Tamar Estuary
River Health Action Plan
33
The reduction in sewage volume discharged to the Estuary was calculated by first establishing a base line for the
combined system. Utilising the validated combined system hydraulic model, a range of design rainfall events was
simulated and the total sewage and stormwater discharged to the Estuary was quantified. The combined system
hydraulic model was then altered to include the proposed infrastructure and once again, the same design rain fall
events were simulated and the discharge quantified. The modelled reductions in sewage volume discharged to
Estuary post-mitigation works could then be readily calculated.
West Launceston Diversion
This diversion is expected to provide the greatest value for money project related to the combined system. Part
of the LSIP, it has support of TasWater which is an important feature for progressing its implementation.
Currently, the West Launceston and Trevallyn sewage catchments, despite being separated from stormwater, are
piped to the join the combined system at the Margaret Street Pump Stations. While under dry weather flow
conditions this does not cause problems, in wet weather events the untreated sewage can bypass the pump
station to Ti Tree Bend and spill into the Estuary.
In order to facilitate this mitigation option, upgrade works will be required to the sewer mains between West
Launceston and the Ti Tree Bend STP. In summary, the works required include:
Diversion of the West Launceston trunk sewer across the South Esk River;
Installation of a new transfer main between West Tamar Road and Ti Tree Bend STP including connection
of West Tamar No. 1 Pump Station and crossing of the Tamar Estuary; and
Connection works at Ti Tree Bend.
This project is estimated to have a capital cost of $4.6 million and lead to approximately a 19 per cent reduction
in sewage loading to the Estuary. This option also has ongoing operational costs in new and increased pumping
costs which would be incurred by TasWater. The present value of costs (capital and operational) of this action
over a thirty year period is $5.6 million.
New Combined Rising Main
The works include the decommissioning of the Old Margaret Street Pump Station (OMSPS) and diverting these
flows to the New Margaret Street Pump Station (NMSPS) and increasing the combined low (sewage) flows to the
STP from approximately 400 L/s to 800 L/s. To accommodate the additional flows, it is proposed that a new rising
main be constructed to connect the upgraded NMSPS to Ti Tree Bend.
In addition to reducing the sewage loading discharged to the Estuary from the Margaret Street site, benefits of
constructing a rising main between NMSPS and Ti Tree Bend include the following:
Reduced flow in the City Rising Main enabling greater flows to be discharged from St John Street Sewage
Pump Station and the Forster Street Pump Station;
Provides the opportunity for a significant area of habitat rehabilitation at Ti Tree Bend;
All flows will be screened prior to discharge at the Margaret Street site (currently CSO from Old Margaret
Street Pump Station are not screened); and
Provides an alternative discharge route (system redundancy) to the STP in the event that the City Rising
Main is "out of service".
Tamar Estuary
River Health Action Plan
34
To achieve the full benefit of this increased flow, it will be necessary to upgrade the Ti Tree Bend STP so that this
additional volume (and the associated pathogens) is not overflowed to the Tamar River after the inlet works at
the Ti Tree Bend STP during high inflow periods. It is proposed that the land adjacent Ti Tree Bend (owned by
the City of Launceston, currently known as the "silt ponds") be converted to a wetland system with additional
buffer undercover storage.
The land available at the silt ponds would enable the construction of a 10 hectare wetland. It is likely that the
wetland would still require some undercover storage to mitigate the effect of odour.
In summary, the project will include:
1.
Works upstream of New and Old Margaret Street PS to divert flows to NMSPS (making OMSPS
redundant);
2.
Installation of new high head sewage pumps to increase the total sewage pump capacity to (nominally)
800L/s;
3.
Installation of rising main works to connect NMSPS to both the proposed storage facility and Ti Tree Bend
STP;
4.
Reconfiguration of St John Street Sewage Pump Station including the required rising main upgrade from the
pump station to the City Rising Main (junction in the vicinity of the Charles Street Bridge) to increase the
pump rate to Ti Tree Bend to approximately 500-600L/s;
5.
Reconfiguration of Forster Street to increase the pump rate to Ti Tree Bend to approximately 500-600L/s;
and
6.
Works to a storage and wetland at Ti Tree Bend as described above.
The capital cost of this option is estimated at $26.8 million (total present costs over 30 years of $34.9 million)
and is expected to result in a reduction in loading to the Estuary of 28 per cent of current load.
Offline storages
The proposed three offline storage projects are all grounded in the same theory. That is, with a large rain event
following a period of dry, there is currently a “first flush” of untreated sewage and highly contaminated
stormwater that may overflow from the system. While overflows may continue with continuing high rainfall, the
amount of sewage in the system is not at the high levels of this first flush.
The off line storages are proposed at three locations that will help capture much of the first flush effluent, such
that it can be bled back into the system when rainfalls subside and can be treated at the Ti Tree Bend STP plant.
The first of these storages is planned for a location adjacent to the New Margaret Street Pump Station and would
be the largest storage planned at 4.2 mega litres. It would preferably be located underground in Kings Park, but
there are heritage issues that would need to be considered in any construction. Its capital cost is estimated at
$10.0 million ($11.4 million present value of total costs) and is expected to reduce the sewage loading to the
Estuary from the combined system by approximately 21 per cent.
The second storage is proposed to be underground on vacant land adjacent to the Forster Street Pump Station
and would nominally be 2.5ML. It is estimated to cost $8.4 million ($9.7 million total present cost) and would
reduce sewage load by approximately 6 per cent.