[Study on optimization gradation of substrates in vertical flow constructed wetlands].

Bio-ceramic, anthracite, zeolite, steel slag and vermiculite were used as substrate according to different kinds of gradation to treat wastewater in vertical-flow constructed wetlands simulation systems. The results show that the removal ability of COD by graded substrates according to particle size are better than single substrates, and average removal efficiency by graded bio-ceramic is up to 72.91%. The removal rate of TN by graded zeolite, which reaches 91.23%, is higher than single zeolite. No significant difference (p < 0.05) is detected with regard to organic matter and nitrogen removal between single and combined use of bio-ceramic and zeolite. The pH values in effluents of all columns filled with steel slag and anthracite are within normal limits, but phosphorus removal of all columns filled with steel slag and anthracite are lower than that filled with single substrates, except for the column filled with anthracite, vermiculite and steel slag from up to down. No difference between planted and unplanted systems can be observed. The present results probably provide a basis for vertical-flow constructed wetland design, among which based on the characteristic of wastewater proper selection of high-efficiency graded substrates, e.g., graded bio-ceramic, graded zeolite, graded anthracite, combined use of bio-ceramic, zeolite and anthracite, is a guarantee of better performance at a high hydraulic loading rate.