|Conceptual plant components (expand image)
|Conceptual plant enclosed within a building (expand image)
As we selected a site, we hired Natural Systems Utilities (NSU), a wastewater treatment design/build/operate firm from New Jersey, and they began designing a plant for the location. We worked with NSU on our Littleton Smart Sewer project. Plant design and green infrastructure are site specific, so system components are adaptable. For example, components can be on different sites but connected. Creative application of design and engineering options for distributed plants is essential when working in challenging environments like dense urban areas, and NSU is among the most creative at designs given site constraints.
For our first district, Site 1, a mined wastewater flow of two million gallons a day was selected given site parameters and available sewer flow. For this district, plant components include a membrane bio-reactor to initially clean the incoming water. Organics are then sent to an anaerobic digester where they are treated to create and capture methane. Cleaned reclaimed water is sent to a heat exchanger to capture the water’s heat energy, and that energy is used to power the plant and sold to heat and cool surrounding buildings. Some of the water is captured for reuse by surrounding facilities requiring process and cooling water. Fully treated organics are captured for sale and reuse.
At Site 1, food waste will also be collected. The design calls for a food waste processing area where packaging and non-food elements are removed. A second separate anaerobic digester is used to create methane from the food waste, which is then combined with the wastewater methane for burning to generate electricity. The treated food organics are dried for sale. These organics are kept separate from the wastewater organics because they produce a higher quality fertilizer, without some of the metals common to wastewater organics.
A list of the environmental benefits of such a plant is quite long. It includes generation of renewable electric and thermal heating and cooling energy offsetting fossil fuel use, reducing greenhouse gas emissions, reducing vehicle miles traveled by trucks transporting food waste, reclaiming water and organics for local use, and a distributed source of energy and water supplying local demand.
CRWA also designed interconnected green infrastructure for the district. As our feasibility study involves a plant in the midst of an existing centralized system, it could potentially sell 100 percent of the water it reclaims. The problem with that, however, is that we are not reserving water for replenishment to the natural water cycle. Because the plant we are currently investigating is intended to demonstrate the value of our Restore Nature approach, we are reserving 25 percent of the treated water for discharge to the local environment.
One reason CRWA retained NSU to do the work is that the firm is in the business of designing, building, and operating such plants around the world. Their proprietary financial models for returns on investment are a key component to their work. Roughly, the plant conceptualized for Site 1 would cost $46 million to construct, and return approximately $6 million in revenue annually on sales of energy, reclaimed water, and organics. This revenue stream also includes the collection of “tipping fees” from haulers bringing food waste to the plant for treatment.
We agreed to examine the finances for distributed plant construction without charging for wastewater treatment. On that basis, the Site 1 plant is viable over a 20 year bond at a Massachusetts State Revolving Fund interest rate of up to 2 percent. At greater interest rates and site acquisition costs, the plant remains viable if it is allowed to collect up to 20 percent of the 100 percent value of wastewater treatment.
There are important reasons we are examining the finances in this way. Currently, about 62 percent of the revenues the Massachusetts Water Resource Authority (MWRA) collects goes to bond debt borrowed to build Deer Island and its associated facilities. Though that debt begins to diminish incrementally in 2018, the entire debt obligation is not met until 2038.
Over 20 years of transformation, our move away from centralized plants to distributed treatment and restored natural hydrology would require careful balancing. From all we know at this writing, however, the transformation CRWA envisions is absolutely possible.