The fermenter is a reactor that uses raw manure as an input to create volatile fatty acids (VFAs) that are precursors to bioplastics. This is also the first stage of what is often referred to as a two-stage aerobic digester (AD). When integrated into a two-stage AD, this is the first phase, known as the acid phase. While the principles are the same whether or not this phase is combined into an AD, for the integrated dairy manure processing, the two stages must be separate in order to properly partition the products after they leave the reactor.
The driving force of the fermenter is a community of bacteria, called acidogenic bacteria, that create VFAs. The two most important operating variables for the fermenter itself are temperature and solids residence time (SRT). The temperature is important because bacterial reactions are highly affected by temperature – reactions generally occur faster at higher temperatures, but if the temperature becomes too high it can inhibit or kill the bacteria. SRT is the amount of the time material stays in the reactor. This is related to the amount of new “food” the bacteria receive and the amount of VFAs that can be produced from a given amount of manure. Each reactor has an ideal operating temperature and SRT to obtain the highest yields, but the ideal values usually vary between reactors. Another parameter that is important for figuring out how well the fermenter processes the added carbon-containing compounds is the organic loading rate (OLR). This quantity tells how much organic matter is added per unit of volume and time, which helps the operator know how much and what concentration of material is being added at a given time.
The desired result of the fermentation process, as mentioned above, is VFAs. There are several different types that contain varying amounts of carbon. The most common types produced are (from highest to lowest concentration) acetate (C2H3O2), propionate (C3H5O2), butyrate (C4H7O2), and valerate (C5H9O2). These are then separated and fed to the PHA reactor. The residual solids are transferred to the AD.