Opportunity
Trenton Water Works, a division of the City of Trenton’s Department of Public Works, provides water service to approximately 255,000 people through 60,000 metered connections in the city and surrounding communities.
The city’s supply of drinking water comes entirely from the Delaware River, and is treated by a water filtration plant with a peak capacity of approximately 60 million gallons per day (MGD). The system also includes an open, unfinished water reservoir, three booster pumping stations, and five storage tanks.
In 1977, the City of Trenton entered into an Administrative Consent Order with the New Jersey Department of Environmental Protection to eliminate discharges to the Delaware River from the filtration plant’s filter backwash and sedimentation basin operations.
Solution
Mott MacDonald has used its expertise to assess and upgrade every aspect of the water distribution system, to model it using sophisticated software, and to prepare a master plan for its continuing maintenance and improvement.
Our services have included plans and specifications, site evaluation, hydraulic modeling, pipe inspection and analysis, detailed design, construction services, plant operations, staff training, cost estimates, loan applications, bid phase services, permitting, and development of strategic plans, asset management plans, and a master plan for the system.
Overall water system
Water distribution system analysis and rehabilitation
For a detailed analysis of the distribution system, we developed a computer model of the entire system, including all pipes of 4 inches (10 cm) diameter and larger. The model included approximately 6,500 pipes and 4,300 junctions, as well as storage facilities and booster pumps. GIS techniques were used to input metered production and consumption records.
The model was calibrated to existing system operating conditions and more than 30 fire-hydrant flow tests conducted by Mott MacDonald. We performed analyses to identify areas of deficient fire flows, low pressure, and potential water quality problems due to poor circulation.
We then developed a plan of improvements to address these deficiencies, and used the model to help the City prioritize its ongoing water main rehabilitation program.
Trenton water system master plan
In 2010, we were retained to develop a ten-year, $35 million Capital Improvement Plan to include improvements to the pumping station and reservoir, and a review of renewal needs for 64 miles (103 kilometers) of distribution system piping. The project also includes a detailed hydraulic investigation to determine the feasibility of selling infrastructure for certain townships to another water utility.
A critical component of the project was the development of an Asset Management Plan. This addresses the improvement of instrumentation to improve day-to-day operations and maintenance and provide better data collecting and information processing for long-term water system planning.
Water filtration
Mechanical dewatering facility
In 1990, Killam Associates (acquired by Mott MacDonald in 1996) was retained by the City of Trenton to determine if residuals from the water filtration plant could be directed to the sanitary sewer system. Killam determined that this would seriously overload the wastewater plant and inhibit the operations of the regional sludge processing facility.
Continuing this work, we were retained to prepare plans and specifications and secure all permits for a mechanical dewatering facility using belt filter presses. Our engineering staff operated and maintained the facility for approximately one year after completion, and provided training and instruction to Trenton Water Works personnel.
Asbestos management services
Mott MacDonald was retained by the City of Trenton to perform an asbestos survey of the water filtration plant before plans were developed for a full renovation of the plant. We performed a detailed survey of the facility, obtained bulk samples of suspect material in accordance with EPA protocol, and delivered them to an accredited laboratory.
Based on the survey and analysis, we prepared specifications for the removal of asbestos-containing pipe and boiler insulation and window-frame caulking. We obtained technical variances from the local building code authority to allow the abatement to occur without closing down the facility.
Water storage
Alternatives for covering Pennington Reservoir
Trenton’s Pennington Reservoir, constructed in 1896, consists of concrete walls and earthen embankments with an average height of 41 feet (12.5 meters). The reservoir has a surface area of 7 acres (2.8 hectares) and a reported capacity of approximately 104 MGD.
Responding to concerns over water quality, Trenton Water Works retained us to evaluate options for covering the reservoir. We analyzed the need for storage in individual pressure zones to determine if the reservoir could be removed from service. Also considered were requirements for fire reserve, emergency reserve, and future growth in demand.
We evaluated options including installing a floating cover or rigid cover, constructing new tanks within the footprint of the existing reservoir, and abandoning the existing reservoir and constructing new storage tanks elsewhere.
The evaluation concluded that installing a floating cover was the most cost-effective solution. We designed temporary storage facilities to be used when the reservoir was out of service. These were two 1.6-MGD glass-lined tanks connected with existing transmission mains and the Central Pumping Station by ductile iron pipes 36 inches (0.9 meters) in diameter. Likewise, the floating cover, including repairs to the reservoir, has been designed by Mott MacDonald.
Brandon Farms water storage tank
Plans for a large residential development in Hopewell, NJ, created the need for a new pressure zone with booster pumping and storage facilities, including an elevated tank. A Mott MacDonald study confirmed that elevated storage was needed. In response to community concerns, we used computer modeling to determine the minimum adequate size of the tank.
When community opposition continued, we identified and evaluated a number of alternative sites. Trenton Water Works chose to proceed with the original site, and expert testimony that we provided helped the utility gain approval for the project. We prepared a detailed design for a fluted-column tank holding 600,000 gallons (2.3 million liters).
Whitehorse water storage tank
One of Trenton’s four central-zone storage tanks, the million-gallon (3.8-million-liter) Whitehorse Tank has a high-water level elevation about 10 feet (3 meters) lower than the other tanks, causing inadequate circulation of water.
We conducted a hydraulic analysis to determine the optimum height of the Whitehorse Tank at its current location and at alternate sites. We also evaluated the feasibility of raising the existing tank by jacking it up and extending its legs.
Water conveyance
Emergency interconnection feasibility study and planning
In the past, outages have severely impacted the Trenton Water Works distribution system. The neighboring New Jersey–American Water, with multiple sources and interconnections, was considered less vulnerable, but in 1999 a major flood severely impacted its supply capability.
The two systems have had a limited-capacity interconnection for many years. A System Reliability Study completed by Mott MacDonald in 2002 recommended upgrading the interconnection. This recommendation, combined with concerns over security and terrorism, convinced the two systems to improve their interconnection.
We were retained to perform a feasibility study for providing two-way emergency transfer capacity of at least 10 MGD, with the capability of increasing to 20 MGD in the future. We used computer modeling to perform hydraulic analyses, identify improvements, and consider alternatives. We also helped develop agreements between the two utilities for the implementation, operation, and maintenance of the interconnection.
Both utilities retained us to perform design, permitting, bid, and construction phase services for two booster stations and associated water mains. The two stations were designed as prefabricated underground facilities, and among the highest-capacity stations of this type ever built in New Jersey.
Cleaning and lining of water mains
Approximately 200 miles (322 kilometers) of Trenton’s water mains are unlined cast iron, some of it more than 69 years old. To address insufficient fire flows, low pressure, and water quality complaints, we were retained to perform a hydraulic study using its computer model of the system to determine a cost-effective program of upgrades and rehabilitation. The model was field-verified in a series of fire flow and coefficient tests.
We recommended a program of main replacement, abandonment of 4-inch (10-cm) mains, and cleaning and cement mortaring of unlined cast-iron mains. The project was divided into multiple construction contracts, and we prepared separate sets of contract documents for each contract.
We also prepared engineering documents for the utility to apply for state revolving-loan funding. We obtained environmental, cultural resources, and road-restoration permits, and provided bid phase service and complete construction phase services. The program, which was started in the 1990s, continues to the present.
48-Inch ascending transmission main
Trenton Water Works’ 48-inch-diameter (1.2-meter) ascending main is one of two main arteries connecting the water filtration plant with the central storage reservoir. Built in 1907 and repaired with cement mortar lining in 1940, the ascending main consists of 5,300 feet (1.6 kilometers) of steel lock-bar pipe and 900 feet (274 meters) of cast-iron pipe.
The ascending main has suffered numerous leaks, some of which have disrupted roadways and created a safety hazard. Leakage was estimated at 300,000 gallons (1.1 million liters) per day.
Mott MacDonald was retained to conduct a field investigation and to design improvements to the ascending main. We provided an internal and external pipe investigation, and examined cracks to see if the main was overstressed. We determined that rehabilitation would be more cost-effective than replacement or no action, and recommended the sealing of all internal pipe joints, installation of reinforced cement mortar lining, and spall and steel-pipe repairs
Central Pumping Station improvements
Constructed in 1957, Trenton’s Central Pumping Station directly serves 80% of Trenton Water Works’ 60,000 customers. Three of its four pumps have constant-speed electric motors, and the fourth runs on natural gas.
Following an incident when two electrical service lines failed, we were retained to design improvements and address vulnerabilities identified in an evaluation conducted in accordance with the Public Health Security and Bioterrorism Preparedness and Response Act of 2002.
In 2013, we evaluated various alternatives and submitted a design for the replacement of the natural gas engine with an electric motor, the use of two generators (one available as a backup), and variable-speed drives on the two larger pumping units.
Outcome
Over a period of 20 years, we have designed capital improvement projects for Trenton Water Works totaling $190 million.
Through a variety of services, we have helped Trenton Water Works maintain fire flows and water pressure, reduce leakage, remediate asbestos hazards, safeguard the distribution system against disruption, address community concerns, obtain permits, manage its assets effectively, and identify and develop cost-effective alternatives. Our detailed model of the entire Trenton water distribution system will continue to save the city time and money into the future.