This $61.6-million project upgraded the facilities and systems that control hydrogen sulfide gases produced during waste breakdown processes. Aside from their unpleasant smell, these gases can be harmful in high concentrations—underscoring the critical need for optimized HVAC systems to provide fresh air in work areas to maintain plant-wide safety.

Along with requiring a strict sequential construction process to maintain seamless treatment operations throughout demolition and construction, all equipment slated for replacement was situated underground. Major elements included four 55,000-cu-ft-per-minute fans, installation of fiberglass carbon-containing tanks, custom air handling units and three new oil-fired boilers with two new 20,000-gallon diesel tanks. Most of the facility’s ductwork required replacement as well due to long-term exposure to the highly corrosive gases. Compounding these challenges were pandemic-related supply chain delays that required frequent sequencing and schedule adjustments throughout the project.

Photo courtesy Walsh Construction

To access the work area, the project team temporarily replaced existing precast roof planks with weatherproof panels that could be removed and reset each day. Radio communication and hand signals helped crane operators overcome the jobsite’s extreme visibility limitations, enabling them to precisely maneuver large sections of materials and equipment into the underground spaces.

Comprehensive daily work plans, contingency strategies and extensive task hazard analyses helped protect workers from risks such as high concentrations of gases, treatment process chemicals and silica dust from demolition of pedestals, bases and slabs. Complex confined space operations, such as installing equipment in effluent channels and work inside new tanks, demanded flawless execution with no room for error.

Photo courtesy Walsh Construction

While the bulk of construction took place underground, the project team was cognizant of its effects on the surface. As Nut Island is part of the Boston Harbor Islands National and State Park, visitors frequently take advantage of well-maintained gardens and walkways surrounding the treatment facility.

Photo courtesy Walsh Construction

Privacy fences kept most of the work out of sight, while strategically locating the crane in a parking lot allowed the walking path to remain open throughout the work. The project team paid strict attention to restoring disrupted landscaping, using only native plantings and adhering to the existing aesthetic.