Make consistent cake solids
In just two years of operation, the city of Middletown, Ohio’s Water Reclamation Facility installed a decanter centrifuge that reduced production time by two days a week, increased efficiency, and saved thousands of gallons of chemicals per year translating to significant cost savings.
The Wastewater Treatment Division of the city of Middletown, Ohio, operates and maintains the city’s 26 million gallons per day (mgd) treatment plant. The division protects the health of the community and environment by reclaiming an average of 14.5 million gallons of wastewater each day from all residential, commercial and industrial sources. From this process, two recycled products are provided: clean water discharged into the Great Miami River and approximately 1,500 dry tons per year of processed biosolids, which are applied to farmland for its nutrient and soil conditioning value. The design flow for the Water Reclamation Facility is 26 mgd and serves a community of 48,000 to 50,000 people. The combined system means the flows could peak at 48 to 50 mgd.
The Challenge & Solution
The Water Reclamation Facility previously used a belt press and a centrifuge that was not performing well. In 2018, the utility removed the belt press because it was difficult for technicians to operate and was only producing 15 to 18% solids. The centrifuge was producing 30% solids.
“We wanted more consistency in our cake solids, resulting in less cost to haul,” said WRF Manager Gerry Burris. “We wanted something to be comparable with the machine we already had, but the machine had a numerous mechanical issues, and we were not getting good service from the manufacturer.”
In 2018, the facility installed a Flottweg Xelletor centrifuge and was able to reduce production time from five days a week to three days a week (8 to 10 hours per day). The older centrifuge was kept as a backup and eliminated the belt press.
“With the reduced schedule, we could still move product without the operating cost and the chemical costs of running five days a week,” Burris explained. “We now operate with better consumption and efficiency than running five days a week, as in the past.”
Burris said the city saved money by installing the machine themselves, which went smoothly.
“We had a few startup problems as the operators were trying to mimic the other machine,” he said. “They were accustomed to the controls of the other machine, but Flottweg did a great job of integrating the screens and making them look similar to the previous machines. This made it an easy transition.”
In addition, the new machine offered the facility an opportunity to make an adjustment in its usage of chemicals.
“It was like night and day,” Burris said. “Flottweg saved us a ton of chemicals. We don’t use nearly as much polymer or ferric chloride.”
In 2016 and 2017, the facility used 71,000 gallons of ferric chloride per year, with 112,00 gallons used in 2018, due to using the belt press and the older machine. In 2019, it used only 49,000 gallons of ferric chloride—a significant drop in chemical usage. As of November 2020, only 39,000 gallons have been used. Ferric chloride is a supplemental conditioner.
“We use an auto-thermal thermophilic aerobic digestion process or ATAD using mixing and high temperature of up to 140 degrees to eliminate pathogens in the waste stream,” Burris said. “The waste is then sent through a heat exchanger to cool it back down to about 100 degrees and send it over to the centrifuge. With polymer, it doesn’t dewater as well — due to the high temperatures — so ferric chloride is added at this point of the in the process. This provides the dewatering enhancement that we need.”
The facility cut the use of ferric chloride in half and the polymer has also been cut significantly.
In 2017, the facility used 102,000 gallons of polymer. In 2018, it used 59,000 gallons and only 40,000 gallons had been used through 11 months in 2020.
How Decanter Centrifuges Work
Sewage sludge dewatering offers enormous potential savings for the operators of sewage treatment plants. The costs of transport and disposal of the dewatered sewage sludge frequently make up 80% of operating costs. Depending on the capacity of the plant, 1% more dry substance in the dewatered sewage sludge can represent cost savings in the five- to six-digit range.
In a traditional centrifuge design, the liquid mixture to be separated is fed through a stationary pipe into the machine. The mixture enters the cylindrical part of the bowl through holes in the scroll body and is then accelerated. Due to centrifugal force, a liquid ring is built inside the machine. The thickness of the ring is called the pond depth. The solids, or heavy phase, form a concentric layer at the bowl wall. The solids are discharged from the machine by the scroll via the conical part. The separated liquids are discharged through a weir at the opposite end.
The cone angle, weir and scroll body limit the depth of the “liquid pond.” A shallow pond is optimal for dewatering mixtures containing granular solids, whereas as a steeper cone is better for neutrally buoyant materials that need more residence time to separate. The majority of wastewater applications are well suited to deep ponds.
The holes in the body of the scroll, where the liquid mixture is discharged into the centrifuge, have hard facings to increase wear resistance. The geometry of the holes is adapted to ensure a soft entry for the liquid, but shear force is still exerted on the mixture which diminishes the effectiveness of the polymer, and therefore also on separation efficiency.
In case of conventional decanter centrifuges, the body of the scroll limits the pond depth. The construction of equipment like Xelletor eliminates these limits. For the first time, a super-deep pond is possible, which has a positive impact on compression, the clarification zone and ultimately separation efficiency.
Flottweg developed a centrifuge concept specifically for high-degree dewatering of sewage sludge. Inspired by the lightweight construction of high-performance sports cars and motorcycles, the German engineering company developed the idea of a centrifuge design and named it Xelletor. The heart of the new design — the rotor and decanter scroll — were redesigned from the ground up. The result is a scroll with no scroll body.
In the Xelletor system, the liquid is discharged directly onto the pond in the centrifuge bowl. The liquid is accelerated with far less shear, which results in a reduced consumption of flocculants.
The fully automatic adjustment of the bowl speed and differential speeds produce optimum dry matter levels in the dewatered solids at any time, even in case of varying conditions in the feed.
The city of Middletown has been using a Xelletor centrifuge since 2018 and has experienced cost savings, as well as a favorable 28 to 30% solid cake.
“We’ve had some bumps, but the Flottweg Team has been very responsive,” Burris said, referring to an instance when the utility required a special part. “We called Flottweg that morning and they had a technician here within an hour. They installed the new part and the machine was running before day’s end. We were very impressed with this.”
While the other machine provided a slightly better cake solid, it required significantly more chemicals to achieve approximately 1.5% solids improvement.
“We had to ask ourselves if we really needed the 32% solids,” Burris said. “We are now averaging between 28 to 30% solids on a consistent basis with the Flottweg. For the 1.5% we couldn’t justify the extra expense in chemicals. It was a good move to switch to Flottweg. They are very responsive to our needs. They will send a tech out whenever we need one, and they stay until they get it right. This is a big selling point for us.”
With regard to maintenance, the Xelletor does not require much.
“After every run, the operators give the bearing end two squirts of grease. That’s it,” he said. “We are very satisfied. We have good solids using less chemicals and running fewer days. Who could ask for more?”
About the Author
Robert Rhea is sales manager for Flottweg Separation Technology Inc.