The Elilination of Nitrate from Urban Storm Runoffs by Multi-Filters Process

Background and Objectives: Over the last two centuries, a substantial increase has been observed in the rate of production and consumption of nitrate, particularly in agricultural sector. Currently, in many parts of the world and even in Iran, the high concentration of nitrate in drinking water has shown to be a serious problem, mainly caused by the introduction of agricultural wastewater and home and industrial sewage runoffs in the water resources, and especially in the groundwater. Use of the contaminated water with nitrate along with some food products containing high levels of nitrate can result in the entrance of excessive amounts of nitrate into the body. The ultimate goal of the present study is to design and investigate the bio-geo-filters in order for the elimination of nitrate from the runoffs.
Materials and Methods: In this research, alternate layers of non-woven geotextile filters and granular soil have been used for reduction and removal of pollution. These layers are of paramount importance in terms of their permissibility and absorption capability. For selection of materials some points have been considered, which include the material capability for pollution elimination, their accessibility, and maximal cost-effectiveness.
Results: After conduction of permissibility tests, the ratio of the weight mixture of the applied materials in PRB has been considered as 25% sand, 20% zeolite, 20% iron borings, and 10% poplar wood sawdust. It has been observed that for pH=7, the maximal nitrate absorption efficiency by zeolite is about 69%, sawdust 29%, and iron borings 12%. As indicated by the results of nitrate absorption through the final mixture of PRB in different concentrations of nitrate under optimal pH conditions while other parameters being constant, maximal absorption is due to the concentration of 150 mg L-1 and occurs in about 83%. The more the original nitrate concentration increases, the more the absorption amount goes up. Moreover, nitrate elimination with equal amounts of absorbent and optimal pH has been performed in different times for determination of equilibrium time, and the maximal elimination of 100% has been obtained in equilibrium time of 96 hours. in the administered test for removal of the pollution, after the growth of biologic mass in its environment, the filter was able to decrease the amount of nitrate up to 99% after the elapse of 9 days, and consequently its final amount was decreased from 100 mg L-1 to 1 mg L-1.
Conclusion: The designed permeable reactive barriers with the percentage of weight mix has the capability of adsorbing a quite large amount of nitrate in a short time. Washing of adsorbent materials and removal of the pollutants result in the increase in the especial surface of the adsorbent, and thus the adsorption power increases.