Landcom Book 4 Maintenance - WSUD

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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 <strong>Book</strong> 4 | MAINTENANCE
Water Sensitive Urban Design Detention pond | Garden Gates Open space | Victoria Park The location of the dewatering facility should consider existing site constraints. 5.3.3 Liquid waste (from dewatering activities) Prior to discharge, the silt removed must be dewatered such that it does not contain any free liquid (that is, the density is 1.5 to 3 times the density of the material if completely dry (Collins, 2006)). The dewatering of collected silt can be accommodated in the design of the sedimentation basin, whereby free liquid is directed back into the treatment train. Alternatively, collected silt can be stockpiled within designated bunded areas offsite or stored in skip trucks brought into the site. The location of the dewatering facility should consider existing site constraints, DECC regulatory requirements, health and safety issues, odours and other community concerns (WBM and Ecological Engineering, 2005). Liquid removed during dewatering is likely to be classified as either Group A liquid waste or non-controlled aqueous liquid waste, depending on the concentration of suspended or dissolved chemicals (WBM and Ecological Engineering, 2005). The level of contamination should be confirmed by laboratory testing; however it is likely that dewatered water will classified as non-controlled aqueous liquid waste. The decantered water must be treated prior to discharge to a receiving waterbody. Treatment can be achieved through many mechanisms, including: •• Pumping decantered water to a settling tank to remove elevated levels of suspended solids. This may require the addition of a flocculent such as gypsum. •• Decanting liquids to pervious areas, ensuring liquids are appropriately treated to meet reuse requirements. Potential implications of reusing decanted water from stormwater treatment measures include high pollutant (in particular heavy metal) concentrations and health risks posed by trapped pathogens. •• Decanting liquids back into the stormwater treatment system. 5.3.4 Equipment requirements For a sediment basin, wetland or pond, cleaning will generally require a backhoe, excavator or eductor / vacuum truck, requiring service from one to two vehicle operators and one to three labourers (HSC, 2001). The removal of waste from a bioretention basin must be done by hand to protect the bathymetry and compaction of the filter media. <strong>Book</strong> 4 | MAINTENANCE 37
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Level 2, 330 Church Street Parramat
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