Operation And Maintenance 6101 Vegetation Establishment

Under ideal conditions, startup of a constructed wetland system would not commence until at least 6 weeks after planting of the vegetation. This time period is required to allow for the new plants to acclimate and grow. In actual practice, startup has sometimes occurred the day after planting was completed at a number of projects. Such emergency responses risk damage to the new plants and may delay achievement of expected performance. Startup procedures are quite simple and involve opening the inlet gates or valves and setting the desired wetland water level at the adjustable outlet or weirs. If the plants have not grown to a height that significantly exceeds the design water level, then the water depth must be increased gradually as the plants grow taller. In northern climates, it is also typical to increase the water depth at the onset of freezing weather to compensate for the expected formation of ice. If the plants are so short that this procedure cannot be implemented, then startup of the system should be deferred until the following spring. Under these circumstances it might also be better to defer planting the vegetation until the following spring.

If startup is defined as that initial period before the system reaches optimum performance, then, in cool temperate climates, the startup period for a wetland system might require in the best case 2 years and in the worst case 3 to 4 years. The system will not attain optimum performance levels until the vegetation and litter are developed fully and at equilibrium. The time required for that to occur is a function of planting density and season. A high-density planting in the spring of the year is likely to be fully developed by the end of the second growing season in cool temperate climates. A low-density planting in late fall just before the first frost in a northern location may require 3 years to reach equilibrium. Start-up time may be more rapid in continuously warm climates. Fortunately, most systems at startup are not faced with the ultimate long-term design flow, and the increased detention time may help compensate for the incomplete vegetative cover. Under these conditions, a spring planting at moderate density (<3 ft) will usually produce a reasonably dense vegetative cover by the end of the first summer, and the system is likely to meet discharge requirements from that point on. A fall planting, in cold climates, would probably not achieve a comparable vegetative cover until midsummer of the following year.

During the startup period, the operator should inspect the site at least several times per week to observe plant growth and health, the integrity of berms and dikes, and the emergence of mosquitoes (from FWS systems only) and to adjust water levels as required. The experience gained during this initial period will then suggest the inspection frequency required over the long term. During the first spring season after planting and startup, the plant coverage in all wetland cells should be inspected carefully. Any large unvegetated areas should be replanted to avoid the risk of short-circuiting the flow.

The water quality performance during this startup period will not be representative of long-term expectations. In some cases, the performance may be better than the long-term expectations. Phosphorus or nitrogen removal in FWS wetlands is an example. The new wetland has freshly exposed soil surfaces (presumably with some clay content) and rapidly growing vegetation. Both conditions provide a rapid but short-term removal pathway for phosphorus and ammonia nitrogen. These systems may not reach equilibrium for these two parameters until the end of the first or second full growing season, and the equilibrium effluent concentrations are then likely to be higher than the results during startup. The opposite results may be expected for BOD5 and TSS. A new wetland system has minimal vegetation and minimal substrate for attached growth organisms. As a result, removal of BOD5 may be marginal and TSS removal poor if algae develop in any exposed open water. The removal of these two parameters can be expected to improve as the plant canopy develops and increases in density.

The operation of a wetland system is very simple and very similar to the requirements for operation of a facultative lagoon. Much of the effort involved is visual observation of conditions and then correction of any problems that develop. The major issues of concern are:

• Water level maintenance

• Uniformity of flow distribution and collection

• Berm and dike integrity

• Health and growth of designed system vegetation

• Control of nuisance pests and insects

• Removal of undesirable vegetation

The key hydraulic requirement is maintenance of uniform flow conditions. The operator must routinely observe and adjust inlet and outlet structures as required, including flow splitter boxes at the inlet end and water level controls at the effluent end. Some temporary surface flow may be observed after surcharge by intense rain storms. If persistent and large-scale surface flow is observed, the operator must then lower the water level an appropriate amount. Surface flow on these systems negates the goal of eliminating the risk of public exposure, and it may also result in the emergence of mosquitoes.

Most municipal FWS wetland systems will have at least two cells in parallel to allow better system control and temporary shut down of one side for maintenance, if required. Seasonal water level adjustments for these systems may be suggested in the operations and maintenance (O&M) manual, even in warm climates. In general, this may require maximum water levels during the winter months and minimal water depths during the warm summer months. The latter is intended to encourage new plant growth and allow maximum dissolved oxygen in the shallow water. The water depths involved might range from 1 to 1.5 ft during the winter and 0.5 to 0.75 ft during the summer. If the O&M manual does not contain such guidance, it is suggested that the operator develop a plan and systematically try similar conditions and observe results. A lag time of several weeks will typically occur before the effects of such a change are observed.

To satisfy their National Pollutant Discharge Elimination System (NPDES) permit requirements, most municipalities only have to measure the specified pollutants in the untreated wastewater and in the final system effluent. Because most systems will have some form of preliminary treatment (e.g., primary, secondary, lagoon, septic tank), these data provide an insufficient basis for the operator to determine if the wetland component is performing to expectations or requires adjustment. It is recommended that the influent to the wetland be sampled and tested periodically for the constituents of concern so the operator can build a record of performance for the wetland component. If problems then develop, these data will be of great assistance in determining the necessary adjustments. Data of this type can also assist the operator in developing a plan for optimized operation (e.g., seasonal water depths, flow splitting to different cells).

A well-designed and properly operated wetland system will not require routine harvest and removal of plant material and litter to achieve water quality goals or sustain expected hydraulic conditions. Harvesting or burning on a few FWS systems has been used to relieve the hydraulic resistance developed in poorly designed systems with very high aspect ratios (length-to-width) and in an attempt to control the habitat for mosquitoes.

The plant litter in these wetlands decomposes over time but leaves a sediment residue that does accumulate (<0.04 in. or 1 mm/yr) over time. When this accumulated sediment and the accumulated refractory solids from the wastewater TSS begin to interfere with the design treatment volume or hydraulics in the wetland, then removal will be necessary. The problem will be most acute near the head of the system, as most of the influent TSS will be removed in the first 20% of the cell length. The access ramp to the cell should, therefore, be located near the head end of the cell. Such maintenance activities have not yet been performed on any operational wetland system in the United States. It is estimated that the need might arise every 50 to 75 years, depending on the wastewater characteristics, the types of plants used in the system, and the local climate.

Maintenance requirements for constructed wetlands are also simple and are similar to those required for facultative lagoons. These include maintenance of berms and dikes (e.g., mowing, erosion control), maintenance of watertight integrity (threatened, for example, by animal burrows, tree growth on berms), and control of nuisance pests and vectors (e.g., muskrats, nutria, mosquitoes). When the wetland is designed to operate at a shallow depth, a special requirement may be the periodic removal of tree seedlings from the wetland bed. If the trees are allowed to reach maturity, they will shade out the emergent vegetation and not provide the necessary substrate for attached-growth organisms. Inlet and outlet structures and water-level control devices must be periodically cleaned and adjusted, including debris removal and cleaning any weir surfaces to remove bacterial growth and other clogging substances. Submerged inlet and outlet manifolds should be flushed periodically and cleaned with a high-pressure hose.

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