Abstract: Subsurface infiltration system (SIS) is an efficient and economic technique in treating decentralized wastewater, but its nitrogen removal is frequently problematic, due tothe complex inner environment of the system. In operating, intermittent running is a positive method to enhance the nitrogen removal of SIS. This paper studied the effects of alternate drying-wetting on the nitrogen removal efficiency of SIS in a laboratory experimental scale for domestic wastewater treatment. The experiment was conducted under the conditions of NH₄⁺-N concentration 30-37.5 mg·L^-1 and hydraulic loading 0.08 m³·m^-2·d^-1. With the increasing drying days, the substrate permeability and nitrifying bacterial quantity increased, specific volume and ammonifying bacterial quantity changed little, and denitrifying bacterial quantity declined. The ORP values at different depths fluctuated with the operation of drying-wetting. It was recommended that the drying-wetting ratio of SIS was 3:1 at the start-up period (20 days), and was 1:1 in the period of stable operation. In these cases, the removal rates of NH₄⁺-N and TN were 86.9% and 79.1%, respectively, and that of organic matter was consider
ably high, with the effluent meeting the standard of water quality for scenic environment use (GB/T 18921-2002).

Key words: Quantificational index, SOM, DOC, δ¹³C, CPMI