Treatment MeasuresWetlands : Maintenance & Costs

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Cost Considerations

Constructed wetlands require relatively large areas of land, so construction costs are high in built-up areas. The cost of vegetation establishment is also high, although maintenance cost are moderate.

Maintenance

An operation and maintenance plan should be prepared for all constructed wetlands. This plan should address the wetland's entire 'life cycle' and can include:

Construction and commissioning: proper environmental management during the wetland construction and commissioning period.
Sediment and litter removal: removal of sediment and litter from the inlet zone or primary pre-treatment will be required on a regular basis. Litter should not be allowed to accumulate excessively after a storm, otherwise breakdown products may pollute the water.
Weed maintenance: this is particularly critical during construction and commissioning.
De-commissioning: a major refit or decommissioning will be required when the wetland reaches its design life.

The frequency of maintenance may be reduced by incorporating pre-treatment upstream of the wetlands system to remove coarse sediment and litter. If a pre-treatment measure is not included in the wetland system design, wetland sediment removal is expected to be required every three to six months which results in unacceptable disturbance. This falls to a frequency of between ten to 30 years in systems that incorporate sediment removal as a pre-treatment which all new wetland systems should incorporate.

The ability to draw down the wetland inlet zone during sediment removal should also be considered during the design phase.

To minimise maintenance costs, the wetland can also be designed for mechanised sediment removal-possibly by incorporating submerged berms.

Macrophyte harvesting is not considered necessary to maintain the long-term nutrient retention capacity of the constructed wetland system. However, occasional harvesting may be desirable to maintain a vigorous vegetation cover.

Inspections should be carried out as part of overall system maintenance program. Along with providing a general monitor of the health and diversity of the vegetation, inspections provide the opportunity to detect specific site problems. These can include the accumulation of sediment, plant debris, litter or oils; infestation of weeds; mosquito and other pest problems; algal blooms and scouring. Inspections will also help assess the wetland systems' performance in achieving its stated objectives.

Many weed species are transported during flood events. As a result, inspection should occur after each major event. This also provides the opportunity to locate any physical damage caused by high flows.