At Biobot, we like to say that everyone who uses the toilet gets counted in wastewater data, even if they don’t get a clinical diagnostic test. But what about people whose homes don’t connect to public sewers? About 1 in 6 American households has a septic system, and in rural areas, this proportion jumps to half of households.
What effect do septic systems have on wastewater-based epidemiology in the US? In a new white paper from Biobot, we tackle exactly this question. In short, we found that septic systems are not a major problem for wastewater monitoring programs, for two main reasons.
Septic users are more advantaged than sewer users
First, wastewater monitoring actually focuses on the populations that are most likely to be underserved and underrepresented in other public health monitoring systems because septic users tend to have higher socioeconomic status than sewer users. While septic systems are more common in rural areas (and rural areas tend to be less advantaged than urban areas), septic-using households actually have a higher median income nationwide, compared to sewer-using households.
This effect is even stronger when looking within individual counties. Septic-using households in rural counties have about $10,000 higher median household income, compared to sewer-using households in those same counties. There is a similar gap between septic users in urban counties, compared to sewer users in the same counties.
Septic users still contribute to public wastewater
The second reason septic systems likely aren’t a major problem for wastewater-based epidemiology is that people use bathrooms outside their home more than one might think. Even if your house is on septic, if you use the bathroom in any building on the public sewer system — a friend’s house, an office, a restaurant, a bar, a public library, a shopping mall — you will contribute to a wastewater monitoring program.
The scientific literature on bowel habits in healthy people suggest that many of us aren’t regular enough to reliably use the bathroom only at home. (Yes, there are such studies, measuring what times and on what days people poop!) And while poop is currently in the wastewater monitoring limelight, we should remember that urine and saliva also get into wastewater. So regardless of whether you are regular, if you are well-hydrated, you could be contributing to a wastewater monitoring program!
Wastewater monitoring can produce equitable community data
None of this is to say that we get equity “for free” out of any wastewater monitoring program. Sampling sites need to be chosen with care to ensure that relevant groups, especially rural populations and tribal nations, are included. But our fact-finding suggests that, if a community’s wastewater system is sampled, the resulting data will give a picture that is equitable or that desirably over-represents populations that have been traditionally underrepresented.
Written by Scott Olesen, PhD
Scott is a computational epidemiologist at Biobot. Trained at MIT and Harvard, his specializations include infectious diseases, antibiotic resistance, and the microbiome.