Watts from Wastewater
Of New Jersey’s 28 wastewater treatment plants employing anaerobic digestion (AD) systems, at least 10 have infrastructure in place to utilize the resulting biogas, many producing heat and/or power.
It may not seem like an overwhelmingly high count, but the number is more impressive when also considering the fact that the entire country has a total of 1,500 wastewater treatment facility AD systems. With only 250 of those wastewater treatment plants using their biogas, New Jersey’s position is suddenly much more progressive.
“It’s certainly in the upper-middle third of all states,” says Patrick Serfass, executive director of the American Biogas Council. New Jersey is close behind its fellow northeastern state of New York, which has 16 wastewater treatment AD systems that use their biogas, and behind Connecticut, at 21. Oregon has 22 that use the methane they produce, and California tips the scale at 50, with several others having one or two, Serfass cites.
The remaining plants with AD systems that choose not to use their methane for energy, simply employ the process because it reduces the volume of the waste, much like a cow’s stomach digests food, Serfass says. In addition, it improves the quality of the waste on the backend. “The active digestion upgrades the product that you’re digesting,” he says. Typically, without an anaerobic digester, the material is classified as a Class B biosolid, which is a hazardous material. “If you take that sludge and put it through a digester, you make heat, you make biogas, and then you make digestate, which is both in a liquid and solid form.” The solid that comes out of the digester, in comparison to the solid that comes out of the primary wastewater treatment, is a Class A biosolid and nonhazardous. It can be sold as compost or fertilizer, in addition to occupying a much lower volume.
“Therefore, depending upon how much biogas they’re making and what the will is of the municipality, they may or may not even want to use the biogas, which we think is a total waste,” Serfass says. “But that’s their own business decision, and we’re trying to encourage folks to use that biogas.”
The New Jersey Board of Public Utilities agrees that unused biogas is a waste and is tasking the state’s newly formed Renewable Natural Gas Work Group with three assignments: identify the number of locations currently producing biogas through AD, identify plants without digesters that would be good candidates for them, and determine how much unused biogas is currently available. The group is divided into subgroups that focus on three potential areas: wastewater treatment plants, landfills, and freestanding locations such as farms or food processors. “These three scenarios are all at different stages of development,” says group co-chair Dave Specca, who is also the assistant director for bioenergy technologies at the Rutgers EcoComplex. Besides the recognition of the potential for increased use of existing biogas, the research is also being conducted based on the understanding that New Jersey could produce much more usable biogas than it currently does, he says. “We also see that there are new technologies being developed on a smaller scale for things like compressed natural gas.”
The work group’s study might even show that more than 10 of the 28 facilities that currently employ AD use their biogas, according to Richard Kunze, member of the wastewater treatment branch of the group, and director of technical services for Ocean County Utilities Authority. Kunze speculates that the study will show biogas use by 60 percent or more.
The study is ongoing, but landfills are likely the furthest along in AD development potential, as most already have gas collection motors to produce electricity, Specca says. But second in line are wastewater treatment plants.
In fact, that enormous potential prompted the League of Women Voters of New Jersey to dedicate an entire track to the hot topic at its April 2010 conference. The league produced a report and presented its findings at the event, spurring a slew of follow-up inquiries from several wastewater officials in the state. “What prompted it was our local sewerage facility recently bought a special digester to speed up the process of sludge digestion,” says Eleanor Gruber, who led the team dedicated to researching the sewage sludge-to-energy industry for the conference. “We looked online to find out that we were not alone in the state in doing this.”
“Wastewater treatment plants are good candidate sites for a lot of this work,” Specca says. “They already have the infrastructure and a way to handle the digestate.” Many have laboratories and crucial on-site expertise, as well. The work group will come up with models to illustrate how the systems would work at each of the three scenario locations, using what the operations already have on-site, as well as additional potential sources such as food waste.
Food scraps, fats, oils and grease produce the most biogas of any digestible feedstock, Serfass says. “If you happen to have a good supply of food scraps or fats, oils, or greases that your wastewater facility is set up to process, you might produce a lot more biogas, and you might get a tipping fee from whoever is giving you their waste because it’s cheaper for them to give the waste to a wastewater facility than it is to put it in a landfill.”
Such opportunities pile on even more potential for AD systems in New Jersey’s wastewater treatment sector, especially for smaller plants that might not have enough input on their own for biogas-production components to be advantageous. The general rule of thumb for the minimum size of a facility to benefit from an anaerobic digester is between 1 million and 5 million gallons per day (mgd) of wastewater input, Serfass says. “So if you’re over that, there’s a very high likelihood that you will significantly benefit from having a digester at your facility.” The benefits to facilities between 1 million and 5 mgd will depend on a number of factors, including electricity and gas prices paid by the plant, as well as access to additional feedstocks. Any facility below 1 mgd, however, likely would not realize meaningful benefits.
The majority of the 10 New Jersey plants Serfass cites that currently use their biogas have inputs of below 5 mgd, but the group also includes facilities that process 75 million, 23 million, and 17 mgd.
Ramping Up Ridgewood
The Ridgewood wastewater treatment plant processes only 3 mgd of raw sewage sludge, but still uses its biogas to heat its own digesters, according to Robert Gillow, the facility’s superintendent. The plant is the only wastewater operation in the northeastern New Jersey village of Ridgewood, and its anaerobic digester has been running for more than 40 years, but now more options are being evaluated for its biogas.
“We have more methane than we can use,” Gillow says. So Ridgewood will explore the potential to further the use of that resource instead of wasting it, looking into an electricity-production component. “It’s been put out for bids just to see what it would cost and how much it could benefit the town.”
Gillow says research into state tax incentives is ongoing and Kunze adds that the state does offer grants for biogas projects through its clean energy program.
Even with an added power-production component, however, the Ridgewood treatment plant would still not be capable of producing excess power for the grid. “We’re probably not big enough to produce enough to sell,” Gillow says. “Basically just enough to run this facility, and maybe not even 100 percent.” Kunze says that’s not an unusual position for New Jersey’s wastewater treatment operations. “Generally speaking, a wastewater treatment plant will not produce more energy than it needs,” he says.
Ridgewood is also the focal point of Gruber’s team report, as it illustrates perfectly the progression toward fulfillment of the statewide desire to increase the use of biogas produced from readily-available wastewater. The Renewable Natural Gas Work Group set a deadline of the end of September for its preliminary report on the topic. A final report will be distributed to state legislators and state departments, Specca says, setting the stage for educational seminars on the findings. Specca, along with the rest of the group and the state’s wastewater treatment plant industry, hopes the results will prompt policy development to help New Jersey take advantage of its existing potential to increase both biogas production and use, fostering a robust wastewater-to-energy industry.
“After all,” Gruber points out, “We never run out of sewage, do we?”
Author: Lisa Gibson
Associate Editor, Biomass Power & Thermal