Q&A: Benefits of a rotary press in dewatering operations

Oct. 18, 2021
A rotary press means less water, more savings for dewatering applications.

Operators who require dewatering systems as part of their manufacturing processes have several choices in equipment. Centrifuges, belt presses, filter presses, screw presses and rotary presses can all get the job done, and each has its strengths. But in recent years, experts at Evoqua Water Technologies have prioritized rotary presses in their dewatering portfolio. Mike Jager, the senior product manager for Evoqua, discusses why the rotary press is the best option in terms of maintenance, safety and efficiency.

Q: What are some of the market challenges influencing your work on rotary presses?

A: What we see out in the market, and the reason that we started looking for a better device, is the fact that there has been a lot of attrition, a lot of retirement, which means skilled dewatering labor has been leaving many of the manufacturers. In the meantime, more stringent regulations and rules have come out from the EPA, states and even local regulators. As a result, domestic wastewater plants are adding all kinds of stipulations around what can be put in the water and what they can receive without manufacturers paying additional surcharges. Disposal strategies like land application, especially in food and beverage and municipal wastewater, are becoming more difficult. There are fewer farms within driving distance, and many farmers are refusing to do land application with waste from manufacturing processes because they're just not sure what's in it and it opens them up to lawsuits.

Q: What about wet hauling? Is that still viable?

A: People have been wet hauling for years. By wet hauling, we mean taking the slurry at 2, 3, 4 or 5% and hauling it away in trucks to dispose of on land or in lagoons. But those avenues are drying up, so to speak. Manufacturers need to get the sludge dryer and safer for the environment. It relates to sustainability, which is huge now. Rather than taking wastewater and just dumping it, people want to reuse what they can.

Q: I understand that a rotary press can result in transportation cost savings. Could you explain that?

A: The amount of savings depends on what you're hauling. If you're wet hauling, you will save money by reducing the number of trucks required to haul away the slurry. You might go from 10 trucks a day to two or even one truck a month. We've seen those kinds of savings get up into the thousands of dollars — especially if you're an industrial customer who's hauling wet trucks to a municipal wastewater facility and dumping them there. You save all that transportation. If you’re using a belt press that operates at 18% dry slurry and you're hauling that to a landfill, going to a rotary press with 25% dry slurry can eliminate four or five trucks a month. And again, every truck you’re not using saves a couple thousand dollars. With a higher rate of dry slurry, you also save on landfill charges. Landfills generally base their charges on weight, and the last thing you want to put in a landfill is perfectly good water. Or if you're using an incinerator, the wetter it is, the higher the charges. In either case, there can be big savings just with a few more percentage points of dryness.

Q: Many of our readers are interested in remote monitoring. How does your product address those requirements?

A: A lot of industrial clients want the systems to talk to each other through SCADA (supervisory control and data acquisition) systems. The rotary press is a high-tech PLC-based unit that can talk to a wide range of SCADA systems Many clients, especially municipal clients, don't want any information leaving their plant. We can do that. The rotary press is completely contained. It can also send faults and alarms to an operator’s phone. This relates back to the attrition issue; you might have one operator in your wastewater plant operating a wide range of that system and they can't always be standing there by the machine if they have to monitor many types of equipment. A filter press, for instance, needs to be emptied. A belt press needs to be inspected periodically to ensure the belts are tracking. If that’s not done, a single problem could lead to the whole area filling with slurry. The rotary press won't do that. If it does fault, it'll go into a clean-in-place setting for five minutes, then shut down and await further instructions. In the meantime, the remote monitoring enables an operator to be somewhere else, get an alarm on his phone and then come back to the machine. And when he comes back, the equipment is not self-destructing. It's just sitting there waiting to be restarted past the fault.

Q: Does the rotary press require much maintenance compared to similar products on the market?

A: We like to compare the rotary press to two other devices: One of them is the belt press and the other is a centrifuge, both of which we've sold over the years at Evoqua. There’s nothing wrong with that equipment, but we see the rotary press as an improvement upon them. In terms of maintenance, a belt press needs its belt replaced periodically. You also must replace bearings and rollers about every four years. Very quickly the costs can get into the hundreds of thousands of dollars to rebuild your belt press. A centrifuge is even more daunting. Doing any work on the inside of the rotating assembly requires returning it to the factory, during which time, you’re completely down. The rotary press doesn't have any of those maintenance issues. Standard maintenance involves replacing scrapers on the inside. In a rotary press, none of the bearings contact the slurry; it's all protected behind seals. With it spinning at only 1 rpm, the wear and tear on the machine is very low. There are no vibrations, no belts to track, nothing to go seriously wrong. There is just a small drive motor with a maximum horsepower of about 13. That means you're not using a lot of power. The maintenance ends up being minimal. It cleans itself. At the end of the day, if you're going to shut down after a shift, it'll go into a clean-in-place setting based on a timer. Or it can run 24/7 and continue to plug away. The biggest maintenance item is having to replace the oil in the main drive after 10,000 hours — and that’s only about an hour's worth of work to drain and fill a drive.

Q: It sounds like there is minimal interaction between the operator and the equipment. Is that accurate?

A: An operator spends about 10 minutes in the morning to start it up and make sure the flock is correct for the slurry that's coming in. As long as there are no radical changes in the slurry, there will be no further interaction during the day. It will continuously bring sludge out of the machine.

Q: What are some of the safety considerations Evoqua has built into its rotary press?

A: Typically, sludge will be conveyed directly to something like a 40-yard container. There are no pinch points. There are no places for an operator to get his hand caught in a belt. There's nothing spinning at high speed. Compared to a filter press where the plates open and close, the rotary press doesn’t have any places where an operator can get caught because everything is closed. And because it's completely closed, there are no odor problems. And there is no issue with high decibel levels like there is with a center feed. All these things are taken care of with one elegantly designed machine.

Q: Many of our readers are setting high goals in terms of sustainability. How can a rotary press help them achieve those benchmarks?

A: The capture rate of a rotary press is the highest of the dewatering devices that we have had over the years. Capture rate is the number of particles removed on the first run through a rotary press — and it’s 98 to 99%. Compare that to a belt press at 75% to 80%. Over the years, operators have told us they would just return the used water back to the head and run it through again. But that just cleans the same water repeatedly, and you can’t use that water for your systems. Whereas at 98%, you only have 2% particles or less left in the water. That filtrate can be reused as non-potable wash water in other parts of the system. You get to use that water again without using a purification system to clean the water. If you're going to use it for potable water, such as in a food and beverage setting, then you will of course need some sort of microfiltration process after the initial filtration. But having cleaner water to begin with relieves that microfiltration system of having to do extensive cleaning of the water. Water is a precious resource. Cleaning your wastewater right the first time lets you reuse it elsewhere. There’s nothing more sustainable than that.

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