Effect of artificial aeration on tertiary nitrification in a full-scale subsurface horizontal flow constructed wetland

A full scale comparison of a newly commissioned artificially aerated horizontal subsurface flow constructed wetland and a non-aerated bed of identical design was conducted to determine the efficacy of artificial aeration on tertiary nitrification. The works serves a population equivalent of 400; each bed is 100 m2 and has a mean hydraulic loading rate of 0.27 m3/m2/d. During the first 9 months of operation the wetlands received inlet loadings of NH4-N: 3.1 ± 2.4 g/m2/d, CBOD5: 2.8 ± 2.0 gO2/m2/d, chemical oxygen demand (COD): 19.4 ± 11.2 g/m2/d and total suspended solids (TSS): 6.6 ± 5.0 g/m2/d (mean ± standard deviation, n = 17). Results demonstrated enhanced nitrification in the aerated bed with 99% mass removal up to the maximum tested loading rate of 10.1 gNH4+-N/m2/d. In comparison, an ammonia removal of 59% was observed in the non-aerated bed up to a loading rate of 1.6 gNH4+-N/m2/d beyond which performance deteriorated. Carbonaceous biochemical oxygen demand and suspended solids removal were seen to be statistically similar between the beds while a significant difference was observed in terms of mixing pattern, quantity and characteristics of the accumulated solids and hydraulic conductivity. The suitability of the technology was also assessed through comparison of cost, carbon footprint and land area relative to alternative upgrading options. Retrofitting existing horizontal subsurface flow wetlands was shown to be the most cost effective solution delivering the required treatment at 38% of the cost of the least expensive alternatives.

► Artificial aeration improved ammonium removal from 11% to 99%.
► Up to 10 gNH4+-N/m2/d treated, without impairment of solids and organics removal.
► Aerated wetlands had improved hydraulic mixing patterns.
► Aerated wetlands are the lowest whole life cost nitrifying technology option.