Assessing the TSS Removal Efficiency of Decentralized Stormwater Treatment Systems by Long-Term In-Situ Monitoring

Decentralized treatment of stormwater runoff from heavily polluted surface can be a good solution for effective source control. Decentralized stormwater treatment systems (DS) and test procedures to monitor their performance, have been developed in recent years. At present in Germany, only lab-based tests are officially established to determine the removal efficiency of Total Suspended Solids (TSS), and in situ monitoring is still lacking. Furthermore, the fine fraction of TSS with particle sizes less than 63 µm (TSS63) have been established as a new design parameter in Germany, because of their substitute characteristics of adsorbing pollutant substances. For research and evaluation purposes continuous data of urban stormwater runoff quantity and quality at the in- and outflow of two different DS at two different sites were collected. Turbidity is used as a surrogate for TSS. Continuous turbidity data and time proportional sampling served to obtain (i) regression coefficients and (ii) to determine the TSS removal efficiency of DS. For a wide range of events the total removal efficiency of DS1 was 29% for TSS and 19% for TSS63 and for DS2 19% for TSS and 16% for TSS63. An event-based data analysis revealed a high variability of the efficiencies and its uncertainties. Moreover, outwash of still suspended or remobilization of already deposited material was observed at individual events. At both sites TSS63 dominates urban stormwater runoff as indicated by the mean ratios of TSS63 to TSS of 0.78 at the inflows and 0.89 at the outflows of both DS. A significant shift of TSS63 ratio from inflow to outflow demonstrates that TSS63 particles were removed less efficiently than coarser particles by DS1, for DS2 data was too heterogeny. It clarifies that common sedimentation methods can only contribute to a small extent to the reduction of solid emissions if the stormwater runoff contains mainly fine-particle solids. The findings suggest that treatment of urban stormwater runoff with high TSS63 pollution requires additional techniques such as a proper filtering to retain fine particles more effective.