Environmental Impacts
Anaerobic digesters are biochemical systems that convert various waste streams into biogas and digestate. While the exact environmental impacts of on-farm anaerobic digestion systems will depend on the manure management system that is replaces and use of the biogas, all on-farm anaerobic digestion systems will have the following benefits:
- Odour reductions;
- Pathogen reductions;
- Improved water quality; and
- Greenhouse gas emission reductions.
Odour
Anaerobic digestion systems provide cost effective odour reduction. Natural fermentation of manure results in production of ammonia, hydrogen sulphide and volatile fatty acids (VFAs). VFAs are responsible for most odour issues. In a sealed and properly functioning anaerobic digestion system, VFAs are metabolized by anaerobic bacteria, reducing the overall odour emissions from the manure. Anaerobic digestion systems therefore result in significantly lower odour emissions compared to baseline manure management practices.
Pathogens
Anaerobic digestion is considered an efficient process to reduce pathogens. In an EPA evaluation of anaerobic digestion systems, a reduction in fecal coliform concentration of more than 99% and a 90% reduction in fecal streptococci concentration was determined.
Water Quality
Anaerobic digestion systems reduce the potential for water quality degradation by removing 70% – 90% of the Biological Oxygen Demand (BOD) in high-strength wastewater by degrading volatile solids available in the feedstock. Anaerobic digestion systems can also enable responsible manure management practices, that in combined with nutrient extraction technologies, could offer practical and sustainable solutions to addressing nutrient management overloading issues and protecting water quality.
Greenhouse Gases
Anaerobic digestion systems reduce greenhouse gas (GHG) emissions in two ways. First, when used in combination with a manure management system that stores manure under anaerobic conditions, anaerobic digestion systems prevent the release of methane, a potent GHG, into the atmosphere. Second, the biogas generated by the anaerobic digestion system can displace the use of fossil fuels that generate GHGs.
Potentially negative environmental impacts of anaerobic digesters that combust biogas include the generation of nitrogen oxides and potentially increased ammonia emissions during digestate spreading.
