Aerated lagoons are a standard method of treating dilute wastewater from pulp and paper mills. During the treatment process, settling and degradation of solids should be avoided. Degraded solids that leave the lagoon are difficult to settle in downstream processes, with the result that they often reach the receiving environment. Mixing within the lagoon is one technique that can be used to improve the solids suspension and therefore, the treatment process. CFD has allowed engineers to determine liquid velocities at any point in the lagoon and to assess the mixing efficiency of the lagoon as a whole.

Aeration and mixing processes in a wastewater pond

CFD-RES recently completed a project at Australian Paper's Maryvale pulp and paper mill site using FLUENT for this specific purpose. The collaborative investigation between Brown & Root, Australian Paper, and CFD-RES engineers was conducted in three stages. The first stage examined the natural flows present in the lagoon. It found that there was a significant degree of short-circuiting occurring, with the majority of the flow proceeding along a dividing wall. This finding mirrored the results of earlier flow studies carried out by Australian Paper using surface flotation devices.

The second stage examined the liquid velocities generated by an individual aerator, typical of those used in the lagoon, and one type of mixer, which may be added to the lagoon in the future. The simulations were done in a rectangular block with the depth of the lagoon, rather than in the lagoon itself. The results showed that the aerator provides very good surface mixing, but is relatively inefficient at the lower depths, while the mixer promotes good mixing at the lower depths, but is less effective on the surface.

The final stage combined the results of the first two simulations to produce the overall flow patterns in the lagoon under the influence of both aerators and mixers. Through the use of the FLUENT analysis, engineers were able to develop potential arrangements of mixers and aerators to reduce short-circuiting, minimize the number of stagnant zones, and increase the average water velocity throughout the lagoon.

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