Renewable Natural Gas (RNG) Project: Anaerobic Digester Biogas Treatment

Photo Credit: McKinstry

Good things come…to those who persevere! The City of Tacoma’s Renewable Natural Gas (RNG) project at their Central Wastewater Treatment Plant (WWTP) took close to a decade to come to fruition and finally began operation in June 2022. Great partnerships, teamwork, innovation, and perseverance, as well as the alignment of stars, came together for this project to be completed. The new biogas treatment system, designed by KJ, treats biogas produced from anaerobic digesters to create RNG. The RNG is injected into Puget Sound Energy’s (PSE’s) pipeline distribution system to be used by the surrounding community.

Reducing greenhouse gas (GHG) emissions and producing less waste has been a major focus for the City, as documented in their 2030 Climate Action Plan. The City initially contracted KJ in 2014 to assess alternative uses for the biogas generated at the Central WWTP, which, at that time, was used in the plant’s boilers for hot water process needs and to heat buildings; unused biogas was flared (burned off through methane gas burners) and emitted into the atmosphere. Biogas, containing mostly methane and carbon dioxide, as well as trace amounts of other gases, is generated from treating sewage sludge within anaerobic digesters. The biogas alternatives posed in KJ’s 2014 assessment included electrical power production using internal combustion engines, microturbines, and fuel cells; fueling the City’s fleet vehicles; and pipeline injection for delivery to natural gas utilities. Accomplishing any of these alternatives would require much coordination and buy-in from multiple stakeholders, logistical planning, and, above all, financing.

In 2018 the City hired McKinstry, an Energy Services Company (ESCO), to conduct an energy optimization and efficiency study to recommend upgrades to City systems, which included energy upgrades at the Central WWTP, as well as revisiting digester gas as a renewable energy source. McKinstry hired KJ to design the biogas treatment system while they identified unique financing solutions, including a large state grant and a federal tax credit program. The City and McKinstry also brokered contracting mechanisms with utilities in accepting RNG and establishing agreements regarding gas quality standards, pressure, and pricing. One huge star that aligned was the Washington state legislature’s passing of a law in 2019, requiring each gas local distribution company (LDC) to offer RNG to its customers. The change in regulations made the injection of RNG into existing, commercial natural gas pipelines a more viable and attractive option.

The biogas treatment system designed by KJ captures and treats biogas by removing hydrogen sulfide, moisture, siloxane, carbon dioxide, and VOCs to create a pipeline-quality methane (RNG) that is compressed to 250 psi for injection into PSE’s distribution system. Additionally, a unique blending system of tail-gas and natural gas was provided to blend leftover tail-gas, which could not be reclaimed for RNG, with natural gas to be used in the boilers. The enriching of the tail-gas for reuse as a fuel source results in less GHG emissions, since the tail-gas would otherwise be flared.

The RNG project also included retrofitting one of the anaerobic digesters with an external pump mixing system. Finally, diesel was previously used as a backup fuel source for the WWTP boilers. The improvements at the plant included the removal of the diesel fuel source and conversion of the boilers to dual-fuel biogas and natural gas, with biogas/enriched tail-gas and RNG now being the primary and backup fuel sources, furthering the reduction in GHG emissions.

Many stars had to align from a regulatory, financial, and technical standpoint for the City to complete this RNG project – surpassing all of those hurdles was a long process and one that can hopefully be expedited by other WWTPs considering utilizing their biogas as an alternate fuel source.