One variable critical to effective and efficient wastewater treatment is the measurement of level and flow. When selecting instrumentation for this type of measurement application, several factors must be considered. For example, an instrument must provide reliable and accurate level or flow measurements within dynamic conditions, withstand a corrosive environment, overcome turbulence, and resist entanglement with floating matter.
Ultrasonic devices have been implemented for many level and flow measurement applications throughout Henderson's treatment facilities. These devices have, by nature and design, many of the characteristics needed for the environments and conditions in which they will be operating. Most importantly, the non-contacting nature of the ultrasonic transducer eliminates all of the problems associated with contacting level and flow measurement devices.
The units are used in one application involving raw water intake to the potable water plant. Initially mag-flow, propeller, and transient-time type flowmeters were tested but failed. These were not viable solutions due to the turbulence of the water flowing through the intake pipe into the system, thus accurate flow readings were not possible with these devices. Finally a non-contacting, ultrasonic level measurement device was installed.
The Probe, manufactured by Milltronics, was placed in a stilling well adjacent to the raw water intake. The problematic turbulence of the water was overcome by installing the unit above the turbulent water stream. Electronic signal filtering, automatically performed in the unit, negates the effects of the agitated water surface and ensures a steady level reading.
While the plant operators wanted to know how much raw water was coming into the plant, the Probe offered only a four to 20 mA signal proportional to level. To determine flow, the milliAmpere signal is fed into the plant's remote terminal unit (RTU) and is then transmitted to a central processing unit (CPU) where the flow calculations are performed. By integrating the unit into their system, the plant saved the expense of a complicated flowmeter while maximizing the use of the plant's existing supervisory control and data acquisition (SCADA) system.
Ultrasonic level and flow measurement devices are also in Henderson's wastewater treatment plant. The Probe is used on a stilling well attached to a Parshall flume. The device measures influent flow into the plant. Like the unit used on the raw water intake to the potable water plant, the milliAmpere signal representing level is transmitted to the plant's RTU, and then to the CPU where flow rates are calculated.
Another application for ultrasonic level measurement within the wastewater collection system is the controlling of lift stations. Of Henderson's 45 stations, seven are currently utilizing the HydroRanger for complete lift station control. Benefits of ultrasonic technology in this application again center around the non-contacting nature of operation. With the transducer located at the top of the well, it is immune to tangles and breakages common with devices such as floats. Personnel need not enter the well for installation or maintenance, and daily maintenance checks are eliminated. Based on past success, Henderson intends to convert several more lift stations with this system.
This unit has many specialized features for lift station control. Five relays may be used for either alarm or control functions. They can control the on/off cycles for as many as five pumps. Each relay can be connected directly to the motor control circuitry of the motor starters. The unit's on/off setpoints are programmable to operate a single pump, or multiple pumps in fixed or alternating sequence. The HydroRanger can also log pump run times, provide volume measurement, and totalize volume throughput from the station.
Outputs from the level and flow measurement devices are transmitted to the RTUs via a SCADA system. Devices located within both of the treatment plants and around the community in lift stations and water storage tanks supply 4p;20 mA signal to RTUs. The data is then converted to a two-way communication protocol and transmitted to the SCADA CPUs via radio telemetry.
Jeff Roberts, GIS/SCADA systems manager for the facilities, said that in addition to the benefits of the non-contacting, ultrasonic technology already mentioned, significant and ongoing cost savings have been realized from reduced maintenance.
