access ways + Operating and monitoring costs + Other expenditures . WETLAND - AN INTRODUCTION … Le Anh Tuan Catholic University of Leuven, Belgium December, 2003 oOo I. WHAT IS WETLAND ? Wetland can be understood. treatment wetland FWS wetland with emergent macrophytes FWS wetland with free floating macrophytes FWS wetland with floating-leaved, bottom-root macrophytes