What do lava and sewer sludge have in common?

BUFFALO, N.Y. – The Buffalo Sewer Authority (BSA), established in 1935, cleans the wastewater produced by more than 500,000 people in Buffalo and several other municipalities. It operates the second largest wastewater treatment facility in New York State, located on Unity Island between the Niagara River and the Black Rock Canal.

Paul Harris, PMP, who earned his bachelor’s degree in mechanical engineering at University at Buffalo, is the superintendent of mechanical maintenance, working to improve the reliability and efficiency of every piece of equipment used to clean the wastewater. The process begins with a series of filtering screens and grit removal equipment, followed by four large tanks called primary clarifiers where solid waste, or sludge, settles to the bottom.

The clarifier effluent goes through a treatment process that includes both aerobic and anaerobic bacteria and a series of additional settling tanks to remove residual solids. Finally, it is treated with hypochlorite to kill any remaining bacteria before being discharged into the Niagara River.

The sludge that settles in the primary clarifiers presents different challenges. A series of pumps move the sludge through a process designed to treat and concentrate it, reducing the amount of sludge that must be burned in the facility’s incinerators. Those incinerators are primarily powered by methane captured from a step in the treatment process in which bacteria “digest” some of the material, emitting methane in the process.

Large molecules called polymers are added to the sludge to thicken it, further reducing the water content. The thickened sludge passes through pumps that require extensive maintenance because some solid materials like sand and grit remain in the sludge, wearing out the pump’s components prematurely. Harris began to look for pumps that could move the sludge more efficiently while requiring less maintenance.

As the search progressed, Harris brought it to the attention of Christopher Lowry, PhD, UB associate professor of geology. Lowry had collaborated with Harris in ongoing BSA efforts to reduce stormwater runoff to the treatment plant. Lowry suggested that Stephan Kolzenburg, PhD, UB assistant professor of geology and an expert in volcanic flows, might be able to help identify some of the specifications for the pumps.

Different fluids, similar properties

The molten rock that lies beneath the earth’s surface is called magma. When a volcano brings it the earth’s surface, it’s called lava. Kolzenburg’s work takes him and his students to volcanic eruptions around the world, most recently sites in the Democratic Republic of the Congo and Iceland.

“What sludge and lava have in common is that they are both a three-phase mixture of gas, liquid and solids,” says Kolzenburg. “Lava contains gases such as carbon dioxide and water-steam bubbles, liquid molten rock and solids in the form of crystals. Sludge also has gas, liquid and solids, but the materials that make up these phases are different.”

Both sludge and lava have a property called viscosity — a measure of how hard it is for a fluid to flow. Centipoise is commonly used as the unit to measure viscosity.

The BSA initially considered a type of centrifugal pump which could handle sludge with viscosity as high as 3,000 centipoises. (For reference, maple syrup ranges between 150-200 centipoise.) But the pump still required excessive maintenance.

After Lowry introduced Kolzenburg and Harris, they decided to use a viscometer in Kolzenburg’s lab. It had been custom-built to measure the flow properties of molten rocks, and Kolzenburg adapted it for measurements to obtain more detail about the sludge’s viscous properties. Because viscosity is a complex property, depending not just on the material itself but also variables including flow speed and temperature, the viscometer — able to characterize flow properties while varying these parameters — revealed valuable information for the pump selection process.

First, the viscometer revealed that the sludge’s viscosity may reach up to 16,000 centipoise at low flow speeds — too thick for the pump under consideration to handle efficiently. Further, the data suggested to Kolzenburg that the sludge was behaving like a suspension that contains a polymer.

“That made sense,” Harris said, “because we use polymers in this phase of the treatment process.”

As a result, Harris and his industrial pump vendor began to search for another kind of pump. They found a possible solution with an enhanced centrifugal pump design that can move fluids that have a higher viscosity. Sean Morrison, one of the shift superintendents, worked with Harris during the initial pump trial that helped to identify appropriate process and equipment modifications.

Results to date have been promising. The initial test showed that this type of pump, with appropriate modifications, works more efficiently and requires less maintenance. The next step is to further tweak the proposed pump and run a pilot test. “We’re very hopeful that we’re moving toward a solution,” said Harris. “Collaborating with UB researchers has been extremely helpful.”