Dodge City’s South Wastewater Treatment Plant in southwest Kansas was built for zero stream discharge because even back in the 1980s, city leaders knew water would become a precious resource.

Today the plant sends effluent from its facultative lagoons to center-pivot irrigation systems that water and fertilize some 3,000 acres of cropland.

More recently, the city has taken to recycling biogas, to substantial benefit. The Warrior Biogas Project at the South Plant produces 1.6 million cubic feet of raw gas per day. After scrubbing, the gas is delivered via utility pipeline to a company that uses it to produce methanol for blending with gasoline for vehicle fuel.

The city expects to achieve full payback on the project in five years or less. After that, the biogas revenue will be available for treatment plant projects, city infrastructure work, development of park amenities like bicycling and walking trails, and tax levy reduction.

The project, conceived just three years ago and built on a fast track, went online in February 2018. It has earned a Project of the Year award from the American Biogas Council, a City Public Improvement Award from the American Council of Engineering Companies of Kansas, and an Infrastructure Innovation Award from the League of Kansas Municipalities.

It also received a 2018 PISCES recognition for excellence and innovation from the U.S. Environmental Protection Agency (EPA).

“Biogas was an energy source that was basically being wasted,” says Ray Slattery, P.E., director of engineering for the city. “In the past three years, we started moving aggressively to utilize it, with help from newly available technologies.”

Lagoon treatment

Dodge City’s two clean-water plants serve a population of about 28,000; both are operated by Jacobs. The North Wastewater Reclamation Facility is a 1.25 mgd (design) membrane bioreactor plant completed in 2011 to serve a growing section of the city. Its effluent is used to irrigate a municipal golf course and to sustain the water level of a lake at the high school that’s used for environmental studies.

The South Plant (7.2 mgd design) isn’t the typical facility producing biogas in concrete circular digesters. Built in 1982 and expanded over the years, it’s a facultative lagoon system that receives on average 2.5 mgd of municipal wastewater and 3.0 mgd of higher-strength wastewater from a National Beef plant that processes 6,000 head of cattle per day.

The flows are treated separately. The municipal influent, at about 60 degrees F, is fed to a pair of anaerobic digester lagoons for a retention time of 14 days and then enters two aerated basins before passing to a pair of facultative lagoons.

The beef plant wastewater goes through screening and then dissolved air flotation for grease removal before discharge to South Plant at about 90 degrees F. There it is treated in the same manner as the municipal wastewater, with one anaerobic digestion lagoon and one aeration basin. All told, the three digester lagoons encompass 7.75 acres, and four facultative lagoons cover 181 acres.

Treated water from the lagoons, some 1.7 billion gallons per year, goes to water alfalfa, corn, sorghum and winter wheat on land owned by Nicholson Ventures, a partnership of Charles Nicholson and his sister Deketa Schuckman.

Gearing up

Starting in the 1990s, biogas from the digester lagoons has been captured under fabric covers, drawn off and flared, at a cost of $10,000 to $12,000 per month for supplemental fuel. Several years ago, those covers were replaced with more durable high-density polyethylene material, 100 mils thick, anchored and sealed to the digesters’ concrete walls.

On deciding to pursue biogas utilization, the city and its consultants moved quickly. The team chose Guild Associates to supply the gas scrubbing equipment and associated vessels. The PEC engineering firm from Wichita designed the facility; UCI of Wichita handled construction under a modified design-build arrangement.

The raw digester gas contains, in addition to 72 percent methane, relatively high levels of water, hydrogen sulfide and carbon dioxide, according to Cody Woods, operations supervisor, who runs the gas project with the Jacobs staff in Dodge City.

Removing impurities

The gas requires significant treatment to meet the requirements of Northern Natural Gas, which serves as the conduit from the treatment plant to the ultimate gas users. The gas first passes through a feed compressor that raises it from atmospheric pressure to 100 psi and removes the moisture. Next comes a Molegate skid (Guild Associates) composed of four separate treatment trains that operate in sequence.

“Proprietary media in each train acts as a scrubber of the gas,” Woods says. “The raw gas goes into the bottom of each train, and methane, due to its molecular size, goes out the top very easily. The H2S, CO2 and the other impurities are captured in the media.”

At the end of each cycle, a valve closes at the top of the tank, and the impurities are drawn off by vacuum compressors and sent to a thermal oxidizer. There, the gases are burned off along with supplemental fuel at 1,400 degrees F, providing full destruction, “basically the same as if we were flaring biogas,” Woods says.

Methane emerging from the Molegate unit is fed to a two-stage product compressor that raises it to 700 psi. “From there, a valve equalizes the pressure of the city’s skid with the gas company line, and the gas is pushed into their system and out for distribution,” Woods says.

Slattery observes, “The line is capable of handling up to 800 psi operating pressure, and that’s why our product compressor is so large. So if someday in the future the utility goes to that higher operating pressure, we will still be able to achieve that and inject our gas.”

On to end users

The gas utility line delivers the gas to OCI Partners, which sends it by pipeline to a location in Beaumont, Texas, where it is converted to liquid methanol. The methanol is then taken by ship to the Netherlands, where it is added to automobile fuel to meet that country’s environmental regulations.

Tanner Rutschman, P.E., city engineer, observes, “When we put the gas into the Northern Natural Gas pipeline, they have all their analyzing equipment right there on site. They’re continuously checking for heat content, oxygen, water, H2S and CO2.

“If our gas were to go out of their spec, they would shut us in. That hasn’t happened since the startup phase. They’ve been good partners with us. They’ve been real helpful. We also have gas chromatographs (Emerson) and flowmeters monitoring the industrial and municipal gas streams so we know the volume and the heat content coming off each side.”

While the OCI Partners business is going strong, another customer waits in the wings. Kwik Trip, which operates service station/convenience stores, plans to use the gas to fuel its fleet vehicles. “We are under contract to sell to them once our facility is certified with the EPA,” Slattery says. “Once that is finalized, the biogas from our municipal digesters will be sold to Kwik Trip, and we’ll continue selling the gas from the industrial side to OCI.”

Paying dividends

At present, revenue from gas sales has the Warrior Biogas Project on track for a payback time of five years on an initial investment of $10 million. “That could decrease once we have our EPA registration,” Slattery says.

Initially, the revenue will be used to retire debt on the gas project and the construction of the North Wastewater Reclamation Facility. “Then it will be used for deferred maintenance of infrastructure, other capital improvement projects pertaining to the city’s infrastructure and quality of life projects for the citizens of Dodge City,” Slattery says.

The Warrior Biogas Project isn’t the only city recycling initiative that’s paying dividends: Water reuse is also beneficial. “The thought was that to recycle and reuse wastewater, instead of using freshwater for crop irrigation, would also help replenish the aquifer,” Slattery says.

“Our potable water comes from the Ogallala Aquifer. The last couple of years have been somewhat wet, and we’ve seen a little bit of an increase in the aquifer water level from Dodge City east. But we still need to be good stewards so the projected life of the aquifer can be extended.”

The solids side

Biosolids management has not been a major concern for Dodge City’s South Wastewater Treatment Plant in Kansas.

“The first part of the South Plant was built in the 1980s,” says Ray Slattery, P.E., director of engineering. “In the early 2000s when we did our expansion, we dredged the solids out of the two lagoons that treat our municipal wastewater. There wasn’t all that much.

“Now we feel we’re getting better treatment of the water, and so our buildup is probably not at as high a rate as before. We put in a bar screen (Fairfield Service Company of Indiana) on the municipal side to help capture larger objects. In addition, National Beef has done a better job of screening its wastewater to catch material that would tend to fill up the lagoons.”

The state permit calls for monitoring only on BOD and TSS because the South Plant is a no-steam-discharge facility. “We are required to do pretty stringent soil testing under the center-pivot irrigators and provide that information to the state,” Slattery says.

“Based on what is measured in the soil, our farmer partner rotates the crops. We add some soil amendments to help maintain the status quo.” That mostly means adding gypsum to offset salt buildup. “The nutrients the crops need are basically supplied through the effluent. It’s been fairly high in nutrient level.”