Coal Tar Detection in Road Binder
In the UK, utilities dig approximately 2.5 million holes in roads and pavements every year. WM3 guidance states that any soil or other material excavated is waste and should be assessed and classified for waste management purposes. Any excavated material identified as hazardous must be disposed of at a licenced hazardous waste receiver and the hazardous waste landfill tax paid. New laws in April 2018 bring significant fines and a possible criminal record if waste is incorrectly classified or disposed of.
Material excavated during utilities works consists of road surface material that contains aggregate and a binder and soil from the subsurface. Before 1980, the majority of the binder was derived from coal tar, an absolute hazardous material under WM3 rules. Petroleum Bitumen, the residue from crude oil distillation, has been routinely used to make road binder since 1980 and is classified as non hazardous under WM3. Bitumen is essentially 100% petroleum hydrocarbon. The soil subsurface may be contaminated with coal tar residue if coal tar derived road binder was used when the original road surface was put down. The majority of roads in the UK contain variable patches of coal tar and bitumen. The coal tar may be in lower layers, but repairs and re-use of excavated material can mix everything up.
WM3 has specific rules for bituminous waste containing coal tar. The waste is considered hazardous if it contains more than 0.1% coal tar or any coal tar present contains Benzo a Pyrene above 0.005%. Bitumen is considered non hazardous, so waste can be 100% bitumen and will be classified as non hazardous.
Chemical analysis is needed to identify if coal tar is present. One regularly used option is PAK spray, a quick roadside test that reacts with poly aromatic hydrocarbons (PAHs) in the tarmac to give a visual colour change to indicate if coal tar is present. Coal tar and bitumen contain the same PAH compounds. Coal tar usually contains more of a certain type of PAH than bitumen. The PAK user guide recommends its use on material that is above 15ºC and dry. In the UK, for much of the year, this is not that common, causing the PAK spray to give false negative results, especially when mixtures of coal tar and bitumen are present. PAK spray also gives false positive results because some bitumen binders contain naturally high PAH concentrations, while vehicle exhaust residue and atmospheric deposition also containing PAHs are embedded in road surface material. The colour change is also subjective, with people interpreting the colour change differently. For regulatory purposes PAK spray is not sufficiently reliable.
Another option is conventional laboratory analysis. The most frequently used analysis is GC-MS for the 16 target PAHs. The “double plot” ratio of these PAHs can indicate if a sample is just coal tar or just bitumen, but cannot reliably measure the proportions of coal tar in bitumen when mixed together. WM3 specifically states that “Assessments based on PAH’s alone are not consistent with the legislation and cannot be used to classify a waste as non-hazardous”. Just using GC-MS for the 16 PAHs is therefore inappropriate for coal tar identification. The more comprehensive conventional laboratory analysis required to get data consistent with the legislation is considerably more expensive because multiple analytical methods are required and it typically takes weeks to deliver results.
Common practice however uses a single GC-MS analysis to quantify Benzo a Pyrene (BaP) included with the 16 target PAHs and use the 50 mg/kg BaP limit to classify utility arisings as hazardous. WM3 guidance specifically states that the 50 mg/kg BaP limit is only for the coal tar content of the waste and not the total waste. PAH results alone cannot give the ratio of coal tar in the bitumen, so the BaP concentration reported is for the entire binder content. If the binder is predominantly bitumen, a below 50 mg/kg value may incorrectly give a non hazardous classification because the bitumen will dilute the BaP content of the coal tar fraction. Some laboratories also include the aggregate content in the final result, also diluting the BaP value, creating an incorrect classification. Many people who get laboratory data are unfamiliar with how the values have been generated which can cause misunderstanding with the interpretation. This can lead to inadvertent misclassification and the costly consequences.
Regulators and various utilities industry associations recommend chemical testing that determines coal tar content to be accredited to UKAS standards. An analysis method is accredited for a specific task. A method that is UKAS or MCERTS accredited for the determination of the 16 PAHs in a silty soil, clay, loam or made ground is not accredited for the determination of the 16 PAHs in a bituminous mixture. The data is only accredited for the determination of coal tar in bituminous waste if the interpretation is included in the accreditation scope. In the UK there are no laboratories that are specifically accredited for the analysis and determination of coal tar content in a bituminous matrix. The simple PAH analysis by GC-MS cannot be accredited for coal tar analysis because the data generated cannot reliably identify if coal tar is present in bituminous mixtures. This means that the majority of utilities contractors are not getting accredited results for coal tar determination in bituminous waste, but may be under the impression that they are.
Local Authorities and the Highways Agency are responsible for UK roads, so jointly liable with the utility and contractor for any asphalt arisings incorrectly classified and disposed of. To minimise this liability, contracts with contractors now make the contractor take full responsibility for identifying if coal tar is present. The potential liability of incorrect disposal, the delay in obtaining results and the uncertainty of analysis pose significant problems for utilities contractors. If they wait several weeks for lab results from analysis that meets the legislative requirements, the arisings must be left by the excavation until a correct hazard assessment is made. The contractor however can get financial penalties if the utility is not connected or traffic congestion occurs because the works are not finished quickly. The alternative is to assume all the arisings are hazardous and pay the increased costs of shipping to a licenced hazardous waste receiver using a licenced hazardous waste carrier. Hazardous waste disposal sites are often a considerable distance from where the excavations are carried out, increasing the number of miles HGV vehicles must travel to dispose of the excavated material. With new low emission zones in city centres, typical “muck away” HGVs will need to pay additional entry and exit fees adding more cost. By assuming everything is hazardous the work is finished on time, but the cost of sending arisings from 2.5 million excavations as hazardous could be as much as £1 billion per annum in management, transportation and landfill tax fees. These significantly increased costs are ultimately passed onto the tax payer or utilities customer.
QED HC-1 Hydrocarbon analyser
The QED HC-1 is a UK designed and manufactured instrument specifically designed to analyse for a wide range of petroleum and coal tar derived hydrocarbons. The QED can be set up in the back of a van, on an office desk or in a laboratory. It does not need expensive sample extraction systems or use environmentally damaging solvents such as dichloromethane, typically used in conventional laboratories. It is easy to use requiring only basic skills and has a comprehensive QC system to ensure valid data is generated.
The QED can, within a few minutes of taking a sample of road surface or subsurface, detect and identify if coal tar, bitumen or bitumen/coal tar mixtures containing as little as 1% coal tar is present. For mixtures, the result will indicate the approximate % of coal tar in the bitumen. If coal tar is detected, the approximate % of benzo pyrene in the coal tar (as required by WM3) is given. The sample cost is at least 10x lower than conventional laboratory analysis that meets the legislative requirements.
Sample extraction and analysis is very simple. A sample pot is filled ¼ full with small pieces of road surface sample. Extraction solvent is added until the layer of binder is covered. Nothing is weighed or measured, so no special equipment is needed. The sample is shaken for about 30 seconds and then a dip stick is put into the solvent and immediately transferred to the QED analysis cuvette and analysed. The QED analysis takes approximately 5 seconds. If the cuvette contains too much or too little sample, the QED QC system detects this and provides a dialogue box that tells the operator what to do to obtain a valid result.
The analysis is unaffected by temperature or sample moisture and only measures the extractable binder component so is unaffected by variability in %aggregate within the binder. If the identification is just bitumen based road binder, the arisings containing binder are non hazardous. If the result is just coal tar, the arisings will be hazardous. Binder content is typically between 3% and 6% in a road surface so even relatively low coal tar content in the bitumen may breach the 0.1% limit. Where a mixture is present, if the BaP content of the coal tar present is above 0.005%, according to WM3, the material is hazardous.
QED analysers provide reproducible results regardless of the operator, location or instrument used. This gives confidence to the contractor and regulator that data generated is reliable and fit for purpose. Results generated are easy to interpret and less open to misunderstanding.
As no accredited methods are actually available, it is possible to use the QED under the BATNEEC principle, Best Available Technique (technology) Not Entailing Excessive Cost, embodied in the Environmental Protection Act 1990. If the proposed technique is sufficiently reproducible, has an appropriate QC process to give confidence the data is valid and allows the user to comply with current environmental regulations, it can be used and the data will be accepted by the regulator. The QED can be used in a conventional laboratory setting and obtain UKAS accreditation as a single analysis method for the specific task of coal tar identification in road binder. Waste contractors, landfill operators and utilities labs in the UK currently operate QEDs and rely on the data that the EA also accepts.
When the QED is used by utility contractors, they are confident any arisings are correctly classified for WM3, minimising their liability. Where unplanned works are required, samples can be analysed on site during the excavation process. For planned works, cores are analysed at the contractor’s depot or sent to a lab that uses the QED. Sample turnaround is fast even if sent to a lab. The low per sample cost allows contractors to analyse individual bands of binder within a road surface without exceeding the usual low analytical budget. Coal tar containing binder is often found in discrete layers. By identifying which layers the coal tar is in, the contractor can specifically remove this material as hazardous. This reduces the overall tonnage of material sent to a hazardous waste facility and allows the rest to be recycled or used on the site as road base or backfill. This significantly reduces transport costs and the associated environmental impact of sending material to landfill. By getting rapid results the contractor can more efficiently manage the process, complete the job faster and significantly reduce costs, minimising the amount the taxpayer eventually pays.
The cost of conventional lab analysis that can reliably identify if coal tar is present is high. The cost of sending approximately 200 samples for this analysis is the same as a buying a QED analyser. The oldest QEDs are 8 years old and working well, proving reliability and cost effectiveness.
Using a QED significantly reduces the costs for utility contractors, speeds up road repairs, reduces the overall environmental impact of utility works and allows full compliance with current environmental regulations. This approach provides a substantial benefit for taxpayers, road users and the environment.