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.
Alleviating CSOsCSOs 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 DetentionBuilt 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 PumpDue 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.
緩解 CSO 問題CSO 發生在較舊的下水道系統中，這些系統通常將生活污水管和雨水管結合在一起。在這些系統中，強降雨產生的流量常常超過污水處理廠的處理能力，然後作為溢流轉移到最近的小溪或河流。
對辛辛那提 MSD 來說，最合理的解決方案是開發系統內的儲存容量，從而均衡流向處理設施的流量。一些城市建造了大型隧道來收集和儲存污水以供後續處理，而其他城市則使用渦流調節器來限制和支持現有下水道的流量。
管道內儲存的另一種方法是將高速流量轉移到衛星儲罐，衛星儲罐充滿後排空到通往主處理廠的截流管線。這是辛辛那提 MSD 在其試點計畫中使用的方法。
在 Muddy-Creek 地點，一條下水道幹線經過，從約 2,000 英畝的郊區流域輸送雨水和生活污水。在試點工廠投入運作之前，每當下雨時，雨水就會流入幹線，直到截流器充滿為止，然後匯流進入泥溪。然後，小溪將接收所有沖入雨水下水道的碎片，更不用說衛生流中的內容了。
「我認為附近的人們對新設施表示歡迎，因為該地區過去常常散發出很多氣味，尤其是在夏天，」辛辛那提 MSD 的首席工程師馬蒂·烏姆伯格 (Marty Umberg) 說。
分流和滯留衛星工廠建在佔地一英畝半的土地上，建築成本略高於 500 萬美元，它在分流室將水流與乾線分開，然後將其流過粗濾網和過濾器。
然後，水流被引導至調節器，在乾燥天氣下，調節器允許水流向下游移動至廢水處理廠。然而，在降雨事件中，隨著流量增加，調節器將水流轉移至滯留池。高速流量平息後，感測器觸發將污水從水箱中泵出至底流管線，然後泵至主處理廠。當滯留池排空時，感測器會依序打開一系列 6 個 5 英尺高的沖洗盆，清除滯留池底部積聚的所有污泥和碎片。在此階段，系統已準備好應對下一次降雨事件。
對於大多數超出罐容量的事件，它仍然可以沉澱固體。處理後的水流經過氯化和脫氯處理，然後通過細篩溢出並送往 Muddy Creek。
切碎機和泵浦由於所有碎片都會進入滯留罐，因此清空罐的工作取決於從 沃恩公司購買的一對堅固的切碎機泵。
當滯留池在正常事件後排空時，底部可能會堆積多達四英寸的污泥。 「污泥中混有紙張、樹葉、啤酒罐、塑膠瓶等大量垃圾，所有這些都必須經過斬波泵，」辛辛那提 MSD 工廠專案經理 Lou LaCortiglia 說道。
兩台 25 馬力Vaughan Chopper泵 (800 gpm) 用於清空水箱，而一台較小的 15 hp Vaughan Chopper泵(300 gpm) 用於咀嚼從精細篩子中耙出的碎片，並將其發送到篩子坑。 「這對泵浦來說可能是更大的挑戰，」LaCortiglia 說。 “令人難以置信的是，裡面竟然有這麼多煙蒂。”
從路過的道路上看去，無人設施就像一棟整潔的小建築。地上結構的長度約為地下儲槽的一半，設有工廠的控制室、化學操作室和風扇室。該設施自動運行，並由數英里外的主要處理廠進行監控。風扇室的作用是循環來自地下作業的空氣，並將其輸送通過活性碳過濾器。因此，該設施幾乎沒有氣味。
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