建立廢水處理濕地系統(tǒng)Constructed-Wetlands-for-Wastewater-Treatment課件

1、Constructed Wetlands for Wastewater TreatmentDesign and Construction1. Types and Functions of Constructed WetlandsConstructed Wetlands (CW) are classified into two types: free water surface (FWS) system with shallow water depths and subsurface flow (SF) or vegetated submerged bed (VSB) systems with

2、water flowing through the sand or gravel.1.1 Free Water Surface SystemsThese system typically consist of basins (盆地) or channels, with a natural or constructed subsurface barrier of clay or impervious (密封的) geotechnical material to prevent seepage, soil or another suitable medium to support to emerg

3、ent vegetation, and water at a relatively shallow depth flowing over the soil surface (Fig. 1.)1. Types and Functions of Constructed Wetlands1.1 Free Water Surface SystemsThe shallow water depth, Low flow velocity (速度), andPresence of the plant stalks and litter regulate water flow and, especially i

4、n long, narrow channels.1. Types and Functions of Constructed Wetlands1.2 Subsurface Flow (SF) SystemsAn SF system typically consists of a trench (溝渠) or a bed underlain by impermeable material to prevent seepage and containing a medium that supportsthe growth of emergent vegetation (Fig. 2).The med

5、ia used have included rock or crushed stone (10-15 cm diameter), grave, and different soils, either alone or in various combinations.The wastewater flows laterally through the medium and is purified during the contact with the surfaces of the medium and the root zone of the vegetation.1. Types and F

6、unctions of Constructed Wetlands1.2 Subsurface Flow (SF) SystemsThis subsurface zone is continuously saturated and therefore is generally anaerobic.However, the plants can convey an excess of oxygen to the root system, so there are aerobic microsites adjacent to the roots and rhizomes.1.3 Advantages

7、 and DisadvantagesMost wetland constructed for MCMD treatment have been surface flow wetlands.Of the two types, surface flow wetlands are simpler to design and construct and require simpler inlet distribution structures1. Types and Functions of Constructed Wetlands1.3 Advantages and DisadvantagesEme

8、rgent wetland vegetation is used in surface flow systems.A water depth can be selected that is optimum for the chosen wetland species.Invasive weeds are unlikely to become established in flooded conditions.Surface flow systems are, however, subject to ice-cover in cold climates.Effective water depth

9、 is reduced and hence retention time is reduced.Some water storage facility may be necessary for the winter months.1. Types and Functions of Constructed Wetlands1.3 Advantages and DisadvantagesSubsurface flow systems provide for greater contact between wastewater and substrate But are prone to clogg

10、ing (堵塞) with precipitated metal hydroxides and may experience problems with invasive weeds.Design is more difficult and a constant flow rate is important to avoid breakthrough.Influent distribution structures require closer attention as does choice of substrate.2. Wastewater Treatment MechanismsWet

11、land system can significantly reduce biochemical oxygen demand (BOD5). Suspended solids (SS), and nitrogen, as well as metals, trace organics, and pathogens (病菌, 病原體).The basic treatment mechanisms include sedimentation (沉淀, 沉降), chemical precipitation and adsorption, and microbial interaction with

12、BOD5, SS and nitrogen, as well as some uptake by the vegetation.2.1 BOD RemovalThe removal of settleable (會(huì)沉淀的) organics in very rapid in all wetland systems and is due to the quiescent condition (靜態(tài)) in the FWS systems, and to deposition (沉積作用) and filtration (過濾) in the SF systems.2. Wastewater Tr

13、eatment Mechanisms2.1 BOD RemovalIn FWS wetlands, removal of the soluble BOD is mainly due to the attached microbial growth.The major source of oxygen for these reactions is reaeration at the water surface.The major oxygen source for the subsurface components (soil, gravel, rock and other media, in

14、trenches or beds) are the gases transmitted by the vegetation to the root zone. The selection of plant species can therefore be an important factor.Table 2 reports the removal efficiencies of BOD5 and SS from CW in Canada, U.S.A, and Australia.2. Wastewater Treatment Mechanisms2.2 Suspended Solids (

15、SS) RemovalSS removal is very effective in both types of CW (see Table 2).Most of the removal occurs within the first few meters beyond the inlet, Owing to the quiescent conditions (靜態(tài)) and the shallow depth of liquid in the system.Controlled dispersion (散布) of the influent flow with proper diffuser

16、 pipe design can help to ensure low velocities for solids removal.2. Wastewater Treatment Mechanisms2.3 Nitrogen RemovalNitrogen is mainly removed by nitrification (硝化作用)/denitrification (反硝化作用).Other removal mechanisms include plant uptake and volatilization (揮發(fā)).In CW, nitrogen removal ranges from

17、 25 to 85%.2.4 Phosphorus RemovalPhosphorus removal in wetlands is not very effectiveBecause of the limited contact opportunities between the wastewater and the soil. The principal mechanisms for P removal are plant uptake or retention in the soil.2. Wastewater Treatment Mechanisms2.4 Phosphorus Rem

18、ovalIf P removal in wetlands is required, clay with Fe and Al content should be considered.However, soils with a high P removal capacity are finer textured, and sand may be added to imporve hydraulic conductivity.Also, Fe or Al added to the substrate or fed into the wastewater can improve P removal.

19、2. Wastewater Treatment Mechanisms2.5 Heavy Metal RemovalMetals are removed in treatment wetlands by three major mechanisms (Kadlec and Knight 1996):Binding to soils, sediments, particulates, and soluble organics by cation exchange and chelation.Precipitation as insoluble salts, principally sulfides

20、 and oxyhydroxidesUptake by plants, including algae, and by bacteria.While the first two mechanisms along with microbial uptake are the predominant pathways of metal removal in the treatment wetlands.2. Wastewater Treatment Mechanisms2.6 Trace Organics RemovalMunicipal and industrial wastewaters con

21、tain variable concentrations of synthetic organic compounds.Adsorption of trace organics by the organic matter and clay particles present in the treatment system is thought to be the primary physicochemical mechanism for removal of refractory compounds (難熔化合物) in wetlands.Other mechanism can be biol

22、ogical degradation of easily degraded organic compounds, sedimentation and volatilization (揮發(fā)).2. Wastewater Treatment Mechanisms2.7 Pathogen (病菌, 病原體) RemovalThe pathogens of concern in CW are parasites (寄生蟲), bacteria and viruses.Pathogenic bacteria and viruses are removed by such mechanisms preda

23、tion (掠奪), sedimentation, absorption, and die-off from unfavorable environmental conditions,Including UV in sunlight and temperatures unfavorable for cell reproduction.Table 3 reports performance data on pathogen removal for both FWS and SF wetlands in the US and Canada.3. Design consideration3.1 Si

24、te SelectionA CW can be constructed almost anywhere.Because grading (坡度緩和) and excavating (挖掘) represent a major cost factor, topography (地形) is an important consideration in the selection of an appropriate site.In selecting a site for FWS wetland, the most desirable soil permeability (滲透性) is 10-6

25、10-7 m/sec. sandy clays and silty clay loams can be suitable when compacted.In heavy soils (難耕的土地), additions of peat moss or top soil will improve permeability and accelerate initial plant growth.3. Design consideration3.2 Preliminary design considerationsPreliminary design considerations include t

26、he chemistry of drainage and the hydrology (水文) of the area.Comparison of drainage chemistry with regulatory requirements will enable the assessment of the degree to treatment required and partially answer the question of whether wetland treatment is appropriate.Possession of background information

27、on mine drainage composition is crucial.3.2 Preliminary design considerationsGood knowledge of the surface and groundwater (地下水) hydrology is the key to success.A complete water budget for the site should be constructed, including investigation of all inflows and outflows of surface water, rainfall,

28、 estimation of losses from evaporation and transpiration and Subsurface water losses and gains.3. Design consideration3. Design consideration3.2 Preliminary design considerationsOther preliminary design considerations include the availability and cost of plants and substrate, The long-term cost of t

29、he wetland compared to other treatment systems.3.3 Configuration (配置, 外形)To minimise the risk of hydraulic (水力的) short-circuiting, and to provide operating flexibility (彈性,機(jī)動(dòng)性), large wetlands are best divided into smaller cells. The topography (地形) of a site will often demand the use of cells also.

30、Cells may be arranged in series, in parallel, or as a combination of the two. As shown in Fig. 9.13. Design consideration3.4 Flow Patterns A CW cell is designed to use one or more of three types of flow patterns: plug flow (栓活塞流), step feed (分級(jí)進(jìn)給), or recirculation (Fig. 3).Plug flow (Fig. 3a) is on

31、ce-through flow down the cell length.Plug flow is now used for most municipal and acid drainage systems and requires minimal piping, energy use, operation, and maintenance.3. Design consideration3.4 Flow PatternsStep feed (Fig. 3b) may benefit pollutant removal by using more of the SW area for solid

32、s removal and by providing carbon for N removal in the lower bed area.Step feed is typically combined with recirculation.Recirculation: Recycling treated effluent will dilute influent BOD5 and SS, decreasing odor potential and increasing dissolved oxygen concentration and retention timeWhich will en

33、hance nitrification and subsequent N removal.3. Design consideration3.5 Size and shapeSize and shape of cells can be determined using theoretical design equations or empirical (完全根據(jù)經(jīng)驗(yàn)的) rules of thumb.There are many empirical design suggestions in the literature.3. Design consideration3.6 SlopeA slo

34、pe of 0.5 or less is recommended for an FWS system.Some slop is needed to drain the cell for maintenance and possible mosquito control.SF bed slopes should be 2% or less based on the initial hydraulic (水力的) conductivity (傳導(dǎo)性) of the substrate.A level substrate surface is recommended for an SF system

35、 to facilitate vegetation planting and control weed by flooding the bed.3. Design consideration3.7 LinersIf groundwater contamination or water conservation is a concern, an impermeable (不滲透性的) liner below the substrate is required.Possible materials are compacted (緊湊的, 緊密的) in situ soil (permeabilit

36、y less than 10-6 cm/sec);Bentonite 斑脫土(火山灰分解成的一種粘土), synthetic butyl rubber (丁基橡膠), or plastic membranes.The liner must be strong, thick, and smooth to prevent root penetration and attachment.3. Design consideration3.8 SubstrateThe provision of an appropriate substrate can be the key to successful p

37、lant establishment.The selection of a suitable substrate is based on horticultural (園藝的) principles, i.e. plants need support, ability for roots to grow downwards and nutrients.In subsurface flow wetlands a specific substrate will be required to support plant growth whilst allowing water movement.A

38、variety of substrates have been used for the treatment of MCMD.These have included straw (稻草, 麥桿) with Sphagnum peat (泥煤, 泥炭塊), straw with clay, mushroom compost (堆肥), forest litter, and manure (肥料).3. Design consideration3.8 SubstrateIt may be advantageous to underlay the substrate with a layer of

39、crushed limestone.As the limestone is under anaerobic conditions, significant neutralization of the MCMD should result.This layer will also improve hydraulic conductivity (傳導(dǎo)性) but care should be taken to avoid hydraulic short-circuiting.Gravel (砂礫) can be incorporated into a substrate to increase p

40、ermeability.The substrate should be analyzed prior to use to assess hydraulic conductivity, pH, buffering capacity, plant nutrient concentrations, and microbial activity.3. Design consideration3.8 SubstrateThe depth of substrate influences retention time in a subsurface flow wetland.In a surface flo

41、w wetland, water depth is the most import determinant of retention time.Factors influencing the selection of depth include:desired retention time (subsurface flow wetland), depth of root penetrationA maximum root penetration of 0.3 m has been observed for Typha latifolia and one of 0.6 m for Phragmi

42、tes.cost of substrate, climate, and design for sulphate reduction.3. Design consideration3.8 SubstrateSulphate-reduction potential can be enhanced by increasing the size of the anaerobic zone and increasing contact of the wastewater with this zone.The former can be accomplished with deeper substrate

43、s, 0.75 m or so, the latter with appropriate inlet design.The United States Bureau of Mines suggests substrate comprising (包含) 0.05-0.10 m gravel and 0.30 0.45 m of compost for coal mine drainage treatment using surface flow systems.3. Design consideration3.9 Inlet and outlet structuresInlet and out

44、let structures for surface flow wetlands are usually simple.Pipes, channels and spillways (泄洪道) are typically used.They should be sited to minimize hydraulic short-circuiting and sized to accommodate (向.提供) the maximum flow envisaged.3. Design consideration3.9 Inlet and outlet structuresInlet and ou

45、tlet structures for subsurface flow systems are need more attention.Uniform flow distribution, across both width and depth of the bed, is important.Inlets can be uniformly spaced holes, slots (縫), or serrations(鋸齒) in pipes or channels across the width of the cell.Inlets above the bed allow for main

46、tenance and adjustment and also provide aeration.3. Design considerationInlet and outlet structures for subsurface flow systems are need more attention.Large, coarse gravel placed beneath inlets will assist uniform distribution of MCMD.Outlets can be perforated pipes (多孔管) buried in coarse gravel (粗

47、砂礫).Inlet and outlet structures for subsurface flow systems can be designed to provide upflow or downflow configurations (配置).Kleinmann et al. (1986) strongly advise the use of open channels rather than pipes for inlet and outlet structures since pipes almost invariably (總是) become blocked (阻塞)by pr

48、ecipitates.3. Design consideration9.10 Flow regimeThe flow path within each cell should be controlled to minimise hydraulic short-circuiting and hence maintain a sufficient contact time between wastewater and wetland.Flow velocity within the cell should be carefully controlledToo great a velocity le

49、ads to erosion and resuspension of precipitated matter, too low may lead to stagnation (停滯).Keinmann et al. (1986) suggest surface flow velocities of between 0.03 and 0.09 m/s.Flow velocity can be controlled at the design stage.A dam or other influent storage facility may be used to restrict flow va

50、riations.3. Design consideration9.10 Flow regimeFlow velocity within the cell should be carefully controlledThe facility (設(shè)備) for water depth control, using adjustable inlet and outlet structures, is important.Seasonal changes in depth can counteract (抵消) the effects of greater evapotranspiration (土

51、壤水分蒸發(fā)蒸騰損失總量) in the summer and ice-cover in the winter.Seasonal changes in flow velocity may also be necessary (Huntsman et al.1985).3. Design consideration3.11 Design optimization (最優(yōu)化)Some wetland water treatment systems have been designed to cope with average conditions, but require constant atte

52、ntion.Others have been designed to accommodate (適應(yīng)) extremes of condition expected during the operating lifespan.Experience shows that reliance on system resiliency (彈性) is often better than reliance on operator intervention (干涉).3. Design consideration3.11 Design optimization (最優(yōu)化)Optimization of w

53、etland design must provide the maximum treatment efficiency and capacity under all scenarios, minimize system costs, achieve the greatest possible flexibility (彈性, 適應(yīng)性), and safeguard (安全裝置, 安全措施) the long-term viability (生存能力) of the system.3. Design consideration3.11 Design optimizationOver design

54、 to cope with conditions more extreme than the average, must take into account predictable and probable perturbations (動(dòng)搖, 混亂).Predictable (可預(yù)言的) perturbations include low treatment during start-up periods andThe effect of seasonal changes (high precipitation, high evapotranspiration, low temperatur

55、e).Probable (很可能的) or unpredictable perturbation includeRecord storm events,Damage to the integrity (完整性) of the structure, orThe loss of electrical power in certain cases.3. Design consideration3.11 Design optimizationMeasures that may be used include designing for additional treatment capacity, al

56、lowing for recirculation, fail-safe structures such as overflows and standby (備用) chemical treatment capacity.Some conventional treatment capacity is required in the United States where constructed wetlands are considered for use at operational mines.4. Vegetation (Selection and Plant Establishment)

57、Good decisions concerning plant selection and incorporation of plant requirements into the design, can prevent (預(yù)防) a wetland from being an engineering success, but a biological failure.Performance of the wetland relies not only on good design, but also on good construction and operation.Healthy wet

58、land plants are a key feature affecting the consistent performance of wetland treatment systems (Adcock et al., 2000).4. Vegetation (Selection and Plant Establishment)4.1 Plant ProtectionPre-planning to ensure planting success is necessary at the design stage of any wetland project.1. Will plants be

59、 dislodged (撞出, 移動(dòng)) or washed away.2. Sediment and gross pollutants must be extracted prior to flows entering vegetated areas of the wetland.3. What potential pest animals can occur in the wetland?4. What pest species occur in the wetland?5. Are there any pollutants that may harm plants?Submerged pl

60、anting, e.g. may be damaged by elevated levels of chlorine (氯) or high turbidity (混濁).4. Vegetation (Selection and Plant Establishment)4.1 Plant ProtectionWhen designing wetlands thought must be given to planting issues. Issues include:Water depth and its control;Flows and velocities (速率); andExpect