motors drives
As nonflushable wipes become more of a nuisance, anti-clogging technology steps in.
Mitsubishi Electric

For wastewater treatment plants, clogged pumps are not just a nuisance—they can shut down a city’s entire water system. One of a plant’s most critical responsibilities is to keep systems running to efficiently process that wastewater. 

Today, advanced technology is helping enhance the wastewater treatment process and make it more efficient at every stage, from mechanical and biological treatment to chlorination and filtration. One component of the process that can be improved by technology is the variable frequency drive (VFD) that controls pumps.

Modern treatment plants have long used VFDs in their processes. Because VFDs can vary the speed (or frequency) of those pump motors as needed and don’t have to run consistently at top speed, wastewater treatment plants can save on energy costs. VFDs also help extend motor life and ease motor startup to minimize damage over time. The result is greater productivity, which can mean a healthier bottom line and more satisfied citizens.

Today’s VFDs can do more than originally thought, including automatically resolving clogged pumps.

vfd

IMAGE 1: The VFD cleaning function uses a programmable timing sequence of forward and reverse
rotations of the pump to remove nondispersible materials from the impellers. (Image courtesy of
Mitsubishi Electric Automation)

Taking on Wastewater Treatment’s Toughest Challenges

Clogged pumps have been a challenge for wastewater treatment plants since the beginning, but the problem is growing for a few reasons. One issue is that up to 90% of materials flushed down the toilet were never intended for wastewater treatment systems. According to Nonwoven Industry’s online magazine, the exponential rise in the use of “flushable” wipes over the last few years has made the problem worse, wreaking havoc on sewage and plumbing systems across the world. A 2020 report by the National Association of Clean Water Agencies (NACWA) reported that North American businesses and households spent an estimated $2.5 billion on personal wipes in 2019—and only a small percentage of those wipes are truly flushable.

Because most wipes (and many other items) do not break down in sewage and wastewater treatment plants, NACWA estimates that they cause approximately $441 million annually in additional operating costs at treatment plants in the United States. That averages out to about $30,467 in extra costs per plant per year.

This big problem got even bigger in 2020, thanks to COVID-19. When areas of the world experienced toilet paper shortages, people turned to baby wipes and other personal hygiene wipes. The use of disinfectant wipes skyrocketed as people focused on sanitizing surfaces. People suddenly had to use, and eventually dispose of, paper face masks and rubber gloves. And much of this waste went down the toilet or into city storm drains.

A recent Bloomberg article shed more light on the problem: “The blockages [wipes] create when flushed—dubbed fatbergs—have become a costly headache…The term fatberg originated in the U.K. and describes the tangled black masses of wipes and congealed cooking fat that accumulate in sewage systems. One 40-metric ton specimen under London was estimated to be the size of a double-decker bus.”

All of these nonflushable wipes and other detritus are clogging pipes and pumps at wastewater treatment plants. When pumps get clogged and are not shut down quickly, they can get overloaded and damage the motors. Even when a pump is shut down immediately, it can take hours for plant technicians to remove the clog and get the system up and running again. That is time and money wasted (not to mention, a dirty job). Besides the manual labor costs, chronic clogging can cause excessive wear on pumps and lead to premature equipment failure and repair or replacement costs.

When even a moderate-sized municipal wastewater treatment plant is responsible for processing millions of gallons of water a day, any amount of downtime can cause serious problems, from lasting equipment damage to shutting down entire sewer systems to a backwash of untreated wastewater flowing into rivers and streams.

Anti-Clogging Capabilities 

The rising problem of clogged pumps is being mitigated in part by advanced VFDs, which offer automated pump cleaning—or anti-clogging—features, which can help boost plant performance.

Using a VFD with built-in anti-clogging capabilities, technicians can preset load limits for the drive and determine when the VFD should begin anti-clogging operations. When the drive detects that the pump motor is overloading to the configured limit, it will kick into a cycling sequence that moves the pump impellers forward and backward at designated speeds to clear the items that are clogging the pump. Once the VFD detects that the pump is clear, it will go back into normal operation mode.

 Beyond the initial presets, the anti-clogging operations happen automatically with no additional intervention needed by technicians. By leveraging a supervisory control and data acquisition (SCADA) system or programmable logic controller (PLC) interface, however, technicians can monitor the pumps and stay aware of any anti-clogging instances, when the feature was activated, how many times and so on.

Automatic Pump Cleaning 

One southern U.S. municipal wastewater treatment plant deployed VFDs with built-in anti-clogging technology in its system that serves a metropolitan area of about 150,000 people and treats 20 million gallons per day. Previously, technicians had struggled with pumps getting clogged primarily with wipes, shop towels, feminine hygiene products and other nonflushable items. Clearing those clogs meant a worker had to open the pump access plate and manually clean out the blockage by hand. For some pumps, clearing the blockage required technicians to remove the back suction part of the pump. The plant experienced about two clogs per week and each clog took several hours to resolve.

After installing the VFDs with anti-clog capabilities, the plant went from manually cleaning out pump clogs twice a week to zero clogs in three months—a boost in productivity and uptime that required no additional manpower. Modern VFDs can make technicians’ jobs easier and processes more efficient. Unplanned downtime, costs and manual labor can be reduced, along with potential risk of serious public health issues that can occur when pumps get clogged. These VFDs are smarter, smaller, easier to manage and install, and deliver higher performance for more efficiency and return on investment.  

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