Removal of chromium from tannery wastewater by vertical infiltration reed beds
The aim of this study was to investigate the potential of a helophytic plant, Phragmites australis (Cav.) Steudel, to remove chromium from concentrated tannery effluent in comparison to unplanted soil, under arid climate conditions. The experiment was conducted from August 2002 to August 2003. Six plots (capacity: 120 L, diameter: 50 cm) were filled to depths of 15 cm and 60 cm with gravel and soil respectively (texture: 3.5% clay, 8.5% silt, and 88% sand). Three plots were planted with Phragmites australis (Cav.) Steudel. Young shoots of plants (36 stems/m2) were taken from local and natural reed stands. Three unplanted plots were used as controls. The soil for the study was from the Tensift River (Marrakech). The experimental plots were supplied exclusively with diluted tannery wastewater, 3 times a week (approximately 10 L each time). The water flowed vertically through the substratum. To investigate the capacity of the systems to increase pH, to decrease conductivity chemical oxygen demand (COD), and to remove total chromium, concentrations in the inflow and outflow of each plot were determined during the experiment. System hydrology indicated that for planted systems, the mean flow varied from 18 mL/min (in winter) to 49 mL/min (in summer), with an average value of 35 mL/min. For the unplanted system, the mean flow varied from 5 mL/min (in winter) to 15 mL/min (in summer) with an average value of 12 mL/min. Water recovery at the outflow of the planted system (PP) and unplanted system (NPP) was 40% and 60% respectively. This may be related to the phenomenon of evapotranspiration from the PP system, which was more significant than simple evaporation from the NPP system. No sign of clogging was observed in the planted systems due to the plant rhizomes that ensure a better aeration of the soil. The study of the hydrological operation showed that mean flow in the planted system was three times higher than that in the unplanted soil, which makes it possible to treat a larger volume of wastewater and to reduce the required per capita surface for treatment. The presence of Phragmites australis ensured a sufficient porosity for the percolation of water for treatment. The pH of the effluent at the outlets of the planted and unplanted systems increased by two to three units in comparison with the raw wastewater at the inlet. The average inflow pH was 5.5; at the outflow of the two systems, the pH was 7.30 and 8.20 respectively for the planted and unplanted soil. The electrical conductivity (EC) of the treated wastewater increased for the two systems. The average inflow conductivity was 54±5.5ms/cm. At the outflow of the two systems, the EC was about 75±14 ms/cm and 68±11 ms/cm respectively for the planted and unplanted systems. The EC removal for all systems varied significantly from season to another (p< 0.05). The planted system was more efficient in removing total COD (74%) than the unplanted one (61%). There were significant differences (ANOVA) between the PP and NPP systems (p<0.05). Total chromium concentrations at the inflow varied from 534 mg/L to 1000 mg/L with an average content of 780±196 mg/L. At the outflow, total chromium concentrations varied from 2.2 mg/L to 3 mg/L with an average content of 2±0.5 mg/L for the PP system and from 2.5 mg/L to 4 mg/L with an average value of 2.4±1.3 mg/L for the NPP system. Total chromium removal of 99% occurred for the two pilots, which provides limpid purified water. Despite the shorter retention time for the planted system, which was three times less than in the unplanted one, total chromium was almost completely removed. The aquatic macrophyte plant seemed to play a physical role by enhancing the hydrological conductivity of the substratum and consequently the operation and management of these systems. Phragmites australis (Cav.) Steudel has the ability to transfer oxygen from the rhizome towards the roots through an internal lacunar system, which offers the creation of an aerobic zone around the roots. This aerobic zone allows the proliferation of micro-organisms, which are the principal decomposers of the organic matter in the root zone. The treatment of tannery effluent by reed beds is a clean approach from an ecological point of view and constitutes a viable economic alternative in comparison to purely chemical approaches and is thus better adapted to the complexity of the systems used in the leather industry.