Hydrodynamics of a hydroponic bed bioreactor with different substrate media

Water flow through hydroponic media beds has seldom been characterized, and little is understood about the effects of different substrate media and flow rates as they pertain to mixing within. The hydrodynamics of four hydroponic substrate media were assessed in a small-scale hydroponic media bed bioreactor using tracer tests and mixing tests. Four media commonly used in hydroponic media beds, including 12.7-mm (0.5 in) fractured rock, 25.4-mm (1.0 in) fractured rock, 25.4-mm (1.0 in) lava rock and 12.0-mm light expanded clay aggregate (LECA) were selected and placed into identical rigid plastic containers to test the mean hydraulic residence time (mean residence time (MRT)) and vessel dispersion numbers (dispersion numbers) at two water levels and two flow rates. Sodium chloride tracers were used in these tracer tests and electroconductivity (EC) was measured every second with an EC probe at the outlet to generate residence time distribution curves for each bioreactor under each set of conditions. Expanded clay had a 16–29% longer mean residence time in the low water level, low flow treatment compared to the other media. It was also the most consistent overall and did not show any significant differences between flow rates or water levels. The mean residence times of the small fractured rock treatments got longer as the water velocity decreased, with the highest mean residence time achieved at an estimated water velocity of 0.38 cm s⁻¹. Dispersion through the expanded clay or large fractured rock was not affected by flow rate or water level. Dispersion in the small fractured rock treatment only appeared to be greater at the lowest water velocity tested. Mixing time was also significantly shorter in expanded clay treatments at the lower water level. In the mixing tests, small fractured rock was comparable to the expanded clay at the low water level, low flow treatments but mixed 59% slower than in expanded clay treatment when the velocity increased. Overall, expanded clay performed better than the other substrate media tested, while the small fractured rock only performed well at lower estimated water velocities. The large fractured rock was substandard to both of those media and the lava rock was inconclusive but appeared to slow the movement of the tracer due to its intricate structure.