The Neuse River Wastewater Treatment Plant in Raleigh, N.C., uses several methods to manage biosolids. The variety of land application and other alternatives keep operations interesting and employees on their toes. It also makes good business sense.
“It’s like diversification in the stock market,” says Tim Woody, reuse superintendent with the City of Raleigh Public Utilities. Woody knows the ins and outs from crop rotations to weather, and he ultimately decides where biosolids will go. “The main role of our Environmental Management System (EMS) is to identify markets three to six months ahead of time,” he says.
In December 2006, the Neuse treatment plant became the 14th utility in the country, the first in North Carolina, and the first in the southeastern United States, to be verified by a third party and receive EMS certification for biosolids.
The EMS has given plant operations a more formal structure to diversify its biosolids program and expand its distribution network. Before EMS certification, solids were solely converted to Class B liquids and land applied. Now, the facility also produces Class A biosolids with lime stabilization and composting, adding flexibility to work around changes in the market.
First stop – Class B liquid
The Neuse plant produces about 40 dry tons per day (tpd) of aerobically digested solids, which are handled one of three ways. About 10 percent is Class B material, land applied as a liquid. About 55 percent is lime-stabilized to create Class A fertilizer. The balance (35 percent) is sent off site and mixed with bulking agents to form a Class A compost.
Regardless of the end result, all of the solids start in the same place. The plant uses the activated sludge process, and waste activated sludge is pumped through a 2-meter Ashbrook belt filter press to one of four 2.5-million-gallon Crom tanks that serve as aerobic digesters. For the Class B liquids, this is the end of the line. From the digesters, liquid 3 to 4 percent solids is hauled via tanker trucks to privately owned farmland within 60 miles of the plant and is used to fertilize corn, soybeans and alfalfa.
“In spring, we made a lot of Class B because the farmers were getting ready to plant a lot of corn and soybeans,” Woody says. “We look at the markets because they’re seasonal. If we’ve got three full tanks of Class B at 2.5 million gallons each and the farmer says, ‘I’m not ready,’ and we can’t send it, that’s costly.” To store unused Class B material at the plant, the digesters must be continually aerated, adding to the electric bill.
Once at the farm, the biosolids are injected into the ground or sprayed on top and disked into the soil. Most farmers use TerraGator agricultural equipment (AGCO).
Some of the 2.5 million gallons of biosolids is ready to leave the plant at a given time. The trucks hold about 6,000 gallons, and biosolids can be applied at 18,000 gallons per acre, or about three truckloads per acre. This means around 400 truckloads make the trip between treatment plant and farm land and close to 140 acres are receiving the biosolids.
“I have to be aware and know the different processes,” says Woody. “We hauled 2 million gallons of stored liquid biosolids last week. We couldn’t process it that quickly, because we couldn’t get it through the gravity belt press. It’s my responsibility to balance what we make.”
For example, once farmers finish planting corn and soybeans, distributing liquid product is difficult. Woody needs to understand the agricultural base. The plant deals with 300 to 400 farmers in a given year.
Laboratory tests are done on samples from each batch out of the digesters for regulatory compliance and to help farmers apply the material at agronomic levels. When the batch is released, so are the results. “We want 100 percent confidence,” says Woody. “We are completely transparent. If you hold anything back, it can take years to win back the farmers’ trust.”
Having your cake
The portion of the biosolids not directed to land application is converted to biosolids cake. From the digester, the material is pumped to sludge holding tanks. It is then dewatered using three 2-meter Ashbrook belt presses operated in parallel, yielding material at about 22 percent solids.
A relatively small portion of that material is transported off site to make compost. For this stream, solids from the belt press go to one of two storage bins (hoppers), each with a capacity of 100 wet tons. The solids are then dropped into trucks parked below, transported to a composting company, and mixed with wood chips. The process converts the Class B material to Class A compost, which is distributed in bulk to local markets.
The majority of solids are kept on site to be converted to Class A material through lime stabilization. These solids are transferred via belt conveyor to a cement mixer. “We used to use fly ash and hydrated lime,” says Woody. “But the arsenic quality was an issue, as well as reliability. A consistent product is more achievable with only one admixture, rather than two.” Two admixtures also double the chances of delivering and scheduling errors.
Now the plant mixes dewatered biosolids and lime kiln dust using a Cemen Tech mixer. The final product is Raleigh-Plus fertilizer, sold and distributed to farmers. “We will deliver and provide the farmer a pull-behind spreader as part of our lease program for $3 a ton,” Woody says. “Costs may be reduced if the farmer transports the material himself or does not need to lease one of our spreaders.”
When the fly ash and hydrated lime were eliminated, so was the previous mixer, which had been in service for 25 years. “The Cemen Tech mixer does such a good job,” Woody says. “We maintain a temperature of 70 degrees C for 30 minutes. Before, we had to overdose to keep the temperature up. It’s been running for about one year, and it mixes so well we’ve been able to cut back on the lime kiln dust.”
When changes were being made to the plant to bring on the new mixers, options for managing biosolids were temporarily limited. “When we put the mixer on line, we had to shut down the Class A operation,” Woody says. “The entire daily production went to the storage hoppers. The composter was only contracted to receive 100 tpd. We landfilled the excess tonnage to avoid exceeding the contract amount.”
Smooth operators
The Neuse River facility typically makes about 120 product tons in an eight-hour shift. “The shift starts making Class A product,” Woody says. “The next eight hours will be spent filling up the hoppers for the composter. We do it on a shift basis because it keeps the quality consistent.”
Even though it’s Woody’s responsibility to read the market and understand the agricultural demands, he acknowledges the other employees involved in producing consistently high-quality fertilizer. It starts upstream in the treatment plant where responsibility rests with T.J. Lynch (Grade IV operator), superintendent of plant operations. Others include Jesse Luper, assistant reuse superintendent; Winslow Davis (Grade IV), operations supervisor; and Jason Waters, land management supervisor.
“We have a great staff,” says Woody. Beyond working in the plant, one employee spends the vast majority of time in the field with the farmers. “Don Edwards is our residuals distribution coordinator,” says Woody. “He is a certified land application operator and is very well-versed in nutrient management and agricultural practices.”
It’s important to connect with the farmers, not just to schedule deliveries, but to verify the quality and consistency of the product. “The farmers all know each other, and they know who gets what.” Woody says. “If they see something on someone else’s land and it’s black and theirs is gray, there’s a problem.”
Beyond the gates
For the Class B liquids, the utility samples the biosolids to stay in compliance with the state-issued permit. With the Class A biosolids, the farmers sign on as a receiver stating that they will properly manage the material.
“They say they won’t eat it or drink it,” Woody jokes. “But we are selective. Back to the EMS program, we have the responsibility beyond the confines of our gate. Fifty miles away, it’s still our responsibility. If a farmer dumps it in a creek, we’ll have to deal with it one way or another. We might as well deal with it upfront.”
Even with the complexities of various routes and processes, Woody is proud that the facility manages its own biosolids. “Some plants simply contract out biosolids — they don’t have to keep up with these details,” Woody says. “Especially with land-based disposal, you need a strong biology and agriculture background and an understanding of economics. If it’s land-based disposal, you need to know the processes, engineering and management.”
Woody manages to stay abreast of current operations with an eye on the future. He’s considering other projects for the plant, including using biosolids to fertilize biofuel crops. That’s just one more way to keep employees on their toes and make good business sense.
