Integrated system for aqueous nitrate removal using Fe0/CO2 reduction, iron precipitation, and ammonia stripping

Abstract

To investige the integrated system for aqueous nitrate removal using Fe[superscript 0]/Carbon dioxide reduction, iron precipitation, and ammonia stripping. Results show that the bubbling of Carbon dioxide flow rate at 200 mL/min was sufficient for supplying H+ into solution to create and acidic environment favorable to nitrate reduction by Fe[superscript 0]. Sigmoidal model equation satisfactorily describes the S-curve behaviors of nitrate reduction, ferrous accumulation and ammonium formation. The t1/2 obtained from the sigmoidal model can serve as a powerful tool for the comparison of nitrate reduction rate. In the system with various water characteristics, it found that humic acid significantly inhibited the reduction of nitrate. Calcium ions also strongly retarded nitrate removal, whereas chloride ion promoted the nitrate removal. Removal of ferrous from Fe[superscript 0] corrosion was investigated using the fluidized sand bed reactor. The resultsshow that the lower air flow rate of 20 mL/min could achieve up to 95% removal which provided a better performance than at higher flow rate. Under the studied conditions, air flow rate and surface area of sand were found to play an important role for iron pelletization onto the sand. Ammonia was the dominating end product from nitrate reduction by Fe[superscript 0]/Carbon dioxide processes. The optimum pH at 12 is recommended for ammonia stripping and the removal efficiency increased with increasing air flow rate. Considering continuous operation, nitrate of 23 mg-N/L was reduced to 2.9 mg-N/L within 2.5 hours of stripping under influent feeding rate, Fe[superscript 0] dosage, and Carbon dioxide gas flow rate of 3 L/hr, 60 g, and 200 mL/min, respectively. However, to maintain effluent nitrate to comply with drinking water quality standard of nitrate, supplement of 40 g of Fe[superscript 0] at every 27 hrs of operation was needed. According to field test, dissolved organic carbons and calcium ion existing ingroundwater strongly inhibited nitrate removal similar to the batch study. Nonetheless, 70% removal of nitrate was still achieved due to enhancement effect from high chloride content of the tested groundwater.