Designing Ecological Habitats - Gaia Education

160 Designing ecological Habitats
Sizing of SSFCWs to
Treat BOB to 15mg/l
after septic tank pretreatment.
Volume
2001/pe
5 pe = 1m 3
BOD
in mg/l
TSS
in mg/l
NH4
in mg/l
Winter
Temp o C
Estimate
Person
Area/pe
Area m 2
/m 3 of
sewage
HRT
in days
BOD
out mg/l
TSS
out mg/l
NH3
out mg/l
1m 3 160 100 35 10 4.0 20.1 5.8 15.0 11.1 9.3
1m 3 160 100 35 15 3.0 15.0 4.3 15.0 11.2 13.2
1m 3 160 100 35 18 2.5 12.5 3.6 15.0 11.4 31.4
1m 3 160 100 35 20 2.3 11.2 3.2 15.0 11.5 13.6
The numbers are approximate. Add 30% for gross footprint plus septic tank area.
is to ensure that the influent does not overload the narrower front of the
wetland with incoming suspended solids that might cause clogging. It is
recommended that the sizing and material specifications for the proposed
wetland are checked by a qualified engineer before building starts.
Typically the land is excavated with earth taken out from the excavation
used to create berms as shown in the sketch below. Enough freeboard should
be allowed above the gravel to take a large storm event. When this happens,
the wetland will flood and gradually drain out as the storm subsides.
A liner will prevent the polluted water contaminating the groundwater.
This is typically made from 1 mm thick high-density polyethylene (HDPE).
This material will normally come in rolls 5 metres wide. Joining sheets will
require specialised welding equipment and is best carried out by a qualified
contractor. Geotextile, such as 1 mm or 1.5 mm polypropylene, should be
installed below and above the liner to protect it from penetration from sharp
rocks and gravel.
The influent pipe from the septic tank will pass through the liner using a
boot, which again should be installed by an HDPE contractor. This influent
is connected to a manifold with cleanouts. This manifold has many holes
along its length to distribute the influent across the width of the wetland. A
similar effluent manifold is installed at the far end of the wetland. Over each
manifold, rocks of around 75 mm are used to help distribute the incoming
sewage across the width of the wetland cell. In between, 25 mm gravel is
installed to a depth of 70 centimetres. A 5 centimetres layer of 10 mm pea
gravel can be laid on top of the 25 mm gravel to facilitate planting.
It is important to protect the liner from sunlight at the edges, as it will
degrade slowly with ultra-violet light. The liner can be covered where it
comes out of the gravel with the outer layer of geotextile and the geotextile
covered with rocks.
A level adjust sump needs to be installed inside the wetland or in the
effluent pipe as it leaves the wetland cell. This device allows the water level
to be set. If the septic tank or tanks are below ground they will have effluent
pumps to feed the wetland. If the septic tank is above the ground, the level
adjust sump will control the level from the septic tank through the wetland.
The cell can then be flooded, preferably all or partly with river or lake
water, which will come with living aquatic life. Wetland plants are installed