Clogging of fractured formations by biocolloids suspended in reclaimed wastewater

Two pilot-scale fractured filters consisting of horizontal limestone slabs were employed to investigate fracture aperture clogging due to deposition of biocolloids suspended in reclaimed wastewater. To better understand the behavior of real fractured aquifers, the filters intentionally were not "clean". The fracture apertures were randomly spread with soil deposits and both filters were pre-flooded with reclaimed wastewater to simulate the field conditions of the Nardò fractured aquifer in the Salento area, Italy, where due to artificial groundwater recharge the fractures are not "clean". One of the filters was injected with secondary effluent from a wastewater treatment plant collected prior to the chlorination step, and the other with exactly the same effluent, which was further treated in a commercial membrane reactor. Consequently, the organic and pathogen concentrations were considerably higher in the secondary effluent than the membrane reactor effluent. The injected wastewater was continuously recirculated. The pathogen removal by the filter was more significant for the secondary wastewater than the cleaner membrane reactor effluent. A simple mathematical model was developed to describe fracture clogging. The results suggest that the hydraulic conductivity of fractured filters can be significantly reduced from the retention of viable and inactivated biocolloids originating from reclaimed wastewater. Fracture aperture clogging is strongly related to the chemical oxygen demand of the reclaimed wastewater injected. Schematic diagram of the experimental setup.