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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


7. MANAGEMENT OF FLOWS IN LARGE RIVERS FOR WATER QUALITY IMPROVEMENT

A. Water quality improvement in constructed and natural wetlands - general information


Fig. 7.1. Digital Terrain Model (DTM) of a potential location above a lowland, drinking water reservoir for a constructed wetland for water quality improvement

Wetland systems have been successfully used for water quality improvement. They may act as reservoirs, maintaining the natural biogeochemical links between land, water and biota, or as a final step in conventional wastewater treatment technologies. There are three basic types of wetland systems: natural wetlands, constructed surface flow (SF) wetlands, and constructed sub-surface flow (SSF) wetlands.

Water levels in the SSF wetlands are usually less than 0.4 m. The purification processes are enhanced by the variety of plant species. SSF are usually vegetated with the common reed (Phragmites australis), cattail (Typha sp.), and/or bulrush (Scirpus sp.). Water treatment occurs as a result of microbiological processes taking place in a substrate that forms the bed of the treatment wetland and is aided by uptake through the roots of the vegetation.

In SF wetlands, treatment is primarily mediated through suspended matter sedimentation and the vegetative uptake of contaminants. SF wetlands and natural wetlands have variable water depths ranging from periodic inundation in the case of wet meadows to a meter or more of water in deep water marshes.

Wetlands have been used for water purification in the following cases:

  • Treatment of municipal, industrial, and agricultural wastewaters;
  • Treatment of acidic mine drainage waters;
  • Reduction of nutrient and pollutant loads in runoff and stormwaters;
  • Reduction of nutrient load in rivers supplying downstream reservoirs.

The advantage of using wetlands as a treatment measure is the variety of processes that occur with the wetlands that combine to contribute to water quality improvement. It is this combination of biogeochemical processes that make wetlands efficient treatment systems. These processes include:

  • Sedimentation, filtration, and sorption of particulate matter within the wetland due to long water retention times and large sediment surface areas;
  • Assimilation and retention of dissolved nutrients within the biomass present in the wetland;
  • Oxidation and microbial transformation of organic matter in wetland sediments;
  • Denitrification of nitrogenous compounds by microbial action within the wetland system.

There are the several advantages and some limitations to wetland systems used in wastewater treatment applications. These appear below and need to be taken into account before such wetlands are created or designated for use in wastewater treatment processes (based on Moshiri 1993).

B. Dynamics of nutrient loads during flood events

The timing of nutrient loading transported to wetlands by rivers is determined by catchment characteristics, climate (especially precipitation), and rivers hydrology. Usually, in a case of highly degraded catchments having a considerable contribution of non-point source pollutants, surface runoff resulting from intensive rain events can transfer large amounts of nutrients from the catchment, via the river, to the wetland. These nutrients can be derived from materials adsorbed on to eroded soils or from dissolved nutrient leaching (Figure 7.2).

Advantages:
  • Low costs of construction and maintenance;
  • Low energy requirements;
  • Limited training requirements for operators ("low-technology" system);
  • Ease of creation using locally-available skills and materials;
  • Greater flexibility and less susceptibility to loading variations than conventional systems;
  • Multiple purpose:
    • Increased habitat and biodiversity;
    • Flood control;
    • Increased aesthetic appeal and value;
    • Energy and marketable plant production for sustainable development
Limitations:
  • Large land area requirements (thus recommended for small- to medium-sized distributed communities);
  • Reduced efficiency during winter (especially in temperate regions)

 

Fig. 7.2. General relationships between increased discharge resulting from precipitation in a catchment area and nutrient concentrations in rivers. Depending upon the pollution sources within the catchment area, the highest concentrations appear during low discharge (1, 2) or high/medium water events (3)

In catchments where point sources of pollution are present, their reduction is the first and necessary step to improving riverine water quality. In such situations, wetlands may be used as a final step in the wastewater treatment process, to reduce pollutant concentrations to a level safe for the receiving environment.

In rivers where non-point sources of pollution prevail, wetland systems may be applied both to purify polluted river water as well as reduce the movement of nutrients and suspended solids from the watershed to downstream reservoirs. In the latter case, the wetlands should be located in the river valley upstream of the reservoir.

There are some general principles related to the pattern of nutrient transport by rivers within degraded catchments. These are summarised below.

  • The highest nutrient concentrations and loads are observed during the first phase of a flood, while the flood waters continue to rise (nutrient-condensing stage);

  • Before river discharge reaches its maximum, nutrient concentrations and loads start to decrease and continue to decrease during the period following the flood peak (nutrient-dilution stage);

  • During high/moderate floods of short duration but with an high amplitude, the nutrients loads transported are greater than during lower amplitude and longer duration events within a given river;

  • Flash floods may result with dilution of concentrations of the transported contaminants, however the loads are still high due to high hydraulic load. This may periodically lower the efficiency of wetlands in water quality improvement.

 

Fig. 7.3. Observed general trend of change of maximum discharge in temperate climate zone. The intensity of flood determines the alternative role of impoundment in water management - water quality or flood control

 

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