Top Performer - Biosolids: Hands-on Contracting

The Everett (Wash.) Water Pollution Control Facility moves biosolids every two years, but the management process remains a year-round concern with a sharp eye on the future

From a day to day perspective, biosolids management at the Everett Water Pollution Control Facility (EWPCF) in northwest Washington is relatively low key. But every two years, it’s an intensive effort that includes a survey and measurement of solids, preparation of bid packages, and oversight of multiple contractors headed out in different directions with the plant’s Class B biosolids cake.

“We’ll have one contract for dredging and dewatering, and maybe two or three contracts or professional service agreements for transportation and application,” says Chris Chesson, biosolids specialist with the City of Everett Public Works Department.

The process starts four to five months before the biosolids handling begins. “For the last dredging and dewatering, we started in February 2008, and the contractor was onsite the first of July,” says Chesson. The schedule gives him time to evaluate site conditions, determine the material volume and percent solids, and review available land and application methods.

Chesson stays on top of biosolids management year-round, always making sure the material will have a home in some form of beneficial reuse. He also looks ahead with newer approaches, which soon may include creation of Class A compost.

Solid improvements

The Everett treatment plant, built in the 1960s, originally included an oxidation pond with an outfall to the Snohomish River. In 1971, aeration ponds were added upstream of the oxidation pond, and a chlorine contact channel downstream.

The aeration ponds are two 15-acre cells that operate in series. These are followed by a 130-acre oxidation pond, a 30-acre polishing pond, and the chlorine contact channel. The aeration cells are separated from the rest of the lagoon system by a public access road leading to nearby Spencer Island Park.

A trickling filter/solids contact (TF/SC) process was built in 1991 and is referred to as the mechanical system. It operates in parallel with the aeration ponds (lagoon system) and includes two secondary clarifiers and a chlorine contact chamber.

The final site modifications came in 2006 when two primary clarifiers were added in an effort — successful so far — to address odor problems. Residents living across the river haven’t voiced any objections. “You can stand right next to the pad and not detect a high ammonia smell,” says Chesson. “It’s earthy, but not as pungent as others.”

Also added in 2006 was a biosolids handling area measuring about three acres, two-thirds of it paved. This elevated area, in the southeast corner of the oxidation pond, is used to treat and stockpile solids removed from the lagoon system.

Plant influent generally receives primary treatment through the bar screen, grit collectors, primary sedimentation tank, and biofilter. After that, it is directed to either the mechanical system or lagoon system for secondary treatment. The method of secondary treatment is based on various factors including influent volume and organic content.

At present, all primary clarifier effluent flows to the first aeration cell, then to the TF/SC. To maintain pond levels and make use of their discharge location, effluent from the secondary mechanical system is pumped back to the lagoon system.

The lagoons also manage solids from the mechanical system. Mixed liquor is concentrated in the secondary clarifier and discharged to the aeration cell. When flow to the mechanical system is 16 mgd, about 15,000 pounds of secondary solids is sent to aeration.

Dredging up solids

Every two years, solids in the second aeration pond are dredged, dewatered, and distributed. Even though Chesson has been through the process many times, he still makes sure everything in the bid package is up to date.

“We do a hydrographic survey of the pond bed before going out for bid,” he says. “We’ll then be able to say, for example, that we have about 4,000 dry tons of biosolids at 8 percent solids. We provide a lot of information to potential bidders. And there’s a pre-bid walk-through.”

Because of a legal challenge, walk-throughs are no longer mandatory, but the specifications strongly encourage bidders to make a physical inspection of the property. “They have the bid documents, and they can ask questions,” Chesson says. “We can answer all their questions, and if we have to follow up on one, we can post a written addendum before they submit their packages.”

Bid invitations are sent to contractors Chesson knows can do the work. Advertisements are sent to industry publications and to the Northwest Biosolids Management Association (NBMA), a regional nonprofit organization for biosolids professionals, on whose board Chesson serves as secretary. The organization is familiar with contractors Chesson may not know, and contractors keep in touch with the NBMA to expand their client base.

Chesson prepares the bid packages, assisted by staff in the city clerk’s office, which distributes the packages, accepts the bids, and checks contractor bonds. When it comes to dredging and dewatering the biosolids, Chesson lets the contractors decide on the best methods. The specifications require contractors to meet a minimum solids content.

“We let them propose a dewatering method, but in our experience, centrifuges work better than belt presses,” he says.” We used high-speed centrifuges in the last three or four dredging cycles. I haven’t had anyone bid a belt filter press. We always specify 28 percent solids because we know it will be fairly easy to manage. It stacks up pretty well and doesn’t have a lot of free liquid.”

In 2008, the biosolids were removed from the aeration pond using a cutterhead dredge that passed back and forth across the lagoon. The rotating cutter apparatus, surrounding the intake end of the suction pipe, excavated six feet into the solids. The suctioned material was pumped to the handling area, where American Process Group had set up two model HDC 970 centrifuges, modified to meet the needs of large-scale dredging. Praestol 855 BS (Ashland Water Technologies), a white granular polymer, was added to the biosolids to facilitate dewatering.

Dredging and dewatering took 30 days to complete with the contractor working 24 hours a day, six days a week. The result was more than 4,000 dry tons of biosolids, stockpiled on site and ready for transport.

Getting a move on

Separate contractors handle transportation and application of the biosolids, but the work is scheduled concurrently. Haulers use sealed, covered, and watertight trailers. Chesson knows the routes and ensures that contractors clean up any spills.

The application contractor is responsible for disposing of the biosolids, and specifications require them to be applied within 10 days of delivery to a site. The city uses four types of land application sites, each with its own nuance, its own method of application, and its own permit requirements:

Silviculture. Material is applied to city-owned land used as a poplar plantation, and to other sites that grow Douglas fir and cedar. Here, the contractor must have equipment to project the solids 80 to 120 feet between the trees. Once dispersed, the biosolids are not incorporated into the soil, but slowly break down at the surface.

Cropland. Large volumes are applied to private land used to grow crops and raise dairy cattle. The material is tilled into the soil.

Wheat fields. Wheat farms in King County in eastern Washington accept biosolids from Everett and other municipalities on what Chesson refers to as a good will basis, meaning the material disposal is free and the city covers transportation costs. Travel through the mountains to these sites is expensive, so the city seldom uses this option.

Municipal projects. Recently, the Snohomish Public Utility District used the biosolids as part of a hydroelectric pipeline construction and land reclamation project. Everett hauled and stockpiled biosolids along the four miles of backfilled trench. Workers then spread the material with a bulldozer and seeded the area.

Planning and paperwork

The state encourages land application of biosolids as beneficial recycling. Administered by the Department of Ecology, the permitting process includes identifying land to receive biosolids to state and local governments. A state coordinator also visits the site. After any concerns are addressed, a report is submitted to the U.S. EPA, the state, and the county health department. The public is notified and allowed to comment before a “determination of non-significance” is issued.

“I’ll talk to farmers or others at conferences,” says Chesson. “We’ll meet on site and I’ll ask what crops are grown. If they’re prepared to accept our quantity and quality, then I prepare the site-specific land application plan. The plans are involved. We have a template, but each one is site-specific.”

Chesson keeps multiple sites in the queue as fields go fallow. Like many biosolids professionals, he tries to plan as far as five and 10 years ahead. For Chesson and other professionals at the Everett treatment plant, biosolids management involves equal parts of paperwork and hands-on skill, with eyes to the future.


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