The Invisible Sewage Plant
Some of the best processes are the ones that go unnoticed. For example, if you go looking for a particular sewage treatment plant in Cincinnati, you might drive right by, dismissing it as just another office building.
Like many cities, Cincinnati is working to find the best ways to meet state and federal requirements concerning combined sewer overflows (CSOs). In order to address this issue, the Metropolitan Sewer District (MSD) of Greater Cincinnati placed the Muddy Creek-Westbourne High Rate Treatment Facility into operation in June of 2001. This unmanned satellite plant is set up to automatically handle high volume, wet weather sewage flow. The plant was also designed to be virtually invisible.
CSOs occur in older sewer systems, where sanitary and storm sewers are often combined. In these systems, the flow resulting from heavy rainfalls often exceeds the capacity of sewage treatment plants, and is then diverted as an overflow to the nearest creek or river.
The most logical solution for Cincinnati’s MSD was to develop storage capacity within the system, therefore equalizing the flow to the treatment facility. Some cities have built large tunnels to capture and hold sewage for later treatment, while others used vortex regulators to restrict and back up the flow in existing sewers.
An alternative method to in-pipe storage is to divert high-rate flows to satellite tanks that fill and then empty into interceptor lines running to the main treatment plant. This is the method used by the Cincinnati MSD at their pilot project.
At the Muddy-Creek location, a trunk sewer passes by, carrying the combined storm and sanitary flow from a suburban drainage basin of roughly 2,000 acres. Before the pilot plant went into operation, whenever it rained, the storm water would pour into the trunk line until the interceptor was full, and then the combined flow would divert into the Muddy Creek. The creek then would receive all the debris swept into the storm sewers, not to mention the contents of the sanitary flow.
“I think people in the neighborhood welcomed the new facility because a lot of odors used to come out of that area, especially in the summer,” said Marty Umberg, Cincinnati MSD’s chief engineer.
Diversion and Detention
Built on a property of an acre and a half at a construction cost of a little more than five million dollars, the satellite plant separates the flow from the trunk line at a diversion chamber and runs it through a coarse screen and over a grit pit.
The flow then is channeled to a regulator that in dry weather allows it to move downstream to a wastewater treatment plant. In a rainfall event, however, as the flow rate increases, the regulator diverts the flow to a detention tank. After the high-rate flow subsides, sensors trigger the pumping of sewage out of the tank to the underflow line and then to the main treatment plant. As the detention tank empties out, sensors open up, in sequence, a series of six 5-ft-high flush basins that clean out all the sludge and debris that accumulates along the tank’s bottom. At that stage, the system is ready for the next rainfall event.
For most of the events exceeding the tank’s capacity, it still serves to settle out the solids. The treated flow is chlorinated and dechlorinated and then overflowed through fine screens and sent off to Muddy Creek.
Chop and Pump
Due to all the debris entering the detention tank, the job of emptying the tank relies on a sturdy pair of chopper pumps, purchased from the Vaughan Company .
When the detention tank empties out after a normal event, up to four inches of sludge may be built up on the bottom. “Mixed in with the sludge are papers, leaves, beer cans, plastic bottles—a lot of junk, and it all has to go through the chopper pumps,” said Lou LaCortiglia, Cincinnati MSD’s project manager for the plant.
While two 25 hp Vaughan Chopper pumps (800 gpm) are used to empty the tank, a smaller 15 hp Vaughan Chopper pump (300 gpm) is used to chew through the debris raked from the fine screens, and sent to the screenings sump. “That might be the bigger challenge for a pump,” remarked LaCortiglia. “It’s unbelievable how many cigarette butts can end up in there.”
From the passing road, the unmanned facility stands as a trim little building. The above ground structure is about half the length of the underground tank and houses the plant's control room, chemical operations and fan rooms. The facility runs on automatic and is monitored from the main treatment plant miles away. The fan rooms act to circulate air from the underground operations and to send it through filters of activated carbon. As a result, the facility is practically odorless.