TECHNIQUE

Reedbed Construction for Water Cleaning (Managing Wetlands for Wildlife - Implementing Management Plan)

Summary Information
Type of technique Health & Management / Managing Wetlands for Wildlife / Implementing Management Plan / Techniques:
Synonyms and Keywords --
Description The following notes are intended as a summary description of some key aspects of reedbeds, based on B93 (Chapter 14). Experts/expert texts should be consulted prior to the construction of a reedbed system for water treatment.

Chains of "cells" may be constructed, with most of the silt settling in the first cell, which may then be periodically de-silted. For treatment of wastewater, at least two parallel series of cells are suggested, to allow flow to continue through one series while maintenance work is being carried out on the other.

In an overland (surface flow) system, a shallow depth of water passes above ground through the litter layer of the reedbed. In a subsurface (horizontal flow) system, wastewater filters through a porous sediment (e.g. gravel) in which the reeds are growing.

Suitable emergent plants for use in reedbed systems have an extensive growing period, produce a deep leaf litter layer, exhibit dense even growth (reduces channel formation) and develop an extensive root/rhizome system (assists in substrate aeration).

Construction with a level surface and a very shallow (about one degree) slope of bed appears best to produce a uniform flow across/through the entire area.

Correct siting of the inflow and outflow should allow the whole system to operate using gravity.

Management requirements should be minimal once the reed bed is established

  • Line each reedbed cell with an impermeable layer (e.g. clay or butyl rubber).
  • Fill with about 60cm of substrate (required for root systems of emergent plants) - graded gravel or sandy soils for subsurface flow systems, native soil or sub-soil for overland flow systems.
  • Establish emergent vegetation across the cell surface: Common Reed (Phragmites australis) frequently used in Europe, Common Reedmace (Cattail) (Typha latifola) frequently used in USA. Reed canarygrass (Phalaris arundinacea) and Reed sweet-grass (Glyceria maxima) may also be used.
  • May incorporate open water areas, such as ditches, perpendicular to water flow direction, in second half of reed bed, not too close to outflow.

(B93.14

Appropriate Use (?)
  • Cleaning/removing nutrients from water before returning water to a watercourse: used in sewage treatment, also used for waterfowl collections prior to returning water to e.g. a river.
  • Cleaning water from a source such as a river before use for e.g. a waterfowl collection.

(B11.33.w1, B93.14)

Notes
  • Size of each cell will vary depending on the characteristics of the water entering the system and the parameters required of the effluent.
  • Large size, incorporation of open water areas, mixed areas of dry and flooded reedbed and the presence of complementary habitats adjacent to the reedbed all increase usefulness for wildlife.
  • Metal removal may be enhanced if there is a deeper mat of dead vegetation: cutting reedbeds and leaving the cuttings in situ may be used to increase the leaf litter layer.
  • Cutting of blocks on a 5-15 year rotation may be used in an established reed bed (at least 5-10 years old) to reduce excessive build-up of reed litter.

(B93.10, B93.14)

Complications / Limitations / Risk
  • Require relatively large land areas.
  • Require relatively flat land.
  • Not suitable for treatment of highly contaminated water.
  • Time (may be a few years) is required to reach full efficiency.
  • Reduced efficiency in extremely cold weather with freezing of water.
  • Reed growth may be inhibited by irregular sudden fluctuations in water level.

(B93.14)

Equipment / Chemicals required and Suppliers
  • Appropriate plants and building materials.
Expertise level / Ease of Use
  • Expertise required for correct development.
Cost / Availability
  • Relatively cheap to construct, unless materials/structures need to be brought from off-site
  • Relatively cheap to operate and maintain
  • Higher costs if slope of land requires the construction of terraces.

(B93.14)

Legal and Ethical Considerations A license for water abstraction may depend on reaching stringent conditions of cleanliness prior to returning the water to the source (B11.33.w1).
Author Debra Bourne
Referee  
References B11.33.w1, B93.10, B93.14

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