Agricultural Mineral Additives for Passive Nitrate Removal in Septic Systems

Processes for removing sewage nitrogen at the individual residential scale must be low-maintenance with only annual service visits. A passive autotrophic process in an upflow configuration, using a 50-50 mixture by volume of: (a) agricultural sulfur for nitrate reduction, and (b) agricultural limestone for pH adjustment, was field-tested at 8 – 34 hours hydraulic retention times (HRT) at one site and at 14 hours HRT at another. An HRT of 8 – 12 hours was found to be the minimum without substantial nitrate breakthrough; pH was fully neutralized and little BOD and TSS addition resulted. About 90 – 95% of total nitrogen was removed in the two field study sites.The positive correlation of ‘mass of sulfate formed’ versus ‘mass of nitrate + oxygen removed’ reflects a simple stoichiometry, though possibly requiring ammonium to be included. Intercepts of the correlation near the origin suggest that gypsum is not precipitated in the process