Characteristics of ORP variation in subsurface wastewater infiltration system (SWIS) under hydraulic loading rate (HLR) fluctuation

Subsurface wastewater infiltration system (SWIS) is a small-scale land treatment technology used for sewage deep purification, especially for nitrogen removal, based on the combined effects of physical, physical-chemical, and biological mechanism. Although it has been accepted widely that the oxidation-reduction potential (ORP) in the matrix bed plays a crucial role in the SWIS purification process, the ORP variation characteristics have not been revealed completely. To make it clear, a SWIS with mixed matrix was constructed in greenhouse. The system worked successfully under a drying-wetting alternate operation with five different hydraulic loading rates (HLRs) for over 4 months. The ORP was monitored in the depths of 25, 40, 65, 95, and 145 cm in the matrix bed using depolarization ORP detection probes. Results indicated that with the increase of HLR, the ORP variation was more obvious. ORP variation of the different matrix layers was different, the ORP of the upper matrix layer would produce a rising-descending variation in drying-wetting alternate operation and the middle matrix layer was on the contrary. The ORP fluctuation of the lower matrix layer was little. Results suggested that the fluctuation of HLR did not affect the periodic variation of ORP in each depth of matrix bed, but it would affect the variation range. And different temperatures can lead to changes in ORP.