Sioux City WWTP to upgrade biogas to renewable natural gas

By Katie Fletcher, Biomass Magazine.

Earlier this year in Iowa, Sioux City Council members unanimously approved a project that will allow the city to capture, clean and compress gas at its wastewater treatment plant (WWTP), then sell it as renewable natural gas (RNG). The council voted 4-0 on Jan. 9 to award West Des Moines-based engineering firm Bartlett & West $1.29 million to perform all services leading up to construction on the project, as well as the engineering construction administration and observation services necessary during construction, startup and commissioning. 

“Bartlett & West has been a key ally during the development of this concept and will act as our design consultant going forward,” said Mark Simms, Sioux City’s utilities director.

Since the plant’s original construction in 1961, anaerobic digestion (AD) technology has served as an integral component of the facility to reliably stabilize biosolids and produce biogas. The facility consists of eight digesters, two sets of four in the East and West complexes, which consist of two primary digesters and two secondary in each set. According to Simms, two of the digesters are not currently operating, but the intention is to eventually convert the two unused into primary digesters in order to maximize biogas production.

Until this most recent announcement, the last upgrade the plant received was in 2008 when measures to improve mixing and digester covers were put in place. Currently, some of the digested sludge is brought to the landfill or is used for land application. According to Simms, the city just recently entered into an agreement with a local land applicator and is now applying digested biosolids to fields that are owned by Sioux City as well as other farm fields in the area that can benefit from the application.

Methane gas is also used within the WWTP as a fuel for building heat and to heat the primary digesters. “The plant currently uses as much of the biogas as possible,” Simms said.  “In the winter, the building heat and primary digester heating demands represent a relatively small fraction of the biogas produced, and the remainder is currently flared. They flare even more during summer months when heating of the facilities isn’t necessary.”

According to Philip Gates, PE with Bartlett & West, they plan to make all of the RNG available for vehicle fueling and will buy natural gas from the local utility as needed in order to maximize the generation of environmental credits. “At this time, we believe it is important to connect to the existing natural gas pipeline infrastructure,” Gates said. “This allows us the flexibility to contract anywhere in the country and to seek the best terms available for the city.”