Bioreactor flow rates combined with the design factors media porosity and bioreactor flow volume dictate the retention time in the reactor. Very low retention times may not be sufficient to reduce the amount of influent drainage dissolved oxygen to a level which would allow denitrification to proceed, whereas very high retention times would provide excellent NO3 - removal, but also the potential for oxidation reduction (ORP) conditions indicative of undesirable processes like sulfate reduction (Blowes et al., 1994; Robertson and Cherry, 1995; Robertson and Merkley, 2009; Van Driel et al., 2006b) and mercury methylation (Hudson and Cooke, 2011). Christianson, Laura Elizabeth, "Design and performance of denitrification bioreactors for agricultural drainage" (2011). Graduate Theses and Dissertations. Paper 10326. http://lib.dr.iastate.edu/etd/10326.
Residence times for water in the bioreactor will change based on temperatures. Chun et. al (2010) observed a 55% reduction in nitrate load with a residence time of 3 hours, while Christianson et al. (2011) reported a 30 to 70 percent reduction with residence times of 4 to 8 hours.
Cooler temperatures reduce microbial action, reducing denitrification processes.
Dr. Richard Cooke, from University of Illinois, recommends designing systems to remove 50% to 80% of the nitrate load based on the tile flow that will not be exceeded 90% of the time.
A design curve has been established through University of Illinois research which has been used to help develop engineering design criteria for bioreactors.
Lake Decatur and Lake Bloomington Watersheds
An interactive bioreactor design program has also been developed by Richard Cooke and N.L. Bell at University of Illinois, and is available through the Illinois Drainage Guide.
The following shows an example of the bioreactor design program: