Use of green inhibitors for hardness stabilisation of tunnel drainage systems

The drainage system is a core element of tunnel construction and operation. Frequently, natural as well as technical boundary conditions lead to the deposition of scales (especially calcium carbonates) in the drainage system. As a preventive measure – in contrast to post-depositional cleaning procedures – the use of scale inhibitors to treat the drainage water (hardness stabilisation) is an option. ‘Green inhibitors’ are tailored green organic substances that delay or prevent scale formation when added in small concentrations. Moreover, green inhibitors can change the material consistency of scale deposits (soft sinter). An additional advantage of their use is the good environmental compatibility compared to conventional inhibitors. Suitable test procedures can be used to select a substance or product and evaluate or optimise its dosage. The application of polyaspartic acid or polysuccinimide products as ecologically harmless and readily biodegradable agents has proven advantageous in tunnel structures. The choice of liquid or depot stone conditioning essentially depends on the scaling mechanism, in addition to the technical considerations, flow rates and water chemistry. Regarding discharge to existing receiving water bodies, the inhibitor concentration in the water can be measured and controlled by DOC analysis and fluorescence spectroscopy. This article presents contemporary and tunnel-specific case studies.