by Anne Watson; photos by Carla Occaso
MONTPELIER — Wastewater treatment is typically one of the most energy-consuming processes for a municipality, but Montpelier’s Water Resource Recovery Facility at Dog River may soon be a remarkable exception. Earlier this month, the Montpelier City Council approved a project development agreement with Energy Systems Group, which outlined a process that may lead the Dog River facility to become not just “net zero energy,” but “net negative.” A net negative facility would mean that it produced more energy than it consumed. Our wastewater treatment plant could end up generating heating, electricity or fuel for vehicles.
How did we get here?
The award-winning facility’s management has already taken some huge steps toward energy efficiency and renewable generation. They added a 62.4 kW solar array onsite in 2010. Since 2008, they have made at least 16 small energy improvements that have had substantial results. Between 2008 and 2014 their electric load was reduced by 34 percent and between 2010 and 2016 they cut their gasoline usage in half.
The plant has always used some of the methane, which comes from fermenting organic waste, to heat its primary digester year round. Some of the excess methane was flared, but some of the excess methane was just vented into the atmosphere. This was bad not only because that methane could have been useful as a fuel, but also because methane is a particularly potent greenhouse gas. In 2015, the flare was upgraded so gas was no longer vented directly into the atmosphere. At this point the methane is either burned for heating or it’s flared.
Now, some of the facility’s regular equipment is at the end of its engineered life and needs to be replaced. Along with necessary equipment replacements, Energy Systems Group will be looking into the feasibility of bringing in additional contracts for organic waste in order to produce biogas, mostly made of methane, which would require an upgrade to their existing digesters. This methane could then be used to heat buildings, power vehicles, dry biosolids (organic matter recycled from sewage) produce electricity, or some combination of those things.
If Energy Systems Group is not able to find sufficient waste streams for the energy generation portion, the City will just go out to bid for conventional equipment replacement, which should also save the City money through increased efficiency. This work by itself is likely to require a bond for $5 million. However, if they are able to find sufficient waste streams to make the upgrade to their digesters worthwhile, then they will come back with a performance based contract proposal that will require a roughly $13 million dollar bond. With either possibility the bonds will be paid off through the energy savings or the revenue from energy generation, and not felt by the ratepayers of Montpelier. If they do upgrade the digesters and for some reason the savings or revenue is not fully realized, Energy Systems Group will pay back the difference between the bond payment and the actual savings and revenue. They have taken out an additional bond to help pay our bond for just that possibility.
In short, here are the gates this project will need to pass through in order for the wastewater plant to be net negative:
- Energy Systems Group will need to find sufficient waste streams to warrant upgrading their existing digesters to produce biogas. They’ll also need to find some use for that biogas. This will all be packaged into a performance based contract.
- The City Council will need to approve the performance based contract with Energy Systems Group.
- On Town Meeting Day, the voters of Montpelier will need to approve the bond required by the performance based contract. Don’t worry, I’ll remind you about this project when the vote approaches.
Montpelier’s Energy Story
This is a part of a larger story about how the City of Montpelier is changing its energy portfolio to be greener. Between 2011 and 2015, Montpelier reduced its greenhouse gas emissions by 56 percent. This was largely due to the improvements at the Dog River facility and from heating our municipal buildings through the district heat plant that burns wood chips. Since then we’ve signed on to a one megawatt net-metered solar array that covers about 55 percent of the City’s municipal electric load. That saves the City of Montpelier about $50,000 a year on the their electric bill. This one megawatt of power is composed of 500kW on Log Road in Montpelier and 500kW from an array in Sharon.
The prospect of generating biogas to make the Water Resource Recovery Facility an energy producer rather than a net consumer, is of course very exciting. The council will need to look carefully at the details of the proposal put forth by Energy Systems Group in December, but with any luck, we’ll have a workable proposal for Montpelier and we’ll be able to take the next step towards becoming a sustainable city.