Looming in the world’s sewer system are contaminants of emerging concern. They’re not the endocrine disruptors, antibiotics or painkillers we’ve been reading about for the past several years (though those are indeed a challenge). These contaminants are bigger, and like the legendary alligators rumored to lurk in the New York City sewer system when I was a kid, they are growing steadily, day by day, to pose a catastrophic threat.
In a dramatic example of the problem, a 15-metric-ton “fatberg” was found in London, England last summer, choking the 70-by-40-centimeter (27.5-by-15.7-inch) sewer line down to just five percent of its capacity. The mass damaged 20 meters of sewer pipe. That means it was roughly the length and weight of a city bus.
At the heart of the problem were wipes — nonwoven fabric impregnated with cleansers. Many of those wipes were likely to have been classed as “flushable” by manufacturers; many more may have been described in eco-friendly ways that consumers could have inferred meant that they could be disposed in the toilet.
Millions of Dollars
Whatever the package said, nonwovens are emerging as a major threat to sewers and wastewater treatment facilities. And the problem is only getting worse.
According to Freedonia Group data cited in a Nonwovens Industry magazine article in January 2013, 552 million pounds of raw materials were used in the manufacture of wipes in the U.S. in 2011. Even just a fraction of that mass flushed down the nation’s toilets has huge costs. According to an Associated Press (AP) article last September, 971 “de-ragging” operations cost Orange County, Calif., $320,000 in a single year. The story notes that the city of Vancouver, Wash., racked up more than $1 million in costs replacing 11 sewer pumps that were routinely clogging, and city engineers said wipes were a significant part of the problem.
AP also noted that the Washington Suburban Sanitary Commission, a Maryland sewer utility, spent more than $1 million over a five-year span installing heavy-duty grinders, in large part to deal with nonwovens.
In an effort to stem the flow of nonwovens into the wastewater stream, the Orange County Sanitation District has created an informative, entertaining and very focused website called www.what2flush.com. Still, for every wastewater utility, nonwovens are going to be an ongoing issue.
In fact, like alligators emerging in childhood dreams, nonwovens are turning into sewer nightmares. Grinding may be of limited use—research such as a 2009 study by Australian engineers Mark Wilson and Michael Thomas indicate that waterborne solids such as hair and fine fibers can re-agglomerate downstream in wastewater treatment plants, forming new masses that could bedevil pumps and filtration equipment.
A more productive approach to dealing with nonwovens is to remove them from the wastewater stream at the headworks of the treatment plant rather than reducing particle size and passing the problem downstream. Appropriately engineered inlet screens can effectively entrain and rake or auger out solids right from the start, without interfering with flow through the system.
Removing nonwovens and other large solids from the intake of this Italian wastewater treatment plant eases disposal and protects valves and filtration equipment downstream.
The Wilson/Thomas study found that in-process screens placed deeper within the wastewater treatment system captured an additional 33 to 100 percent of the volume caught by the intake screens. Clearly, first-rate intake screens are vital to prevent pass-through problems like that; further in-process screening could provide vital backup.
Clearly, not all inlet screens are created equal. Wilson and Thomas point out that the volume, velocity and fluctuation in flow through the inlet screen all impact the performance of the screen. At Bilfinger, we have perfected computer models to gauge those parameters and aid in sizing and specifying screens for water treatment facilities. We’re working now on weaving nonwovens into our modeling.
Augers in the Bilfinger Mid V screen remove nonwovens from the wastewater stream at the headworks.
More to Come
Wastewater trade groups and manufacturers of nonwovens are grappling over standards for what is “flushable,” and we can expect the battle to continue. Meanwhile, tons of nonwoven materials are flushed down the world’s toilets daily, so the need for removal won’t abate.
In fact, expect even more problems as demand for “convenient” wipes grows. According to the Freedonia Group data cited by Nonwovens Industry, the wipes industry will grow steadily, representing 637 million pounds of raw material by 2016. Demand for “green” materials will guide product developers toward greater use of cotton and viscose fibers, they predict.
The bottom line — municipal wastewater treatment operators will need to adapt to a growing, changing challenge, and manufacturers will have to continue developing more effective screening solutions with nonwovens in mind.
It will be hard not to feel like we’re up to our waists (or is it wastes?) in alligators.
Image credit: "Brighton Sewer Tour," © 2009 Dominic's pics, used under an Attribution-ShareAlike 2.0 Generic license: http://creativecommons.org/licenses/by-sa/2.0/