In-State Facilities Incinerate R.I.'s Wastewater Sludge

By TIM FAULKNER/ecoRI News staff

Did you know Rhode Island has two large-scale incinerators? Located in Cranston and Woonsocket, both are sludge incinerators that burn most of the state’s residue from wastewater treatment plants.

The two municipally owned incinerators accept sewage waste from across New England, in both liquid and dry form, known as sludge cake. The main benefit of incineration, according to the state Department of Environmental Management (DEM), is keeping the sludge from taking up space at the Central Landfill in Johnston.

The DEM regulates sludge shipments and emissions.

States without sludge incinerators, such as Maine, New Hampshire and Vermont, spray their sludge on crops as fertilizer.

The Woonsocket facility opened in 1973. The main hearth was replaced in 2007 and now receives about 2,780 tons of wet and dry sludge each month, most of it from outside the city. It also accepts municipal sludge from neighboring states and some commercial waste, such as waste from dairy farms.

The Cranston incinerator opened in 1985 and today processes about 1,225 tons of liquid sludge monthly. Most of the sludge arrives from its adjoining wastewater treatment plant. Both the sludge incinerator and the wastewater facility are run by the French company Veolia Water.

Houston-based Synagro runs the Woonsocket sludge incinerator. The company, which recently received approval to emerge from bankruptcy protection, was reprimanded in July for shipping 127 tons of wet and dry sludge to the Central Landfill while work was being done to the Woonsocket plant. DEM said that waste should have been sent to another Synagro facility in a nearby state. The company wasn't fined by DEM or the Rhode Island Resource Recovery Corporation, which manages the state landfill, for the violation.

It’s worth noting that this illicit sludge wasn't the raw soup shipped to the incinerators in 9,000-gallon tanker trucks. Instead, it was a moist, compost-like material.

Sludge shipping and burning is highly regulated, and facilities are tested at least annually. Emissions and fly ash are monitored for 200 compounds, including toxins. The DEM’s Office of Air Resources oversees the monitoring and testing program, which is conducted independently and paid for by incinerator operators. DEM hasn't found significant violations in the past three years.

There were several odor complaints earlier this year from neighbors of the Woonsocket incinerator, but complaints dropped after new odor scrubbers were installed in May. To minimize odors, trucks are also required to unload in a closed garage.

Woonsocket officials say many of the odor complaints are caused by trucks venting gas from their tank as they drive through residential areas. The city doesn't totally hold its nose about the odor, however, as it receives annual royalties of about $550,000 from the facility — its share of the revenue from sludge shipment fees.

The city runs a hotline for odor complaints.

The Woonsocket facility uses a sand-fluidized bed reactor — a system that is considered more efficient through its use of natural gas combustion. By the end of the year, Synagro plans to open a 2-megawatt electric generator running off the incinerator's heat. The energy is expected to power the incinerator and new treatment systems at the wastewater treatment plant.

The Cranston facility operates a six-level, multiple-hearth incinerator. The burned sludge creates an ash that is hauled to the Central Landfill for daily cover.

What is sludge? Up to 95 percent is water. But it starts as wastewater, which is a mix of food, paper, diapers, plant mater, feces, condoms, sanitary napkins, paints, pesticides, bacteria, pathogens, pharmaceuticals, sand, metal particles, road salt, insects and gases. The inorganic material is removed at the wastewater treatment facility and sent to the Central Landfill.

Rhode Island businesses and residences with sewer connections generate about 100 million gallons of wastewater a day, according to DEM. This waste is treated at 19 publicly owned treatment facilities in the state. After treatment, the effluent discharges into a nearby waterway. The Blackstone River for Woonsocket and the Pawtuxet River for Cranston. RI wastewater treatment facilities generate some 30,000 dry tons of sludge annually. On average, 75 percent of the sludge is burned in the state’s incinerators, 9 percent is composted as fertilizer, 2 percent goes to the Central Landfill and 13 percent gets treated and is shipped out of state as fertilizer.

Medical waste incinerators run by hospitals were closed by DEM in the 1990s. Hazardous medical waste is shipped out of state for disposal.