UTILIZATION SUCCESS

COMPOST PLAYS ROLE IN RIVERFRONT RESTORATION

Pittsburgh project uses biosolids compost to build soil capable of withstanding flooding and providing nutrients for plant growth.

Dave Block

Last year, work began on a new park designed to reconnect Pittsburgh, Pennsylvania residents with the Allegheny River. Bounded by the river, a highway, a bridge and embankment walls, the park was conceived to link Pittsburgh’s Convention Center and Point State Park while furthering its reputation as one of the country’s “most livable” cities. The lower of two levels was completed in the fall, and when all work is done, the park will boast dense groves of trees and ground covers, native plants, clusters of large indigenous boulders, a pair of 350-foot-long concrete wheelchair ramps and unknown to most who enjoy the park is the biosolids compost incorporated into the soil mix used for much of the plantings.

The mile-long park is only 35 feet wide, which makes having the right soil instrumental to success, notes Laura Solano, senior associate at Michael Valkenburgh Associates, Inc., the landscape architectural firm that led the project. “Having plantings, walkways and accessibility while still providing meaningful places is difficult,” she says. “Proportionally, the landscape has very little space to do a lot of work, so the viability of the planting areas is essential.”

Soil for the lower level of the park had to be manufactured because there was only slag underneath the existing parking area. “There was no growth medium there,” she says. “It was a parking lot filled and paved over in the 1940s. When excavation for borings was done, we discovered absolutely no soil available for plant growth.” Other challenges include the severe sectional grade changes that led to a two-level park plan, flooding caused by the location of the lower tier just three feet above the Allegheny River, and an adjacent four-lane highway.

A NEW GROWTH MEDIUM

The task of designing a new soil profile was entrusted to Dr. Phil Craul, a land
scientist based in Manlius, New York. He needed material that would drain at a high rate and handle the large amount of silt
left by waters that overflow by five feet several times annually and 25 feet during a 100-year flood. “I like biosolids compost because it has nutrients and trace metals, like boron, that contribute to sustainable soils,” he says. “Composted sewage sludge also is very cost competitive with other sources of organic matter. Peat moss is getting very expensive and has a very low nutrient content.”

Solano wasn’t surprised at the choice of biosolids compost, since Craul had used the product before on other projects of theirs. “I knew he would have to create a fairly sandy soil profile because of drainage problems,” she says. “There would be very few nutrients, so organics would have to be added... Use of biosolids compost isn’t ‘experimental’ anymore because it’s been tried and true on other projects.”

Biosolids compost as a feedstock suited Craul because of its consistency. Municipal yard trimmings compost can be more coarse, he notes, potentially including particles that could interfere with root establishment and germination of grass seeds.

One of the main elements Craul included in the compost specifications was an obvious one. “It had to meet EPA Part 503 standards because this was a public site. Pennsylvania standards are the same as the EPA’s,” he notes. Craul insists on using compost only from facilities that consistently generate batches of product that test under heavy metals limits.

Another required characteristic of the compost related to the location of its application. “Because these soils would be subject to flooding on an annual basis, we had to ensure the compost was very mature and stable,” Craul explains. “That means getting it up to level four or five according to stability tests developed at Woods End Research Laboratory. The reason for having it stable is that when the soil containing compost is flooded, we don’t want it to start decomposing again and producing compounds that would kill tree roots. We also wanted to ensure introduction of high organic matter. If the soil were not submerged, it wouldn’t be so critical to get to four or five on the stability test. Three is what I usually shoot for.” Particle size was another important consideration. “Most producers want to go with a half-inch size, but I always insist on three-eighths at the most — preferably a quarter-inch,” adds Craul. Samples of the compost showed it was a little coarse, but when it was mixed into soil, it worked out well. The amount of coarse material in the soil was less than five percent.”

COMPOST SOURCE

The Butler Area Sewer Authority in Pennsylvania produced the biosolids compost used for the riverfront project. Located adjacent to the city of Butler’s wastewater treatment facility, the compost site operates at about 50 percent of its 34 wet tons/day capacity. Using two workers five days/week, 4,800 cubic yards (cy) of compost were produced in 1998.

Before composting, about 15 to 20 percent lime is incorporated into the biosolids for stabilization. “Lime stabilization has a big advantage,” says John Schon, manager of the sewer authority. “There are no odors in any part of the process or in utilization.” Biosolids are dewatered to 36 to 38 percent dry solids in a plate and frame press, then loaded into a trailer-mounted SSI batch mixer for blending with amendments at a ratio of about one-third biosolids, one-third wood chips from a paper mill and one-third sawdust and finished compost. (The finished compost is needed to stimulate microbial activity curtailed by lime stabilization.) The sawdust is brought in a walking floor trailer by the authority’s main customer, which then backhauls compost.

A side conveyor on the batch mixer discharges the materials on a pile of wood chips in a 180-by-100-foot open-ended facility with a concrete floor. Static piles are formed at about ten feet wide, eight to ten feet high, and 25 feet long. Three eight-inch HDPE aeration pipes spaced about three feet apart operate primarily in negative mode during the winter and positively in the summer. “If they run positively in the winter, the exterior of the pile tends to freeze from condensation,” notes Schon. The aeration system operates automatically according to temperature. Some misting is done when necessary to bring temperatures up. After 21 days of composting, the 65 to 70 percent dry solids are taken to a 60-by-100-foot building enclosed on two sides for screening down to one-quarter or three-eighths of an inch. A minimum 30-day curing period takes place in a 180-by-100-building with one open side.

Because of the lime stabilization process, the compost “is a little low on nitrogen,” says Schon, which is why it is marketed as a soil conditioner (versus for fertilization). It is sold to topsoil manufacturers at $2/
cubic yard.

MAKING A BLEND

The Butler compost was blended for the Allegheny riverfront project by S&S Processing, a processor and marketer of soils and bark mulches (see sidebar). S&S does not accept any compost without the proper data to back it up, notes Chris Collier, company president. “On my end, I want a compost that is a tested and approved Class I product,” he says. “It has to come in very clean and be limited in smell. It also must be fine because the soil materials are rescreened and a high percentage of compost needs to stay within the mix.”

When the Butler compost arrived, it registered only a two on the Solvita test. As Craul advised, it was placed in windrows and turned every few days until it matured. “Producers move compost out as soon as possible because they don’t want to store it,” Craul says. “They may cure for 21 days, but if it’s not stored properly, it goes back to anaerobic respiration and is unstable again.”

S&S prepared three soil layers. The bottom was 500 tons of USGA-quality sand used as a fine drainage layer. The middle was a subsoil of 1,500 tons of coarse sandy loam. “I had to alter my existing sandy loam with more sand to make it coarse enough,” Collier explains. The top layer was comprised of about 200 cy of biosolids compost and 500 tons of soil.

A computer spreadsheet designed by Collier calculated how many tons of each material were needed for the soil blend. Operators meticulously scooped loads of five cy (about 6.5 tons) in metered buckets attached to loaders and dumped them on the ground in a large field. The materials were blended with the loaders before being run through a Screen Machine Maximum plant. The soil was fed through the machine’s hopper into a large hammermill with the capacity to process 300 tons/hour. From there it went up a conveyor to a five-by-ten-foot vibratory screening deck. After screening, the soil was ready for transport to the Allegheny site.

APPLICATION AND RESULTS

In October, 1998, the soil was applied by Mele and Mele & Sons of Rankin, Pennsylvania on 7,000 to 8,000 square feet to a depth of 2.5 feet. Because of a highway close to the other border of the park site, the material had to be brought in from the river. “It had to be placed in careful layers,” notes Solano. “Boulders were included; it was really tricky.” Application took about a week.

Craul visited the site as the process occurred. “I insist on being there when application takes place,” he says. “I have to be present with the authority to stop the project... It’s like an engineer. He’ll inspect the beams and everything else to make sure nobody slips in a ‘cheapie.’ I look for the amount of wood chips and the stability of the compost, which, believe it or not, you can tell with the old sniff test. If it smells like ammonia or hydrogen sulfide, that tells me it isn’t cured.” Fortunately, the compost passed muster.

Meadow plants, vines and over 500 small shade trees were planted in the soil, which has performed well despite an instance of flooding. “The growth of the plants has been phenomenal,” says Solano. “All the seeds we put in have taken off. We also used the material on the highway side of the project because we knew we would get salting from winter snow treatment and wanted well draining soil. The vines there have done unbelievably well — they have literally grown ten feet.”

In all, the soil component comprised $200,000 of the $7 million project. “For relatively little money, the soils have had one of the largest impacts because of the success of the plants they support,” says Solano. “Nobody would ever point to the park and say ‘wow, look at what those soils did,’ yet it is the soil that has allowed the park plants to thrive and give a human scale and inviting appearance. The experience of the park has been improved greatly.”