The White Wave Treatment Plant in the city of Boulder (Colo.) pretreats wastewater from a variety of different industrial operations before discharging its outflow to the citys main treatment facilities.
The discharge line is a 3-inch PVC pipe connecting the pretreatment tank to the city sewer line. Flow conditions vary between open channel and full pipe depending on the industrial process feeds and the operational hours of the processes. During low flow conditions (evenings and weekends) the flow could be as low as fractions of a gallon per minute (gpm).
An in-line electromagnetic flowmeter was being used for discharge rate measurement in conjunction with a datalogger to record and total the cumulative daily flows. The cumulative totals often were overestimated due to the magmeters inability to discern partial-pipe flow from full pipe flow. A solution was sought to solve this overestimation and hence overpayment of treatment charges.
Renaissance Instruments, a flowmeter manufacturer based in Austin was approached by Glenn VanBlaricom of the White Wave Plant to provide a 3 inch, in-line, DataGator? Flowmetering system for this application. The flowmeter was installed in May 1999 and flow rates and cumulative daily discharge volumes were collected. The magmeter was left in place during the initial period and a comparison was made of the flow rates at various times of the day.
With a thorough knowledge about the process flows during the day and also with a quantitative measurement of the influent to the various processes, it was shown the devices flow measurements were accurate under both open channel and full pipe conditions. Flow data comparison clearly showed the overestimation was mainly during the open channel flow condition.
The flow measurement technology is a patented flow measurement technique that uses a modified Venturi concept to measure sewer flow. The primary element, the flow tube, functions as a Venturi flume under open-channel conditions and as a modified Venturi tube under full pipe conditions.
The patented feature of the flow tube permits simultaneous filling of the throat and the upstream pipe. This design eliminates the inaccuracies in flow measurement during the open channel to full pipe transition.
The flow tube design is based on hydrodynamic energy considerations and the continuity equation that eliminates velocity profile dependent errors, a problem with other existing technologies. The flow tube is pre-calibrated and the measurement has NIST traceable accuracy. Since there is no velocity "puck" at the bottom of the pipe there is no low velocity threshold in the flow measurement. This is a critical factor in the White Wave application where the pipe diameter was only 3 inches.
The flow metering system at White Wave also included a datalogger that was user programmed to sample and log the flow rate data at one minute intervals. This data was used to generate trend charts to study the daily treatment volume rates and discharge rates. Also, cumulative discharge rates were generated for city billing purposes. (Sampler trigger was provided by the datalogger for flow-proportional sampling of the discharge stream).
The flow measurements, averaged over a few months of readings, showed a daily cumulative flow of about 33,000 gpm. This was more than a 40 percent reduction over the previously estimated volumes using the mag meter. This has reduced the White Wave wastewater discharge bills by almost half.
