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Shaping processed biosolids into pellets has been deemed the optimal way to create a fuel product and fertilizer at a new system in Philadelphia.

June 4, 2014
Brian Taylor

Among the many waste and residual streams being prepared as a fuel product are human biosolids flowing into wastewater treatment plants throughout the world.

Few outside observers may consider human waste as a product with inherent value, but the organic constituents that comprise it can yield both energy as well as soil additives with horticultural or agricultural value.

A recent installation at a Philadelphia Water Department (PWD) plant features a system designed by Baltimore-based Synagro Technologies that converts the biosolids into pellets that can be used in both waste-to-energy and soil additive applications.

Diverted and dried

The PWD facility in southwest Philadelphia that hosts the new Synagro Technologies system served as the site of an open house tour in early May 2014, co-hosted by Philadelphia Mayor Michael Nutter and Synagro Technologies President and CEO Eric Zimmer.

Both Nutter and Zimmer touted the green credentials of the new waste-to-energy system, known as the Philadelphia Renewable Bio-Fuels Facility.

Doing well by doing good

The drying and pelletizing system installed by Synagro Technologies, Baltimore, at the Philadelphia Water Department (PWD) waste treatment plant in the southwestern part of that city will not only score sustainability points for the city of Philadelphia but also should yield solid financial returns.

In a news release announcing an opening ceremony for the PWD treatment plant, the city of Philadelphia says it expects more than $200 million in savings during the 20-year contract.

An advantage the plant enjoys compared to one that focuses solely on disposal is the production of a “Class A biosolids product” suitable for the fuel and fertilizer markets.

Other benefits, says the city, include the elimination of off-site odors, improvement of the site’s aesthetics, minimized noise and traffic impacts and reduced greenhouse gas emissions.

“I’m proud to participate in the public opening and tour of this modern, ecofriendly facility,” said Nutter. “The processes within this plant turn waste into quality biosolids pellets for organic fertilizer and renewable fuel—an environmentally friendly solution for managing the PWD’s biosolid waste,” added the mayor.

Synagro Technologies, Baltimore, was selected by the Philadelphia Water Department (PWD) as a partner after the department chose to upgrade its biosolids management program. “We’re proud of the achievements that our team, working with the city and local community, have made to make this center a reality,” said Zimmer at the event. “Together we are creating an environmentally and economically sustainable solution to the ongoing challenges of recycling organic wastes.”

Synagro Technologies, founded in 1986, describes itself as the largest recycler of organic byproducts in the U.S. The company says its processes range from beneficial reuse to renewable energy and are designed “to provide sustainable solutions for communities across the nation.” The company currently works with over 600 municipal and industrial water and wastewater facilities in the U.S.

The new facility features a thermal heat drying system that produces biosolids pellets without the need for pellet forming equipment. The pellets are ready for sale in organic fertilizer and renewable fuel markets, ensuring a “consistent recycling of PWD biosolids for many years to come,” says the PWD in a press release.

John Goodwin, Synagro vice president of engineering and operations support, says Synagro’s thermal drying system has been configured to create pellets that can be used by both end markets.

Subtraction and addition

According to Goodwin, the system in place at the PWD facility involves a thermal drying process that goes beyond the norm.

“Not all thermal dryers produce pellets, so there is a difference between thermally dried biosolids and end use marketability that is technology-based,” he comments. “The granulite product made in Philadelphia is screened to size guide number (SGN) standards specific to meet fertilizer industry requirements.”

The round pellets produced in Philadelphia meet fertilizer standards but also can work in waste-to-energy applications. “The process of using recycled particles to form the pellets produces a relatively round particle that is ideal for blending and spreading as fertilizer or use as a fuel source,” says Goodwin.


While some residual materials, such as the biosolids being converted to pellets by Synagro Technologies and the Philadelphia Water Department, are subject to several forms of regulatory oversight, the U.S. Environmental Protection Agency (EPA) may soon classify several other key feedstocks as “non-hazardous secondary materials” (NHSM).

In late March 2014, the EPA issued the latest iteration of its proposed NHSM rule, expanding the list of materials recognized as “non-waste fuels” to include processed construction and demolition (C&D) wood, paper recycling residuals and creosote-treated railroad ties.

Operators of boilers and other facilities that can use fuel pellets made from waste streams must abide by the NHSM listings to determine whether the materials are regulated under EPA’s boiler maximum achievable control technology rule or the agency’s commercial and industrial solid waste incinerators rule.

Perhaps most notable about the Synagro system at the PWD plant is that pellets are created without any pellet formation equipment. Rather, the drying and screening stages that comprise the system break down and then build up particles until they are the desired pellet size and shape, according to Goodwin.

“At Synagro’s Philadelphia facility the thermal drying equipment itself forms the pellet and is the basis by which the process works, utilizing smaller dried [recycled-content] material to mix with wet material and form layer on layer, which is subsequently dried and the pellet enlarged as it cycles through the system until it is of the correct product size,” says Goodwin.

Goodwin says waste treatment plant operators have other options when it comes to producing pellets from biosolids that involve the use of pellet formation and shaping equipment.

“With lower-tech thermal dryers that do not form a pellet as part of the drying process there is add-on equipment (pelletizers) available to manually compress dried material into a pellet,” he comments.

Goodwin says the Synagro system’s ability to avoid pellet-shaping machinery provides a floor space advantage. “The entire thermal drying process forms the pellet, so the footprint [consists of only] the drying trains itself,” he remarks.

The entire process also can take place relatively quickly, he says. “At the Synagro Philadelphia facility, we process biosolids that we receive (about 2 to 4 percent of the total inflow volume) via two main process components, known as centrifuge dewatering and thermal drying,” says Goodwin. “The entire process of dewatering and drying takes from two to four hours from liquid storage tank to product silo loading.”

The size and shape of the pellets produced by the Synagro process also provides the company access to premium markets, says Goodwin.

Installations that use pellet mills to make larger pellets may end up with a product that is “larger than the higher-end fertilizer and fuel markets demand,” says Goodwin.

Synagro’s system in Philadelphia, he says, caters to market segments “where very uniform-sized pellets would be used” such as “in the fertilizer blender market in retail bag blends and within the citrus fertilizer market, along with the fuel market.”

The typical size of these higher-end market pellets is in the range of 0.5 to 2.0 millimeters (one-fifth to three-quarters of an inch), according to Goodwin.

Ready for market

Although dried animal manure has been used as fuel for millennia (and lightly treated or even untreated animal manure has been used in agricultural applications for just as long), the use of human biosolids raises red flags in some people’s minds and does entail regulatory compliance.

“First and foremost, the Synagro Philadelphia thermal drying process is required to comply with U.S. Environmental Protection Agency 503 regulations related to pathogen and vector attraction reduction,” says Goodwin, “so monitoring temperature and the percentage of total solids of the dried pellets exiting the process is the primary consideration.”

He continues, “The temperature is monitored via online thermocouples and the percentage of total solids is analyzed routinely through the operating day within the facility laboratory, as well as outside labs for compliance documentation.”

To make pellets that meet all regulatory requirements for fuels and soil additives, the right people have to be on hand to monitor a system such as the one installed at the PWD facility, says Goodwin. “Improper knowledge and understanding of the thermal drying equipment is the primary opportunity of mistakes and bad results,” he states.

“Synagro Philadelphia has qualified and well-trained operating staff [who are] very familiar with the thermal drying concept and have a specific understanding of the thermal drying equipment that is utilized there,” says Goodwin.


The author is editor of Renewable Energy from Waste and can be contacted at