Evaluation of deniotrification in two shallow-placed low pressure distribution systems

Nitrate removal by biological denitrification was examined in two shallow-placed low pressure distribution (LPD) systems located in soils that were unsuitable for conventional disposal systems because of shallow depth to restrictive soil horizons in an Edom soil or to fractured shale in a Penn-Bucks soil. Four independent LPD subsystems were installed in the Edom soil with actual loading rates of 2.3, 2.9, 4.6, and 8.9 L d^–1 m^–2 while three subsystems were installed in the Penn-Bucks soil with actual loading rates of 3.6, 7.2 and 15.3 L d^–1 m^–2. Maximal rates of nitrate loss through denitrification were determined in the laboratory based on the acetylene block procedure, while actual field denitrification rates were estimated based on nitrate:chloride ratios. In the Edom soil, all four LPD subsystems demonstrated the same potential rates of denitrification in laboratory tests, while field estimates of nitrate loss ranged from 2% to 21% over the four subsystems. These low field denitrification rates were attributed to lower-than-designed loading rates which maintained aerobic conditions. In the Penn-Bucks soil, the subsystem dosed at 15.3 L d^–1 m^–2 failed within six months of installation and was not used further. The subsystem loaded at the rate of 7.2 L^–1 m^–2 yielded higher rates of denitrification under laboratory studies as compared to the subsystem loaded at 3.6 L d^–1 m^–2, and a similar trend was observed in field nitrate losses through denitrification which were 71% and 65%, respectively. These high denitrification rates (compared to the Edom subsystems) indicated that anaerobic conditions were present in the Penn-Bucks subsystems.