The effect of gas to liquid ratio on carbon dioxide removal and heat loss across a forced ventilated trickling filter

Abstract The objective of this study was to investigate the effect of the gas: liquid ratio (GLR) on carbon dioxide removal and heat loss across a forced-ventilated trickling filter (TF) used as bioreactor. The effect of 8 different GLRs (1.2 - 15.7) on carbon dioxide removal and heat loss from water passing a TF were tested in random order. The TF was part of a pilot scale fresh water intensive recirculating aquaculture system and had a media bed height of 1.8 m, a diameter of 0.9 m, a specific surface area of 200 m2/m3 and was operated at a fixed hydraulic surface load of 9.42 m3/(m2.h), a mean inlet water temperature of 27.0 - 27.5 °C, and a mean inlet pH ranging from 7.02 - 7.12. For each GLR, CO2 removal from water was calculated from CO2 measurements in the inlet and outlet air of the TF using an online infrared probe (developed for agriculture and greenhouses) and from 12 water measurements done once every 3 h during 2 subsequent days. The water inlet CO2 concentrations varied between 4.5 – 9.0 mg/L (calculated from total inorganic carbon and pH). The results showed that the CO2 removal from water to the air in a TF increased from 3.1 mg/L at a GLR of 1 to 6.1 mg/L for a GLR of 15. The relationship between GLR and CO2 removal could be described as an asymptotic curve with the following equation C O 2 r e m o v a l f r o m w a t e r ( m g L ) = 6.44 - 5.82 ( 0.648 G L R ) (R2 = 0.91). A linear relation between total heat loss across the TF and the applied GLRs showed an average total heat loss of 33.2 kJ per cubic meter of air for all GLRs. Of the total heat loss on average 91% was latent heat loss and 9% sensible heat loss. This study showed that: (1) GLRs above 5 will not result in significant additional CO2 removal; (2) energy loss at a GLR of 5 is, under our experimental conditions with a mean inlet air temperature of 22.6 °C and an mean inlet air relative humidity of 66.9%, 26.0 kJ per g CO2 removed.