عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Nowadays farmers often use N-fertilizers to increase crop yield. On the other hand, in agricultural lands nitrogen fertilizers, usually lost quickly by leaching, cause contamination of water resources and soil that reported as the most important source of pollutant. Also, treating and reusing drainage water is necessary because of a shortage of water resources, especially in arid and semi-arid regions. Although biofilters are useful for removing nitrate from drainage water, but many researches show that they need an external carbon source to sustain denitrification. Many researchers have studied on different method, but most of them need equipment or material that becoming use in the laboratory experiment. For example, mixing these biofilters and soil in large-scale underground drainage systems is very difficult and costly. So this study describes laboratory experiments that investigated nitrate removal from drainage water with barley straw in different percentages layered or mixed with soil in columns. The columns’ nominal diameter and height were 9 and 30 cm, respectively. There were two groups of treatments in this study. In the first group, the columns were filled with 3 layers of soil and 2 layers of barley straw between them (layered treatments). In the second group, the columns were filled with a mixture of soil and barley straw (mixed treatments). In both groups, barley straw was used in three levels of 10, 20 and 30 percent of volume. In layered treatments, the thickness of the 2 layers of barley straw was equal and proportionate to the percentages of its usage. The experiment lasted for 77 days. The influent drainage water contaminated with nitrate of 80 mg/L concentration.
According to the results, the concentration of nitrate from all effluent of treatments decreased with the passing of time. The highest percentage of barley straw was more effective in removing nitrate than the treatments. The control treatment (without barley straw) reduced less than 25% of the nitrate concentration of the influent. So the lump of earth cannot often reduce nitrate denitrification significantly due to a shortage of carbon source while water passes through it. Ammonium concentration of the effluent in all the treatments has decreased with the passing of time. On the other hand, the results show that a small amount of ammonium could be consumed under the anaerobic condition owing to the growth of anaerobic bacteria cells. The pH and EC values didn’t have a particular procedure during the experiment, the effluent values almost followed the influent procedure, but the pH effluent values have been always less than the influents. These values fluctuated between 7.18 and 7.95, which are close to the pH of soil saturation extract. This may be due to unexpected variation of the influent pH and soil buffering characteristics that did not allow dramatic changes in pH. Also maximum nitrate removal by denitrification occurred in the pH range of 7.2-8.4, that was about the pH of the soil in this study. The results showed that as long as pH was between neutral to a little alkaline denitrification effect was excellent. On the other hand, high acidic and alkaline environment was not fit for denitrification, and pH value played an important role in nitrite accumulation. The main reason was that pH influenced the enzyme activity of bacteria.
In this study, biological nitrate removal was investigated applying 10, 20, and 30% volume of barley straw (as layered or mixed treatment) in soil columns, under water flow for 11 weeks. Layered and mixed treatments were compared because mixing barley and soil in large volumes needed in underground drainage systems is sometimes difficult or impossible, costly and requires advanced equipment. Results proved all the treatments to be profitable. Also, the results showed that the optimum treatment with maximum nitrate removal efficiency (80%) was the 20% layered treatment. Ammonium concentration of the effluent decreased in all treatments with the passing of time, but it was almost close to influent values. Furthermore, variation of pH levels of the treatments didn’t have a particular procedure during the experiment and almost followed the influent procedure. So the application of the barley straw in layering method is a practicable inexpensive solution to achieve higher nitrate removal efficiency compared to mixing method or other bioreactors without having to be worried about of side effects and dangers of pH in effluent drainage water that can be used in artificially recharge systems or reusing wastewater as irrigation water.