Opportunity
Drinking water for Jersey City, considered some of the best in the state, flows 26 miles from the Boonton Reservoir. In 1908, water drawn from the reservoir was the first in the US to be chlorinated for a municipal water supply. The chlorination facility was designed by George W. Fuller, a pioneer in the field of sanitary engineering.
During the 1970s, Jersey City was ordered by the New Jersey Department of Environmental Protection (NJDEP) to provide filtration for its water supply system to comply with the requirements of the Safe Drinking Water Act. The 102-inch diameter aqueduct was intercepted below the reservoir and a raw water pump station was built, using six 20 million per gallon day (MGD) pumps with 250 horsepower motors. The water is pumped back uphill to a treatment plant with a capacity of over 80 MGD. The pumps consume approximately $500,000 in electricity each year.
Solution
In the 1980s, several studies were performed to investigate the potential for generating hydropower from the head of the reservoir and to investigate the ability to flow water from the reservoir to the treatment plant by gravity. At the time, the economics did not favor investments in hydropower nor gravity flow options, and the control operations for gravity flow were considered complex.
In 2011, Mott MacDonald revisited the past studies, and evaluated the economics of the solution with the current low interest rates for loans and significantly higher electrical costs. We worked with the JCMUA to review trends in reservoir elevation since 1955, and found that for eight or nine months of the year, the head in the reservoir could supply water by gravity alone to the treatment plant, using an 84-inch-diameter bypass pipe. Advancements in Supervisory Control and Data Acquisition (SCADA) to provide flow pacing control using modulating valves increased the feasibility of the project.
In 2012, JCMUA retained Mott MacDonald to provide engineering design, permitting, bid, and construction phase services for the Gravity Supply Project. United Water, the operator of the system, provided critical assistance and support for the project. Scafar Contracting of Newark was the successful bidder for construction services.
Mott MacDonald performed computational fluid dynamic (CFD) modelling for the complex geometry of the upper and lower gate houses, to further refine headloss calculations developed using empirical methods. We designed 72-inch-diameter header piping and valving for the lower gate house to allow a flexible supply of water to the treatment plant. Based on seasonal flow demands and the elevation of the reservoir, pumped water, gravity-fed water, or a mixed supply can be provided.
Mott MacDonald designed a treatment component with a firm capacity of 100 MGD, a three-process-train rapid mix basin with variable-speed mixing capability, and a new traveling screen system. We also designed a major upgrade to the treatment plant’s instrumentation. This includes a redundant, fiber-optic, Ethernet-based communication protocol to provide chemical flow pacing, valve modulation, and pump station control. The front-end Supervisory Control and Data Acquisition System (SCADA) was also enhanced.
The new gravity system required 2,000 feet of 72-inch- and 84-inch-diameter prestressed concrete cylinder pipe. A temporary bypass system was used to maintain the construction schedule while ensuring partial flow to the treatment plant.
Outcome
Because the project reduces the use of electricity and the plant’s carbon footprint, we were able to secure principal forgiveness for 20% of the loan from the New Jersey Environmental Infrastructure Trust. This resulted in a grant of $1.8 million on a $9 million project.
Average savings in electricity are calculated at $375,000 per year. For the main components of the project, an 84-inch-diameter pipeline with an estimated life of more than 100 years, the return on investment was immediate, since the electrical savings will exceed the debt service on this portion of the 20-year project loan. Initial trials of the new system showed better than anticipated results, indicating an even greater potential for energy savings.
The project provides additional flexibility in operations. During low-demand periods, when the reservoir level is high, portions of the existing aqueduct and raw-water pump station can be taken offline for routine maintenance. The project also extends the life of the existing pump and motor equipment. The introduction of chemical flow pacing for coagulants optimizes raw-water treatment chemistry.
Mott MacDonald and JCMUA are currently collaborating on a project to harness hydropower from the spillway of the reservoir’s dam. Regulations enacted in July 2013 make this a financially attractive plan, and it will allow the treatment plant to have a “net zero” carbon footprint in the future.