While most digestate produced by on-farm anaerobic digestion (AD) plants in the United States is separated into liquid and solid fractions, further treatment before application to land or sale for use as a soil conditioner is usually limited to composting the dry fraction. However, in Europe pasteurization is commonly used as part of the AD process to allow digestate produced from foodstuff or animal by-products to be used or sold as a fertilizer and soil improver. Some farm plant owners also pasteurize their feedstock or digestate in order to reduce the risk of plant pathogens and weed seeds carrying through in the digestate.
There are a number of factors which suggest that, going forward, the pasteurization of digestate produced by on-farm AD plants in the United States will become more attractive to plant operators and therefore more widespread. Some of these include:
• The increasing use of anaerobic digestion: There are currently around 250 on-farm AD plants in the United States, but there is believed to be the potential for as many as 8,000 on-farm plants nationally1. Such an increase in the use of this technology will also increase the amount of digestate produced, meaning that more land will be treated, and the greatest levels of safety will be required.
• The increasing use of food waste for anaerobic digestion: As AD becomes more widespread as a method of food waste treatment, the amount of digestate produced from non-crop sources will increase accordingly. Such material will need suitable treatment if it is to be applied as a fertiliser or used for other purposes such as landscaping, soil improvement or manufacturing.
• While research suggests that certain plant pathogens and weed seeds, such as tomato or black-grass (Alopecurus myosuroides), are not always destroyed by mesophilic anaerobic digestion at 99oF, pasteurization has been shown to effectively control most plant pathogens and seeds2.
In Europe, pasteurization is more or less the standard treatment for food- and waste-based AD plants. Some farmers also choose to go beyond what is required by waste legislation and use pasteurization as part of their anaerobic digestion process even where crops from their own holdings are used as the feedstock. One such plant is Allen's Farm near Colchester in Essex, England. The company grows around 300 acres of onions, 300 acres of potatoes, 1,000 acres of cereals and 500 acres of corn for its biogas plant.