A New Centrifuge Technology Delivers Drier Material. That Means Lower Costs Downstream.

A new approach to decanter centrifuge dewatering of biosolids offers drier end product, lower polymer usage and energy savings.

A New Centrifuge Technology Delivers Drier Material. That Means Lower Costs Downstream.

In innovative design replaces the conventional solid scroll body with a tubular space frame configuration.

The biggest expense in dealing with biosolids has to do with water — specifically, separating it from the solids. The less water content, the lower the cost to haul, landfill, or incinerate the material.

That’s why makers of dewatering presses and centrifuges strive constantly to improve their technologies to yield a drier end product. A few percentage points improvement in solids content can make a big difference in a clean-water plant’s operating expense.

Flottweg Separation Technology, a company specializing in separation technologies, introduced a new decanter centrifuge at the Water Environment Federation Annual Technical Exhibition and Conference in October. It uses innovative technology to improve dewatering performance, reduce polymer consumption, save energy and maximize capacity.

Decanter centrifuges aren’t new, and the physical principles by which they operate are simple: Spin the material fast and the solids are pushed to the exterior of the machine, where they are collected and removed. In an interview with Treatment Plant Operator, Frank Scriver, environmental market manager, explains how Flottweg Separation Technology’s Xelletor series centrifuges have improved on conventional technology.

TPO: What was the rationale for creating this new technology?

Scriver: Our municipal customers are always looking to get a drier cake. The owner of a wastewater treatment plant is interested in performance. How much will the technology cost me to operate? How much will it save in terms of managing my biosolids? That’s where the benefit of the Xelletor design comes into play.

TPO: At the most basic level, how does this technology improve drying performance?

Scriver: The key dewatering performance metrics are dry cake, clear centrate, polymer consumption and energy consumption. The major variable that significantly affects performance is the depth of the liquid pool inside the centrifuge. A deeper pool gives you more volume, which means higher throughput. In addition, by having a super-deep pool, we get solids that are drier on average by 2 percentage points. So, if we were getting 25 percent solids using conventional technology, we’re now getting 27 percent.

TPO: What is the function of that deeper liquid pool?

Scriver: First of all, the super-deep pool gives the solids more time to settle, and that improves solids capture efficiency. Another benefit is lower consumption of polymer, which is also a significant operating cost.

TPO: How does the technology help plant operators save on polymer?

Scriver: By having the super-deep pool and a new design of the scroll, we significantly reduce the amount of shear force that’s applied to the flocculated feed. A lower shear force means you’re not breaking up the floc, so you actually need less polymer in the centrifuge to achieve the same result. With this technology, we’re seeing an average 20 percent reduction in polymer. It’s a significant cost saving.

TPO: How has the scroll been refined in this centrifuge series?

Scriver: The scroll that removes the settled solids from the centrifuge is a shafted screw. What has limited the depth of the liquid pool in the machine is the hub of the scroll. The liquid pool can’t touch the scroll body because that creates turbulence, hinders settling, and increases power consumption because the liquid is dragging on the scroll. We’ve replaced the conventional solid scroll body with a tubular space frame design.

TPO: What is the benefit of this alternative design?

Scriver: We’ve kept the same structural integrity as the solid scroll body, but now we can run the liquid pool inside that tubular space. That means the only limit to the depth of the liquid pool in the machine is the diameter of the bearing housing. We’re basically running the depth of the liquid almost right up to the diameter of the feed tube inside the centrifuge. With the tubular space frame, we’ve also eliminated the feed zone in the scroll body from the conventional design. With the tubular space frame, we’re feeding directly onto the surface of the liquid pool. So we’re eliminating a shear point and a point of wear.

TPO: What is the impact of the design improvements on energy savings?

Scriver: The closer the liquid is to the axis of rotation, the lower the energy consumption. By running a super-deep pool, we reduce the distance to the rotating axis.  That has a major impact — it requires about 20 percent less energy than our conventional design. This is in addition to a feature of our conventional design that reduces energy consumption by directing the liquid discharge to help rotate the bowl — in the same way that a rotating lawn sprinkler uses the force of the water to spin the arm. That yields about 25 percent savings. 

TPO: What have you done to prove out this technology?

Scriver: With all of our new technologies, we run the equipment for a minimum of one year, both to vet it mechanically and to collect performance data. In the case of Xelletor, we have been running it at three plants in Germany for more than three years, each at commercial scale. We’ve also done some comparative testing at facilities where we have our existing C series decanter centrifuges installed.

TPO: Is there a particular size of treatment facility for which this technology is especially well-suited?

Scriver: We offer three models with a total throughput range of 50 to 525 gpm. Any treatment plant 5 mgd and up would be a good application for this technology. Historically, centrifuge sizing in the engineering phase has been based on bowl diameter. With the new scroll design, this is no longer an accurate way to predict performance.


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