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3.9 Hydrobotanical or Wetland Treatment

Technical Description

Hydrobotanical treatment is based upon the natural water purification ability of wetland vegetation. Hydrobotanical treatment requires mechanical wastewater pretreatment, which is typically provided by means of a three-chamber flux settlement tank made of plastic (for ease of transport and construction). The biological treatment unit is specifically designed for each individual case, but typically can consist of ponds populated by various bulrushes, reeds, and cattails; land-based vegetative (cattail) filters with lateral and gravity-fed wastewater applications; or, cascades populated with bog vegetation (e.g., bulrushes, reeds, cattails, and willows). It is also possible to install a wastewater recirculating system for complex nitrogen removal. The plant beds generally consist of shallow trenches, shaped to conform to the land slope (which should not exceed 1% to 2%) and lined with an artificial (having a membrane thickness of at least 0.5 mm) or natural (e.g., clay or poorly drained loam) isolation lining. The trench should be about 0.5 m to 0.6 m in depth and filled with well drained grit. Wastewater is applied by distribution pipes equipped with a system of tees to ensure sheet flow across the vegetation bed. The level of wastewater is kept under loading bed surface by controlling the rate of application. Treated waters are released through a drainage, positioned perpendicular to the direction of wastewater flow. There are usually two vegetative filters operated as a dual system. The reeds used in combination with the other vegetative elements provide oxygen translocation to the rhizosphere, allowing for nitrification and decomposition of organic matter, maintenance of proper hydraulic conditions, and, in winter, good thermal insulation.

The technology is capable of removing BOD5 through aerobic and anaerobic decomposition and sedimentation; nitrogen through nitrification and denitrification, and vegetative utilization; phosphorus through vegetative utilization and accumulation in the soil; and, bacteria through sedimentation, filtration, and natural degradation as a result of exposure to unfavourable environmental conditions. The biological treatment can be enhanced by dosing with bioadditives to reduce odours near the mechanical treatment plant, BOD5, and the amount of wastewater sludge. Sludge occurs as a result of the reduction of organic solids, and can interfere with the movement of the wastewater through the canals, pipelines, and outlet.

Treatment of this kind requires suitable terrain, which allows natural laminar flows. The treatment area required is 10 to 12 m²/person served. For installation purposes, an area with a flat surface, having a shallow slope (1% to 2%) in the direction of outlet and a sandy undersoil, is recommended. Such terrain permits the system to be operated by gravity; however, in cases where the recirculation of wastewater or unfavourable relief occurs, power requirements average about 1.5 kW.

Extent of Use

This technology has been used on a limited basis in Poland.

Operation and Maintenance

These systems have few requirements for their operation and maintenance. The materials needed for their construction (e.g., pipelines, interunits, well regulation controllers, moulders, drainage systems, PVC couplings) are readily available in the region.

Level of Involvement

This technology is implemented at the level of a local administration, generally as a public works project.

Costs

The investment cost is about $40 to $60/per inhabitant, depending on land relief, soil conditions, quality of the wastewater to be treated, size of the wastewater treatment plant required for pretreatment of the effluent, and the availability of filter bed materials. Operating costs in a gravity-fed scheme are low, including the cost of periodic control of flow rates, removal of sediments (once per year), and the eventual replacement of vegetation.

Effectiveness of the Technology

The technology can produce an effluent with a BOD5 of 30 mg O2/dm³, an organic matter content of <50 mg/dm³, a total nitrogen concentration of <30 mg N/dm³, and a total phosphorus concentration of < 5 mg P/dm³.

Suitability

Treatment facilities of this kind are suitable for use in rural areas, small settlements, and recreational centres, and as a secondary treatment in conventional wastewater treatment plants. Better performance of this technology is achieved in the southern parts of Europe than in north due to unfavourable climatic conditions in the latter; in winter, the biological processes are slowed down.

Advantages

This technology can achieve an high rate of BOD5 and suspended matter reduction (up to 90% to 95%), as well as nitrogen and phosphorus removal. It is simple and offers a reasonable degree of treatment at a competitive price. The technology has few requirements for operation and maintenance, and, in most cases, have no or low energy demands. The wetland systems can enhance the natural landscape and are generally harmless to the environment. There is no need for protection zones because the treatment occurs in the undersoil.

Disadvantages

This is a land-intensive technology.

Cultural Acceptability

For climatic reasons, this technology is not well accepted as a wastewater treatment technique in Poland.

Further Development of the Technology

There is a need for to enhance this technology by improving the capacity of the biogenic elements to effect pollutant removal, and the winter performance of the technology.

Information Sources

Contacts

Zak ad Gospodarki, Wodno- ciekowej, FIN-SKOG Geomatics International, ul. Ja kowa Dolina 59, 80-286 Gda sk, Poland, Tel./fax: (48-58) 476771.

Bibliography

Ministry of the Environmental Protection, Natural Resources and Forestry 1993. Water Protection and Waste Water Treatment. Ministry of the Environmental Protection, Natural Resources and Forestry, Warsaw.

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