The wastewater treatment sector has used both aerobic and anaerobic treatment for many years; therefore, it is not surprising that it is one of the largest generators of biogas from anaerobic digestion in the U.S. The solids from more than 1,200 plants are treated via anaerobic digestion (AD).1 Despite these impressive figures, the sector has not yet maximized the potential of this technology.
While 48 percent of all wastewater is treated with AD, only 5,127 water resources recovery facilities (WRRF) from nearly 15,000 facilities across the country are listed as producers of biogas or sending materials to other AD plants for treatment.2 These tend to be the largest facilities serving large metropolitan areas. It is estimated that only 85 percent of the facilities that are using AD actually utilize the biogas produced, with only 16 percent producing electricity and just 1 percent producing upgraded biomethane for further use.
Benefits of anaerobic digestion
The U.S. Environmental Protection Agency (EPA) stated that the use of AD can help it meet a number of goals including its target to reduce food waste by 50 percent by 2030.3 Some of the strategic EPA policy goals supported by the increased use of AD include:
- Taking action on climate changes — capturing and combusting methane via AD reduces greenhouse gas emissions
- Protecting America’s waters — the anaerobic digestion of sewage sludge from wastewater treatment facilities plays an important role in cost-effective wastewater solids treatment and management at thousands of facilities
- Supporting sustainable development by reducing fossil fuel consumption, producing local sources of energy and conserving scarce resources such as fertilizers and soil amendments
- Reducing pollution by diverting organic wastes from landfill and agriculture
The EPA also indicated that as the AD sector grows, it may provide additional benefits and help deliver cross-agency strategies on food waste and agricultural efficiency, for example.
Anaerobic digestion from wastewater potential
According to the Water Environment Federation (WEF), Hawaii, Washington and California have the highest percentages of WRRFs that process solids through anaerobic digestion, while New Mexico, Illinois, California and Hawaii have the highest percentages of wastewater flow being treated by AD. The most common use of produced biogas is for digester heating (some 48 percent of WRRF anaerobic digestion facilities use gas in this way), and then building heating, power generation or driving process machinery (8 percent), with just 1 percent used for pipeline injection.1
The National Renewable Energy Laboratory (NREL) suggests the wastewater sector could produce as much as 2.3 million metric tons of biomethane a year.4 Other estimates suggest the U.S.’s aging water infrastructure will require an investment of about $600 billion over the next 20 years if it continues to reliably transport and treat wastewater.5 Many older wastewater AD facilities are now in a position to upgrade and take advantage of the latest technology. All of this means is, there is a huge potential to increase energy production from America’s wastewater sector.
When the potential codigestion of food waste is considered, the economics of green gas production become even greater. Research at California State University suggests that mixing 25 percent food waste with 75 percent municipal wastewater sludge can increase biogas production by as much as 60 percent compared to digesting sewage alone. With only a fraction of sites currently codigesting other wastes, the potential for the sector to capitalize is significant.
Take the heat
The use of AD for wastewater treatment has traditionally been driven by the treatment needs of the plant rather than the economics of gas production, energy generation or digestate sales. However, even if this remains the key driver for the installation of AD technology at WRRFs, there are other reasons to make any investment in AD as cost-effective or profitable as possible.
In many cases, the economic decision about whether to invest in AD depends on the overall efficiency of the final project. Techniques such as heat recovery using heat exchangers enable a resource — that would otherwise go to waste — to be used to preheat feedstock or digesters to improve gas production efficiency. Equally, this heat can be used for pasteurization and concentration processes, increasing the value of the digestate coproduct produced.
A well-designed system could recover and use 40 percent of the heat produced by a wastewater AD plant. This free heat provides a useful bonus when evaluating any investment in AD, and it is something that potential developers should consider from the beginning of their planning process.
- Biogas Production and Use at Water Resource Recovery Facilities in the United States (2013). http://www.casaweb.org/documents/8-5-2013_wef-phase1_biogas_data_results.pdf.
- Water Environment Federation Biogas Data. http://www.resourcerecoverydata.org/biogasdata.php.
- Anaerobic Digestion and its Applications. https://www.americanbiogascouncil.org/pdf/AD%20and%20Applications-finalcls.pdf.
- National Renewable Energy Laboratory (NREL), Energy Analysis Biogas Potential in the United States. https://www.nrel.gov/docs/fy14osti/60178.pdf.
- Energy-Positive Water Resource Recovery Workshop, April 28-29, 2015. https://www.energy.gov/sites/prod/files/2015/10/f27/epwrr_workshop_
Matt Hale is the international sales and marketing director for the HRS Group. He is responsible for overseeing and developing the company’s strategies for business development and marketing. Hale has a background of more than 20 years within process and mechanical engineering and aims to provide effective heat transfer solutions to overcome industry challenges. With a focus on renewable energy, he works closely with the global sales teams to achieve their goals.