Effluent quality improvement of two pilot-scale, horizontal subsurface flow constructed wetlands using natural zeolite (clinoptilolite)

The performance of natural zeolite is presented in providing additional treatment of the effluent of two pilot-scale, horizontal subsurface flow constructed wetlands. The two constructed wetlands operated initially for 2 years without the zeolite-filters (period A). Then, the zeolite-filters (fine- and coarse-grained clinoptilolite) were added at the outflow of each wetland unit, and the two systems operated for an additional year (period B). During period B, the two constructed wetlands showed a comparable performance in organic matter, nitrogen and phosphorus removal to that in period A. The two zeolite-filters removed 60.6% and 63.2% of wetland effluent BOD5 and 52.5% and 62.0% of the wetland effluent COD, respectively. The removal rates of nitrogen pollutants were higher, reaching 75.1% and 83.2% for TKN and 78.3% and 85.8% for NH4+-N. Mean removals of influent ortho-phosphate in the two zeolite-filters were 56.4% and 39.2%, while mean removals of total phosphorus were 56.8% and 40.5%. The comparison of the two working periods (with and without the presence of the zeolite-filters) showed that the addition of the two zeolite-filters improved significantly the effluent quality of the two constructed wetlands, since nearly 95% of the organic matter, 80% of nitrogen and 70% of phosphorus from the wastewater applied to the systems was removed. The performance of the zeolite-filters appeared to be relatively unfluctuating throughout their entire operation period; nevertheless, higher removal rates were observed at higher temperatures. Furthermore, the fine-grained zeolite proved to be more effective in organic matter and nitrogen removal than the coarse-grained one. On the other hand, the coarse-grained zeolite retained phosphorus at a higher level.