The Old with the New

The City of Portland biosolids management program combines beneficial use with sequestration of older, contaminated material

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Over half a century old, it’s no wonder that the Columbia Boulevard Wastewater. Treatment Plant in Portland, Ore., has been upgraded, expanded and modified. Even so, there are legacies and leftovers the operators must account for.

 

The prime legacy operation is Triangle Lake Lagoon, a 37-acre storage area that holds older biosolids and also serves in today’s treatment train. The lagoon stores three types of biosolids: older material too contaminated to land-apply (generally the oldest of the lagoon solids); older material suitable for land application; and new solids pumped from the plant.

“Some of the older solids in the lagoon have been around for decades,” says Greg Charr, who recently took over as biosolids and residuals manager when Mark Ronayne partially retired. “Pollutant levels tend to be lower in the new solids, but newer solids are less stabilized than older lagoon solids.”

 

And what’s to be done with 20,000 tons of the oldest solids deemed unsuitable for land application because of elevated levels of lead, cadmium, and PCBs? They’ll be encapsulated in a monofill next to the lagoon.

 

“To accomplish this, one half of the lagoon will be drained while the other half remains in operation,” says Charr. “The solids in the drained portion will be allowed to air dry, to about 40 percent solids, before being placed in the monofill.” The cost of managing the material on ­site is half of what it would be to transport it to a landfill. And when the project is complete and the solids safely entombed, the monofill will be closed.

 

Other solids are treated using anaerobic digestion (newer solids) or lagoon stabilization (older solids and a portion of the newer solids). The two streams are mixed together before being dewatered and trucked off to a farm for land application to produce a variety of crops, including canola, whose seeds are processed into biodiesel fuel.

 

Twists and turns

The solids treatment train at Columbia Boulevard takes many twists and turns. It includes eight digesters (some not used exclusively for digestion), and processes that include thickening, dewatering, and blending.

 

Charr, with help from Ronayne, oversees the biosolids management team. Operations specialist Willy Park and five operators work with the plant’s six mesophilic digesters and two solids blending tanks. Operations specialist Richard Brixey and six operators control solids thickening processes, which include gravity belt thickeners, belt filter presses, and lagoon dredging operations. Newer solids come from three sources:

• Primary sludge from the Columbia plant is processed through gravity thickeners (Eimco Water Technologies, 55305-1 and 55305-2).

• Waste activated sludge from Columbia is processed through a gravity belt thickener (Ashbrook Simon-Hartley Type 6093) to a solids content of 19.3 to 22.5 percent.

• Primary solids and thickened waste activated sludge from the city’s Tyron Creek Wastewater Treatment Plant are trucked in 6,600-gallon tankers to the plant and pumped in just upstream of the anaerobic digesters.

 

As solids are brought into the system they are routed for treatment one of two ways: two-stage mesophilic digestion or lagoon stabilization. Primary sludge is never sent to the lagoon; it is treated solely in the three anaerobic digesters (concrete structures equipped with WEMCO Torque-Flow pumps from Weir Specialty Pumps).

 

Treatment takes place in the digesters and lagoon, and the material is ready to be combined in what the plant calls digester 3, which acts as a mixer. In addition to the blending lagoon and digested solids, polymer (ClariFloc WE-686 from SNF/ Polydyne) is added in digester 3.

 

For every load of solids entering digester 3, about one-third is from the lagoon and the remaining two-thirds are from the mesophilic digesters. From digester 3, the mixture is sent to the belt filter press (Ashbrook Simon-Hartley Winkle-press with a 4-foot extension).

 

Working the lagoon

Thickened waste activated sludge not treated in the digesters goes to the lagoon for stabilization. These solids are sometimes mixed with older solids, or given a place of their own.

“Most of the older solids in the lagoon are in the south cell,” Charr says. “New solids can be pumped into either cell, but the majority of newer solids are pumped into the north cell. Dredge crews avoid the section of the lagoon with the heavily contaminated materials.”

 

Dredging is a daily operation — two dredges work in the lagoon. The pumping of thickened waste activated sludge to the lagoon and removing more stabilized solids from the lagoon is an organized operation. Surveys over the years have mapped the location, quality, and quantity of solids in the lagoon. The information was last updated in fall 2009.

 

While it may be a relic in some respects, the lagoon still serves a function: solids from the lagoon are pumped back into the plant, to digester 3, and blended with solids from the second-stage anaerobic digesters to provide additional solids stabilization. “If not for the lagoon solids, our cake density would be in the 16 to 17 percent range, as opposed to the 21 percent we see now,” Charr says.

 

The lagoon also serves an alternate purpose: filtrate disposal and weather-related emergency storage.

 

“Filtrate solids from our belt presses and gravity belts are periodically pumped out to the lagoon,” says Charr. “Cleanings from any of our digesters can be pumped out to the lagoon as well. The lagoon also serves as emergency storage. When winter road conditions are too hazardous for trucks, all of our digested solids can be discharged to the lagoon.”

 

A long haul

From the belt filter press, the mixture of stabilized lagoon and anaerobically digested biosolids are stored on site in two 100-wet-ton cylinder silos. Trucks pull in below the silos onto platform scales and are filled from above without assistance from plant staff. Gresham Transfer Inc., a private hauler, collects triplicate tickets with tare, gross, and net weights. The firm transports the solids 200 miles to Madison Farms, keeping a ticket for its own use and giving the others to personnel at the farm and to the city. Trucks make morning and evening trips to the farm six to seven times a day.

 

Each truck carries 35 wet tons of biosolids for application to perennial dryland pasture, canola, alfalfa, corn, snap peas, potatoes, rye grass, and wheat. It’s typical for 20 to 30 acres to receive biosolids every day. At an application rate of 9.7 dry tons per hour, a week’s worth of biosolids can be land-applied in about 26 hours, given level terrain.

 

Even with the long haul, 400 miles round trip, Madison Farms has a number of advantages as the city’s land application site. One is the climate: It is in a semi-arid region that receives only 7 to 9 inches of rainfall a year, and the depth to groundwater is more than 300 feet. That means land application can take place year-round, helpful for a plant that generates 16,000 dry tons per year.

 

Another is that Madison Farms is designated as Exclusive Farm Use property, meaning it is dedicated farmland. The designation never expires, and it offers tax incentives to the owner as long as the land is farmed.

 

The benefits of land application are evident at Madison Farms. Since land application began, forage production per acre has increased, and so has the cattle-stocking rate (the number of cattle per acre). Along with that, cattle on the farm weigh more than they used to.

 

Madison Farms also grows crops that benefit from land application, specifically canola. The farm owns a biodiesel production facility that crushes the canola seeds. The final step in conversion to biodiesel can be completed at the farm or by an off-site processor. Either way, the city offsets carbon emissions by using the biodiesel-to-fuel fleet vehicles.

 

Looking at options

Charr cites a study performed by the University of Iowa in partnership with Madison Farms showing that even when accounting for the drive from the treatment plant to the farm and the fuel needed to spread the biosolids, “Land where biosolids are applied sequesters more than eight times the carbon dioxide released during solids transport and land application.”

But no option goes without continued evaluation. Charr and his group review biosolids management alternatives. After a recent request for proposals the city shifted handle of 25,500 wet tons of biosolids per year — about one-third of what is taken to Madison Farms — to a contract with another hauler.

 

“We will partner with K&S Madison to land-apply biosolids to dryland small grains in the Sherman and Wasco county areas of Oregon,” Charr says. “The truck haul is about half as far as Madison Farms. This will result in an annual savings of about $280,000.”

 

Charr enjoys almost everything involved in managing biosolids. “I like to interact with different people in different roles: the engineers, the public, private contractors,” he says. “And the wastewater industry is so dynamic. I’m curious to see where biosolids goes from here. It’s exciting that new technologies can present new biosolids management options.”

 

The Clean Water Act requires the EPA to review biosolids regulations every two years. Charr believes that opens the door to changes, and will require identification and study of additional pollutants, including pharmaceuticals and endocrine disruptors. All this, he feels will lead to additional research to assess the safety of and possibly change current biosolids land application practices.

 

It’s an ever-changing field, just the way Charr likes it.



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