When the Hampton Roads Sanitation District ran short of storage space for biosolids, it had a choice: Add capacity for Class B biosolids or produce Class A material requiring less space.

The district went with Class A, and now the Atlantic Treatment Plant in Virginia Beach, Virginia, uses a high-temperature, high-pressure thermal hydrolysis process. The result is a drier, easily stored and more marketable product.

“We could just build another shed,” says Christopher Wilson, chief of process engineering and research. “Or we could do a process to make a better aesthetic cake and create opportunities for other beneficial-use outlets.” The Class A product is easier to handle and stacks higher, so the existing storage is more than adequate.

The storage issue arose when the district decided to divert the flow from another treatment plant to Atlantic. The plant was operating far below its 54 mgd capacity but did not have the capacity for additional biosolids.

The district installed a Cambi reactor that heats the solids to 338 degrees F, under pressure so that the mixture doesn’t boil. The process sterilizes the solids and breaks them down so that they dewater more easily after going through the digester.

Although the Cambi process uses substantial energy, much of the heat is recovered, and more biogas is produced in the anaerobic digesters. Therefore the process is energy neutral and could become energy positive. The Class A biosolids have potential applications in landscaping and gardening.

Extensive infrastructure

The thermal hydrolysis reactor is just one part of the new biosolids process; extensive pre-reactor and post-reactor infrastructure is required. “It’s not that thermal hydrolysis is overly complex or extremely expensive, but it takes a lot of solids handling around that process to make it work,” Wilson says.

“Heating up a lot of water to 338 degrees takes a tremendous amount of energy. So, the efficient way to do it is to dewater the materials initially, pull out as much water as possible, and then only heat up what’s left. That dewatering step is a significant adjunct facility. There is a whole building associated with that.”

The plant uses belt presses (Huber Technology Strainpress) and older centrifuges to bring the material to 18-22% solids. The dewatered material then enters a pulper tank where it is warmed to 190 degrees F before being pumped to the reactor.

After the reactor, the material is cooled and sent to the digesters. That process captures heat that is used to warm up the pulping tanks. “It’s a relatively efficient process,” Wilson says.

Sterilized solids

The solids going into the digester are sterile, and that presents another challenge. “In most treatment plants you always have live bacteria coming into the digester,” Wilson says. “Here, you’ve sterilized everything. You have to be very protective of the bacteria already growing in the digester, because you are not constantly re-seeding it. You definitely don’t want to overheat the digester, or you would lose those bacteria, and there’s no easy way to re-seed.”

Before launching the thermal hydrolysis process in 2020, the Atlantic plant was producing Class B biosolids spread on farm fields. That arrangement is continuing with the new product for the short term.

“We still have the same partners, the farmers, and they still need the nitrogen and phosphorus from biosolids,” Wilson says. “Until we figure out our upgraded product, which is more suitable for landscape use, we’ll still use the bulk spreading.”

At one time, the district worked with an industrial company to produce a compost called Nutri-Green. The company still makes the product, but not under the Nutri-Green brand, which the district retained and will use on the new product: “It has very good name recognition.”

The district still needs to decide how the new product will be distributed. “There’s always the possibility that from a public relations and exposure standpoint, some of it ends up in bags,” Wilson says. “It helps for the public to see it as a commercial product.”

The Atlantic plant produces about 20 tons of the product per day (dry weight), but that could increase by adding solids from other district treatment plants to the Cambi system.

A grease solution

A district as large as Hampton Roads (16 wastewater treatment plants with a 249 mgd total design capacity serving 1.7 million people) can undertake more than one major sustainability project at a time. The 30 mgd Nansemond Treatment Plant in Suffolk is installing a Greasezilla grease-processing system developed by Downy Ridge Environmental.

The system heats the grease and separates it into a commodity-grade fuel, a lower-grade fat and water. “If you heat up a water-grease emulsion, the water and the grease tend to separate really nicely,” Wilson says. “You heat it up and drain off the water; you take what’s left and heat it and drain it off again. You keep on doing that until what’s left is highly refined with very little water in it.”

The water is returned to the wastewater treatment process; the top-grade fat known as brown grease can be traded on a commodity exchange. The district expects to use some of that material to fuel a boiler to heat the process, but the rest will be sold through a broker. The other component of the grease, known as batter, will go into the anaerobic digesters.

Dual motivation

The plant takes in about 20,000 gallons of grease-trap waste per day at about 95% water; that will yield about 1,000 gallons of brown grease. Generating revenue from the grease is a plus but is not the only motivation for the project, which is to go online in December 2022.

“We view this as part of our core business, to manage environmental protection in the most cost-effective and most environmentally positive way possible,” Wilson says. “Producing a renewable fuel that has an economic and environmental benefit, while preventing that material from causing blockages and overflows in the collection system, is kind of a win-win.”

Wilson is confident that the Cambi process and the Greasezilla system are good fits for their plants, but he notes every plant has unique characteristics that make finding workable solutions complicated.

“When you add something that’s intended to solve a problem, you want to make sure it’s not creating another problem,” he says. “There are a lot of good technologies out there, and each of them solves a set of problems. The challenges are specific, and the technological solutions are specific as well. A really good process for one plant could be a poor fit at another plant.”