Mathematical modeling of constructed wetlands for hexavalent chromium removal.

Constructed wetlands (CWs) have been extensively used in Cr(VI) removal and have proven their ability to achieve high removal efficiencies. Although, numerous studies have been published in the past years presenting experimental results of CWs treating wastewater with Cr(VI) concentrations, a mathematical modeling describing the processes for Cr(VI) removal in CWs is lacking. In this work a mathematical model was developed, able to accurately describe the main mechanisms and reactions (i.e. biological reduction, plant biomass uptake-sorption) which are responsible for Cr(VI) removal in a wetland system. The model was calibrated and validated using data from a previously reported experimental study of horizontal subsurface CWs. Mathematical simulation indicates that in an unplanted wetland Cr(VI) was mainly removed through the diffusion/reduction process inside biofilm, attached on the porous media, while in the planted unit Cr(VI) was mainly removed through the sorption process to the root system of the plants. The developed model's simulations showed high correlation between predicted and experimental data, indicating that the proposed model can be used to design and predict full scale constructed wetland process for Cr(VI) removal.

Integrated Cr(VI) removal using constructed wetlands and composting.

The present work was conducted to study integrated chromium removal from aqueous solutions in horizontal subsurface (HSF) constructed wetlands. Two pilot-scale HSF constructed wetlands (CWs) units were built and operated. One unit was planted with common reeds (Phragmites australis) and one was kept unplanted. Influent concentrations of Cr(VI) ranged from 0.5 to 10mg/L. The effect of temperature and hydraulic residence time (8-0.5 days) on Cr(VI) removal were studied. Temperature was proved to affect Cr(VI) removal in both units. In the planted unit maximum Cr(VI) removal efficiencies of 100% were recorded at HRT's of 1 day with Cr(VI) concentrations of 5, 2.5 and 1mg/L, while a significantly lower removal rate was recorded in the unplanted unit. Harvested reed biomass from the CWs was co-composted with olive mill wastes. The final product had excellent physicochemical characteristics (C/N: 14.1-14.7, germination index (GI): 145-157%, Cr: 8-10mg/kg dry mass), fulfills EU requirements and can be used as a fertilizer in organic farming.

Biological Cr(VI) removal using bio-filters and constructed wetlands.

The bioreduction of hexavalent chromium from aqueous solution was carried out using suspended growth and packed-bed reactors under a draw-fill operating mode, and horizontal subsurface constructed wetlands. Reactors were inoculated with industrial sludge from the Hellenic Aerospace Industry using sugar as substrate. In the suspended growth reactors, the maximum Cr(VI) reduction rate (about 2 mg/L h) was achieved for an initial concentration of 12.85 mg/L, while in the attached growth reactors, a similar reduction rate was achieved even with high initial concentrations (109 mg/L), thus confirming the advantage of these systems. Two horizontal subsurface constructed wetlands (CWs) pilot-scale units were also built and operated. The units contained fine gravel. One unit was planted with common reeds and one was kept unplanted. The mean influent concentrations of Cr(VI) were 5.61 and 5.47 mg/L for the planted and unplanted units, respectively. The performance of the planted CW units was very effective as mean Cr(VI) removal efficiency was 85% and efficiency maximum reached 100%. On the contrary, the unplanted CW achieved very low Cr(VI) removal with a mean value of 26%. Both attached growth reactors and CWs proved efficient and viable means for Cr(VI) reduction.