Swept Under the Carpet? Road Derived Sediments: Friend Or Foe?
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SWEPT UNDER THE CARPET? ROAD DERIVED SEDIMENTS: FRIEND OR FOE? Peter Mortimer Technical Services Manager, Downer NZ Ltd, Dunedin, New Zealand Kirsty Marlow Divisional Manager, Downer NZ Ltd, Queenstown, New Zealand Abstract It is common practice to dispose in landfill the material removed from road surfaces and transported into sumps as part of rain runoff. The material contains contaminants including heavy metals, especially Lead, Copper and Zinc, which has detrimental effects to the receiving environments from landfill leachate. However, a landfill tends to be acidic, and this environment encourages the heavy metals (especially Zinc) to go into solution forming a neutral salt, and leach into the surrounds. Better, if possible to encapsulate the heavy metals elsewhere in a useful product and avoid adding to landfills. Downer undertook research and commissioned laboratory testing as part of a project development for Queenstown Lakes District Council in order to investigate this possibility. This paper will describe the development of a sampling method, and results of testing compared with industry solid waste management limits. Uses to date in the Queenstown Lakes District are discussed, as well as the potential for further use as an economic commodity. Key Words Road Sump Waste, Street sweepings, Road Derived Sediments, Hazardous waste, Landfill, recycling Introduction Road maintenance contracts with an urban component commonly have a requirement to remove the build up of detritus from the road surface and kerb and channel and the sediments and deposits within surface drainage sumps. It is usually the contractors responsibility to dispose of this material in a responsible manner. The cost of this disposal (and thus the price allowed for in tenders) is directly related to the disposal method which is obviously dependant on the definition of the materials being disposed. Within the context of the Queenstown Lakes District Council (QLDC) road maintenance contract, these road sweepings were not specifically defined as a substance requiring special treatment and information about the degree of toxic contamination was not provided in the contract documents. Standard practice of the time was to remove the refuse from the sweepings and take the material to landfill by the truckload as cover material or clean fill. This worked well up until the point that the landfill operator of the time refused to take the street sweepings on the ground that they had test results to show that the degree of contamination breached the limits set by the Ministry for Environment for their class B landfill rating. The cost for disposal rose from under $10/tonne to over $200/tonne. So now what were we to do? Time was of the essence as the drainage maintenance budget was being rapidly consumed by the landfill provider. The nearest class A landfill was Dunedin and to take the material to Dunedin was equally cost prohibitive and just didn‟t make sense. After all, this isn‟t nuclear waste, right? About the QLDC Road Maintenance Contract Downer has undertaken road maintenance on behalf of the Queenstown Lakes District Council since 2004. The roading network is approximately 817 km in length, 470km of which is sealed road. There is a reasonably diverse road user profile, with a high percentage of tourist traffic, but a relatively low number of heavy vehicles as farming activity is limited, and there is currently no forestry activity. The district has a low rate payer base relative to road user numbers. The Queenstown Lakes District roading network is diverse in nature Whilst the district contains many historic and relatively low volume roads, for instance Macetown Road, Tobins Track, and the Skippers Canyon Road, it is also home to built-up residential and commercial centres, particularly in the Queenstown and Wanaka townships. To give an indication of the scale of the RDS generating activity, in the 2011-2012 financial year, 1900km of road sweeping was undertaken, and 1334 sumps cleaned. Annual quantities of RDS collected are variable and at least partially dependent on the severity of the preceding winter and therefore how much ice grit has been spread. Network wide, this would total anywhere between 1000 and 2000 tonnes. Ice grit and sealing chip make up the majority of RDS recovered. The emerging problem When this problem was realised, it was accepted by QLDC that the reclassification of the RDS from general material that has historically been treated as clean fill, to a hazardous, contaminated material, was outside of the scope of the current contract. Therefore any additional cost associated with either interim or long term solutions would be borne by QLDC. An agreed interim solution was put in place which was: Decant any liquid component in to the foul water system Continue to dispose of the resulting dry solids to landfill, with the landfill operator treating them prior to disposal within the landfill As indicated, the cost of treatment and disposal of the predominant solid portion presented QLDC with an unsustainable activity. Therefore the Downer and QLDC team commenced a project to find out as much as possible about Road Derived Sediments (RDS) so that we could avoid any breach in resource consents and environmental certification, whilst providing QLDC with a practical and cost effective solution to the management of RDS. We developed a sampling methodology and laboratory testing regime designed to learn more about the type of contaminants and their variability. We also completed testing on samples treated using an alternative method identified through a literature search. Landfill legislation As road maintenance contractors, landfill management and legislation is not our specialty, however we needed to understand more to know how to provide a solution to this problem. Landfills within New Zealand require resource consents to operate under the Resource Management Act 1991 (RMA). The basis for the development of resource consents is provided in a set of “guidelines” published by the Ministry for the Environment (MfE) particularly Module 2: Hazardous Waste Guidelines – Landfill Waste Acceptance Criteria and Landfill Classification. Figure 1 provides a simple workflow describing the acceptance criteria for waste at any landfill. Figure 1, Waste acceptance decision process Source; MfE Module 2: Hazardous Waste Guidelines, landfill waste acceptance criteria. Using this workflow and assuming from the start that RDS is not a “Prohibited” substance then this should be relatively straight forward. We get down to question four where the user is asked if the waste is “asterisked on the NZ Waste List?” At this point after doing a search for the NZ Waste list we identify code “20 03 03 Street Cleaning Residues” under the sub category of “Other Municipal waste” as being the closest description to our waste. It does not have an asterisk against it therefore deemed acceptable for disposal? There is absolutely no requirement to complete screening testing to prove otherwise and nor does the guideline state that and “special treatment” is required. However, the QLDC landfill operator had completed screening testing and deemed the wasted “prohibited”. In the interests of gaining more knowledge we undertook quite extensive Toxicity Characteristic Leaching Procedure (TCLP) and Particle Size Distribution testing to determine the extent of our (QLDC and Downer) problem. Development of testing regime After completion of a literature review the Land Transport New Zealand (Now New Zealand Transport Agency) research paper 345 “Contaminant Characterisation and toxicity of road sweepings and catchpit sediments: Towards more sustainable reuse options” (Depree, C) was identified as being cornerstone research for the New Zealand context. The main premise of this work suggests Road Derived Sediments (RDS) are characterised as a prohibited substance. They sampled RDS from various locations within Auckland, Hamilton and Christchurch. The findings of this research showed there was potential reduction in the leachability of heavy metals through stabilisation with compost and also perhaps a reduction by allowing the materials to “cook”, meaning leaving them to weather or cure in the elements prior to disposal. Some aspect of “Passive bioremediation” may also be at works here. With these results in mind we agreed with QLDC to obtain samples from several different locations within Queenstown and Wanaka which were representative of what we considered high to low stress loading areas. We would assess the individual initial toxicity level using the TCLP method. We decided to test the affect of “cooking” the sample and also mixing the sample with locally produced compost as a means of stabilising the heavy metals. We used Hills Laboratory in Hamilton to advise on testing methodology and undertake the tests. The goal of the testing was to firstly identify if we are in fact dealing with a material with toxicity that does breach the class B landfill guidelines and if so by how much. Additionally we wanted to understand how much in-organic material we were collecting as well as particle size distribution of this material fraction to be able to determine if the in organic component could be reused or recycled. Testing for heavy metals (Lead, Copper and Zinc) was undertaken as these are the main heavy metal contaminants present and of concern to the land fill operator. This is backed up by the findings within research report 345. Sampling methodology Based on the agreed testing regime, above, samples were taken from each the following locations, in both the Queenstown and Wanaka areas: Mudtank in “high stress” area such as at the bottom of a hill Mudtank in CBD Mudtank in other urban area Roadside sweepings (collected from kerb and channel or road surface) Typical “high stress” mudtank location A further sample was generated by combining RDS from each of the four locations above to simulate the actual process of combining sweeper truck loads at a handling facility to cart the waste in larger quantities to landfill.