Landcom Book 4 Maintenance - WSUD

Water Sensitive Urban Design
5.3.1 Dewatered silt
The NSW Department of Environment
and Climate Change (DECC) has recently
amended the rules governing waste
disposal. Silt collected from sediment
basins is now defined as general waste
(non-putrescibles). For general waste
(non-putrescibles), there is no requirement
for the silt to be tested, unless the Council
(or owner of the asset) believes that the
nature of the catchment could cause the
silt to have:
••
A Specific Contaminant Concentration
(SCC) higher than the guideline values
given by DECC; and/or
••
A leachable concentration of any
chemical contaminant (as determined
through the Toxicity Characteristics
Leaching Procedure (TCLP)) higher than
the guideline values given by DECC
Guideline values for General Solid
Waste for both measures are given in
the following document http://www.
environment.nsw.gov.au/resources/
waste/08202classifyingwaste.pdf
Disposal of dewatered silt is accepted
by WSN Environmental solutions, but
only at their Eastern Creek and Lucas
Heights locations. WSN Environmental
solutions classify General Solid Waste as
Special Waste, which has a disposal cost
of $220 per tonne. The waste service
facility at Belrose will also accept General
Solid Waste; however due to a limited
capacity, it will only accept waste from the
surrounding LGA (Ku-ring-gai).
Although in theory general waste (nonputrescibles)
can be accepted by private
waste disposal operators, very few facilities
within the Sydney Metropolitan actually
have the capacity to accept sludge / silt
type material. One of the few exceptions
is Blacktown Waste Service who accepts
general solid waste (non-putrescibles) at a
cost between $75 and $110 per tonne.
5.3.2 Filter media
The filter media for bioretention basins
may need replacing in the following
situations:
1) Filter media has reached full capacity
for retaining metals as identified
through pollutant breakthrough.
2) Surface of filter media is clogged.
Recent research conducted by FAWB has
shown for a bioretention basin sized at two
percent of the impervious catchment area
and a filter media depth of 0.5 metres,
pollutant breakthrough will occur within
15 years. The results were considered
conservative (ie lower estimate) as the
soils had a low pH. Soils with a neutral
pH will have a greater capacity to attract
metal pollutants, hence further delaying
breakthrough from occuring. When
breakthrough occurs, the entire filter
media will need replacing.
Surface clogging can be observed
through poor plant growth, or when
ponding times exceed the design
specifications. The replacement of filter
media is not required if plant growth is
poor. The clogging of the surface media
could be remedied by re-establishing
plants to ensure density is sufficient in
maintaining surface porosity. If, however,
plant growth is adequate, clogging
is related to failure of the filter media
and the top 200 to 300 millimetres of
filter media will require replacing.
Removed filter media may be
contaminated and should be tested
accordingly. Filter media classified as
contaminated should be disposed of
at a certified waste disposal centre.
Alternatively, there may be options to
bio-remediate the soil and reuse it.
36 Book 4 | MAINTENANCE