Excessive using of nitrate fertilizers in agriculture causes harm to humans and the environment. The most suitable nitrate removal process, is heterotrophic biological denitrification. The purpose of the present study was to evaluate the performance of three shapes of denitrification bioreactors including triangular, rectangular and semicircular. The main element that was used to removal nitrate was beech wood chips. The concentration of inlet nitrate was 75 mg/l and the salinity of solution were 1 ds/m and 5 ds/m, for a period of six months. The results showed that the efficiency of triangular bioreactor with salinity level of 1 ds/m was 90%, which is more efficient than rectangular and semicircular bioreactors with performance 55.8% and 53.8%, respectively. The results also indicated that in the salinity of 5 ds/m, semicircular bioreactor with performance of 50.8% nitrate inlet removal is best among three tested bioreactors shape, the efficiency of triangular and rectangular were 49.9% and 48.6% respectively. Also, it was observed that in the salinity level of 1 ds/m, high hydraulic retention time had a high positive effect on denitrification, on the other hand in the salinity level of 5 ds/m, denitrification had better performance if hydraulic retention time is ignored.

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