Influence study of constructed rapid infiltration system constructedby purple soil on sewage Nitrogen removal performance

The nitrogen removal performance of a constructed rapid infiltration system (CRI) constructed using the common purple soil and sand from the Three Gorges Reservoir Region in the P.R. China was studied. In order to optimize the construction and the nitrogen removal performance of the CRI, three 1.5 m high columns were examined using mixed media with soil/sand ratios of 2∶1, 3∶1 and 4∶1, respectively. Four soil columns with heights of 0.3, 0.6, 1.0,1.5 meters subsequently were tested using media ratio of 3∶1. It is shown that the CRI constructed by the medium of sand/soil ratio being 4∶1 possesses better removal performance for the removal of NH3 N, NO3 N and TN. The removal rates for NH3 N and TN are 71.7% and 48.8% respectively, close to the nitrogen removal performance of unmixed purple soil. Hence the 4∶1 medium of sand/soil ratio is appropriate. It is also shown that, when considering the functional area of CRI treating the sewage, the upper 0.6 m of CRI is the main area that removes COD and NH3 N, whilst 0.6 m~1.0 m is the main area that removes TN. Increasing the soil layer thickness does not ensure better nitrogen removal performance. The 1.0 m soil layer thickness of a CRI system is appropriate for treating sewage in the Three Gorges Reservoir Region.