Personnel at Johnson County (Kan.) Environmental and Wastewater departments devise a sampling arm that enables accurate waste strength measure

Huge numbers of large paper rags flushing into the Johnson County wastewater (JCW) collection system in Olathe, Kan., were playing havoc with sample collection lines and strainers.

The 6-foot sewer carries 15 mgd across the state line to Missouri for treatment, and Johnson County Environmental Department (JCED) staff pulled samples every 30 minutes. Sampling accuracy was critical to Water Quality Laboratory Division director Tony Holt, as the agency was charged for treatment based on waste strength as well as volume.

Holt designed various devices to catch the rags and hold the strainer inserted in a weir 20 feet underground, but none were successful. Clogging or strained samples that didn’t represent the true strength of the flow remained his personal migraine headache.

Meanwhile, JCW personnel used an ultrasonic transducer to measure the volume of the flow through a shaped channel in a below-ground vault. Although the vault had lights, a fresh air supply, and electrical power, it remained a hazardous confined-space entry. The two men below and the safety man above were driven out occasionally by hydrogen sulfide monitor alarms. Steam fog during winter made it difficult for the safety man to see the other workers.

When JCW prepared to introduce sludge from its 4 mgd trickling filter plant instead of trucking it 10 miles to another treatment facility, Holt worried that the flow might sometimes carry the equivalent of a bed load, since the injection location was 50 yards upstream from the sampling point. With the heavier solids staying closer to the bottom, attaining well-mixed samples would be im-possible. Furthermore, the hydrogen sulfide from the sludge would make the confined-space work untenable.

Holt and his team found a new sampling site and designed an apparatus through which they ran a strainer for pulling samples. Based on the possible cost of samples overstating the waste strength, the ragless sampler arm saves the utility about $400,000 or more annually.

Head ’em up, move ’em out

“Our goal was to find a new site where one man could take samples without a confined-space entry,” says Holt. “They chose a manhole in a local park.”

Peak flow conditions there were 6 feet per second with extreme diurnal changes in the levels. The pipe had 20 feet of head and contained the never-ending rags. Since it was important that the samples be accurate and represent true BOD and TSS levels, the sampler needed its own refrigerator to maintain samples at the required 39 degrees F.

“That was the easy part,” says Holt. “We purchased a Glacier portable refrigerated sampler from Teledyne ISCO, and a 130-watt solar panel to power its deep-cycle marine battery. The unit maintains the necessary temperature during the 24-hour compositing cycle.”

Tim Lawrence from JCW built the solar panel’s seasonably adjust-able frame and support from recycled plastic. Teammate Lloyd Newman did the wiring. They surrounded the manhole with a privacy fence.

Holt mulled over the rag problem until he thought of an apparatus through which a strainer could fit. A hollow arm would gather rags until the pressure of the flow behind pushed them down and off the arm as it pivoted upward. He took his idea to Dave Robinson, the plant’s master welder.

Holt was thinking aloud about how to engineer a smooth, uniform curve at one end when Robinson walked over to his scrap heap, dragged out an old aluminum street light support, and asked, “How about this?” The angle at the end of the support was close to the angle of intersection Holt wanted with the water to enable the rags to slide off.

The men reshaped the arm’s submerged end (nose) into an ellipse and polished the final 18 inches to reduce drag. “The hollow arm had to withstand the high flow and vertical head, while supporting and protecting the sampling tubing and strainer inside,” says Holt. “Besides making the arm pivot up and down, we had to get the angles and weight correct so that the strainer would ride 5 inches deep in the center of the flow. That was tricky.”

The 3/8-inch I.D. aluminum sampling tube had to weigh enough to keep the strainer submerged during high flows, but not so much that it would sink and catch rags. When extended, the strainer would “swim” downstream of the nose, which would intercept the rags. They tested and fine-tuned the device at JCED’s Tomahawk Treatment Facility.

Moment of truth

To deploy the sampling arm, Robinson fabricated a stainless steel frame, which he anchored in the manhole ring a foot above the sewer. When mounted, the nose hung 7 feet below the frame, pivoting up to release accumulated rags.

At low flows, a counterweight enabled the arm to pivot easily as the force of the sewage pushed off the rags. Peak flows, however, pivoted the arm to an angle that forced the counterweight up. This exerted maximum downward pressure on the nose, holding it 5 inches below the flow.

Although the arm caught most rags, a few were still driven down its polished end to interfere with the strainer. “We replaced the 3/8-inch I.D. tubing with reinforced 1/4-inch I.D. tubing,” says Holt. “The strainer was replaced with 1/4-inch aluminum tubing extending in a taper 2 feet beyond the nose. The Glacier sampling tube fits inside.” A press-fit held the parts together, as a clamp could provide purchase for the rags.

Since these final adjustments, the arm worked flawlessly. Holt heard that the U.S. Geological Survey has a new invention to sample stormwater inside large-diameter pipes. “The man who told me about it was describing what I’d built, only on a larger scale,” says Holt. “I got a kick out of that.”


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.