Recent comments by the American Biogas Council (ABC) on the first Renewable Volume Obligations (RVOs) for the Renewable Fuel Standard (RFS) for 2020, 2021 and 2022 show that the Unites States is finally recognizing the huge potential that anaerobic digestion and biogas has to contribute to a renewable, low carbon economy.
Although the final fuel volumes set out by the Environmental Protection Agency (EPA) may be disappointing and lower than the industry was expecting, there ABC statement contains much to be positive about, not least the high levels of investment and growth in the American biogas sector. According to the ABC, ‘new renewable natural gas (RNG) projects have increased by 47% in the last year, on—farm projects have increased by 21%; and major companies have announced investments in new RNG projects totaling over $2B – twice the industry’s total investment in 2020.’
This level of investment provides a clear message about the technology and it’s potential. However, it is important with the focus on the production of RNG not to forget about the other output of anaerobic digestion – the production of digestate. Frequently in the U.S. digestate has been viewed as an after-thought and frequently used as livestock bedding. However, such an approach greatly underestimates the environmental and financial benefits that digestate has when used as a high-quality biofertilizer and soil conditioner for crop production.
The use of digestate as a sustainable biological fertilizer can be improved with pasteurization to prevent the spread of pathogens, weed seeds and crop diseases. One of the most energy-and cost-efficient methods to pasteurize digestate is the HRS Digestate Pasteurization System (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurization is possible using surplus heat while allowing additional thermal regeneration levels of up to 60%.
The standard 3-tank DPS provides continuous pasteurization, with one tank being pasteurized while one is filling, and another being emptied. The DPS uses a double-tube heat exchanger to heat the digestate to 75 °C above the required pasteurization temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurized. The tanks can also be used individually, for example to allow for routine maintenance.
Another system to improve the management and handling of digestate is the HRS Digestate Concentration System (DCS), which uses patented technology to reduce the volume of liquid digestate up to 90%, raising the concentration of total solids to 20% solids while also maximizing the nutrient content, using heat from the AD plant’s CHP engine. Lower water content also means reduced transport costs and field traffic, bringing further benefits in terms of reduced compaction caused by the application of the digestate to land.
Another benefit of the DCS is odor and ammonia control, which helps increase the nutrient content of the digestate. The high temperatures and vacuum conditions needed to concentrate digestate can cause the release of ammonia, largely responsible for the odors associated with digestate. The DCS overcomes this by acid-dosing the digestate with sulfuric acid, thereby decreasing the pH levels. This turns the ammonia into ammonium sulfate, which is not only less odorous, but is also an ideal crop nutrient.