While there are many types of wastewater sources that can use constructed wetlands as a chief component in the treatment system, recently their use for treating combined sewer overflows (CSO’s) has shown to be very cost-effective.
The term ‘constructed wetlands’ has many terms used to define it, but they essentially all mean the same thing:
Constructed treatment wetlands are engineered/artificial/man-made wetland systems, created/designed and constructed to utilize/use/emulate the natural functions/natural processes/interactions and combination of wetland vegetation/plants, soils/media and associated microbial life/populations/assemblages to reduce contaminants/pollutants/anthropogenic discharges and improve water quality in surface water, sanitary/municipal/industrial wastewater, groundwater, stormwater runoff or various waste streams. Constructed wetlands can be generally classified according to the path of the water through surface flow systems, and subsurface horizontal and vertical flow systems. Surface flow systems typically have standing water flowing above a soil-based bottom while subsurface horizontal and vertical flow systems have water running through the soil, or most likely a gravel media bed. Each of these general types can be operated in various modes such as ‘fill & draw’ and ‘pulsed flow’ and recirculation of the effluent flow back to be combined with the influent flow. Knowledge of these various biological, physical and chemical processes that occur in nature are used to select the type of wetland as well as control the operation and efficiency of the constructed wetlands.
Although these systems can provide many benefits related to sustainability and the typical environmental, economic and social benefits from large scale green infrastructure, as with any engineered system that is used in a process, the key reason for its use is costs. Both capital and, most certainly, operation and maintenance costs are typically much lower than conventional mechanical, concrete and steel, wastewater treatment systems. However, constructed wetland systems do take up more land area which can restrain their use due to land availability and budget.
Many older cities, particularly east of the Mississippi River have sewers that carry both sewage and stormwater during wet weather. These cities were required to develop and implement long-term control plans (LTCP) typically with a completion schedule of 20 years or more. The cost of doing so can be staggering and have resulted in projected monthly sewer bills exceeding $100.
Although fairly common in Europe, constructed wetlands have not been used for CSO treatment in the U.S. until recently and then, surprisingly only, in the state of Indiana. As the projects in the approved LTCP’s actually began to be designed, bid and constructed the reality of the cost impacts caused several cities to re-evaluate their plans. As an alternative to common wastewater treatment technologies the use of constructed wetlands became attractive in part because they can provide both storage and treatment in the same volume and therefore have a much lower capital cost than conventional storage and treatment systems. Additionally, the operation & maintenance cost for constructed wetlands are a fraction of the cost for conventional systems, typically less than 25 percent.
The use of constructed wetlands for CSO treatment seems to be most applicable to cities with populations of less than 100,000. The key is available undeveloped/agricultural land located a reasonable distance from the existing wastewater treatment plant and/or the location of larger CSO discharge locations. There are currently four significant CSO projects in Indiana (1) that I have been associated with; two of which have been constructed (town of Akron and city of Washington), one in the city of Muncie where construction began this year, and the last, which is in advanced facilities plan development (city of Evansville). A fifth project, for the city of Peru, will be a key revision of the city’s current LTCP scheduled for 2017. The only other significant system is a pilot installed near Syracuse, New York that was installed a few years ago.
Constructed wetlands for CSO treatment will typically have most of the same components, including:
Pretreatment Components – such as screening to remove floatable solids and grit removal to remove inert settable solids. Their function is based on whether or not most of the first flush volume from the CSO is directed to the constructed wetland.
Forebay – an initial deep water area with or without an energy dissipator cascade aerator for the removal of solids and for reducing the energy in the influent flow prior reaching to the constructed wetland.
Effluent Filter – recommended as a final step to primarily reduce solids in the effluent that can limit the effectiveness of the downstream disinfection system. The filter is composed of washed river rock of variable sizes and essentially functions as a vertical flow constructed wetland.
Constructed Wetland Effluent Control System – to control both the water elevation and the effluent flow rate and allows the operator to control the hydraulic residence time of the wastewater in the wetland. Constructed Wetland Recirculation System – used to increase pollutant removal efficiency and reduce potential odors.
Wetland Hydrology System – to provide another resource of water to discharge to the wetland during long dry weather periods in order to keep the system stable and ready to treat future CSO discharges.
Liner System – Indiana Department of Environmental management now requires a synthetic liner under the wetland soils.
Drainage System – to dry out the planting soil for a short period to allow access into the wetland for various maintenance purposes.
Soils for Planting – excavated soils during the construction of the wetland basin are used to provide the planting medium over the synthetic liner for the plants on both the bottom and the interior side of the berms.
Plant Regime – a mix of plants classified as wet-mesic prairie and emergent wetland plants which can tolerate various water depths and survive extended dry periods. These are specifically selected for water inundation adaptability and plant body density.
Habitat Islands – added for the multiple purposes of controlling the velocity of the flow thru the wetland, minimizing hydraulic short-circuiting and providing wildlife habitat.
The Indiana projects should serve as a catalyst for promoting and receiving approval to pursue projects in other states. Most, if not all, will be based on the need for a revision to existing LTCP’s due to communities/utilities realizing they cannot afford to implement the original plans for implementing CSO control.
(1) A very small system was installed by the City of Elkhart, Indiana in the 1990’s but was very small approximately 1/8th of an acre with the CSO discharge pipe being 6 – 8 inches. The system has no record of monitoring results that were submitted to IDEM and apparently the agency has forgotten about the system.