True Grit (Removal) at DC Water

By Christopher French

At the largest treatment plant of its kind in the world, it’s no surprise that DC Water uses some of the biggest grit classifiers available, with the final two of 16 newly installed units now commencing operation.

For this simple yet important technology, DC Water set the bar extremely high to ensure that its new grit classifiers would work reliably and effectively for many years to come at its 150-acre Blue Plains site.

Averaging 300 million treated gallons per day (and 847 million gallons per day at peak flow through the headworks), this highly advanced wastewater treatment plant had seen its existing grit classifiers (installed in 2002) outstay their welcome. Rising levels of downtime, maintenance and cleaning required fresh investment.

The classifiers had structural defects, causing leaking and a housekeeping problem. Each time, a unit had to be taken off-line to open up the hatch, take the screw out, get inside to clean it, buckle in a new liner and, ultimately, weld it.

Via 1,800 miles of sewers from around the District and from the Potomac Interceptor, wastewater arrives at Blue Plains from a large sewer that begins at Dulles Airport, bringing with it wastewater from the suburbs of Maryland and Virginia along the way.

Following a series of screens and a grit removal system that removes objects and large particles, the grit classifiers are at the sharp end of conveying grit and other non-degradable particles.

With the grit classifiers having become one of Blue Plains biggest maintenance issues, DC Water set out an open bid, determined to find the best possible long-term solution.

After a comprehensive investigation, particularly into the quality of manufacturing, a shortlist of eight - and then later three - suppliers was compiled, from which Lakeside Equipment Corporation was finally chosen.

“DC Water was working with a simple design, which they liked, but that they knew needed modifying,” explained Lakeside’s Dan Widdel. “They knew it was imperative to find a system that would be stronger and much easier to maintain. Sixteen grit classifiers each handling a grit slurry flow rate of 400 gallons per minute is a big deal.”

Lakeside prides itself on being able to respond quickly and professionally to questions, Widdel explained. “From the outset,” added Widdel, “our design proposal went into great detail about what we could offer and how we could best work together.” A grit classifier might be considered a simple piece of equipment, he added, “but that doesn’t mean compromises should be made on its construction.”

With standards of construction so essential to the very detailed bid package, Lakeside was able to demonstrate not only vast experience with grit classifiers but also, more importantly, the flexibility to adapt to DC Water’s design. After the deterioration of the existing grit classifiers, DC Water was keen to see the quality of welding and structural integrity of Lakeside’s work.

Unlike the previous shaftless screw design, which sits on an interior liner and can wear through, Lakeside’s helical screw design is shafted all the way through, supported by a bearing at the bottom and a reducer on the top, so that a build-up of sand or grit provides a bed for the screw, eliminating tank lining wear.

As grit slurry is discharged to the classifier, the grit and water mixture separates to allow grit particles to settle in the tank. A baffle reduces turbulence and prevents short circuiting when the effluent is discharging over the weir. Settled grit is then removed by the helical screw conveyor, equipped with steel flights. Important for long life, the sectional flights are fabricated with hardened welding on the leading face for durability and strong wear resistance.

According to Widdel, DC Water wanted a grit classifier that matched up to its existing classifier envelope, which differed from Lakeside’s standard classifier design. “This was never a problem though,” he explained, “because we custom-build to meet our customers’ needs.” Using 3D modeling, Lakeside’s engineering department worked alongside DC Water to match up the grit classifiers with the existing inlet and outlet pipes to keep the installation process to a minimum while not sacrificing on the performance.

Delivering the new grit classifiers two-by-two over a three-month period was also geared to meet DC Water’s needs.

“We listened very closely to DC Water’s issues,” said Widdel, “and can see that maintenance is now much easier for their personnel.” Access to maintain the unit has been greatly improved, and rather than having to replace a plastic liner, there is now an automatic lubricator on the end of the bearing.

Widdel recalled how, before, DC Water had cracks appearing on its previous grit classifiers. “Now,” he said, “they have complete peace of mind that the new Lakeside structures are sound.”

About the Author: Christopher French is an independent writer and PR consultant who specializes in the water, waste and renewable sectors. Learn more about Lakeside Equipment Corporate at WEFTEC.17, Booth 2025.