Wastewater Treatment System Renovation

Question:

Due to its age, we are renovating our existing wastewater treatment system. The system contains only air -operated diaphragm pumps and our team is arguing over whether or not to replace them or keep them. I propose that we replace them with centrifugal pumps due to their high maintenance. Those who want to keep them say it is cheaper since we already have the compressed air and do not need to bring additional electrical power. What are your thoughts? J.W.

Answer:

In terms of maintenance costs and energy costs, with all other things equal, in my opinion, well specified and selected centrifugal pumps win hands down.

An air-operated double diaphragm (AODD) pump has a number of moving parts, especially the air handling system, diaphragms and ball checks.

Furthermore, the air handling system can be plagued with water and lack of lubrication that will increase maintenance frequency. Also, for those of us who experience cold winters, AODD pumps can “freeze” due to excessive moisture in the compressed air. Not only does one need to evaluate replacement parts costs but also labor costs and wastewater pretreatment system downtime on production.

An industry rule of thumb is that an air compressor will generate four cfm at 100 psig per horsepower. So when you make a comparison, an AODD pump will use about 2.5-3 times the energy of an efficient centrifugal pump to move the same volume of water; this does not take into account leaks and line losses in the compressed air system, which would make the AODD pump even less favorable from an energy standpoint.

For example, a one-hp, 230-volt, three-phase electric centrifugal pump will use about 2,900 kilowatt-hours of electric power if operated eight hours per day, 250 days per year; its electrical cost would be $150 and $200 per year with electrical rates of $0.05 and $0.07 per kilowatt-hour, respectively. An AODD pump, doing the same amount of work, would cost an additional $225-400 per year in electricity.

AODD pumps are selected over centrifugal pumps with electric motors for reasons other than energy conservation. These reasons can include:

• Pumping of high solids, abrasive waste streams that would quickly wear away a centrifugal pump’s impeller and seals
• Pumping of large solids that would clog a centrifugal pump
• Pumping of thick, viscous fluids, such as anionic polymer emulsions
• Pumping of flocculated particles to a wastewater clarifier, sludge holding tank or filter press since a centrifugal pump would “homogenize” these particles to the detriment of the downstream unit
• Pumping of wastewater from sumps, tanks or pipes with insufficient storage volume, which results in frequent but short pumping cycles. Our plumbing designers insist on a three-minute, but prefer a five-minute, minimum pumping cycle for centrifugal pumps in order to protect the electric motor from a short life.
• Pumping of concentrated waste streams or treatment chemicals (acids, caustics, reducers, oxiders and polymers) where small, controllable and easily adjustable flow rates are needed to introduce these waste streams and chemicals into a waste-water pretreatment system.

I hope these factors assist your team in evaluating what is the best pump selection at the various points of your wastewater pretreatment system.