Abstract
Low-cost aerators relying on the venturi principle to entrain air into flowing water have the notable advantage of contributing both to water mixing and oxygen transfer, making them attractive for wastewater treatment in low-resource settings. This study aimed to characterize the performance of such aerators by describing the impact of different design characteristics, including water flow rate, the number of nozzles used, and the nozzle depth. The study also explored the effect on aeration performance of temperature, total dissolved solids (TDS) concentration, and addition of the archetypal surfactant sodium dodecyl sulphate (SDS). Tests were conducted in a 200 L reactor with 2, 3 or 4 nozzles, at depths of 20, 40 or 60 cm, while circulating water through the aeration device at a rate of 400, 600 or 800 L/h. The configuration that yielded the highest mass transfer coefficient (KLa20 of 20.8 h-1) had both the highest flow rate (800 L/h) and the smallest number of nozzles (2). Nozzle depth had no detectable effect on performance. The configuration with the highest standard aeration efficiency (SAE) had a low flow rate (400 L/h) and 4 nozzles. The effect of TDS concentration was not detected in the concentration range typical of domestic wastewater (300–1 250 mg/L). The effect of temperature on KLa followed a first-order exponential curve, as reported in the literature (θ = 1.02). Addition of SDS was found to increase the KLa20 of the tested aerator design by up to 60% of its value in tap water, in contrast to results from literature. The performance data obtained herein was compared to other types of aerators. Though venturi nozzles were found to be less efficient than other available technologies, it is proposed that using plunging rather than immersed venturi nozzles could increase performance to an attractive level for low-resource applications.
Highlights
Aeration is a fundamental component of aerobic biological wastewater treatment, as it allows microorganisms to populate the wastewater and consume unwanted pollutants
While KLa20 allows for the comparison of the relative speed of oxygen transfer, it is not the most appropriate way to describe the efficiency of aeration based on the total volume of air entrained or based on energy input
Since this information could be highly pertinent in contexts where energy is in short supply or cost-prohibitive, this study uses standard oxygen transfer rate (SOTR), standard oxygen transfer efficiency (SOTE) and standard aeration efficiency (SAE) as additional metrics to describe aeration performance
Summary
Aeration is a fundamental component of aerobic biological wastewater treatment, as it allows microorganisms to populate the wastewater and consume unwanted pollutants. Common aeration technologies include bubble diffusers, which use compressed air to produce large numbers of bubbles inside reactors, and mechanical aerators, which disturb the water surface and entrain air bubbles into the bulk of the process water In both cases, the goal is to increase the oxygen mass transfer rate by increasing the air-water interfacial area, which is where oxygen transfer occurs. Reactors fitted with bubble diffusers may need to supplement their reactor design with mechanical mixers to ensure sufficient mixing for aerobic wastewater treatment The reliance on such bulky and expensive devices is a problem in low-resource areas where the availability of service parts or qualified maintenance workers is difficult to guarantee. Aeration through a device making use of the venturi principle is an interesting option, as it is relatively cheap, requires few mechanical parts (only a pump is required), and creates both the air bubbles and the mixing required for aerobic wastewater treatment
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