Introduction

Steel industry is one of the most important water consuming industries in the world and Iran. steel industry Water consumption is about 17% of country's industries total consumption in Iran. The steel industry wastewater quality is different and depends on the type of process. Chromium wastewater is one of the most dangerous wastewater produced in steel industry then if it releases into the environment causes will be dangers. Therefore, removal of Chromium from water is very important and can reuse if treat completely. Constructed wetlands (CW) are receiving special attention due to rising environmental issues and growing importance of natural and sustainable wastewater treatment techniques. CW are popular because of their low maintenance, less energy consumption.

Methods

In this research aim is the design of a horizontal subsurface flow constructed wetland on laboratory scale with slag bed and plant Typha latifolia for treat Chromium wastewater from water resources .The innovation concept used in this paper is addition of bioabsorbent from agricultural waste to slag in the wetland bed. In order to, ponds were built with length, width and height 150, 50 and 50 cm respectively. Two baffles be installed in the wetland to provide flow distribution and ensure that the pollutant passes through the entire bed. The input concentration of Chromium contaminated water was used 100(mg/L) which was similar to the output wastewater of galvanized sheet production. Experiments were designed in 3 different phases. In the first phase, Corn straw with particle diameter about 1-2 cm selected as preferred bioabsorbent for chromium removal. In the second phase, the wetland bed system was divided into two layers and was investigated optimum mix of Corn straw with slag. Finally, in the third phase, with the placement of the selected bedding (chosen from second phase) in the wetland system, experiments were carried out to study the effect residence time on the chromium removal efficiency. residence times of experiments selected were 3, 5 and 7 days.

Results and Discussion

The results showed that if the bed filled only with slag and the residence time was 7 days, after a week, removal efficiency obtained about 80%. But after 12 weeks, the removal efficiency reached less than 10% and the wetland failed. By adding bioabsorbent to the slag in the bed with the retention time of 7 days, the chromium removal efficiency after 12 weeks reached above 99.97% and the exiting chromium concentration was 0.02 (mg/L).Because wetland performance was good, the project was continued. The results showed that the wetland efficiency was still high and the chromium removal efficiency after 7 months was 99.7% and the exiting chromium concentration of the wetland was 0.35(mg/L). In the third phase, the results showed that the percentage of chromium removal for retention time of 5 days after 12 weeks was about 99% and the exiting chromium concentration of the wetland was 1(mg/L). Also for the retention time of 3 days, the percentage of chromium removal was about 87% and the concentration of chromium of the wetland was 13 (mg/L).