Deep bed underflow process for enhanced nitrogen removal advanced treatment of low-pollution sewage

The invention relates to a deep bed underflow process for enhanced nitrogen removal advanced treatment of low-pollution sewage. According to the process, sewage is enabled to flow through a surface flow constructed wetland and a deep bed horizontal subsurface flow wetland in sequence; the surface flow constructed wetland comprises a first-stage surface flow wetland and a second-stage surface flowwetland; a nitrifying bacteria inoculant is added into the first-stage surface flow wetland; an aeration drop weir is arranged at the tail end of the first-stage surface flow wetland; wherein the deep-bed horizontal subsurface flow wetland is sequentially paved with a breathable impermeable sand layer, a filter layer, a denitrifying bacterium agent composite carbon source layer and an ecological gravel filler layer from bottom to top; tail water of the surface flow constructed wetland enters the deep bed horizontal subsurface flow wetland through a water collecting weir and a water passing culvert pipe and then is discharged through a water distribution channel, a subsurface flow wetland main body and a water collecting channel of the deep bed horizontal subsurface flow wetland. Accordingto the invention, the problems of nitrification reaction aeration of the subsurface wetland, lack of a carbon source in denitrification and low hydraulic load and large occupied area of the subsurfacewetland are solved, the stable operation of artificial constructed wetland system is ensured, and the low-pollution sewage is subjected to deep enhanced denitrification treatment.