From Point B to Point A

The City of Philadelphia biosolids program works with a contractor to make the leap from composting to Class A pelletized material

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As home to the Liberty Bell and the country’s most revered cheese steak sandwiches, Philadelphia continues to blend historical relevance with modern life. When it comes to wastewater, the city is progressing toward a more sustainable biosolids program.

 

At one time, biosolids from the city’s three wastewater treatment plants, (Southwest, Southeast and Northeast) were dewatered with centrifuges, mixed with wood chips, and composted in a windrow process. Today, the composting facilities are being demolished. Class B cake is temporarily being land-applied on farms and mine reclamation sites, and by the end of next year, material will be dried and pelletized to create a Class A/EQ (exceptional quality) product.

 

The composting operations were expensive and had caused some odor problems. City Water Department staff also wanted a more sustainable way to manage the biosolids. In need of technical support, they decided to delegate the work to a contractor.

 

“The city ended up composting in 2007, in order to reduce costs and stockpiles of material while the Synagro contract was being negotiated,” says Mary Ellen Senss, Water Department manager. “We were also looking to move toward being green and getting back to a Class A product without expending the city’s capital funds. Synagro would have the expertise to run the pelletizer and then be responsible for the product’s end-use.”

 

Senss’s office is located adjacent to Synagro’s facility to ease communication and oversight. “By contract, we are required to hold monthly meetings,” says Senss. “And I visit the site several times per week. I walk through the facility to observe the operation and any equipment upgrades as well as the construction progress of the Class A facility.”

 

Moving from composting to pelletizing has been exciting for everyone involved. “The whole transition, the interim, the timeliness, it’s exciting, for Synagro, and for the City of Philadelphia,” says Owen Sheehan, plant manager, Synagro Central LLC.

 

By land or by river

The city’s wastewater treatment plants are designed to process a combined 522 mgd, all using activated sludge. The Northeast plant (210 mgd) was built in 1923 and the Southwest (200 mgd) and Southeast plants (112 mgd) were built in the 1950s. All three plants discharge to the Delaware River.

 

Solids from all three are managed at the former Public Works Department Biosolids Recycling Center (now the Synagro facility) next to the Southwest plant. Primary and secondary solids from the Southeast plant are pumped five miles through two parallel pipelines. Once they arrive at the Southwest plant, primary and secondary solids from both plants are combined, generally at 60 percent primary to 40 percent secondary. After mixing, the solids are anaerobically digested and pumped to one of two 1.25-million-gallon storage tanks at the Synagro facility.

 

Biosolids from the Northeast plant are anaerobically digested on site and transferred to the Synagro facility via barge. The barge has a nominal capacity of 930,000 gallons and averages six loads per week. It unloads at one of two piers, pumping material to another 1.25-million-gallon storage tank at the Synagro facility.

 

“K-Sea’s tankerman deals directly with the operators,” says Sheehan. “He announces that they have arrived and asks if our tank is ready. The barges can come at any time during a shift. The tankerman handles everything: the barge valves, pier pump, and valves at our end during loading. We monitor the process as they fill tank number three. As soon as the barge is empty and it leaves, we dewater the storage tank so it’s ready when the barge comes back.”

 

Here responsibilities change hands. “Responsibility is transferred right after digestion, at the storage tanks,” says Sheehan. “The city is responsible for supplying Synagro with anaerobically digested Class B material at 2.0 to 2.5 percent solids at 60,000 dry tons a year.”

 

Synagro then dewaters the material and uses subcontract truckers to move the biosolids to land application sites and spread it. “We have a goal of using 50 percent minority companies on this contract, but as of April 1st we are at 71 percent,” says Sheehan.

 

On the road again

Synagro operates the plant’s 10 centrifuges. As solids are pumped to the centrifuges, Ciba ZETAG (part of BASF Corporation) liquid polymer emulsion is injected. “We have found a good polymer with Ciba and have been using it since we took over operations,” Sheehan says. The recycling center uses four 500 gpm Humboldt centrifuges (Andritz) and six 400 gpm Bird centrifuges (Andritz). It takes about four hours to dewater one 1.25-million-gallon tank using six or seven centrifuges.

 

“As solids are scrolled through the centrifuge, the liquid that’s separated, called centrate, drains to two wet wells,” says Sheehan. “Ultimately it’s pumped to the head of the Southwest plant. I am pleased with what our centrifuge operators have been able to accomplish.”

 

At 30 percent solids, material drops from the centrifuges onto a screw conveyor and then onto a belt conveyor. Solids fall from the belt conveyor into 25-ton shuttle trucks that transfer the material to an on-site covered storage area that can hold up to 10,000 wet tons. Material is stored in two rows, one for the combined Southeast and Southwest solids and one for Northeast.

 

“We usually take it as we make it and clear out the storage every day, but in the winter or during bad weather, we have to wait for a window,” says Sheehan. Another contractor does the hauling, and travel distances to land application sites range from 50 to 225 miles.

 

Most of the biosolids cake goes to sites in Pennsylvania and Maryland for spreading and tilling into row crops like corn and soybean, and in Virginia for surface application on hayfields and pasture. “Because the program is permitted in three states, we have flexibility of different planting seasons, depending on the time of year,” says Sheehan. “We don’t have a lot of farm storage, so it’s delivered and applied the same day. Synagro handles all permitting and oversees all application and reporting to permitting agencies.”

 

About 17 percent of the solids are delivered to mine reclamation sites to build up organic matter in the soil. “The biosolids are combined with a long-term seed mixture, like clover mix,” says Sheehan. “This is usually done during winter and summer.”

 

Under its agreement with the city, Synagro must beneficially use 50 percent of the biosolids, but the company has achieved over 90 percent. When weather prevents land application, the material is landfilled. “It’s a matter of timeliness,” says Sheehan. “Last winter was stressful. Once Christmas hit, we didn’t have a lot of opportunities for land application. And having the landfill gives us some flexibility.”

 

Toward pelletizing

To help reduce the city’s carbon footprint, Synagro’s contract includes modifying the biosolids process to produce Class A material. Their proposal to incorporate a thermal dryer and pelletizer aims to do just that.

 

Construction of the dryer system is expected to be completed in October 2011. Biosolids will still be processed through the centrifuges, but then will be routed to two Andritz dryers (Model DDS-110).

The dryers consist of three concentric drums that rotate. The solids will be conveyed pneumatically from the inner drum to the outermost drum. As the cylinders roll and warm air is forced through, biosolids will roll around, forming pellets that eventually will be blown from the cylinder. Pellets will be sorted by size and the larger and smaller ones will be fed back to the dryer. The pellets will contain a minimum of 90 percent solids.

 

While both the city and Synagro are excited about the pelletizer, they are still thinking about the farmers. “The farmers are very happy with the biosolids we produce,” says Lisa Williams, division director of technical services for Synagro. “That’s why we’re working to make sure we have a comparable product to replace the biosolids after the pelletizer comes online.”

 

A primary advantage of pelletizing will be volume reduction, which will significantly lower transportation costs. Another advantage is that biogas can be used to operate the furnace for the thermal dryer. Staff expects all excess methane from the 12 anaerobic digesters that is not utilized at the Southwest plant to be used for drying biosolids. If need be, fuel oil can be used, but natural gas and digester gas will be the primary fuels.

 

Rainwater from all roof areas of the drying facility will be captured in a 62,000-gallon cistern, filtered, and used as cooling water for the dryers, reducing demand for potable water.

 

Dried pellets will be stored on site in three silos. From there, pellets will be trucked or shipped via rail for use as a renewable fuel at cogeneration plants or at nearby cement kilns. The minimum use of 120,000 decatherms of digester gas from Southwest will be used per contract year to fuel the thermal dryers.

 

“There are roughly 20 of these dryers operating in the United States, and eight of them are run by Synagro,” says Sheehan. “We clearly have in-house experience and we have a great working relationship with the City of Philadelphia.”

 

The company is working to fit in well with the neighborhood. Besides eliminating odors and reducing truck traffic, the project will help support the local economy. “Our local community got behind us,” says Sheehan. “Once we came in, we began hiring and even sponsoring interns. We’re trying to be good neighbors.”



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