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Pumps & Systems, October 2007

Clean Water Services is the sanitary sewer and surface water management utility for more than 500,000 people located mostly in urban Washington County in northwest Oregon. We operate four wastewater treatment plants and 39 pump stations, construct and maintain flood management and water quality projects, and manage flow in the gently meandering Tualatin River to improve water quality and protect fish habitat.   

Within the Tualatin watershed, we maintain nearly 800 miles of sanitary sewer line and 450 miles of storm sewers. Our facilities clean an average of 58 million gallons of wastewater and recycle 28.5 dry tons of biosolids daily. We must meet extremely stringent discharge permit limits because we are a major source of water for the Tualatin - our effluent makes up about 25 percent of the river's volume in the summer.    

We are able to provide a higher level of wastewater treatment than 98 percent of the treatment facilities in the nation because two of our facilities, Durham and Rock Creek, perform advanced biological and tertiary chemical treatment of effluent. The cleaned wastewater that leaves these facilities nearly meets drinking water standards and is discharged into the Tualatin River or used to irrigate school playgrounds, fields, parks and golf courses.

The treatment of sewage involves the initial capturing of waste material through primary settlement. At the secondary stage, where biological treatment begins, carbon is removed. Next, nitrogen nutrients and phosphates are removed biologically at the advanced secondary level. Tertiary treatment involves chemical and physical processes, where secondary effluent is fed into tertiary treatment tanks to be chemically clarified. Here, we carefully dose aluminum sulfate into the flow to entrain dirt particles and to precipitate phosphate as aluminum phosphate. It is then filtered, and finally chlorinated and de-chlorinated before it is discharged into the Tualatin.   

This high degree of performance is the result of our engineers overcoming several challenges through the years.

First, at the tertiary Rock Creek facility (which was named EPA Plant of the Year for 2006), we experienced great difficulty in finding parts for eleven obsolete 15-year-old 3-phase VFD diaphragm alum pumps and hypochlorite pumps used for chemical dosing. Sourcing these components was a very serious issue. After all, our Durham and Rock Creek facilities were the first to meet the state's phosphate TMDL (total maximum daily limit) of 70 parts per billion (ppb). Testing showed that some rainwater samples had[RB1]  more phosphate than we were allowed to discharge. (The state has since revised the regulation to permit 100-ppb).

A digital dosing pump is used for alum delivery in the Rock Creek facility.

Our need to optimize dosing operations was demanding. Because most of the chemical dosing pumps were chosen and installed by the lowest bid contractor - without our staff's ability to specify the pump - we ended up with different pumps on each plant expansion. The diaphragm pumps selected by the low bid contractor used 3-phase motors and VFDs. The SCADA system matched the speed of the pump to the water flow and the chemical dose was set by manually adjusting the stroke setting on the pump, meaning the operators had to physically attend to each pump to make the adjustment.

The motor had to maintain a minimum speed setting to prevent overheating, which resulted in overdosing during low flow periods. As we optimized operations over the years, we continually reduced the dose of chemicals. But because engineers typically size a pump for maximum demand five to ten years in the future, this often leads to the pump being too big for normal operation. The pump then runs at low speed and a low stroke setting. 

The next challenge was to design and build a system that met our tight operating limits with a pump that addressed these issues. Pacific Service & Supply Co., Inc. (North Plains, OR), a local systems supplier, brought us a Grundfos DME 150-4 (150-lph/39-gph) digital dosing pump to test prior to its general release to the market.

After inspecting the internal construction of this pump, we operated it satisfactorily on a test panel and then installed it as a temporary replacement for a peristaltic hypochlorite pump on the critical disinfection system. The 110-VAC single-phase pump performed an 800:1 turndown that matched our minimum low flow demand - something the peristaltic pump couldn't turn down enough to meet. The peristaltic also required a tubing change during the test period, but the trial pump required no attention.

The output of this alum pump can be sized to match current and future plant demand by simply pushing a button.

The entire range of this digital dosing pump is available via a 4-mA to 20-mA control signal, allowing operators to modify the dose from any SCADA terminal without having to physically go to each individual pump and adjust its stroke. As such, we changed our SCADA controller from VFD flow matching to a dose controller.