Nutrient removal from on-site domestic wastewater in horizontal subsurface flow reed beds in Ireland

In order to investigate the nutrient (namely nitrogen and phosphorus) removal efficiency and the governing internal dynamics of the most widely used wetland type, the horizontal subsurface flow reed bed, in receiving domestic septic tank and secondary effluent in a temperate climate such as Ireland, two systems were designed, constructed and rigorously monitored for a period of over 2 years. Nitrogen removal, as expected, was found to be poor across both reed beds, with only 29% removal of TN across the secondary treatment bed and 41% removal across the tertiary treatment bed, with little distinctive seasonal change. A 15N stable isotope tracer study revealed, in line with the results from the chemical analysis, that nitrogen kinetics in the secondary treatment bed were dominated by continuous plant litter decomposition and mineralisation processes converting stored org-N to NH4-N indefinitely. Similar analysis on the tertiary treatment bed indicated that only limited denitrification of the oxidized forms of N was occurring in the anoxic environment of the bed, while NH4-N and org-N were merely changing form on a cyclic basis. Removal of PO4-P from the secondary and tertiary treatment beds was equally poor at rates of 45% and 22%, respectively. While at their maximum growth in the third year of operation, the total phosphorus in the stems and roots of the Phragmites australis in the secondary treatment bed equated to only 10% of the total P removed over the duration of the bed's operation. In the tertiary treatment bed, more seasonal variability was recorded with intermittent negative removal found during winter periods. This was somewhat more reflected in the P-uptake study for this bed with the roots and stems of the Typha and Iris containing phosphorus, which accounted for 31% of the overall mass removed.