A Graphite-Metal Alloy Bushing Comes to the Rescue of a Problem Clarifier

A North Carolina wastewater treatment plant resolves a clarifier issue with a self-lubricating bearing that boosts reliability and cuts maintenance costs.

A Graphite-Metal Alloy Bushing Comes to the Rescue of a Problem Clarifier

A rake in the clarifier at the Durham treatment facility. The wastewater has been drained to enable maintenance. 

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A North Carolina clean-water plant was having difficulty with a rectangular primary clarifier.

The scraper flight boards and scum collector boards had gone out of sync causing the chain to jump the sprocket and fall off the guides.

An investigation found that the drive system needed a properly lubricated rolling element to minimize drag on the drive and chains so that they would not jump off the track or sprocket.

The plant in Durham, North Carolina (12 mgd average flow) found a solution in a graphite-metal alloy bearing for the fiberglass-reinforced plastic chain drive.

A trusted source

In March 2020 Michael Richmond, reliability engineer with the Durham Department of Water Management, called a Graphalloy sales representative for help with the clarifier issue. Biomass from the clarifier is returned to the plant’s biological nutrient removal process for further treatment.

“I had previous experience using Graphalloy bushings in a vertical turbine pump when I worked at a former employer in the oil and gas industry,” Richmond recalls. “I was extremely happy with the results, so Graphalloy was the first idea that came to mind when we started experiencing drive bearing issues with the clarifier.”

In chain-and-flight-type sludge and scum collectors, the FRP chain acts like a large rake that scrapes the sludge from the bottom of the clarifier. The flights move settled solids from the clarifier to a collection pit, while surface debris is pushed into a scum removal device. 

When the chains jumped off the track, the clarifier became inefficient in removing the scum and primary solids. Richmond understood that downstream equipment could be negatively affected. “We wanted to avoid any process upset and unwanted downtime,” Richmond says.  

Two attributes made the Graphalloy bearings well suited for the Durham application.


The Graphalloy bearings are made from a self-lubricating material that does not require grease or oil. “The original drive systems had Babbitt bearings that were submerged in the wastewater, and they would start to drag over time from lack of lubrication,” says Richmond.

“Since proper lubrication is difficult to achieve in submerged applications, the self-lubricating properties of the Graphalloy sleeve bearings would allow for smooth operation of the drive shaft and sheaves for the scraper flight boards and scum collector boards.” 

Long life and reliability

Chemically resistant and dimensionally stable, the alloy bearings are engineered to operate submerged and have been shown to operate reliably for years without maintenance. A Graphalloy representative noted that bearing failure can be costly. For example, a bearing failing in a 25-foot-deep clarifier can severely sap a maintenance team’s time and budget.

The clarifier has to be drained and cleaned before the maintenance technicians can get in to replace the bearings and get the chain drive systems back into working condition. Bearing failures every three months could cost hundreds of thousands of dollars. The new bearings would enable the maintenance and reliability teams to focus on other critical tasks and make their budget stretch farther.

Making the switch

After a thorough engineering review, the Durham team decided to install the Graphalloy bearings into the FRP chain drive system. The problematic clarifier was drained and cleaned in a planned shutdown. “For me, the self-lubricating properties of the Graphalloy make this product stand out,” says Richmond.

Installed in May 2021, the new bearings have work without any issues. This was not surprising in that the Graphalloy representative cited a wastewater treatment plant in the Northeast where a bushing from the company worked for more than 20 years. The company’s bearings have been used successfully in several other wastewater treatment applications, including pumps, screens, mixers and aerators, and more.

About the author

Chris Speer (caspeer@graphalloy.com) is a territory sales manager with Graphite Metallizing Corp., a supplier of graphite/metal alloy bearings and bushings based in Yonkers, New York.  


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