REED BEDS

TACKLING POLLUTION BY WORKING WITH NATURE

It has long been known that wetlands have a natural capacity to clean water. American Indian folklore indicates reeds were considered magical and water flowing downstream of such plants was considered pure.

Time progresses and today these plants offer an effective and alternative way of treating polluted wastewater.

ENGINEERED WETLAND SYSTEM – REEDS IN A BOX

Systems can be self-contained, artificially engineered wetland ecosystems. Designed in a way to optimise the microbiological, chemical and physical processes naturally occurring in the wetland. The reeds transfer atmospheric oxygen down through their roots in order to survive in waterlogged conditions. This creates both aerobic and anaerobic soil conditions, allowing extraordinary microbial species diversity to flourish. Such bacteria and fungi use organic chemical products provided in the waste.

So chemicals are not simply stored in the reed bed, they are actually degraded into harmless components.

Other contaminants, such as metals, are transformed from a toxic state and fixed in the soil via complex chemical reactions. Soil adsorption capacity also provides a buffer for peak and shock effluent loads.

FUNCTIONS OF THE SYSTEM

  • Provides an environment for aerobic and anaerobic microbiological breakdown of organic and nitrogenous pollutants and pathogens
  • Provides a filtration of suspended solids
  • Provides a buffer for fluctuating loads
  • Provides a sink for metal ions, sulphate and phosphate

The reeds provide all of the above by dealing with the dissolved organic matter and nutrients through the action of the microbes associated with the substrate media in the bed. The fine soil hosts very large populations of bacteria, fungi and algae - all of which are instrumental in removing the dissolved pollutants in the wastewater. The fine media of the reed bed acts as a filter to entrap floating particles, which are then broken down over time by the micro-organisms.

WHY A REED BED

The basic premise of design is that the reed bed system has the capacity to protect surface and groundwater from the polluting effects of human activity and industry. The system has the ability to adapt quickly to variation in loads and types of flow, in general due to the exponential nature of the bacterial growth and the potential for the soil to absorb pollutants.

Microbes (bacteria, fungi, protozoa) have the ability to utilise a wide range of carbonaceous compounds for respiration and other metabolic activities. It is the promotion and utilisation of these activities within the environment by reed beds that leads to the breakdown of a wide range of compounds by microbial activity. It is also the kinetics of these reactions that determine the area of reed bed required to effect the necessary breakdown.

STRENGTHS

The greatest strength of this technology is that it has the capacity for self-adaptation; it is fair to assume when designing that there will be microbial populations in the soil that can and will adapt to exist by eating the chemicals in the wastewater. A period of 3 weeks to a month is often quoted, as being sufficient time for an indigenous bacterial population to arise that will feed on the chemicals in the water.

CONCLUSIONS

Reed beds are accepted as a cost-effective alternative to conventional treatment methods in a widening circle of industries. This means that more and more performance and design data is available and that the technology is moving from being perceived as experimental into the mainstream market of pollution control.

Reed beds complement the trend towards implementation of effluent control regimes, which minimise emissions and promote the recycling of water. This is due to the fact that they operate most effectively on relatively concentrated effluent streams and because they are designed to treat the effluent to conform to discharge standards.

To achieve successful plant growth initially and to sustain growth subsequently, it is important to note the following:

  • Gradients to the floor of the bed need to be correct (1% slope to outlet) to encourage saturated soil
  • Plants should be established in alternate series rather than straight rows, because this reduces short-circuiting of wastewater between rows
  • It is essential that the roots are flooded and that at least 50% of the shoot is out of the water
  • It is usually important that ponding does not occur, otherwise this will result in reed die-off in newly established plants. Dry conditions may encourage weeds that will compete if the soil is too dry
  • Ensure the support medium, soil, gravel, soil etc. are kept wet, (watered), sufficiently during extended plant down times and drought conditions, especially for newly planted, establishing plants
  • After the planting of new plants they should be monitored for disease, necrosis or chlorosis, diseased and dead plants should be replaced
  • Care should be taken not to cut developed plants below the bed surface, otherwise it will drown
  • Deep root growth can be encouraged by slowly lowering the water level over several weeks during the dormant vegetation period
  • Natural seasonal changes will occur in aquatic plants because of changes in day length, hours of sunlight and temperature
  • Replacing plants, Phragmites australis and most other reed species, appears to yield best results when done between April and June.

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