Page 1770 - Week 07 - Tuesday, 29 May 1990
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(2) Refer to Table 1.
(3) Refer to Table 1.
(4) The major proportion of the volume of pollutants retained in water pollution control structures (approximately 90%) is generated from storm,runoff up to the 1 in 1 year annual rainfall intensity event (approximately equivalent of a heavy thunderstorm). Therefore, the frequency of cleaning is dependant on the frequency of significant storm flows in a given time period. The traps are not cleaned at regular predetermined time intervals.
Depending on land usage, size and type of development within the catclunent some traps are cleaned more frequently than others. During periods of low rainfall it may be necessary to clean traps to prevent bad odours or potential health risks.
Throughout the life of the structures it is estimated that cleaning frequencies for the removal of sediment would be:
In-ground Gross Pollutant Traps 4 times/year
Open Gross Pollutant Traps 3 times/year
Trash Racks 6 times/year
Water Quality Control Ponds every 5 to 10 years
(5) The full capacity of each trap is identified on Table 1.
(6) At present all waste material collected from traps is transported to either Belconnen or Mugga Lane tip. Recycling of sediments such as coarse sand has been considered but is at present impractical due to the high cost of removing rubbish, decomposed vegetation, petroleum based pollutants and domestic pesticides.
(7) At present no contracts for cleaning traps have been awarded due to the uncertainty in annual cleaning frequencies and related costs. A11 cleaning of in line pipe traps is carried out by day labour and plant from ACT Electricity and Water contracted to the Department of Urban Services. Cleaning of open traps is carried out by the Department of Urban Services. Over the last 12 months a number of mechanical cleaning units have been retailed to increase efficiency of the cleaning gangs and existing machinery.
Refer to Table 1 for average annual costs.
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