Keeps Getting Better

The operations team in Tacoma, Wash., has played a critical role in producing and marketing high-quality and popular biosolids products
Keeps Getting Better
Daily shift operator Bill Shaw at the treatment plant’s Mahr bar screens (Headworks).

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Like any successful private business, the Tacoma Central Wastewater Treatment Plant practices good customer service and product diversity as it markets its biosolids products to area customers. It offers top dressing for lawns, potting soil to retail outlets, and liquid fertilizer for farms and a unique elk habitat area.

And the plant’s operators have had a lot to do with the program’s development, working out kinks in the biosolids processing train, developing new products, and delivering the material to thousands of customers throughout the area.

The program, known as TAGRO, has won numerous accolades. Most recently, the program won the 2011 Excellence in Biosolids Award from the Northwest Biosolids Management Association.

Division manager for wastewater operations Dan Thompson gives much of the credit to the ingenuity of his team. “We’ve relied on the creativity of our operators,” he says. “They’ve really got us ahead of the game.” And now, as the Tacoma plant works to turn its biosolids operation from a cost center to a profit center, operators will no doubt play a role again.


Three basic products

The Tacoma Central Treatment Plant is an advanced secondary facility. The original operation dates to 1953, and the plant has seen three major expansions since: primary treatment in 1967, secondary treatment and the TAGRO biosolids operation in 1989, and increased hydraulic capacity in 2009.

Today, the plant serves 250,000 people in the Tacoma area. After screening (Mahr bar screens from Headworks), grit removal and primary treatment, wastewater is treated in a four-deck oxygen activated sludge system (supplied by the Union Carbide Environmental Systems department, later acquired by Lotepro, now part of Mixing and Mass Transfer Technologies).

Clarified effluent, meeting a 30/30 permit, is disinfected with sodium hypochlorite in a ProMinent system and discharged to Commencement Bay of Puget Sound. Flow averages 20 mgd — 88 percent domestic and 12 percent commercial-industrial.

Biosolids removed from the process follow several paths to final disposition. Dissolved air flotation units thicken the material ahead of a Dual Digestion process of aerobic and anaerobic digestion (aerobic digestion equipment from the Lotepro Environmental Systems Group of Mixing and Mass Transfer Technologies). Thompson says the system reduces odors but does not hinder gas production or volatile solids reduction. The gas is captured and used for building heat (boilers from Cleaver-Brooks).

About 12 percent of the biosolids are removed from the plant as a liquid (5 to 7 percent solids) and trucked to nearby farm fields or to an elk habitat reclamation area near Mount Rainier, about 40 miles away. The plant manages a fleet of 3,000-gallon tank trucks equipped with spray guns to “paint the pastures,” as Thompson puts it. “If we have more than 7 percent solids, the material can get pretty stinky,” he says. In some applications, the liquid is mixed with grass or wheat seed to promote ground cover.

The remainder of the digested biosolids moves on to an Envirex (Siemens) belt filter press. Polymer is added, and cake solids average 22 to 24 percent. About 85 percent of the cake is mixed with sawdust and sand to make TAGRO top dressing for lawns. The sand comes from a local quarry, and the sawdust from a nearby woodworking company.

The mix ratio is four parts biosolids cake, two parts sand, and two parts sawdust. Customers pick up some of the top dressing at the plant. The TAGRO team delivers the rest via city-owned trucks in 1- to 5-cubic-yard shipments. Users pay $10 a cubic yard.

The remaining cake forms the basis for potting soil. The plant mixes Douglas fir tree bark from a local mill with the cake, bags the material, and sells it to retail establishments like greenhouses and nurseries. Altogether, the TAGRO program has about 4,000 customers, and it seeks new ones through marketing efforts that include demonstrations, partnerships with other organizations, and word of mouth.


Fine-tuned process

On the surface it would appear that the TAGRO program has had smooth sailing over its 20-plus-year history. Not necessarily. Thompson says the dual digestion process posed challenges early on.

“We weren’t getting up to the required temperatures in the aerobic phase,” he says. “It’s designed to be autothermal, but it wasn’t. We were unable to reach the reaction temperature. Plus, we had odor issues in the anaerobic stage. So we involved our operations team and worked out solutions.”

So, the team sat down together and brainstormed ideas? “Exactly,” says Thompson. “A lot of these dual digestion systems are no longer in operation, but we agreed that adding a pre-heating step ahead of digestion might be the answer. We discussed it during our operational staff meetings. It seemed if we could get our material up to temperature, that would be the answer.”

Based on that idea, the team installed a spiral heat exchanger ahead of the aerobic digestion basin, increasing the temperature of the input and getting the aerobic digestion process up to the required temperature of about 170 degrees F.

To reduce odors in the anaerobic (thermophilic) system, the team devised a step-down process that lowers digestion temperatures from 130 degrees to 90 degrees F in three stages using spiral heat exchangers. “At higher temperatures, the digested solids were the source of odors,” says Thompson.

Cake solids are critical to a good biosolids product, Thompson maintains. For that reason, an operator tends the belt filter press operation around the clock, making sure the feed solids are adjusted so that the final cake ranges between 22 and 24 percent solids.

Russ Muncey is the senior operator overseeing the biosolids dewatering process. Thompson notes that cake solids in many similar operations might be in the 16 to 18 percent range. “We need to have our feed solids at 1.8 percent, but it can vary as low as 1.46 percent,” he says. “That’s why we need to watch the press operation very closely. That’s the big thing.”

Once the cake comes off the belt press, dump trucks haul it to the biosolids preparation area, a covered building, open on the sides. Framed beds are already filled with the correct ratio of sand and sawdust for the TAGRO lawn-topping product and with bark for the potting soil product. The biosolids cake is dumped onto the beds, mixed, then loaded onto a belt that takes it through a shredder (Royer) to break up clods.


Help with marketing

Operators have also made a big difference on the marketing and distribution side of the TAGRO program. “They’re the secret to our success,” says Tom Amundsen, biosolids coordinator. “They all buy into the program. It’s not just to make a living. They understand and accept that this is a pretty good thing to be doing.”

Amundsen says his team’s enthusiasm and support for the Tacoma biosolids program translates into customer support, as well. “A lot of wastewater plant operators are afraid of biosolids reuse,” he says. “But our staff understands it, and understands people’s reservations surrounding the concept. Biosolids reuse is cheap, safe, and it really works. We’ve seen the benefits, and when we communicate those to people, with a little coaching, we can really turn a lot of gardeners on.”

Amundsen adds that a wide customer base also helps. “It’s better to have 4,000 customers using a ton of biosolids each than one user taking 4,000 tons,” he says. “With 800,000 people in our metropolitan area, there are enough people here to use it all up.”

Thompson adds that the successful product mix itself is the result of operator ideas. “In the early days,” he says, “we were spreading liquid biosolids on farm fields, but as time went on these fields became full of houses.

“We wondered what we could put there, and Gordon Behnke, our biosolids supervisor, started experimenting, adding sawdust and making a product that would be appropriate for homeowners and their lawns.” The result was the TAGRO mixture the plant now widely markets to the community. The operators also came up with the potting soil idea.


Future improvements

As successful as the TAGRO biosolids program has been, Thompson and his staff continue to look for improvements. “We’re operating cost effectively right now,” he says. “We’re one of the lowest-cost biosolids operations in the area. But we want to be a profit center, not a cost center.”

To do that, the Tacoma team believes it needs to produce and sell more of the potting soil, which sells for $30 a cubic yard, or three times the price of the lawn dressing. “Making a profit is not just minimizing costs,” says Thompson. “We need to get more retail outlets.”

The plant has purchased a mechanical Rotochopper bagging machine, which will help the team bag and produce as much as 60 times more potting soil than before. “We were bagging the material by hand, and were capable of putting out about 50 bags an hour,” Thompson says.

In addition to producing more bags, Thompson says Tacoma wants to at least triple the number of retail outlets for the potting soil. “It’s a big mountain to climb for us,” he says. But given the history of the TAGRO program, you get the feeling they’ll meet that goal as well.


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