University of California Water Resources Center

The rapid infiltration (RI) land treatment process is a reliable, cost effective method for secondary and/or tertiary treatment of municipal wastewaters. When properly designed and operated, RI systems can achieve a significant level of nitrogen removal via coupled biological processes, namely nitrification-denitrification. Generally, it is believed that lower overall nitrogen levels can be achieved when influent wastewater is fully nitrified. However, at a specific RI facility located in Colton, CA higher nitrogen removals were observed when non-nitrified influent wastewaters were introduced. As a result, it was first hypothesized that an abiotic mechanism, ammonium adsorption, to the soils was occurring. This hypothesis, led to the conduct of an initial effort to evaluate the sorptive phenomenon that was occurring at this site. As a result of that initial effort, it was determined that ammonium adsorption was not occurring and that no nitrogen removal was observed under abiotic (sterile) conditions. Nitrogen removal was observed only under biotic conditions.

Subsequent to that initial effort, a second study was conducted in an effort to confirm and better understand the biological nitrogen removal mechanisms that are occurring at the Colton RI facility. In addition, experiments were conducted to evaluate whether nitrogen removal could be enhanced at the facility via organic carbon amendment to the influent wastewater. For design purposes, a 2:1 mass ratio of organic carbon to nitrogen is recommended for nitrogen removal in RI systems. The normal organic carbon to nitrogen ratio at the Colton RI facility is 1:3, highly organic carbon deficient. Experimental systems were amended with additional organic carbon in the form of methanol. Additional organic carbon in the Colton RI facility influent water may improve the denitrification rate within some portions of the soil column.