Evaluation of an «extended contact» pilot-scale aerator
Aeration represents the major operating cost in the various aerobic treatment processes intended for wastewaters or other effluents showing a high dissolved organic load, such as swine manure; it is therefore necessary to maximize the utilization of the oxygen provided to any treatment system. Various types of air injectors and turbines have been studied or built in order to achieve an adequate oxygenation of liquid wastes to be treated : porous and non-porous injectors, immersed and surface turbines, the latter rotating vertically or horizontally. The aeration performances of these systems appear to be variable; moreover, these systems are not always well adapted to a substrate such as swine manure. With a view to possible application to swine manure treatment, we have designed an "extended contact" pilot-scale aerator (3000 L). The strategy involved injecting air at the basal end of long coiled pipes in which the manure to be treated was circulated. The objective was to increase the time of contact between the air bubbles and the liquid and, consequently, oxygen transfer to the latter. The internal surface of the pipes was large and could become an effective physical support for microbial attachment, thus promoting further digestion. We evaluated the performance of the system with tap water and compared it with a more conventional aeration system, i.e. a porous-type injector also known as a mini-bubble diffusor. Eight oxygenation parameters were evaluated for each of the two systems oxygen transfer coefficient (KL-a), swelling (SWEL), standard oxygenation capacity (SOC), specific standard oxygenation capacity (SSOC), physical efficiency of oxygen transfer [ɛ, cf. Equation (1)], power consumed (P), specific power injected (SP) and energy efficiency (E). A theoretical simulation of the performances of a hybrid system combining the two aeration principles was carried out. The hydraulic dynamics of the extended-contact system were established through the evaluation of the recirculated flows due to the aerator. The best results obtained were finally compared to those taken from the literature. The main results show the following positive aspects of the extended-contact aerator : 1 - high oxygen transfer coefficients inside the pipes; 2 - high air retention capacity; 3 - good energy efficiency for low air flows. Inside the pipes, values of 230 h-1 were obtained for KL.a. The new prototype retained up to 546 L of air which corresponds to a swelling of 20 %. A raw energy efficiency of 1.9 kg O2/kWh was obtained, and the net value (based open the net power) was 9.2 kg O2/kWh. The prototype, as constructed, gave oxygenation performances globally similar to those obtained with the porous type air injector despite the fact that the internal volume of the pipes, which is of special interest for oxygen transfer, was only 26 % of the total liquid volume. The optimization of the extended-contact system therefore calls for a maximal increase of the internal volume of the pipes as compared to the total liquid volume. Ideally, the internal volume should represent 100 % of the total volume. By doing so, oxygenation performances should be improved because of : 1 - increased total oxygenation capacity due to a better overall KL.a; 2 - increased physical efficiency of oxygen transfer (ɛ) in relation to the length of the pipes (in theory, ɛ could reach 100 %); 3 - decreased injected specific power (SP); 4 - increased energy efficiency of oxygen transfer (E). With such improvements in view and considering the requirement : of the aerobic treatment of highly loaded affluents such as swine manure, the extended-contact aerator appears to be a promising aeration technique that still has, however, to be tested under practical conditions.