Field performance of two on-site wastewater treatment systems using reactive media layers for nutrient and pathogen removal

Abstract

Sparsely populated or peri-urban areas commonly lack access to public water supply and sewerage systems. The inhabitants must make use of excavated wells to meet their water needs and septic systems or pit latrines for domestic wastewater disposal, which can release pathogens and nutrients (nitrate and phosphate) into shallow groundwater. This study compares two on-site sanitation systems improved with permeable reactors for removal of nutrients and pathogens: a lateral flow design at Maryhill site (Canada) and a downflow design at Parelheiros site (Brazil). Both alternative latrines employed two reactive materials: BOF (Basic Oxygen Furnace) slag for pathogen removal and sawdust for nitrate removal. At Maryhill, the sawdust tank was emplaced before the BOF slag tank, and at Parelheiros, the BOF slag layer was emplaced before the sawdust layer. Maryhill latrine was able to oxidize up to 98% of ammonium to nitrate and to remove 99% of nitrate, phosphate, and E. coli from the effluent. Parelheiros latrine was more efficient in promoting nitrification, with almost complete ammonium oxidation, but nitrate removal by denitrification ranged between 13 and 57%, while phosphate and E. coli were completely removed. The BOF slag overlying sawdust layer in Parelheiros design may have negatively affected the performance in denitrifying bacteria due to the high pH created in BOF slag. The Maryhill latrine proved to be a more satisfactory design for nitrate removal. The combined use of sawdust and BOF slag in differing subsurface environments has shown considerable potential for mitigating impacts of on-site sanitation systems on groundwater.