Sewer Overflows assesses the number of untreated waste events from combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs). These overflows are mostly associated with heavy rains that overwhelm the capacity of the wastewater management system, although some SSOs may result from things like pipe ruptures. This untreated waste poses a threat by potentially exposing people, animals, and plants to harmful bacteria, viruses, parasites, and toxins. Additionally, excess organic matter can harm wildlife by lowering dissolved oxygen levels and excess nutrients can contribute to eutrophication in river and coastal areas.
How is it scored?
We scored each permitted CSO or SSO site as pass/fail for each year to get an average score for that site across the three years of data (2019, 2020, 2021). This scoring system is based on the goal of having no untreated wastewater discharges occur at a CSO or SSO site. Site scores for both CSOs and SSOs sites within a watershed were averaged to calculate a watershed score. We used the 10-point grading scale for this indicator because of the human health concern associated with the discharge of untreated waste (Table 1). Watershed scores were population-weighted to calculate a region score. We gave the Rouge River watershed a 0 because of the alarmingly high number of CSO overflows (470 in 3 years) in this watershed.
We considered using the volume of untreated wastewater as a metric in this indicator, but decided not to because of permit-specific differences in allowable discharge amounts and unclear guidance for what is considered an “acceptable” amount for protecting human and environmental health. We also did not include releases from Retention Treatment Basins (RTBs) because these receive at least partial treatment to acceptable levels for protecting human health.
Table 1. Scoring for sewer overflows indicator and equations used to adjust scoring from 10-point scale to the more typical Report Card Scale. The equation for an F was y = 0.3x, while all other grades used y = 0.5x + 50.
Adjusted Scale | Report Card Scale | Grade | Equation |
0–60 | 0–20 | F | y = 0.3x |
60–70 | 20–40 | D | y = 0.5x + 50 |
70–80 | 40–60 | C | |
80–90 | 60–80 | B | |
90–100 | 80–100 | A |
Data sources: The Michigan Department of Environment, Great Lakes, and Energy Combined Sewer Overflow (CSO), Sanitary Sewer Overflow (SSO), and Retention Treatment Basin (RTB) Discharge annual reports from 2019, 2020, and 2021.