When wastewater comes into the Carnation Wastewater Treatment Plant, it first passes through a process that removes larger particles like trash and grit from the water. Next, the water flows through a process that
involves tanks containing membranes. These membranes are fine filters (like spaghetti) that only allow water molecules to flow through. When staff at the Carnation Plant began noticing damage to the membranes, they investigated. What they found were metal strands causing the damage. They made a call to our utility’s Unusual Occurrence Program.
The Unusual Occurrence Program is a subset of King County’s Industrial Waste program. An unusual occurrence is when something is identified in the sewer system that wouldn’t normally be found – such as an odor, an oily sheen, an odd coloration, a high pH, or a large amount of solid material. These are all things that have the potential to impact worker safety, damage sewer infrastructure, interrupt operation of a treatment plant, and negatively impact the quality of our recycled products. When the treatment plant reports an unusual occurrence and the source is not known, a member of the Industrial Waste team begins investigating. They start by looking into businesses or industries that might have created the unusual occurrence.
In this case, Ryan Salem, Industrial Waste Compliance Investigator, was called in. Since the treatment plant had already identified the cause of the damage to the membranes (the metal strands), Ryan’s first move was to search for companies that work with metal in the Carnation wastewater system area. At the same time, treatment plant staff began looking inside the plant for potential sources of metal strands. Ryan identified a number of metal machine fabrication shops and machinery businesses in the area. After visiting and inspecting one of these businesses and contacting another, he concluded that these operations were not the source of the metal strands. The mystery remained unsolved.
After striking out on the metal machine shops, Ryan suggested that the treatment plant staff examine the solids that are captured by the screens as wastewater enters the plant. They found similar metal strands, strengthening the idea that the metal culprits were coming from outside the treatment plant.
Channeling the famous detective Hercule Poirot, it occurred to Ryan that stainless steel scrub pads used for washing dishes could be the culprit. To help test this theory, Ryan bought a scrub pad and spread some of the metal filaments on a business card, replicating the original image of the offending material. He used a magnet on the strands to verify that they displayed the same magnetic properties as those found at the treatment plant.
He passed this information along to the Carnation plant staff. Working with City of Carnation, Tyler Stiltner, Wastewater Treatment Operator at Carnation, identified a nursing home with the similar metal strands in their sinks. Since other facilities like schools and restaurants might use similar scrubbers, the city will send out information to business to either use different scrubbing tools or use sink drain baskets (pictured right) to catch any strands that have broken off scrub pads.
The team working on the ‘case of the metal strands’ determined that scrub pads are likely to be the source of the material. Thanks to this great investigative work, stainless steel scrub pads are now on the growing list of items that are bad for our sewer systems right next to “flushable” wipes and grease. Kitchen helpers should make sure baskets are in place on sink drains to collect any broken fibers so they don’t wash down the drain!