Sewage Treatment
What you can expect a reed bed to achieve.
- Decreased biological oxygen demand
- Reduction of suspended solids
- Reduction of nitrogen concentrations
- Removal of metals
- High level of bacterial and viral removal
Treatment can typically be separated into a primary, mechanical stage where solids are separated from the liquids, followed by biological treatment in the reed beds. If required, further polishing can take place for high treatment standard. Treated water can be reused for non-contact purposes such as irrigation and process water.
Primary treatment by Separation
A sedimentation tank is arranged to settle organic substances. The tanks are designed so that even at maximum flow a good sedimentation efficiency is achieved. The retention time in is generally about 1.5 days.
Secondary Treatment by Reed Beds
Biological treatment of the wastewater occurs in the reed bed. The main treatment process is by the microbial and fungal decomposition of organic matter in the rooted soil or substrate. In addition, 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.
To construct a reed bed, a void is excavated to approx 1.0 m deep. It is lined with polyethylene, rubber or bentonite and filled with soil or other media. The lined and refilled basins are planted with helophytes like Phragmites australis.
Typically for sewage treatment, the wastewater percolates the filter substrate vertically to the bottom drains.
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.
Additional oxygen transfer into the rhizosphere occurs by exchange from the aerial plant stems to the roots.
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