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Biosolids Drying Facility
Recycled process gas adds safety measure and reduces fuel costs

The Greater Lawrence Sanitary District (GLSD) owns and operates a 40 mgd wastewater treatment facility that serves a population of about 200,000 in five communities. GLSD had relied on short-term private contracts to transport and dispose of its wastewater sludge at off-site facilities.

In the late 1990s the District completed an evaluation of its long term biosolids management needs and selected three new processes: anaerobic digestion of the raw biosolids, centrifuge dewatering, and thermal drying of the digested biosolids.

Wright-Pierce was the engineer for the design-build-operate team, led by New England Fertilizer Company, that developed the new biosolids drying facility. GLSD’s new biosolids management system is cost-effective an has minimal environmental impact. The facility uses gas from the anaerobic digesters as the primary fuel for the drying operations reducing greenhouse gas emissions and fuel costs.

The process consists of two dryer trains. Each triple pass rotary drum dryer is capable of evaporating up to 6,000 pounds of water per hour. The anaerobically-digested biosolids are conveyed directly from the centrifuge dewatering operation to the feed hoppers for the drying operation. The product is granular with an average solids content of 96%, and suitable for use as fertilizer.

Another unique feature of the facility is the recycling of 80% of its process gas. Recycled gas adds a safety measure by generating an inert (oxygen depleted) atmosphere in the dryer system. In addition, the resulting hot condensate is used to heat the raw influent sludge to the digesters. With this energy sharing between the digester and drying,
the facilities require no supplemental fuel during the summer and only a low level during the winter.

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Project at a Glance

Background

Following an extensive evaluation of long term biosolids management for this 40 mgd WWTF serving five suburban Boston communities, the Greater Lawrence Sanitary District (GLSD) selected three new processes for managing their biosolids needs:

  • anaerobic digestion of the raw biosolids
  • centrifuge dewatering
  • thermal drying of the digested biosolids

Highlights

  • Design-Build-Operate Team
  • At design capacity, the GLSD facility generates 38 dry tons per day of a Class A biosolids product.
  • It is housed in its own building, with a relatively small footprint, on the site of the wastewater treatment plant.
  • The facility uses digester gas from the anaerobic digesters as the primary fuel for both the dryer and the RTOs.
  • Process gas is recycled resulting in lower air emission, reduced fuel costs and smaller equipment footprints.