The water cycle, essential knowledge for any licensed civil engineer, is an ongoing, uninterruptable natural process that contributes to atmospheric conditions, weather, agriculture, nutrition, biodiversity, vegetation, sanitation, hygiene, etc.; principal variable in the energy cycle. Besides obscure and implicit use, humans use water on a daily basis (i.e. cooking, plumbing, hygiene, watering, etc.). Water resources and wastewater treatment are civil engineering fields that are relevant in any developing region in any climate. In the recent decades wetland systems have been extensively studied and researched due to their role in the water cycle, biodiversity, carbon cycle, and storm surge impact. With the projected arrival of a warmer world, scientists, engineers, architects and design and decision making professionals have shifted their focus on these simple but incredibly complex ecosystems. Sea levels have been proved to be rising, subsidence is taking impact in coastal marshes and the recent drought in the nations Midwest has raised the value of large quantities of water as an essential resource and threat to current lifestyles, businesses, economy, population growth, development, urbanism, etc. Environmental awareness has transitioned modern engineering towards a more sustainable approach, where externalities on ecological systems are highly considered. The objective of this report is to mention design, technical, legal and cost considerations of implementing constructed wetlands as alternative of secondary water treatment in municipal wastewater systems, similar to the Tres Rios Project in Phoenix, Arizona. Throughout the text the development of the particular project will be discussed. Wastewater treatment technology applicable for various types of wastewater treatment and constructed wetland will be explored in full and compared to the case study. Disclaimer: Complete detailed description of design, measurements, codes, construction, water treatment, permit, and operational requirements will not be stated in full, but discussed towards development feasibility purposes. In addition, the legality, code, policy and technical issues met by case study projects will be discussed and should serve as guidelines for future developments. These should not be considered universal and proper research should be performed for regions of interest.
Understanding the Market
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Project Goals
Constructed wetland systems may help communities to conserve water resources, and even increase regional biodiversity. They are developed around communities with sufficient land. These systems are engineered to resemble and emulate natural wetland ecosystems. They are designed to utilize natural processes native to the unique ecosystem within a systematically controlled environment. These processes have proven effective in the removal of organics and suspended solids through native properties of wetlands. They consist of phytoremediation, microbiological mineralization, and filtration by gravel and gravitational sedimentation. These processes occur naturally in their respective environments and are less energy intensive than conventional wastewater treatment methodologies. Case studies suggest that a small, simple 20’ x 20’ x 4’, constructed wetland system has the capacity to treat black and grey water from an average household. The systems have been implemented, on this principal, in the secondary treatment of municipal scale wastewater treatment plants. Arid regions and communities like Phoenix, Arizona, are constantly stressed with water availability limits and have implemented various infrastructures that offer options for water consumption and reuse.
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Design
Project Tres Rios constructed wetland complex is located on the water basins of the Salt, Gila and Agua Fria rivers, which the city of Phoenix, Arizona thrives on. This particular region is characterized by its dry and arid atmosphere and sparse biodiversity compared to more forested regions in the nation. It is increasingly implied that water resources in these types of regions come in the limited forms of ground water, surface water, rare precipitation, and most essentially and evident, recycled and reclaimed existing water. In order to maximize the potential of these sources, Phoenix along with the surrounding communities, such as Glendale, Mesa, Scottsdale, and Tempe, collaborated with engineering forces such as the Environmental Protection Agency (EPA), Bureau of Reclamation, Arizona Department of Environmental Quality and the United States Army Corp. of Engineers (USACE) to design and construct a supplemental water treatment system that would contribute to the existing systems. The motivation for the design of the project originated from the environmental requirements to meet upcoming National Pollutant Discharge Elimination System (NSDES) wastewater treatment effluent requirements. The project was mainly driven as an advanced method to remove nitrogen levels in secondary treatment in a cost-effective applicable technique.
Project timeline 1994 - Design began as an alternative water quality treatment. 1995 – Spring. Primary phase construction began. 1997 - NPDES requirements of effluent treatment became enforced. 2003 - Vegetation status assessment. 2006 - Construction of operational facilities completed. 2010 – Spring. Stabilized steady water flow began.
Design Design criteria of constructed wetland systems may vary with the landscape, climate or politics pertaining to the region implementation but three principals drive the process. Engineers must consider hydrology, types of macrophythis growth and flow path. Two kinds of constructed wetlands exist: Free water surface (FWS) wetlands and Subsurface flow (SFS) Constructed wetlands (CW). Design Components:
• Siting, terrain and soils • Ecological Impact • Natural structure • Buffer zones • Vector control (mosquitos) • Hazing/exclusion devices • Dedicated water sources* • Biodiversity and physical heterogeneity • Vegetation Selection • Seasonal variability • Multiple cells • Maintenance access • Public awareness of project • Public use
Water Source A preliminary task in the design of such constructed wetlands is considering its sustainability and dedicated water source. Projects envisioned serving as aquatic and terrestrial wildlife habitat and shelter should be purposed to meet long term hydrological needs. Water quality should me constantly monitored and controlled through the influent. Historically, water treatment for the 91st avenue wastewater treatment plant, the largest treatment facility in the region, has serviced these communities. Data concludes that this plant has a treatment capacity of 230 million gallons a day and a diurnal average flow of 140 million gallons. A large portion of the project was designed on the premise that the plant discharged highly treated effluent directly into the Salt River strip. Currently the existing plant acts as the main supply of water of the Tres Rios constructed wetland, even though the wetland serves as a supplemental mechanism of the facility. The plant has the capacity of delivering 50 MGD to the supplemental system considering it meets requirements to other facilities. Of the total amount of water delivered from the treatment plant, 2 MGD are purely nitrified wastewater effluent.
Componential Sites The Tres Rios constructed wetland complex consists of various sectors, each with specified treatment processes. Three sites make up the complex: the Cobble site, Hayfield site and Research site. These each contain cells in which monitored natural treatment takes place. Cobble site: The constructed wetland structures that pertain to this region are two 2.1-acre in-stream wetland cells. This section is situated in the bed of the Salt River and consists of two basins that run parallel to each other. They differ in their lining material; while one is lined with native material (no liner), the other’s lining consists of soil form the local farm field. Based on the construction of these liners, infiltration rates and water quality metrics were studied. Hayfield site: This site is made up of two 3.0-acre upland wetland cells, which were constructed on farmland that functioned as literal hay fields. These are set on the upper bank of the Salt River basin, above the ephemeral flow path. The paralleled cells discharge their water into a riparian area below. This sector aided the development of vector management such as mosquito mitigation research. Research site: The wetland components of this are consist of twelve 0.5-acre fast-flowing cells. The twelve cells are divided into four sets of three cells each with different amounts of open-water areas. These pilot cells act as fundamental sections to the system and made clear that natural wetland systems encompass land from the soil of he wetland floor to the trees in the ecosystems vicinity. Additional post-demonstration development was realized by the partnership of the Army Corp of Engineers. These finalizing phases concentrated on a flow regulating wetland (FRW) and an overbank wetland (OBW) sectors. The focus of these additions revolved around water quality, localized flood management and vector control. After the design and construction of these, the system performance greatly improved. The configuration of multiple functional cells allows for residual removal, maintenance of flow control and the ability to manage certain effluents without disruption of a system. Individual cells could be assigned specific purposes and processes. From a circuit point of view, use of multiple cells at different distance from the influent and effluent allows the opportunity to complete required treatments and provide habitat and food for specific biodiversity groups.
Vegetation Selection and Adaptation True to any climate, regional vegetation is most often referred as any plant life that has adapted to temperature, humidity, precipitation, and sunlight and climate variations of the studied area. During the construction phase of the artificial wetland project certain native species primitive to the Sonaron southwest region of the United States were selected and integrated to the system. These include freemont cottonwood, goodings willow, cattail, bulrush, giant and alkali sacaton grasses, mesquites, desert screwbean and saltbush. An assessment of the adaptation of selected vegetation was realized in 2003. Results stated that a level of plant stress and failure was present in certain species. The evaluation considered the species, native climate, depth regime, soil requirements, and topography in which the plants were set. Depth resulted as the main determining variable, contributing most to wetland vegetation prosperity. The average depth of vegetated portions of the wetland was approximately eight inches, in difference; there was an increase plant density pertaining to areas with six inches or less depth. After the evaluation, it was determined that the topography also contributed most to the vegetation resilience. As a result, areas were reconfigured with a fluctuating topography of 0.0 ft to 1.5 ft with features such as hummocks, submerged and surface breaching mounds.
Biodiversity and Vector Management Implementation of any constructed emulation of natural systems must account for the biodiversity pertaining to the specific ecosystem. During the design phase, the project must implement vegetation patterns, wildlife and the food and energy web in which they interconnect. Planning and management of the Tres Rios Constructed Wetlands have been successful in introducing and attracting such biodiversity. Species diversity has been recorded since the design stages and shown an increase in densities. Some examples of wildlife that has been witnesses are heron rookeries, beavers, and bobcats, coyotes, frogs, bald eagles, and fish life which have adapted to the artificial ecosystem. In addition, mitigation mechanisms and techniques of invasive and damaging species should be present since the project’s conception (e.g. vector control). One can minimize mosquito problems by controlling the potential for stagnant water formation and other biological mechanisms. These include biological larvicides, mosquito fish, bats, etc. Dataset records of these kinds of pest mitigation could be found in local abetment districts. Various biological larvicides were introduced in to the Tres Rios water flow, Bacillus thuringiensis israelensis and Bacillus sphaericus have been functional due to alternating application in order to reduce genetic resistance development. Certain disadvantages became present when various vector management techniques were established. Wetland cells bred excessive mosquitos which here mitigated by a percent of the birdlife, yellow-headed blackbirds and marsh wrens. After an effort to manage such vectors that consisted of reducing 50 to 60 percent of the emergent zones, a level of success was achieved resulting in an increase of biodiversity present on the river basins. In turn this minor modification had a negative effect on the water treatment results. Water quality decreased as algal species populated an increased surface area that was exposed to sunlight. This resulted in high levels of dissolved oxygen and TSS.
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Costs
Project Costs In totality, the development involved the construction of approx. 128 acres of wetlands, 38 acres of riparian corridors, 134 acres of open water/marsh areas, and about 69 acres of habitat. Towards the finalization of the design phase, the total project costs were set to reach a sum of $99,320,000.00. This original estimate is mentioned in an official letter towards the chief engineer of the US Corp or Engineers elaborating on the project description and cost breakdown. The sum of $99,320,000.00 is initially divided into federal costs and non-federal costs. These are then broken down in order to specify what the financial support cover in relevant detail. A total of $88,580,000.00 was allocated towards the environmental restoration, which includes design and construction of the wetlands, riparian corridors, etc. Additional funds were required for other implementations of this progressive project. A flood damage reduction, which includes funding for a 5-8 ft. levee, was set to cost $5,380,000.00. The environmental education and recreational development components were processed as non-federal cost responsibilities and this totaled $4,860,000.00. Much of these cost responsibilities are determined by legal policy and code, such as Public law supported by the Water Resources Development Act (WRDA). For example, the Cultural resource mitigation costs of $500,000.00 falls under federal cost due to policy specifying that such components are attributed to such federal sources as long as they are less than 1% of total cost estimates. This level of project funding is strictly enforced by policies, acts, and legal code, documentation in financial books and cost/benefit ratios. For example, the Tres Rios project cost benefit ratios were calculated for the annual costs such as flood control costs (1.65) and average recreational costs (2.54), these included the operational and maintenance costs. Similar assessments achieve a more-for-less perception when project feasibilities studies are performed. The measuring of multiple benefits, such on saving on energy and equipment costs, not to mention projected industry costs, helps a project seem logical and approachable. Projects that revolve around any improvements to established infrastructure through established engineering fundamentals are likely to be considered and ultimately funded. Cost Analysis The focus of the cost analysis was on nitrogen and phosphorous removal compared to other wastewater treatment plants, although BNR processes involve a number of parameters and targets. Total nitrogen removal includes ammonia and organic nitrogen and is not completely removed by secondary treatments. This biological process is achieved through various biochemical reactions that alter molecular structures by nitrification and denitrification. Similarly, phosphorous removal is generally low, less than 20% of total phosphorus is removed from treated waters. It has been established that is can be difficult to mitigate through conventional biological wastewater treatment processes and this was assumed to be in favor of the contracted wetlands BNR mechanism. It should be noted that chemicals added to facilitate the removal and precipitation have risk of resulting in sludge by products and this could be costly to dispose. Advantages of constructed wetlands include the boost to regional biodiversity, recharge of local aquifer, no significant chemicals required for BRN (compared to conventional methods) and these systems treat bother TN and TP. The disadvantages consisted of significant vector control requirements, large land area use and cover, and seasonal temperature variation in BNR removal rates. And functionality. It was concluded that the various disadvantages of constructed wetland systems overshadowed the various advantages of implementing such projects and that the cost implications would ultimately determine their ranking in wastewater treatment plant.
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