Wetlands are those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to maintain saturated conditions. These can be either preexisting natural wetlands (e.g. marshes, swamps, bogs, cypress domes and strands, etc.) or constructed wetland systems. 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. Constructed wetlands are either free water surface systems (FWS) with shallow water depths or subsurface flow systems (SFS) with water flowing laterally through the sand or gravel. Both types utilize emergent aquatic vegetation and are similar in appearance to a marsh. In this report, we will focus on subsurface flow constructed wetlands.
VIETNAM NATIONAL UNIVERSITY HANOI UNIVERSITY OF SCIENCE SCIENTIFIC REPORT Research topic: Design a sub-surface flow CWs to treat domestic wastewater of a residential area of 1000 people Students: Nguyen Tuan Anh - K55 Advanced Program of FES Le Nam Thanh - K55 Advanced Program of FES Instructor: Associated Professor Nguyen Thi Loan, Faculty of Environmental Science, Hanoi University of Science Hanoi, 05/2013 Contents I Overview of wetland Natural wetland and constructed wetland Wetlands are those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to maintain saturated conditions These can be either preexisting natural wetlands (e.g marshes, swamps, bogs, cypress domes and strands, etc.) or constructed wetland systems 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 Constructed wetlands are either free water surface systems (FWS) with shallow water depths or subsurface flow systems (SFS) with water flowing laterally through the sand or gravel Both types utilize emergent aquatic vegetation and are similar in appearance to a marsh In this report, we will focus on subsurface flow constructed wetlands Subsurface flow constructed wetland Subsurface flow constructed wetlands first emerged as a wastewater treatment technology in Western Europe based on research by Seidel (1) commencing in the 1960s, and by Kickuth (2) in the late 1970s and early 1980s The SFS concept developed by Seidel included a series of beds composed of sand or gravel supporting emergent aquatic vegetation such as cattails (Typha), bulrush (Scirpus), and reeds (Phragmites), with Phragmites being the most commonly used In the majority of cases, the flow path was vertical through each cell to an underdrain and then onto the next cell Excellent performance for removal of BOD5, TSS, nitrogen, phosphorus, and more complex organics was claimed Pilot studies of the concept in the United States were marginally successful, and it has not been utilized in recent years in this country Kickuth proposed the use of cohesive soils instead of sand or gravel; the vegetation of preference was Phragmites and the design flow path was horizontal through the soil media Kickuth’s theory suggested that the growth, development and death of the plant roots and rhizomes would open up flow channels, to a depth of about 0.6 m (2 ft) in the cohesive soil, so that the hydraulic conductivity of a clay-like soil would gradually be converted to the equivalent of a sandy soil This would permit flow through the media at reasonable rates and would also take advantage of the adsorptive capacity of the soil for phosphorus and other materials Very effective removal of BOD5, TSS, nitrogen, phosphorus, and more complex organics was claimed As a result, by 1990 about 500 of these “reed bed” or “root zone” systems had been constructed in Germany, Denmark, Austria, and Switzerland Commencing in 1985, a number of “reed bed” systems were constructed in Great Britain based on Kickuth’s concepts, but in many cases gravel was used as the bed media rather than cohesive soil (5) due to concerns regarding soil hydraulic conductivity Many of these beds were built with a sloping bottom (0.5 to 1%) and a flat surface The purpose of the sloping bottom was to provide sufficient hydraulic gradient to ensure subsurface flow in the bed The flat upper surface would allow temporary flooding as a weed control measure to kill undesirable plants Some of these systems also had an adjustable outlet which permitted easy maintenance of the desired water level in the bed These systems are essentially horizontal trickling filters when they use rock media They have the added component of emergent plants with extensive root systems within the media Systems using sand or soil media are also used Soil media systems designated as the Root-Zone-Method (RZM) were developed in West Germany Unlike the FWS system equation, in which the specific surface area is important but not critical, the media porosity is critical to predicting the required area for a given level of treatment Media porosity has a direct mathematical relationship with the microbial degradation rate constant Today, the subsurface flow wetland consists of a basin or channel with a barrier to prevent seepage, but the bed contains a suitable depth of porous media Rock or gravel are the most commonly used media types in the U.S The media also support the root structure of the emergent vegetation The design of these systems assumes that the water level in the bed will remain below the top of the rock or gravel media The flow path through the operational systems in the U.S is horizontal In a vegetated subsurface flow system, water flows from one end to the other end through permeable substrates which is made of mixture of soil and gravel or crusher rock The substrate will support the growth of rooted emergent vegetation It is also called “Root-Zone Method” or “Rock-Reed-Filter” or “Emergent Vegetation Bed System” The media depth is about 0.6 m deep and the bottom is a clay layer to prevent seepage Media size for most gravel substrate ranged from to 230 mm with 13 to 76 mm being typical The bottom of the bed is sloped to minimize water that flows overland Wastewater flows by gravity horizontally through the root zone of the vegetation about 100-150 mm below the gravel surface Many macro and microorganisms inhabit the substrates Free water is not visible The inlet zone has a buried perforated pipe to distribute maximum flow horizontally through the treatment zone Treated water is collected at outlets at the base of the media, typically 0.3 to 0.6 m below bed surface Advantages of subsurface flow constructed wetland in waste water treatment The high cost of some conventional treatment processes has produced economic pressures and has caused engineers to search for creative, cost effective and environmentally sound ways to control water pollution One technical approach is to construct artificial ecosystems as a functional part of wastewater treatment Where wetlands are located conveniently to municipalities, the major cost of implementing a discharge system is for pumping treatment plant effluent to the site Once there, further wastewater treatment occurs by the application of natural processes In some cases, the wetland alternative can be the least cost advanced wastewater treatment and disposal alternative In locations where poorly drained land that is unsuitable for land application is available, wetlands can often be constructed inexpensively with minimal diking Wastewater has been treated and reused successfully as a water and nutrient resource in agriculture, silviculture, aquaculture, golf course and green belt irrigation The conceptual change that has allowed these innovative processes is to approach wastewater treatment as “water pollution control” with the production of useful resources (water and plant nutrients) rather than as a liability The interest in aquatic wastewater treatment systems can be attributed to three basic factors: Recognition of the natural treatment functions of aquatic plant systems and wetlands, particularly as nutrient sinks and buffering zones In the case of wetlands, emerging or renewed application of aesthetic, wildlife, and other incidental environmental benefits associated with the preservation and enhancement of wetlands Rapidly escalating costs of construction and operation associated with conventional treatment facilities Two converging trends encourage engineers to consider natural processes such as constructed wetland systems and aquatic plant systems The first trend is the ever increasing demand for water at a time when the least cost water sources have already been used The second trend is the increasing volume of biological and chemical wastes that potentially enter the surface water system from wastewater treatment plants The SFS type of wetland is thought to have several advantages over the FWS type If the water surface is maintained below the media surface there is little risk of odors, exposure, or insect vectors In addition, it is believed that the media provides greater available surface area for treatment than the FWS concept so the treatment responses may be faster for the SFS type, which therefore can be smaller in area than a FWS system designed for the same wastewater conditions The subsurface position of the water and the accumulated plant debris on the surface of the SFS bed offer greater thermal protection in cold climates than the FWS type II Pollutant removal mechanism in subsurface flow constructed wetland Phosphorus removal Phosphorus removal in most constructed wetland systems is not very effective because of the limited contact opportunities between the wastewater and the soil Some experimental and developmental work has been undertaken using expanded clay aggregates and the addition of iron and aluminum oxides; some of these treatments may have promise but the long-term expectations have not been defined Some systems in Europe use sand instead of gravel to increase the phosphorus retention capacity, but selecting this media results in a larger system because of the reduced hydraulic conductivity of sand compared to gravel If significant phosphorus removal is a project requirement, then very large land areas or alternative treatment methods will probably be required Nitrogen removal Nitrogen is limited in drinking water to protect the health of infants and may be limited in surface waters to prevent eutrophication Nitrogen can be removed in pond systems by plant or algal uptake, nitrification and denitrification and loss of ammonia gas to the atmosphere (evaporative stripping = volatilization) Nitrogen removal in constructed wetlands ranges from 25-85 percent, primarily due to nitrification/denitrification 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 Fecal coliform removal These SF wetland systems are, in the general case, capable of a one- to two log reduction in fecal coliforms, which in many cases is not enough to routinely satisfy discharge requirements which often specify < 200/100 ml Peak flows in response to intense rainfall events also disrupt removal efficiencies for fecal coliforms III Plant in subsurface flow constructed wetland Overview About 40 percent of the operational SFS systems use only Scripus Phragmites, which is the most widely used species in the European systems A number of systems in the Gulf States also used a number of flowering plants for aesthetic reasons These soft tissue plants decompose very rapidly and can affect water quality in the effluent Many locations adopted a routine fall harvest to remove these plants before they died or suffered frost damage There have been some attempts to create a plant diversity similar to that present in a natural marsh; this approach is more expensive and the intended diversity can be difficult to maintain Any of the three species listed in Tables are suitable for use in SF systems If the plant is expected to provide a significant treatment function, then the depth of the bed should not exceed the potential root penetration depth The Phragmites used in many European systems offer several advantages for a low maintenance treatment system They will grow and spread faster than bulrush; their roots should go deeper than cattails; and they are not a food source for muskrats and nutria which have been a problem for cattail and bulrush wetlands However, the habitat values for a Phragmites system are probably less than for other plant species A number of systems in the Gulf States utilize an annual harvest, regardless of the plant species used In contrast, routine annual harvesting is not practiced in Europe or at most other systems in the U.S It may be useful to remove undesirable weeds during the early part of the growing season for the first few years of operation Flooding of the bed surface after the initial planting can help reduce weed infestation A routine annual harvest of the entire system provides minimal benefits and is not recommended It is also suggested that the use of soft tissue flowering plants be avoided and thereby eliminate the need for their annual harvest and related maintenance Water level management in the SFS bed is not only helpful for weed control, but can also be used to induce deeper root penetration Based on experience in Europe, it is claimed that if the water level in the bed is gradually lowered in the fall of each year the roots will penetrate to greater depths A three year period is considered necessary for Phragmites roots to reach their 0.6 m potential depth Although this approach has not been tried in the U.S it should be successful, but it may have to be repeated every year for the operational life of the system The alternative is a root zone where the major masses of roots are limited to the top ± 0.25 m in the portions of the bed where nutrient concentrations are high Plant functions The roles of wetland plants in constructed wetland systems can be divided into categories: Physical - Macrophytes stablise the surface of plant beds, provide good conditions for physical filtration, and provide a huge surface area for attached microbial growth Growth of macrophytes reduces current velocity, allowing for sedimentation and increase in contact time between effluent and plant surface area, thus, to an increase in the removal of Nitrogen Soil hydraulic conductivity - Soil hydraulic conductivity is improved in an emergent plant bed system Turnover of root mass creates macropores in a constructed wetland soil system allowing for greater percolation of water, thus increasing effluent/plant interactions Organic compound release - Plants have been shown to release a wide variety of organic compounds through their root systems, at rates up to 25% of the total photosynthetically fixed carbon This carbon release may act as a source of food for denitrifying microbes (Brix, 1997) Decomposing plant biomass also provides a durable, readily available carbon source for the microbial populations Microbial growth - Macrophytes have above and below ground biomass to provide a large surface area for growth of microbial biofilms These biofilms are responsible for a majority of the microbial processes in a constructed wetland system, including Nitrogen reduction (Brix, 1997) Plants create and maintain the litter/humus layer that may be likened to a thin biofilm As plants grow and die, leaves and stems falling to the surface of the substrate create multiple layers of organic debris (the litter/humus component) This accumulation of partially decomposed biomass creates highly porous substrate layers that provide a substantial amount of attachment surface for microbial organisms The water quality improvement function in constructed and natural wetlands is related to and dependent upon the high conductivity of this litter/humus layer and the large surface area for microbial attachment Creation of aerobic soils - Macrophytes mediate transfer of oxygen through the hollow plant tissue and leakage from root systems to the rhizosphere where aerobic degradation of organic matter and nitrification will take place Wetland plants have adaptations with suberised and lignified layers in the hypodermis and outer cortex to minimise the rate of oxygen leakage The high Nitrogen removal of Phragmites is most likely attributable to the characteristics of its root growth Phragmites allocates 50% of plant biomass to root and rhizome systems Increased root biomass allows for greater oxygen transport into the substrate, creating a more aerobic environment favoring nitrification reactions Nitrification requires a minimum of mg O2/l to proceed at a maximum rate It is evident that the rate of nitrification is most likely the rate limiting factor for overall Nitrogen removal from a constructed wetland system Aesthetic values - The macrophytes have additional site-specific values by providing habitat for wildlife and making wastewater treatment systems aesthetically pleasing As root development commences, nitrification should be enhanced; this should be rapidly followed by denitrification (as long as a carbon source is available) The resulting loss of nitrogen to the atmosphere further reduces the availability of nutrients in the bed and may promote further progressive root development in the portions of the bed where the nitrifying organisms can successfully compete for the available oxygen It is likely that root development would still be limited in the front part of such a bed where the oxygen demand for BOD5 removal would limit the development of the nitrifiers In this area much of the nitrogen would still be in the ammonia form and the plant roots would not have to penetrate deeply to obtain sufficient nutrients Deep penetration of the roots in these short detention time beds may be possible if most of the flow is actually occurring on top of the bed In this case there would be minimal flow through most of the bed profile, resulting in low nutrient levels and deeper root penetration The deeper root penetration in this case would not result in improved treatment since the roots are not in contact with the bulk of the wastewater Hydraulic improvements are necessary for such systems to maintain flow throughout the full bed profile, but the short detention time will still be a limiting factor So we choose Normal reed (Phragmites Australis) with root penetration = 0.6 m for our design IV Subsurface flow constructed wetland design The major costs and energy requirements for constructed wetlands are associated with preapplication treatment, pumping and transmission to the site, distribution at the site, minor earthwork, and land costs In addition, a constructed system may require the installation of a barrier layer to limit percolation to groundwater and additional containment structures in case of flooding (6) Possible constraints to the use of constructed wetlands for wastewater treatment include the following: Geographical limitations of plant species, as well as the potential that a newly introduced plant species will become a nuisance or an agricultural competitor Constructed wetlands that discharge to surface water require to 10 times more land area than aconventional wastewater treatment facility Zerodischarge constructed wetlands require 10 to 100 times the area of conventional wastewater treatment plants Plant biomass harvesting is constrained by high plant moisture content and wetland configuration Some types of constructed wetlands may provide breeding grounds for disease producing organisms and insects and may generate odors if not properly managed We choose the design for our wetland as follow We consider following properties for our design: a Aspect Ratio The aspect ratio (L:W) of the wetland bed is a very important consideration in the hydraulic design of SF wetland systems, since the maximum potential hydraulic gradient is related to the available depth of the bed divided by the length of the flow path Many of the early systems designed with an aspect ratio of 10:1 or more and a total depth of 0.6 m (2 ft) have an inadequate hydraulic gradient and surface flow is inevitable The hydraulic gradient (S factor in equation 2) defines the total head available-in the system to overcome the resistance to horizontal flow in the porous media b Bed Slope SF systems in Europe (29) have been constructed with up to percent slope on the bottom of the bed to maintain an acceptable hydraulic gradient However, it is not practical and probably not possible with SF systems to precisely design and construct the bed for a specific hydraulic gradient due to variabilities in the media used and in construction techniques, and the potential for longer term partial clogging In addition, the construction of a bed with a sloping bottom provides no flexibility for ‘future adjustments Greater flexibility and control is possible with an adjustable outlet which permits control of the water level over the entire design depth of the bed In this case, the bottom of the bed could be flat or with a very slight slope to ensure drainage, when required However, because of the hydraulic gradient requirements, the aspect ratio (L:W) will have to be relatively low (in the range of 0.4:1 to 3:1 ) to provide the flexibility and the reserve capacity for future operational adjustments So we choose aspect ratio = 3:1 for our design 10 c Media Types Table presents a summary of typical characteristics for the types of media which have been used in SF constructed wetlands Essentially all of the operational SF constructed wetlands in the U.S have used media ranging from medium gravel to coarse rock The values in Table are intended as preliminary information only Following selection of a media type and size; the hydraulic conductivity and porosity of the material should be determined in the field or laboratory, prior to system design The use of smaller rock sizes has a number of advantages in that there is more surface area available on the media for treatment, and the smaller void spaces are more compatible with development of the roots and rhizomes of the vegetation, and the flow conditions should be closer to laminar When turbulent flow occurs in the coarser media listed in Table 5, the “effective” hydraulic conductivity will be less than the values listed in the table The hydraulic conductivity (ks) values in Table assume that the media and the water flowing through it are clean so that clogging is not a factor As discussed in a previous section, some clogging can occur in these systems, especially near the inlet zone where most of the suspended solids will be removed As noted previously, the observed clogging represented less than percent of the void spaces in the systems investigated The majority of the material (>80%) was inorganic and believed to be the residue from construction activities, and should not, therefore, have a cumulative impact on hydraulic conductivity It is, however, necessary to provide a large safety factor against these contingencies and 11 adoption of an approach similar to that used in the design of land treatment systems (30) is proposed It is therefore recommended that a value