Evaluation of new carbon dioxide degassing equipment in RAS

Abstract

High carbon dioxide concentrations have adverse effects on fish physiology in recirculating aquaculture systems, making carbon dioxide removal essential for an economically sustainable production. Dissolved carbon dioxide is more difficult to remove from water than other dissolved gases due to its complex liquid reactions and high solubility. Commercial degassers remove high loads of carbon dioxide from intensive land-based fish farms; however they are energy demanding due to energy draining pumps and blowers. This study aimed to design a novel carbon dioxide degassing technique. The design was based on the principal of the old Inka aerator. A degasser without lifting height, comprised of perforated plates with carbon dioxide rich water layers on top and air coming from directly underneath. Two perforated plates were used, an R1 T2 plate with an open area of 23% and an R3 T5 plate with an open area of 32%. Due to fragility, the R1 T2 plate was provided structural support from the R3 T5 plate. The plates were jointed together and placed in a 60 L, polyethylene container. The container was slightly angled, stopping the plates about 10 cm from its bottom. An airflow inlet connected to a blower and an outlet for drainage were drilled in the container, below the perforated plates. A waterflow channel was created with metal pieces and duct tape. Perforated area was 0.0551 m2 and total surface area was 0.0841 m2 within the channel. The plates were sealed with duct tape, ensuring that air only escaped through the perforated area during degassing. The airflow was controlled with a diaphragm valve and calculated from measured air velocity and pipe diameter, while water was weighed on a scale, and the degassing time assessed with a stopwatch. Tests were performed on three different airflows; 12.5; 9.4; and 6.2 L/s, at three water volumes; 1 L, 2 L, and 4 L, and two degassing durations; 2.5, and 5 seconds. 20 ± 1 mg/L dissolved CO2 batches were prepared for all tests and poured onto the running degasser. Measurements for dissolved CO2 and pH were performed before and after degassing each test. Three tests were performed for every combination of airflow, water volume, and degassing duration for increased result reliability. Degassing 1 L samples at 12.5 L/s airflow for 5 seconds illustrated the highest CO2 removal, removing 19 mg/L out of 20 mg/L. The lowest CO2 removal was obtained from degassing 4 L samples at 6.2 L/s airflow for 2.5 seconds, removing 8 mg/L out of 20 mg/L. By increasing the airflow, decreasing the water volume, and increasing the degassing duration, the CO2 removal increased and vice versa. A strong linear relationship was exhibited between the decreasing CO2 concentrations and increasing pH levels from 6.0 – 7.1 at concentrations from 20 mg/L down to 1 mg/L.