Free Water Surface CW's: Operation
Free water surface wetlands are made up of ponds or channels. Water flows through the system at a relatively shallow depth. The wetlands can include densely vegetated areas or open water areas. If emergent macrophytes or rooted submergent macrophytes are used, a suitable substrate is needed to support the vegetation.
Since the water flows through the system at low velocities, particulates and suspended solids will settle in the wetland. The wetland vegetation can enhance this sedimentation.
The microbial communities attached to the vegetation and suspended in the water column, will be responsible for the removal of soluble organic compounds. The degradation can proceed both aerobic and anaerobic.
Nutrient removal is mainly based on plant uptake. But nitrogen can also be removed by ammonia volatilization and nitrification and denitrification. Nitrifiers grow attached to the roots which provide oxygen. If the wetland is densely covered with macrophytes, oxygen diffusion to the water is limited thus decreasing the dissolved oxygen level. Anaerobic conditions will prevail.
Free water surface CW's are often used as tertiary treatment, polishing stage.
FWS with floating macrophytes
FWS with emergent macrophytes
Stabilization ponds are man-made shallow basins comprising a single or series of anaerobic, facultative or maturation ponds.
Anaerobic ponds are used as a pretreatment. They receive high organic loads. These high organic loads produce anaerobic conditions throughout the pond.
Facultative ponds are used as a secondary stage. Remaining biodegradable organic matter is removed through the coordinated activity of algae and heterotrophic bacteria.
Maturation ponds are used as tertiary treatment. The main function is the removal of pathogens and nitrogen.