SIZE CHARACTERISATION OF METALS IN TUNNEL WASH WATER AS A FUNCTION OF TIME AND DETERGENT

Summary

Contaminated water from roads, including water from washing of tunnels, can be a threat to lakes and rivers. The tunnels in Norway are regularly cleaned in the form of sweeping and washing with water to avoid unfavourable conditions in the tunnel such as bad air quality, accumulation of dirt, and corrosion on the tunnel walls and other technical infrastructure. The washing frequency is dependent on the traffic. Hence, tunnels with annual average daily traffic (ADT) above 15 000 vehicles are washed at least 6 times/year while tunnels with ADT below 4000 vehicles are washed once every fifth year. In brief, tunnels are washed by using detergent and high pressure cleaning leading to large volumes of highly contaminated water. The detergent concentrations in the discharged tunnel wash water typically ranges between 0.2 % and 0.5 % volume concentration. The washing process often leads to emissions of partly highly polluted wastewater. Due to the high pollutant content in the tunnel wash water there is now common practice to treat the wash water before it is discharged to a nearby recipient. The most frequent applied method is to build sedimentation basin inside or outside the tunnel. This is reasoned by the fact that a lot of the contaminants are associated to particles. Hence, the treatment of tunnel wash water is based on sedimentation processes removing particle-associated contaminants, and therefore, the removal of dissolved contaminants, i.e. the mobile and assumed bioavailable fraction, is considered low. Currently there is little information on how the detergent affects the sedimentation processes, and the present study aimed to investigate how the detergent influenced this process by size fractionating metals in tunnel washing water as a function of time and detergent concentrations. It is important to obtain knowledge of the effect of detergent on fractionation of metal in tunnel wash water to optimize procedures for operation of the sedimentation basin and thereby minimize the impact on the natural environment. The cleaning process was simulated in a lab and the metals were separated depending on particular and molecular size using filters and a centrifuge. The conditions of the sedimentation basin used to remove contaminates from the wash water was simulated in the laboratory using 15 L rectangular plastic water tanks (Asaklitt) stored at 3 – 4 °C for three weeks. Wash water was obtained during washing of the 3.8 km long Nordby tunnel situated along E6 south of City of Oslo (Akershus county). The tunnel has two separate tubes with four lanes. The ADT is approximately 40 000 vehicles/day. Varying amounts of the detergent (TK 601, Teknisk Kjemisk Produksjon AS) was added to the wash water, resulting in 0, 0.5 and 3.0 % volume concentrations. Each treatment was conducted in triplicates. There were therefore a total of 10 water tanks, including a water tank with distilled water.