A portable vacuum degasser was developed to satisfy seasonal hatchery pretreatment needs. Dissolved-gas pressures in water exiting a packed column were regulated automatically with a unique feedback control loop incorporating a gasometer, pressure transducer, electronic (PID) controller and an electrically actuated pump discharge valve. The flow capacity of the system was 160 l/min with a total power requirement of 0·81 kW. Following controller tuning, field tests demonstrated the ability of the feedback loop to set appropriate column vacuum levels quickly in response to varying inlet dissolved-gas pressures or changes in selected controller set points. The degasser's ability to reduce dissolved nitrogen (DN) and increase dissolved oxygen (DO) concentrations was also assessed at four or five pure-oxygen feed rates under each of four column vacuum levels (−4·5, −9·0, −14·6 and −20·0 cm Hg). Performance was then compared with that predicted with a multicomponent gas transfer model. Relative error of model projections averaged just 7·7% for DO and 2·3% for DN (n = 19). Effluent DN ranged between 60·6 and 96·4% of saturation concentrations at 8·8°C with the influent DN at 135% of saturation. The high DN desorption rates achieved allow side-stream pretreatment with blending.
