COMPOSTING: Baltimore City Turns Biosolids into Composting Success

July 1, 1999

5 Min Read
COMPOSTING: Baltimore City Turns Biosolids into Composting Success

Robin Davidov

While some communities are just starting to dabble in composting programs, Baltimore, Md., City Department of Public Works head, George Balog, was an early pioneer of successful composting. Beginning in 1985, he lay the groundwork for a biosolids composting program, constructing a facility designed to reduce waste volume, conserve landfill space and diversify its biosolids management system.

Baltimore city operates two wastewater treatment plants servicing approximately 1.4 million people in Baltimore and surrounding communities. Together, they produce approximately 170,000 wet tons of sludge per year, which prior to the composting facility, were used to improve marginal land or mixed with soil for use as a landfill cover and in landfill closures. For example, in one project, sludge was mixed with dredge soil to help grow grass on a canal's embankment.

In the early 1980s, most jurisdictions relied on land application to manage their biosolids. However, Balog did not think this approach was completely reliable - the uses were seasonal and during the winter, the city had to store the sludge in city lagoons. Furthermore, in 1989, the city began having a shortage of land application outlets. An alternative was needed, but the options - lime stabilization and land application - was a costly $75 per ton, and heat drying - the latest technology at the time - had not been proven effective. Europe and Asia had had success with in-vessel composting, so Balog chose it as the most reliable technology.

To finance and manage the new composting facility, designed to accept and process up to 210 tons (at 25 percent total solids) of cake biosolids per day, and receive sludge five days per week, Baltimore worked with the Northeast Maryland Waste Disposal Authority, Baltimore. Balog convinced the city to provide eight acres adjacent to the Hawkins Point municipal waste landfill to build it.

Research-Cottrell Inc., which later became a subsidiary of Aqua Alliance Inc. (AAI), Wakefield, Mass., formerly Air Water Technologies Inc., was selected to design, build and operate the facility. As the owner, AAI financed a $3.5 million equity contribution toward the construction project, while the Authority issued $10.6 million in bonds. Commercial operations officially began in March 1988.

Biosolids are transported from the city's wastewater treatment facility and unloaded into bins at the Baltimore City Composting Facility where they are mixed with various wood amendments.

Then, the material undergoes 14 days of in-vessel composting in two large concrete channels - each channel is divided into cells equipped with individual air flow systems. Oxygen, moisture and temperature are monitored to accelerate decomposition. The compost is turned once within the reactor during this period to further agitate and aerate the mix. A two-state, chemical mist scrubber also reduces odor and ensures the facility meets air permit requirements.

Following the in-vessel process, the compost is placed on an aerated curing pad where the temperature is increased to provide sterilization and ensure the end product meets Class A composting standards. The U.S. Environmental Protection Agency (EPA), Washington, D.C., sets the limitation (Class A) standards for compost quality, including pathogen destruction standards and vector attraction reduction.

After completing the composting cycle - a minimum of 44 days from introduction into the reactors to screening - the product is sold to topsoil blenders, nurseries, landscapers, golf courses and retail distributors.

From February 1988 to April 1988, 7,101 tons of wet sludge were processed and 13,825 cubic yards of compost were distributed. Five years later, from November 1992 to January 1993, 10,395 tons of wet sludge were processed and 5,833 cubic yards of compost were distributed. In 1995, the facility was refinanced, and the city and AAI invested an additional $5 million in the plant for capital and operational improvements.

Professional Services Group (PSG), the subsidiary of AAI that manages more than 150 water, wastewater and biosolids management facilities throughout the United States, took over the operation, maintenance and management of the facility in 1996. It implemented a Process Improvement Plan, which is the cornerstone of the facility's technical and financial success.

The most notable change was with the in-feed compost mix. PSG eliminated fresh wood chips and added double-ground pallet wood, sawdust and high carbon wood ash as amendments. This helped PSG maximize the facility's biosolids capacity and increase the percentage of biosolids within the compost mix from 45 percent to 55 percent by weight.

Although PSG invests an additional $250,000 annually for the various wood amendments, changing the mix helped reduce operating and maintenance costs from $2.5 million in fiscal year (FY) 1995 to $2.3 million in FY 1998. Process improvements, better equipment and materials procurement, and preventive maintenance also reduces costs.

Approximately 85,000 cubic yards of compost are sold annually with the brand names Eckology in Maryland and BioCom in Virginia. It also is sold as Orgro to several outlets in New York to manufacture topsoil, sod and turfgrass; propagate nursery shrubs and trees; and blend with commercial potting soils.

The cost of the compost varies, depending on the quantity purchased, from approximately $5 to $9 per cubic yard during peak season, and $3 to $7 per cubic yard during off season. It meets all requirements for Exceptional Quality Compost, which includes tests for nine metals, polychlorinated biphenyls (PCBs) and salmonella. Air quality inside the facility meets Occupational Safety and Health Administration (OSHA), Washington, D.C., and National Institute for Occupational Safety and Health (NIOSH), Washington, D.C., standards.

PPG also has focused on a marketing program that focuses on price incentives and dealership networks to accommodate for its limited storage space and seasonal demand. For example, purchasers receive volume discounts to promote high-quantity sales, and prices are reduced for consistent high-volume customers. Additionally, prices are reduced during the off season (November through February).

By focusing on its dealer network, PSG has been able to decrease its number of clients while improving sales, customer service and billing efficiencies. Furthermore, the marketing effort and product quality improvements appear to have paid off. Compost product sales volumes have soared from 20,000 cubic yards in FY 1995 to 83,000 cubic yards in FY 1998, while processing a relatively consistent quantity of biosolids. Product revenue sales also have increased more than 300 percent over the same period.

Overall, Baltimore has created a biosolids management system - including sludge composting, sludge pelletization and land application - that conserves landfill space and diversifies environmental risk.

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