by Mike Riley
October 5, 2006

New digital dosing technology is redefining the expectations of water and wastewater utilities.

Metering pumps, also called dosing pumps, are found in virtually every facet of water and wastewater treatment, municipal and industrial.

Metering pump users are witnessing the introduction of a significantly new design platform that improves accuracy, control, and costs, while simplifying operation. The robotic premise of these advanced metering pumps creates nearly limitless potential for new user benefits.

The Progression Towards Digital Dosing

Late 1930s: Milton Sheen and son Robert of Betz Laboratories develop the first commercial metering pump. The Sheens added easily serviceable check valves (step valves) to the pump head.

1940s: Dosing pumps first produced commercially as chemical feed pumps.

1940s and 1950s: Modifications to the mechanical adjustment of the stroke length.

Late 1950s and early 1960s: Diaphragm added to dosing pumps. The diaphragm eliminated the need for the plunger to be exposed to the chemical being pumped, making the pump sealless. Mechanically and hydraulically actuated versions available.

1970s: Solenoid drive, which made metering pumps less complicated and considerably more affordable, introduced. Solenoid Drive design still used a diaphragm, but the diaphragm was actuated by energizing and de-energizing a solenoid.

1980s: Constant speed synchronous AC motor and a mechanically actuated design concept becomes alternative to solenoid. Incorporated electronic controls could have power to pump turned on or off, accept a pulse signal, or accept a 4-mA to 20-mA signal to cause the pump to turn on or off.

1990s: Manufacturers begin to incorporate variable frequency drives, stepper motors, and servo motors into metering pump designs.

2000s: State-of-the-art stepper or brushless DC motors, in combination with software to vastly improve the intelligence of the electronic control, developed to allow digital dosing pumps to always operate at full stroke length. 

By definition, metering or dosing pumps are positive displacement pumps that incorporate an internal means of adjusting capacity and inject accurately controlled volumes of liquid. Metering pumps began as rudimentary machines that incorporated crude mechanical adjustments to control capacity. Flow-rate confirmation and pulsation dampening have been supplied as external accessories, often more than doubling the cost of a metering installation. Innovation has been focused on improving capacity control, reducing leakage, and handling difficult liquids. 

Richard Faulkner of Siemens with a Grundfos dosing pump.

After decades of design changes, this convenient form of chemical injection is evolving into the instrument that pioneers of metering pumps foresaw: a level of intelligence within the pump, where flow inducement is less a part of design advancement and more a matter of controlling and measuring the amount dispensed.

Highlighted below are case studies involving digital dosing technology and its benefits at four water and wastewater utilities.

Salt Lake City Public Utilities

Metering pumps found in water and wastewater treatment (alongside cooling towers and boilers), commercial and municipal swimming pools, and in virtually every manufacturing plant are usually injecting expensive additives. The ability of these pumps to meter precisely and conveniently is critical.

Like most public utilities, the Salt Lake City Public Utilities (SLCPU) wants to ensure that new technology provides measurable benefits before it makes wholesale conversions. The Water plant manager for SLCPU, Ken Hibbert, is satisfied that the digital dosing pump they have evaluated over the past two years has proven to be superior to the solenoid pumps used in the past.

SLCPU plans to convert other metering applications to this new technology. The water utility originally purchased a digital dosing pump as a back-up metering pump, allowing them to take older solenoid pumps out of service for maintenance. They could also “test drive” the digital dosing pump in a variety of metering applications.

The first pleasant surprise for the utility was that they were able to use the same pump, with no material changes, to handle several polymers, sodium hypochlorite, caustic soda, and ferric chloride. Secondly, the inherent variable speed capability in the new pump permits viscous products to be more completely drawn into the pumping chamber than the fast moving diaphragms on SLCPU’s solenoid metering pumps.

Hibbert is most excited about the high turndown ratio (accurate capacity range) of the digital dosing pump.

With the huge variation of stream flow and turbidity with which the water utility deals throughout the year, their need for treatment chemicals varies widely. Consequently, in the past, they have required two different sizes of solenoid metering pumps to meet their treatment chemical flow range. The 800:1 turndown ratio of the new digital dosing pump allows this to be done with a single pump. The same pump can even be used to replace the much larger and more expensive roto dip feeders that SLCPU has used for high turndown chemical feed applications.

Fresno Water Department