Seasonal Pathogen Removal by Alternative On‐Site Wastewater Treatment Systems

Abstract

Subsurface-flow constructed wetlands, sand filters, and peat filters near Duluth, Minnesota, were studied to determine their seasonal performance for removing pathogens from wastewater. Influent was a high-strength septic tank effluent (mean values of 5-day biochemical oxygen demand, total nitrogen, and total phosphorus were 294, 96, and 15 mg/L, respectively) at the Natural Resources Research Institute's alternative treatment system test facility in northern Minnesota. Each treatment system was inoculated with cultures of Salmonella choleraesuis (serotype typhimurium) for 5 to 7 consecutive days in summer and winter during 1998 to 1999. After the seeding, outflow samples were taken until Salmonella counts were sustained at background levels. The removal of Salmonella was calculated for each system, although the exact removal mechanisms were not determined. During the summer, the wetlands removed 99.6 to 99.999 4% (2.4 to 5.3 log 10 reduction) of the culturable Salmonella. The sand filters demonstrated a greater than 7 log 10 removal of Salmonella cells, whereas the peat filters were responsible for a greater than 8 log 10 loss of cells. Fewer Salmonella cells were removed by all of these systems during the winter, although the pattern of removal was similar to their summer operation. During the winter, the wetlands and sand filters removed greater than 1 log 10 of culturable cells, but the peat filters were responsible for a greater than 5 log 10 loss of cells. Fecal coliform removal patterns reflected those for Salmonella by treatment systems for summer and winter periods. Based on Salmonella and fecal coliform removal, the peat filters operated most effectively followed by the sand filters and the constructed wetlands. Water Environ. Res., 73, 204 (2001).

Evaluation of full-scale vertical peat biofilters for treatment of septic tank effluents

“…This is a mean removal efficiency of 98% (range 90-100%), with the 75th percentile removal efficiency 98.2%. Although this level of pathogen removal is consistent with the removal rates reported for experimental peat columns treating septic tank effluent [14], it is far greater than the level available by either sand or gravel filters [33].…”

Pathogen removal during wastewater treatment by vermifiltration

“…[1] One of the greatest concerns about groundwater contamination is the occurrence of enteric pathogens, which have been found to survive in septic tanks and then move along with percolating wastewater through the disposal field and soil to reach groundwater. [2,3] The need to remove pathogens from wastewater is long recognized. Pathogenic contamination causes disease outbreak and contributes to background disease rate around the world and most seriously impacting the developing world.…”

Constructed Wetlands

“…In fact, it has been reported that wetlands constructed with peat media removed a larger amount of Salmonella than a wetland constructed with sand media (Pundsack et al, 2001). Ushijima et al (2013) used a filter bed of fine soil (1-4 mm diameter) in a horizontal subsurface flow wetland and reported removal of 5 log 10 units for E. coli and 3 log 10 units for MS2 phage, while coarse soil could not remove these microorganisms.…”

“…However, due to high bacterial nitrification of ammonium in aerobic filter beds and to presence of high natural humic and tannic acids, a very small number of constructed wetlands have reported pH as low as 3.7 to 4.7 (Pundsack et al, 2001). This low pH was reported to contribute to greater Salmonella removal (Pundsack et al, 2001).…”

“…However, due to high bacterial nitrification of ammonium in aerobic filter beds and to presence of high natural humic and tannic acids, a very small number of constructed wetlands have reported pH as low as 3.7 to 4.7 (Pundsack et al, 2001). This low pH was reported to contribute to greater Salmonella removal (Pundsack et al, 2001). importance of dissolved oxygen in pathogen removal in a constructed wetland (e.g., Green et al, 1997) was discussed in a previous subsection on Sunlight and Water Clarity.…”