Determining if Renewable Natural Gas is Right for Your Wastewater Treatment Plant

Renewable natural gas (RNG) is an increasingly significant commodity for municipal and industrial wastewater plants. Economic opportunity and regulatory policies are major drivers for the recent popularity and successful implementation of RNG projects. If you’re weighing whether an RNG project is right for your wastewater treatment plant, start by understanding the basic components, benefits and drawbacks of these projects. Based on these considerations, RNG production may be a sustainable approach to reduce your plant’s carbon footprint while creating many other benefits.

What is RNG?

RNG, also known as biomethane, is biogas that has been cleaned to a methane concentration of 90 percent or higher. Biogas is produced by methanogenic bacteria during the process of anaerobic digestion of organic waste. Biogas from municipal wastewater plants consists primarily of methane but also contains carbon dioxide and other impurities. Lower-quality biogas can be used in boilers or flared, and biogas that has been cleaned but still contains carbon dioxide may be used in engines to produce electricity.

When methane is cleaned to a higher quality and carbon dioxide is removed, it becomes functionally the same as the natural gas used in our homes and in power plants. At high purities, the biogas has its highest value since it can be injected into the utility pipeline as RNG and used as a vehicular fuel.

Is RNG a Viable Option?

The following list is comprised of some basic factors to consider when determining if an RNG project is right for your wastewater treatment plant:

1. Does the anaerobic digestion infrastructure exist or would new facilities be required to process the waste? Many larger wastewater plants already have anaerobic digestion as a means of treating biosolids produced at the facility. Construction of new digesters can be costly and would greatly impact the viability of a potential project.
2. Is the organic material being digested consistent in quantity and composition for the anaerobic digester to operate and produce biogas effectively? Alternative feedstocks such as fats, oils and grease (FOG) or food wastes can also be added to significantly increase the gas production of the digester, although their use can impact the value of the gas produced.
3. Have you considered the disposal of the digested biosolids material? Digested biosolids are the end product of anaerobic digestion and are often used in agriculture applications as a valuable and renewable substitute for commercial fertilizers.
4. Do you have an end user for the RNG? This would be vital, since the gas must be sold or used by the producer to maximize the value. While RNG can be used locally to fuel fleet vehicles, an increasingly common pathway is through coordination with a natural gas utility to access its pipelines and use as vehicular fuel elsewhere.
5. Have you assessed the economic feasibility of an RNG approach? Capital expenses, operational costs and revenue all must be evaluated and balanced to secure financing and determine the feasibility of a biogas recovery project.

Benefits of RNG Projects

There are many reasons to consider an RNG project as an investment. RNG, by its definition, is an entirely renewable fuel and is a green initiative. Harnessing the biogas as a fuel source eliminates methane — a harmful greenhouse gas — as a waste, preventing it from being flared. Emissions from other types of fuels can be offset when replaced by RNG in the fuel market; natural gas, for example, is a cleaner burning fuel than gasoline or diesel.

Federal and state regulations have been established to promote the creation of renewable fuels markets. The Environmental Protection Agency (EPA) developed the Renewable Fuel Standard in 2007, and in 2010 expanded the rule to include fuels from cellulosic, nonfood ethanol. The associated credits from the program are known as renewable identification numbers, or RINs. RNG from a municipal wastewater treatment process qualifies as a D3 RIN, which is currently one of the most valuable categories in terms of credits.

Combining the RIN value with other potential credits, as well as the energy value of natural gas, results in RNG being several times more valuable than non-renewable natural gas. While the movement of fuels, credits and funds can be relatively complicated, RNG projects can result in short returns on investment and generate an ongoing source of revenue for a community.

Barriers to Consider

There are potential barriers to implementing an RNG project. Capital costs for the cleaning technologies and process improvements must be weighed against potential revenues from selling RNG. These projects are highly reliant on government mandates, which could be redefined in the future. The high purity of RNG requires cleaning technologies that can require operation of equipment unfamiliar to plant staff.

Reliability of the feedstock also can be a concern for some plants due to variations in the strength of the typical influent or because of a reliance on waste streams from commercial entities. The classification of the RINs may be impacted by the addition of alternative feedstocks, resulting in more complex management requirements and a reclassification of the RNG that lowers the value of the end product.

While renewable biogas has long been used at facilities for various purposes, the more widespread use of RNG is a relatively new development in the field of wastewater treatment. The potential economic benefits from RNG production can potentially offset some of the costs of maintaining a larger facility and the approach aligns with green efforts currently being promoted in many municipalities and industries.

Sustainability, efficiency, decentralization and innovative generation of revenue are all key factors in a successful RNG project. Plants currently flaring or using methane for heating or electrical generation should consider upgrading biogas to RNG as a way of potentially enhancing the benefit of this valuable renewable resource.

Learn about infrastructure options to consider when implementing renewable biogas at your wastewater treatment plant.