Natural wetlands, permanently waterlogged areas populated by hydrophytic plants such as reeds, comprise a variety of sub-surface micro-habitats of differing oxygenation and redox potential, and support a diversity of microorganisms – bacteria, fungi, actinomycetes, protozoa – which degrade organic and inorganic substrates entering the system. Constructed wetland systems are increasingly being employed for treatment of wastewaters, sludges and industrial effluents as a cost-effective, low energy and robust alternative to traditional engineered biological treatment such as the activated sludge process.
Constructed wetlands (CWs) are classified according to their mode of operation as surface flow, horizontal flow, vertical downflow or vertical upflow type. They have been used successfully in the treatment of domestic sewage, urban, highway and stormwater runoff, acid mine drainage, agricultural waste, industrial effluents, landfill leachate and meat processing effluent. BOD and solids reduction occurs through microbial activity and removal of nitrogen and phosphorus through the processes of denitrification, plant uptake and sorption.
- Artificial wetlands for wastewater treatment were pioneered in Germany and this natural treatment technology is now used all over the world, not only for domestic waste waters but also for industrial effluents.
- A lot of research has been done in the last twenty years and adequate design guidelines for the different kinds of constructed wetlands are available.
- The natural ability of soils to filter suspended solids mechanically and chemical reactions (precipitation) with sewage constituents are other forms of cleaning mechanisms, which interact. So pollutants are metabolized, settled and absorbed in sequential anaerobic and aerobic processes.
The reed bed treatment spray nozzle is a device to facilitate the dispersion of water or wastewater over a given area (3 m diameter throwing circle) by break-up of the fluid into bigger droplets (drop size should be high to avoid aerosol generation).
The fluid rotation within the nozzles provides a self-cleaning effect of the nozzle. The outflow at a given diameter of 10 mm is roughly 0,1 l/s at a necessary pressure head of 0,19 bar (1,9 m water column height).
Reed bed treatment system
The reed bed treatment system combines aerobic and anaerobic decomposition processes in a 1.0 m thick soil or substratum layer. The polyethylene lined and refilled basins are planted with helophytes like Phragmites communis, Typha latifolia, Typha angustifolia or other aquatic macrophytes.
The wastewater percolates the filter substrate vertically to the bottom drains.
Besides the microbial and fungal decomposition of organic matter and pollutants in the rooted soil or substrate matrix, chemical and physical precipitation, adsorption and filter processes occur due to soil constituents like clay minerals and humus particles. This is most important for phosphate and ammonia binding. Some of the wastewater nitrogen is released out of the artificial ecosystem to the atmosphere as nitrogenous gases (denitrification).
Through intermittent loading of the reed beds a radical change of oxygen regime is achieved. After water saturation by feeding with the distribution system a drainage network at the base collects the purified water. The pore space of the substrate is refilled with air thus enabling aerobic decomposition processes.
Another part of oxygen transfer into the rhizosphere happens through a special helophyte tissue in the plant stems and roots (aerenchym) from the air.
Clogging effects of the filter substrates (soil, sand, gravel) are prevented by the continuous growth and decay of roots and rhizomes of the aquatic macrophytes and the thereby remaining soil macropores. In this manner, long-term water transport into the soil matrix is guaranteed.
The substrate, which is filled in the sealed earth basins, is a sitespecific mixture of selected components determined by aspects of hydraulic conductivity and physico- chemical properties. The soil and substratum mixture is a single case decision of planning further depending on the composition of sewage, whether municipal or industrial.
By means of high evapotranspiration of the marsh plants the wastewater tends to increase pollutant concentrations, thus improving the efficiency of the microbial degradation process. These artificial wetlands reduce both pollutant concentrations and the water volume. Their performance has to be described by - pollutant load parameters (kg BOD / [m² x d]), not just comparing (pollutant)concentrations (mg/I) between inlet and outlet.
Download 1: constructed wetlands