Inlet and Outlet Structures

Uniform influent distribution and effluent collection over the full width of each wetland cell in the system are absolutely essential. Uniform distribution is typically accomplished with perforated manifold pipes for both inlets and outlets.

The size of the manifold and the orifice diameter and spacing are a function of the intended flow rate. For example, cell 1 at the FWS wetland in West Jackson County, Mississippi, is designed for an average flow of 0.6 mgd (2271 m3/d) and utilizes a PVC manifold 12 in. (300 mm) in diameter and extending the full 250ft (76-m) width of the cell. This manifold is drilled with 2-in. (50-mm)-diameter orifices on 10-ft (3-m) centers. This pipe rests on a concrete footing to ensure stability and discharges to a 6-in. (150-mm) layer of 2-in. (50-mm) coarse aggregate; the coarse aggregate is underlain with a geotextile membrane to protect the underlying soil and prevent weed growth. A single manifold pipe with one central inlet would not be suitable for a very wide wetland cell as it would be difficult to ensure uniform flow from all of the outlets. Multiple manifold pipes (in pairs) could be used for this purpose. Sequential sets of splitter boxes could be used to uniformly divide the flow from the main influent line to whatever number of manifold sets is required.

In northern climates where extended periods of freezing weather are possible, it is necessary to protect these manifold pipes. These manifolds are placed at the bottom of the bed, below the design water surface. In these cases, the water level can be raised at the onset of winter to allow for ice formation at the water surface. Operational adjustment of these submerged manifolds is not possible, so great care must be taken during construction to ensure that the manifold pipe will remain level for the life of the system. At a minimum, some extra efforts at compaction and careful grading in the inlet and outlet zones will be required. In some cases, with potentially unstable soils (e.g., clays) it may be necessary to support the manifold on concrete footings. A clean-out on each end of these submerged manifolds is also recommended to allow flushing if clogging should occur over the long term.

In warm climates it is possible to install the inlet manifold in an exposed position to allow access for maintenance and adjustment. Alternatives to the simple drilled orifice holes allow the operator greater control over flow distribution. Gated aluminum irrigation pipe has been used but is susceptible to clogging, depending on the influent water quality. The TVA has used a nonclogging alternative originally developed in Europe. In this case, the manifold contains a series of pipe "Tees" of the same diameter. These "Tees" are connected to the manifold with O-rings on each side, with the open end of the "Tee" discharging to the wetland bed. Because of the O-rings, the open end of these "Tees" can be rotated vertically. The operator can then adjust each "Tee" as required to maintain a uniform flow distribution along the entire length of the manifold. The perforated effluent manifold in these cases is still placed at the bottom of the bed. Where the local climate permits, the use of an exposed, accessible inlet manifold is recommended for both SSF and FWS wetlands, except in cases where public exposure is an issue.

When submerged inlet and outlet manifolds are used for FWS wetlands, encroachment of the adjacent emergent vegetation must be considered. If the manifold is placed at the same grade as the main wetland bed, the vegetation may encroach on the inlet and outlet zones, and the plant litter and detritus could clog the orifices in the manifold. Several techniques are available to eliminate this problem.

A deep-water zone (approximately 2 ft deeper than the bottom of the main bed) can be incorporated at both the inlet and outlet to prevent growth of the emergent plants. The manifolds can also be placed on top of large rip-rap underlain by a geotextile membrane, or the manifolds can be enclosed in a berm composed of coarse rip-rap (3 to 6 in. in size); the large-sized stone will not support the growth of emergent plants or weeds. The open water configurations do allow easier access to the manifold but also allow algae growth.

Submerged effluent manifolds must then be connected to an outlet structure containing a device for controlling the water level in the wetland bed. This device could be an adjustable weir or gate, a set of stop logs, or a swiveling elbow, in which case the elbow is attached to the effluent pipe with an O-ring to permit rotation, and a riser in the open end of the elbow sets the maximum water level in the bed. Because the elbow can be rotated at least 90°, the operator can set the water level at any position desired or can drain the bed if necessary.

An alternative to manifolds for inlet and outlet structures is the use of multiple weir or drop boxes. These are usually constructed of concrete, either cast in place or prefabricated. Several boxes along the width of the cell must be used to ensure uniform distribution. Spacing might range from 15 to 50 ft (5 to 15 m) center to center, depending on the width of the cell. In the FWS system at West Jackson County, these boxes are used to transfer water from cell to cell and for final effluent discharge. Box spacing at this site ranges from 70 to 90 ft (21 to 27 m), depending on the width of the cell. These boxes have an advantage as a discharge structure, as the contained weir or gate can be used to adjust water levels and a separate structure is not required for this purpose. They do require an adjacent deep-water zone to prevent vegetation encroachment and in northern climates are at greater risk of freezing as compared to a submerged manifold.

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