Which Positive Displacement Pump Is Best For Wastewater Sludge?

Sometimes, years after the design of a municipal wastewater treatment system, the needs of the plant change and opportunity for better efficiency is born. That's what happened at a wastewater treatment facility in northwestern Wisconsin.

The facility had a centrifugal pump used for both transferring sludge from a tank to a thickener, and loading trucks. They no longer needed the pump to assist with load out. Transferring the sludge to a tank didn't require such a high flow rate, or such a large pump. Operators knew reducing the pump and motor size could mean hundreds of dollars per year in energy savings. They talked to Travis Walker, their Crane Engineering account manager, to investigate options.

There are 4 different pump options Walker would recommend for sludge transfer. Each of them have their own pros and cons:

Centrifugal

Centrifugal pumps are good for low viscosity sludge with very little solids. This type of pump will generate far less pulsation than the positive displacement pumps that make up the remainder of this list.

Centrifugal pumps do have a high shear rate, unfortunately. As you probably know, imparting shear on sludge is counterproductive. When treating sludge, wastewater facilities want to keep the sludge highly concentrated. When sheared, more surface area is created, dispersing the solids, making it harder to dewater.

AOD

The air-operated diaphragm is a popular option for sludge. The simplicity of this pump is one reason why.

It has a single diaphragm powered by compressed air. It is very simple to control the speed of the pump, and it has a predictable fluid displacement result. Check out the video  below to see how an AOD pump operates.

The AOD pump is good for producing a consistent sludge blanket for more consistent percent solids sludge.

Air operated diaphragm pumps have high pulsation rates, however. As stated before, they use compressed air to operate. If a large air compressor doesn't already exist at the facility, it's more expensive to operate this pump. If compressed air is already available, this type of pump becomes more workable.

Progressive Cavity

Progressive cavity pumps are great for sludge. They impart little to no shear on the sludge, and generate low pulsation rates.

As great as they are in application, maintenance and repair are big issues. Ask anyone who has ever repaired one! Some progressive cavity pump manufacturers have realized this and created new pump designs that make servicing easier. Check out the Moyno EZ Strip for example.

These pumps also take up a large footprint. When replacing a centrifugal pump (or other type) with a progressive cavity pump, expect to retrofit some piping.

Rotary Lobe

Rotary lobe pumps are great for high viscosity fluids. Like the progressive cavity, there is low shear and low pulsation. They have a small footprint and easy to access the wetted parts through the cover plate in front of the pump.

This type of pump is more expensive upfront, but over time, the amount spent on parts, maintenance, and repair time is far less than a progressive cavity pump.

At the wastewater treatment facility in northwestern Wisconsin, the rotary lobe pump was the clear winner. Because of its' footprint, the pump would fit the piping layout and require the least amount of retrofit to the system.

There are definitely a lot of options when it comes to pumping wastewater sludge. The right choice comes down to where the sludge is in the treatment process, the amount of footprint available, availability of air at the facility, and budget.

Need help selecting the right positive displacement pump for your sludge treatment? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.