Monday, August 11, 2014

DWSD and CSOs: Bad Choices, Second Chances

Heavy rainstorms cause sewer overflows that pollute lakes and streams. Attempting to control overflows in the Rouge River watershed, officials are constructing or expanding a series of impoundments called retention treatment basins that are intended to hold back a small, initial surge of polluted stormwater for subsequent primary and secondary treatment.  The remainder of the deluge is to be screened, dosed with chlorine (primary treatment) and released to the river, still laden with many pollutants.

The question raised here is whether building one or two small, full-service wastewater treatment plants in the Rouge watershed would provide greater amounts of clean water while simultaneously reducing the occasional overflows that overwhelm the massive treatment plant in Detroit at the same or even lower cost than the retention treatment basins.

Don’t confuse the following with expert analysis.  I’m struggling to understand by what reasoning federal, state and local officials think they are implementing clean water legislation concerning combined sewer overflows (CSOs) by building new or expanding existing retention treatment basins (RTBs).  It doesn’t make sense to me.

The Detroit Water and Sewerage Department (DWSD) central wastewater treatment plant is a mammoth facility intended to serve a large swath of metropolitan Detroit.  Difficulties arise when heavy rainstorms inundate the sewer system.

Major portions of metropolitan Detroit’s sewer system consist of combined sewers that carry both storm runoff and sanitary sewage.  Every time there’s a long, heavy downpour, hundreds of millions of gallons of untreated or partially treated wastewater are diverted to tributaries of Lake St. Clair and the Detroit River in order to avoid flooding basements and swamping the treatment plant.

That means that harmful levels of toxins, pathogens, nutrients and sediment contaminate natural bodies of water.

One would think that the simple solution to combined sewer overflows would be to expand the Detroit wastewater treatment plant.  There are three reasons why that wouldn’t  work:

  • The plant is way too big and unmanageable already.
  • There’s no more room on site.
  • Detroit can’t afford it.

Several years ago, federal and state officials proposed the construction of huge tunnels to hold CSOs temporarily, but Detroit couldn’t afford that, either.

Next, city and state officials came up with a plan to build new or expand existing retention treatment basins, nine in total, in the Rouge River watershed to control CSOs.  

Each RTB would have two tanks.  After screening debris, the first tank would be filled with the initial surge of polluted stormwater, expected to contain the most pollutants (the so-called first flush), and held for eventual pumping to the Detroit plant for primary and secondary treatment.

The second tank would hold subsequent, less polluted flow, which would be chlorinated (primary treatment) and, if the inflow continued beyond the second tank’s capacity, would be discharged, still containing (1) toxins, (2) such sediment as hadn’t settled in the tank, (3) so much of the pathogens as survived the chlorine, and (4) so much of the nutrients as hadn’t clumped up with chlorination, into the Rouge River.

The cost of these nine new or expanded CSO control facilities was expected (2009) to be $479 million.

If each of the nine facilities diverts from the plant to the river nine million gallons per day (mgd) of partially treated overflow, that would trim the treatment plant’s overload by 91 mgd.

But wait just a minute!   An Oakland County study in 2007 suggested an intriguing series of water-related alternatives, including wastewater treatment.  

One alternative deemed cost effective was to build a new wastewater treatment plant on the Clinton River near Mound Rd., north of 22 Mile Rd.  It would reduce both normal flow to DWSD’s plant in Detroit and diversion of polluted CSOs to the Clinton River.   

The proposed plant would handle an average 52.8 mgd with a maximum of 102 mgd.  The cost was estimated (2007) at $275 million.

A new plant like that, built on the Rouge River, would (a) not be limited to diverting partially treated stormwater from the Detroit treatment plant to the river, as would the nine RTBs, but would relieve the Detroit plant daily of up to 102 mgd, and (b) at about 60% of the cost.

Imagine the reduction of CSOs and the Detroit plant’s ordinary load if new plants as described in the Oakland study were built on both the Clinton River and the Rouge River.

Coulda, shoulda, woulda?  Why bring this up now?  Because, these are recurring opportunities. The equipment that goes into these plants takes a beating 24 hours a day, 365 days a year.  It wears out fast.  It can be repaired only so often before it has to be replaced.

As we move toward a regional water authority, we would be well advised to gradually replace segments of the Detroit wastewater treatment plant with new, smaller, more efficient and better managed treatment plants throughout the region.

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