Sunday, September 21, 2008

How the local waterways fared from last week's downpour

It has long been common practice for rainwater to be diverted to city drains. This is largely a consequence of urbanization and the accompanying pavement. Rather than have rainwater be absorbed back into the ground, it goes into the sewer system and becomes the cities' problem and must be dealt with by the local wastewater treatment plant. These systems get overloaded when large storms occur and the end result is that the sewers overflow sewage into the waterways. There are two types of overflows, Combined Sewer Overflows (CSOs) and Sanitary Sewer Overflows (SSOs).

CSOs are when sewer systems use the same pipe for raw sewage and rainwater. When there are heavy rains these pipes get overwhelmed and the sewage goes into our local waterways or goes into a CSO retention basin and is partially treated and then released into our waterways.

A good explanation of CSO retention treatment basins comes from the MDEQ Annual CSO Report (2000-2001):

“Many combined sewer systems have recently installed treatment facilities (called retention/treatment basins), which are designed to capture the combined sewage and rainwater long enough to provide initial treatment and disinfection. This initial treatment often involves allowing solids to settle, the skimming of floatable materials such as oils; and disinfection of disease causing organisms, often accomplished through the addition of chlorine. This combined rainwater and sewage wastewater, with chlorine disinfection is the typical treated CSO discharge in the state of Michigan; therefore, many CSO releases are considered partially treated sewage. The treatment provided significantly reduces the amount of pollutants discharged and the associated public health risk.”

SSOs are generally a broken pipe or equipment malfunction, unlike a CSO these are illegal and you cannot get a permit for these. SSOs generally releases raw sewage that gets no treatment at all.

Some areas in Metro Detroit received over 5 inches of rain last week, so how did the local waterways fair?

All of these incidents occured between September 13-15, the counties that were looked at were Macomb, Oakland, Wayne, and St. Clair.

This list is for releases of partially treated sewage into our local waterways:

Lake St. Clair: 208.05 million gallons
Milk River: 86.199 million gallons
Rouge River: 71.67 million gallons
St. Clair River: 14.75 million gallons
Clinton River: 1.16 million gallons

This list is for releases of raw sewage:

St. Clair River: 3.75 million gallons
Clinton River: 1.92 million gallons
Rouge River: .517 million gallons

The Detroit Waste Water Treatment Plant released 62.41 million gallons of blended effluent into the Trenton Channel.

1 Million gallons of diluted raw sewage was released into the Detroit River.

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