The Application of Permeable Pavement Aggregate Layers in Runoff Water Treatments

Urban runoffs usually contain a large variety of pollutants such as heavy metals, organic compounds, nutrients, solids, and de-icing agents. These are normally accumulated on impervious urban surfaces over time. Hence, the runoff itself becomes a wastewater that could create substantial degradation of water quality in receiving area. There are many alternative management strategies for treating these contaminants. Most of the approved stormwater management measures are difficult to be implemented on a wide scale (due to infrastructure and space/cost constraints). Permeable pavement is one of the urban runoff management methods that are widely used in order to reduce storm runoff flow and volume, and minimize pollution conveyance to receiving waters. Pervious pavement systems consist of a permeable pavement surface layer and one or more underlying aggregate layers designed to temporarily store storm-water. Runoff treatment using three aggregate layers, namely steel slag, limestone and silica aggregates were applied both as filter and pavement base layers. The research was conducted at laboratory scale and in continuous mode. All the experiments were conducted in cylindrical reactors of 0.6 m height and 0.2 m diameter. Each column was filled up to an average depth of 0.5m (0.1 m for filter layer and 0.4 m for the base layer). In order to determine the lifespan of the media, synthetic runoff in successive cycles was injected into the column continuously. Results from the study showed that the base and the filter layers of the permeable pavement can reduce the total range of runoff pollutants effectively with high removal percentages. In all experiments the rate of pollutant removal at the initial time of reaction was faster. However, these were gradually decreased and after 120 hours approximately the maximum removal efficiency was achieved. Comparing the effects of the three aggregates types, the steel slag aggregates exhibited better performance. The treatment process showed that the maximum removal of COD, phosphate and total solids from runoff in 3 hours, were 61, 59 and 70 percent respectively. These were increased to 98, 96 and 99 percent after 120 hours. In addition, the total capacity of slag aggregates for removing COD, P-PO4 and TS parameters were estimated to be 3.43, 0.21 and 22.10 g/Kg respectively. The testing results indicated that after the slag aggregates, limestone materials showed a high ability to remove pollutants from runoff waters as compared with the silica aggregates. The kinetic study resulted that the pseudo-second order kinetics equation, compared with the pseudo-first order and intra-particle diffusion models, described better the removal of organic compound absorption (COD removal) from the storm water. In this study the rate constant of the reaction (K) for the COD removal via steel slag, limestone and silica aggregates were estimated to be 0.31, 0.31 and 0.30 g mg−1 min−1 respectively. The correlation coefficients (R2) under different conditions were also calculated to exceed 97%. Since steel slag is a byproduct of steel production factories, its application as a road-building material, would be an appropriate alternative pavement layer in protecting the environment and conserving the natural resources.