Sub Surface (SS) Wetlands

Sub-Surface (SS) Flow Treatment Wetlands are ecological engineered systems that comprise planted gravel/sand substrate with flows pass through the system below the ground.

The mechanisms underpinning treatment with Sub-Surface Wetlands are dominated by physical filtration and biofilms growing on the gravel (or other) media and plant roots.

SS Wetlands are broadly divided into two groups:

  1. Horizontal Flow Sub-Surface wetlands (HFSS Wetlands):
    water flows horizontally from one end of the planted gravel basin to the other.
  2. Vertical Flow Sub-Surface wetlands (VFSS Wetlands):
    water flows from vertically from the top to the bottom of the planted gravel basin.

Both HFSS and VFSS wetlands have different advantages and are used for different applications.  Key areas that each of these systems are applied include:

VSSF wetland diagramVertical Flow Sub-Surface Wetland schematic | Horizontal Flow Sub-Surface Wetland schematic

VFSS Treatment Wetlands Advantages

  • Lower footprint than SF Wetlands.
  • Aerobic reaction kinetics if fully drained.
  • Organic removal / Nitrification.
  • Efficient suspended solid removal – high level filtration.
  • Ability to incorporate additional media types to provide enhanced treatment (e.g. zeolite, activated carbon).
  • Good for applications where public cannot be excluded as wastewater is kept below ground.
  • Don’t have to wait for plants to grow before treatment is provided.

HFSS Treatment Wetlands Advantages

  • Lower footprint than SF Wetlands.
  • Low hydraulic requirements – flat hydraulic grade line through treatment train.
  • Anoxic environment for denitrification.
  • Suspended solid removal.
  • Good for applications where public cannot be excluded as wastewater is kept below ground.
  • Don’t have to wait for plants to grow before treatment is provided.

Design considerations

It is important to ensure that the solid loading to Sub Surface wetlands are carefully managed, otherwise blocking can occur.  This can be done by either providing suitable pre-treatment (e.g. primary settlement), or by incorporating “resting” periods within the regular operation to enable organic particles within the media to mineralise.

IMG_6003

Applications of Sub-Surface Wetlands

Sub-surface wetlands have a number of beneficial applications in industry, including:

  • Small scale sewage treatment (e.g. <1,000EP) – post primary tank treatment.
  • Landfill leachate treatment (VFSS for nitrification/organic reduction;  HFSS for denitrification)
  • Situations where wastewater needs to be excluded from public access e.g. caravan parks
  • Stormwater filtration and treatment – particularly as water can be held for longer periods within the media to reduce evaporation.  This is an advantage for stormwater applications where flows can be infrequent.