Constructed Wetlands Controlling E. coli 0157:H7 and Salmonella on the Farm

Thomas A. McCaskey, Nar K. Gurung, Sarah P. Lino, Sidath V. Panangola, Allison K. Witherow, and Tracy L. Shadell

Foodborne bacterial pathogens such as E. coli 0157:H7 and Salmonella are a big concern for consumers, the food industry, and regulatory and health agencies. Controlling these pathogens on the farm is one of many strategies being pursued to ensure the safety of foods. Research conducted through the AAES has shown that an on-farm biological treatment system for swine manure is highly effective in reducing on-farm spread of these enteric pathogens.

Though fresh meats have been blamed for harboring these pathogens, even the most modern and sanitary processing plants are not exempt from these bacteria. Because they live in the gastrointestinal tract of all warm-blooded animals and humans, pathogen-contaminated feces of animals and humans is the main source of the bacteria in foods. Intervention strategies to stop the spread of these pathogens to consumers are focusing more on animal husbandry practices at the farm. If the incidence of E. coli 0157:H7 and Salmonella can be reduced in animals and in the environment where the animals are reared, perhaps the incidence of these bacteria in meats also can be reduced or even eliminated.

Because animal manure management practices are likely to come under more scrutiny in the future relative to enteric diseases that can be contracted from food-producing animals, a study was conducted to determine the fate of E. coli 0157:H7 and Salmonella in a liquid swine manure system. An earlier USDA study of fecal samples from 4,229 swine in the top 16 swine-producing states revealed no detections of E. coli 0157:H7. However, Salmonella was detected in 6% of 6,655 fecal samples collected from finisher pigs.

More recently, an AAES study evaluated the fate of E. coli 0157:H7 and Salmonella in a liquid swine manure system that uses a constructed wetland to treat effluent from a two-stage lagoon system. The swine manure treatment system has been operational at the Sand Mountain Substation in Crossville for the past seven years and the treatment system has been highly successful at meeting environmental guidelines required for discharge of effluent from constructed wetlands treating municipal sewage.

Figure 1. E. coli 0157:H7 and Salmonella are isolated on agar plates.

E. coli 0157:H7 and Salmonella were not detected on four attempts to isolate the bacteria from the liquid manure treatment system. Therefore, researchers inoculated these bacteria into the manure in a laboratory setting using wastewater samples collected from four stages of the swine manure treatment system (figure 1). Samples were taken from the primary lagoon, secondary lagoon, the detention pond (that stores water after it has been treated through the wetlands), and the recycle water (the final treated water used to flush manure from the swine houses).

In a replicated design, wastewater samples were inoculated with each bacterium. The inoculated samples were held at 90^oF and viable bacterial counts were determined from the first day of the study and at two-day intervals for 10 days. For each trial, 12 microbial analyses were performed.

E. coli. 0157:H7 and Salmonella typhimurium population levels declined more rapidly in the primary lagoon and secondary lagoon water samples and reached lower populations than in the detention pond and recycle pond water samples (figures 2 and 3). The lagoon samples represent highly polluted waters with much higher ammonia concentrations and higher competitive microbial populations, which are believed to limit the growth and survival of enteric pathogens. Water from the detention pond is lagoon water that has been treated by passing it through the constructed wetland system, then capturing the treated water in a pond for temporary storage. The recycle pond water is detention pond water that has been pumped to a pond of higher elevation from which treated wastewater flows by gravity back to the swine houses for use in cleaning the facilities. The primary lagoon wastewater is the most polluted and the recycle pond water is the least polluted.

Figure 2. E. coli 0157:H7 dieoff in swine wastewater.

Figure 3. Salmonella dieoff in wastewater.

Studies with both of the pathogenic bacteria showed that survival of the pathogens is curtailed more in the two lagoon water samples than in the water after wetland treatment. E. coli 0157:H7 survived longer in the primary lagoon water than S. typhimurium (see table). This trend was apparent for all four of the wastewater samples. About 20 days would be required for treatment of wastewater to achieve safe reduction for E. coli 0157:H7, whereas 18 days would be required for S. typhimurium.

Decline of E. coli 0157:H7 and S. typhimurium in Waste Water

	Time to achieve 1 and 8 long reductions
	E. coli 0157:H7		S. typhimurium
	1 log*	8 log**	1 log	8 log
	days	days	days	days
Primary lagoon	2.45	19.6	1.85	14.8
Secondary lagoon	1.94	15.5	1.81	14.5
Detention pond	2.28	18.2	1.94	15.5
Recycle pond	2.44	19.5	2.25	18.0
* Days required for bacterial count to decline 1 log or 90%.
** Days for count to decline from 1 million bacteria/ml to 1/100 ml.

These results indicate that, to eliminate both pathogens, the wastewater should be stored in lagoons and/or treated in the wetlands for a combined period of 20 days. Any combination of treatment schemes that results in a total of 20 days of treatment would be satisfactory to eliminate both bacteria, e.g. primary lagoon eight days, secondary lagoon five days, detention pond two days, and recycle pond five days. There is a considerable margin of safety that the treated water from the liquid swine manure treatment system will be safe from E. coli 0157:H7 and Salmonella. Therefore the lagoon/wetland-treated wastewater can be recycled and used for cleaning livestock facilities or the water can be applied to land without the risk of spreading enteric infection to animals or humans.