No wastewater treatment plant has the perfect formula for saving energy, but the Gloversville-Johnstown Joint Wastewater Treatment Facility has a strategy that comes close to the ideal.
This 30-mgd (peak) activated sludge plant, 35 miles west of Albany, N.Y., is cutting energy consumption, more than doubling its capacity for on-site electric power generation, and boosting production of digester methane to fuel its combined heat and power (CHP) system. The net result: energy independence by early 2010.
Necessity, says plant manager George Bevington, is the mother of invention. “In 1990, less than 5 percent of our operating budget was for energy,” he says. “A couple of years ago, energy costs consumed maybe 15 percent of the budget. That means energy became a much more important economic driver when you’re trying to run a plant in a cost-effective manner.”
Cutting consumption
The need to hold down costs became critical in the late 1990s when industrial use of the facility was at an all-time low after several area plants closed. Meanwhile, energy costs continued to climb. With the help of funds from the New York State Energy Research and Development Authority (NYSERDA), plant staff hired a consultant to identify ways to reduce power consumption.
“The aeration system was the biggest energy user,” Bevington says. “When two-thirds of your electricity is used in an aeration system, it’s the obvious first place to look.” The system had been sized when industrial loadings were high and energy was relatively cheap. In addition, the plant no longer saw industrial discharges 24 hours every day. That meant the plant needed to automate the system to cope more efficiently with fluctuations in organic loading.
In 2002, the plant invested more than $1 million to upgrade the aeration system. The upgrades include a 450-hp blower from Turblex Inc. and new ceramic fine-bubble diffusers from Sanitaire. The system also uses sensors to monitor dissolved oxygen and control the operation of the air valves as oxygen demand fluctuates with loads.
When it was all said and done, the project reduced annual electricity usage by 30 percent, saving more than 1.3 million kWh per year ($195,000). It achieved payback in five years, rather than 10 years as anticipated.
Optimizing digestion
After a second study funded in part by NYSERDA, plant staff focused on improving anaerobic digestion and capitalizing on biogas. The existing digesters had been built when natural gas and electricity costs were low.
As a first step, the plant eliminated gas leaks in the secondary digester by repairing a misaligned floating steel gas-holder cover. To do so, workers floated the cover to the top of the digester and fixed it in place with steel I-beams and anchors.
Although that resolved the leakage, it eliminated the ability to store gas because the cover no longer floated up and down with fluctuations in gas production. The solution was to build a separate 50,000-cubic-foot dual-membrane gasholder. Inside this sphere, a fabric membrane expands and contracts with the influx and outflow of gas.
In the primary and secondary digesters, the plant installed bubble gun mixers from JDV Equipment Corp. and replaced older mixing systems. The mixers, fed by a compressor that operates on biogas, thoroughly and aggressively mix the digesters’ contents.
The plant also built a 90,000-gallon equalization tank for high-strength waste, which is used as feedstock for the primary digester. Two feed pumps allow operators to control the feed to the digester. The steady, around-the-clock feeding results in steady, more efficient and less variable digestion.
“We do a very slow, continuous feed rate to the primary digester for a more steady input and a more steady output,” Bevington says.
To take advantage of the improved digester performance and increased biogas production, plant staff next addressed the existing 20-year-old CHP system. The project rebuilt the two 150-kW engine-generators so that both could operate continuously, instead of one engine operating at a time.
With the modifications, the unit now generates 1.8 million kWh annually, up from 800,000 kWh previously. By generating 1.8 million kWh on-site, the plant produces $273,000 worth of its own electricity each year. The system meets 40 percent of the plant’s electrical needs and also provides process heat for the digesters and the facility’s Energy Recovery Building. It also eliminates the need to purchase natural gas to heat the digesters.
Adapting to change
Investments totaling $3.5 million in recent years paid off. By 2005, the facility cut its use of purchased electricity by 50 percent. Had the projects not been implemented, the plant would have had to increase its operating budget by $330,000 just to cover the rising costs of electricity and natural gas.
As often happens, the situation evolved, and the operation staff adapted. In spring of 2008, plant loadings began to pick up significantly with increased activity in the food manufacturing sector. With higher loadings and more biogas, there was little question that the plant could generate 100 percent of its own electricity. So the staff made plans to move closer toward its goal of energy independence.
A significant part of the plan is to increase anaerobic digestion efficiency to capitalize on the increased loadings. One step involves a new gravity belt and high-pressure rollers to remove as much water as possible before primary and secondary sludge enters the primary digester. The plant will also recycle a portion of the liquid from the gravity belt process and feed it back to the digester. The new technology will replace a gravity thickener and a rotary drum thickener.
“If the digesters can have more microorganisms inside, they should be able to do more digestion. There should be organisms in there to create gas,” Bevington says. “Hopefully, we’ll be producing biogas more thoroughly. If you’re running too much loading into a digester, some of it isn’t getting digested. Now we’ll be converting more of the sludge to gas, and the less sludge you have, the less you need to haul to a landfill.”
Better technology
To use biogas even more effectively, the plant plans to change out key components of the CHP system. The existing engine-generators will be replaced by two 350-kW units to handle the peak electric load of 700 kW. Waste heat from the engines will continue to supply heat to the digesters. The plant will still purchase natural gas to heat its administration and garage facilities.
“I have much more biogas right now, but not enough generators,” Bevington says. “In 2007, we flared 10 percent of our biogas. By midyear 2008, 50 percent of it went to flare with our two engine-generators running at full production.”
Plans are also under way to construct two dissolved air flotation (DAF) tanks to pretreat the food manufacturers’ wastewater. That will reduce loadings to the aeration system. The food companies’ wastewater is now piped directly to the aeration tank, imposing a large organic load. In the DAF units, fats, oils and grease (FOG) will float to the top, where it will be skimmed for transport to the digesters. The clarified effluent, with greatly reduced BOD, will be fed to the aeration tanks for further treatment.
“What we’re trying to do is treat the solids in an anaerobic environment where it belongs and the liquid in an aerobic environment where it belongs,” says Bevington. “The DAF unit helps us separate those two streams.”
The goal is to reduce the loadings to the aeration system by as much as 70 percent, significantly reducing energy usage.
Everything in place
The plant will have invested some $7 million to improve the process and create more energy savings. The primary components of the investment include the gravity belt and high-pressure rollers, CHP system and DAF units. Bevington is sure the plant will achieve its goal to be 100 percent energy-independent.
“If you can take wastewater that nobody wants, effectively treat it, and discharge it in an environmentally acceptable manner — and do it without the use of purchased electric power — that’s a pretty neat goal,” he says.
Gloversville-Johnstown’s efforts are gaining attention. The plant won the New York State Department of Environmental Conservation’s 2007 award for environmental excellence. In spring of 2008, it won the department’s Operations and Maintenance award.
Plans are to complete the plant upgrades and energy initiatives by early 2010. Bevington and his team look forward to taking energy conservation to an even higher level. “Generating 40 percent of our power is not a bad number, but if we can get to 100 percent that would be even more exciting,” he says. “Everything seems to be falling into place.”
