The novel treatment system combines additional treatment steps with the anaerobic digestion systems promoted by the U.S. Environmental Protection Agency and Innovation Center for U.S. Dairy in order to create value-added by-products with economic value. This system splits the usual two-stage anaerobic digester into two physically different steps. The main purpose of this is to be able to capture volatile fatty acids (VFAs) that are produced in the first treatment step (the fermenter) and divert them to a separate reactor. This reactor converts the VFAs to a plastic precursor called PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)), which can have chemical properties similar to the plastics in milk jugs (HDPE – high density polyethylene) or flexible films (LDPE – low density polyethylene). Byproducts from the anaerobic digester may be extracted and sold, such as fibrous material suitable for animal bedding and nitrogen and phosphorus-containing fertilizer products.
This system also produces algae as a source of added biomass for the fermenter and a way to dispose of the excess nutrients (nitrogen and phosphorus) associated with manure treatment. Once nitrogen is present in soils and water bodies, the microbial processes of nitrification and denitrification create nitrous oxide that can be released to the atmosphere. Nitrous oxide is a concern since it has 310 times the global warming potential of carbon dioxide. The nitrogen in the soil may also convert to nitrate, which can leach into groundwater. Phosphorus runoff into surface water is also a pollutant concern. The algae are grown with CO2 from the second step of anaerobic digestion (also called the anaerobic digester) as the carbon input. This integrated manure treatment system provides a way to reduce GHG emissions and adverse impacts to surface and groundwater quality, while generating additional revenue. The benefits of this integrated system are twofold: (1) create more high-value products, especially bioplastics, and (2) create a GHG-negative system by sequestering carbon in algae biomass and bioplastics. The system modeled by DAIRIEES is designed to be a comprehensive way to treat dairy manure, create high-value products that can be sold or reused, and reduce greenhouse gases from the dairy industry. This integrated system is currently being tested and optimized at the laboratory scale. The DAIRIEES model developed for this project provides a way to estimate of GHG emissions reduction and costs/revenues associated with implementing the treatment system at full-scale.
