The wastewater treatment industry faces new challenges every day. Technologies are developed to meet compliance standards in the face of these new difficulties. Sometimes these technologies can present unexpected side effects, or even exchange one set of problems for another. 

Back to the beginning

In the mid-1990s, a 1.2 mgd conventional activated sludge treatment plant located southeast of Columbus, Ohio, struggled to resume proper treatment after high flow events. Located on two acres in a residential neighborhood, the facility treated mostly domestic wastewater. 

Operators successfully pinpointed I&I, but as in many cases, extreme weather still caused occasional plant upsets, and solids washout forced operators to revert to a near startup treatment phase following high flow cessation. This lead to compliance issues as operators waited for the biomass to mature and proper treatment to resume. 

Engineers tried a new technology developed specifically for small plants to help remedy the problem. Suspended growth media and variations of the process are available from several manufacturers. In this case, thousands of feet of a fuzzy nylon string were suspended in several racks, and submerged in the aeration basins. A portion of the biomass then attaches itself to this string material to prevent washout during hydraulic surges. 

The racks are installed over conventional fine-bubble air diffusers. Installation was part of an overall plant upgrade, and on-site testing showed this new media provided a bio-growth equivalent of about 250 to 500 mg/L, which could speed recovery after a washout, acting as seed to get the plant back to normal in as little as one day.

Symptoms develop

Operations progressed as usual with one exception: 1/4- to 1-inch-long redworms were appearing in the mixed liquor sample. Operators had seen similar worms in other plants, but very few, and they were a result of a long sludge age. 

The worm population seemed to grow exponentially as days progressed. MLSS began to diminish rapidly as the worms consumed the biomass as their food source. This peaked as huge softball-sized writhing balls of worms began to slough off into the tanks, which plugged flumes and telescoping valves. Decaying worms also created odors. 

Finding a cure

Operators tried to identify the worms in the hopes of developing a control methodology. They took samples to the Ohio State University entomological department where scientists determined the worms were nematodes or roundworms from the phylum Nematoda, but the scientists were not able to offer specific information as to origin or control. Other research sources gave no leads either. 

Trial and error

Operators hoped that a worm larvacide such as those used with midge flies existed that could be used for pretreatment, but they found none available. Consultations with engineering and peer groups suggested the plant introduce an anoxic phase at the beginning of the treatment train to choke off the nematodes. Operators tried with little success, but the reduced air presented the plant with problems meeting ammonia limits. 

As time went on and the nematodes went unchecked, the diminishing biomass was beginning to impact CBOD removal. Something more radical had to be done. 

Operators decided the last option was to remove the suspended growth media and deprive the nematodes of a means of attachment. The plant brought in cranes and removed the media racks. As the cranes lifted the racks from the aeration tanks, millions of nematodes wriggled in clusters, attached to the media, and impregnated throughout the racks. 

With racks removed, the remaining nematodes were at the mercy of hydraulic currents, and deprived of their place of attachment, quickly died off. 

Lessons learned

Many conversations among operators and plant supervisors ensued regarding money spent on the now unused media racks, but the end results were undeniable. Without a control methodology, the nematodes were unmanageable and could not be sustained as part of the treatment train. 

Engineers and designers are now building in a scrubbing technology that deters nematode attachment to help to prevent future problems. 

About the Author

Steve Smith is water reclamation manager for the City of Canal Winchester, Ohio.

Contact him at ssmith@canalwinchester.gov. 

Have you ever had a worm infestation at your plant? What did your operators do to stop the problem? Leave a comment below.