Typical Design Criteria and Expected Effluent Quality for Free Water Surface Constructed Wetlands

Item Unit

Design parameter:

Detention time d

BOD loading rate lb/ac-d

Hydraulic loading rate in/d

Water depth ft

Minimum size ac/mgd

Mosquito control —

Harvesting interval yr Expected effluent quality :a

Biochemical oxygen demand (BOD5) mg/L

Total suspended solids (TSS) mg/L

Total nitrogen (TN) mg/L

Total phosphorus (TP) mg/L

Value

2-5 (BOD); 7-14 (N) <100 1-5 0.2-1.5 5-10 2:1 to 4:1 Required 3-5

a Expected effluent quality based on a BOD loading equal to or less than 100 lb/ac-d and typical settled municipal wastewater.

Source: Adapted from Crites, R.W. and Tchobanoglous, G., Small and Decentralized Wastewater Management Systems, McGraw-Hill, New York, 1998.

at the top to permit access by service vehicles. Each wetland cell should contain an access ramp for maintenance equipment. If possible, it is desirable to balance the cut and fill on the site to avoid the need for remote borrow pits or spoil disposal. If agronomic-quality topsoil exists on the site it should be stripped and stockpiled. In the case of a FWS wetland, this topsoil can be utilized as the rooting medium for the emergent vegetation and for revegetation of the berm surfaces.

If the wetland system is to meet its performance expectations it is critically important for the water to flow uniformly over the entire surface area provided for treatment. Severe short-circuiting of flow can result from improper grading or nonuniform subgrade compaction. Tolerances for grading will be given in the construction plans and specifications and in general will depend on the size of the system. Very large FWS systems incorporating several thousand acres cannot afford the effort to fine grade to very close tolerances and is not cost effective so the design will typically incorporate a safety factor to compensate. It is usually cost effective, for smaller wetland systems of a few hundred acres or less, to utilize close grading tolerances for construction. Uniform compaction of this subgrade is also important as subsequent construction activity (e.g., liner placement, soil placement for FWS systems) might create ruts and low spots in the subgrade which then result in short-circuiting of flow.

Fine grading and compaction of the native subgrade soils also depend on the liner requirements for the project. If the native soils are sufficiently impermeable (e.g., high clay content) and a liner is not required, then the soil surface should be graded to the specified tolerances and uniformly compacted to the same levels typically used for the subgrade soils in road subgrades. The same procedures should be followed if a membrane liner is used. If a clay liner is used, the native soils should be excavated to the specified depth and any new clay material placed and then compacted and graded to the specified elevations. Generally, all wetland cells are graded level from side to side and either level or with a slight slope in the flow direction. FWS wetlands are often constructed with a small bottom slope (<0.2%) in the flow direction to assist in drainage when cell maintenance is required. Construction activities with either native clay soils or with installed clay liners should only occur in dry weather when the soil moisture content is on the dry side of optimum.

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