Aeration to promote nitrification in constructed wetlands

As part of a three year study of the use of reed beds for the treatment of wastewater, two pilot scale constructed wetlands (30m x 5m), one planted wi th Phragmites australis and the other serving as an unplanted control, were used to treat primary domestic sewage. This paper describes an experiment conducted to test the hypothesis that increasing available oxygen by aerati on of the wastewater in the treatment beds could increase nitrification and hopefully nitrogen removal rates. Prior to aeration, the influent nitrogen load in the wastewater passing through the beds was reduced by approximate ly 45% and 10% in the planted and control beds respectively. With aeration of the wastewater as it passed through each bed, nitrogen removal was appro ximately 51% and 20% for the planted and control beds respectively. High ra tes of nitrification were recorded for the planted bed with aeration, but r emoval of nitrogen by denitrification was limited, probably because of the absence of a suitable carbon source (indicated by low BOD5 concentrations). High nitrification rates were not recorded in the unplanted control bed. T he higher BOD5 concentration measured in this bed may have caused increased competition for available oxygen between heterotrophic bacteria and the re latively slow-growing nitrifying bacteria, resulting in lower rates of nitr ification but more efficient denitrification.