The largest single construction project ever undertaken by the City of Portsmouth, New Hampshire, has passed the halfway mark as Methuen Construction transforms the outmoded wastewater plant on historic Peirce Island from a primary treatment plant to a modern secondary treatment facility meeting strict federal environmental standards. 

Methuen Construction was awarded the four-year, $72 million contract ($74.2M with change orders) and began work on the island in September 2016. Terry Desmarais, P.E., City Engineer, is the project contact.

Some of the upgrades to the 50-year-old plant include constructing the following processes and the structures to house them: new headworks, rotary screws for solids dewatering, and biologically active filter (BAF) using Kruger BIOSTYR technology for secondary treatment. 

As designed by engineering consultant AECOM, the upgraded facility will provide secondary treatment and nitrogen removal for an average daily wastewater flow of about 6 million gallons.

While some of the existing plant components will remain under the new design, the additions are significant in order to bring the plant up to secondary treatment standards that employ sophisticated technology and equipment, thus the $74 million cost. 

 

What’s the Difference?

In primary treatment, wastewater flows through large sedimentation tanks or primary clarifiers where heavy solids settle to the bottom while oil, grease and lighter solids float to the surface and are removed. The remaining liquid is usually discharged or flows to secondary treatment facilities. 

In secondary treatment, dissolved and suspended biological matter are removed from the wastewater by water-borne micro-organisms. Most municipal plants like the upgraded Portsmouth facility treat settled wastewater using an aerobic biological process. This may require a separation process to remove the micro-organisms from the treated water before it is discharged into a water course or piped to further processing called advanced or tertiary treatment. Treated water is often disinfected, usually with chlorine, before it is discharged though an outfall sewer into a watercourse.

According to the city’s Department of Public Works, the existing Peirce Island treatment facility operations include grit removal, chemically enhanced primary treatment, disinfection with liquid chlorine, and de-chlorination. Treated wastewater effluent is discharged to the Piscataqua River through a 24-inch-diameter single port outfall. Sludge is removed from the primary clarifiers, condensed in a gravity thickener, and stored in tanks. The thickened sludge is dewatered by a rotary press and disposed of at a landfill.

 

Sophisticated Secondary Treatment

Compare the city’s primary treatment plant with the more complicated processing that will take place due to the upgrades underway by Methuen Construction. This includes a two-stage, biological aerated BAF Kruger BIOSTYR filter system that will not only remove dissolved and suspended biological matter but also eliminate nitrogen, a major nutrient present in wastewater. This filtration system consists of a number of large cells containing billions of polystyrene granules – tiny, white beads resembling “peanut packing foam” used in shipping boxes. A film of micro-organisms treating the wastewater sticks to the granules, and since the combined surface area of all these beads is very large, treatment proceeds at a very high rate.

The new treatment system will also have raw wastewater screening, aerated grit removal, primary clarification, disinfection with liquid chlorine, and de-chlorination. Sludge produced by the upgraded treatment processing facility will be thickened in two gravity thickeners, stored in aerated storage tanks, dewatered on rotary screw presses, and finally disposed of at a landfill.

 

Protecting the River

The capability of the BIOSTYR BAF to remove nitrogen from wastewater is essential for protecting the quality of the Piscataqua River, which receives treated effluent from the plant. Elevated levels of nitrogen help cause eutrophication, a process in which an overabundance of nutrients stimulates excessive plant growth such as algal blooms and cyanobacteria. The decomposition of the algae by bacteria uses up the oxygen in the water, causing other organisms to die. This creates a Catch-22 cycle whereby the death of other organisms adds still more organic matter for the bacteria to decompose, and so on. 

By preventing nitrogen from entering the river, DPW notes, the upgrade of the wastewater treatment facility will actually improve the quality of effluent discharged to the river. Furthermore, stormwater runoff from the plant site will be improved since stormwater treatment capability is being added as part of the upgrade. 

 

Historic Island Location        

The current plant shares the 27-acre Peirce Island with the remains of Fort Washington, built in 1775 to control the Piscataqua River at "the Narrows," and to provide crossfire with Fort Sullivan directly across the river on Seavey's Island. The fort was reactivated during the War of 1812 when British warships blockaded the New England coast. 

Most of the fort site was cleared in 1963 to allow construction of the present-day wastewater plant, with the remaining portion of the military site later enshrined by historical markers for the fort as well as markers for the Portsmouth Naval Shipyard, established in 1800 across the river.

Despite the scope of construction at the island site, the upgraded treatment facility is being built within the limits of the existing plant. As part of the contract, two existing facilities were demolished and several new buildings were constructed. Most of the buildings have a brick façade and architectural features. Part of the existing peripheral fence has been temporarily relocated to expedite construction, and extensive landscaping will take place with layered configurations incorporating trees and shrubs and exterior crawling ivy type plants.

The new BAF building is the most visible, standing approximately 25 feet above existing grade. What’s more, some 15 feet of the structure is below grade. The walls of the building are massive 2-foot-thick concrete structures measuring about 40 feet tall, with the longest running more than 200 feet. A large portion of the 18,000 cubic yards of concrete required for the job was used to construct this filter building. 

 

Massive Concrete Walls

VR Concrete Inc. based in Columbia, New Hampshire, is responsible for forming and placing all concrete, with crews supervised by Project Superintendent Bob McKinnon and Project Manager Rob Ayotte. Ready mix concrete is supplied by J.G. MacLellan Concrete Co. Inc. with 90 percent of ready mix being pumped to crews and the remainder delivered using buckets and either Delmag or Grove hydraulic cranes.   

Independent Concrete Pumping is pumping ready mix with several of its Schwing pumps including a 42-meter rig.

According to VR Concrete’s Ayotte, they are using AH Harris Series 1500 gang forms for the 40-foot-tall walls, and EFCO shoring towers to support any suspended floor slabs. About 1,600 tons of rebars up to #12 are needed for the job. Rebars are manufactured by Gerdau Long Steel North America.

Ayotte said crews are waterproofing walls that enclose areas containing liquids with PENETRON, a material said to penetrate deeply into the concrete structure, filling micro-cracks, pores and capillaries with an insoluble crystalline formation. The manufacturer says the material prevents water and water-borne chemicals from entering, even under high hydrostatic pressure. In addition, any cracks that do develop during the lifetime of the concrete are resealed by the waterproofing agent, resulting in permanent concrete protection, according to the manufacturer.

VR Concrete had completed about 65 percent of its contract at the time of this report, according to Ayotte. 

 

Final Completion

The entire Peirce Island Wastewater Treatment Facility Upgrade is scheduled for substantial completion by May 31, 2020, and final completion by August 30, 2020. The final estimated cost, including construction, engineering and contingencies, is expected to be $92 million. 

The updated plant is the responsibility of the Department of Public Works’ Sewer Division. The Division is also responsible for the Pease Wastewater Treatment Facility, 120 miles of sewer, 20 wastewater pump stations, and three combined sewer outflows permitted by the Environmental Protection Agency.