Australian Paper's Maryvale mill is Australia's largest integrated pulp and paper producer. As part of the process, 55 Million liters/day of wastewater are produced and treated in a secondary system. An aerated lagoon forms the main component of the wastewater treatment system and a major problem with the lagoon is settlement of solids due to inadequate mixing. Historically, mixing effectiveness in aerated lagoons was calculated from emperical rules based on power input.
In order to gain insight into the mixing process in the lagoon, a
CFD simulation was performed. The initial investigation examined the flow field generated by the wastewater inlet flow. The results showed that the Coanda effect (whereby a jet will "stick" to a wall) caused the inflow jet to be redirected towards a dividing wall setting up a recirculation back to the inlet along the outer bank of the lagoon.
The second investigation examined the effects of aerators installed in the lagoon. Aerators sit at the surface and draw water up from the depths and expell it radially. The investigation showed that the aerators were providing good mixing at the surface, but were ineffective near the bottom of the lagoon.
A solution was proposed which used a combination of aerators and mixers. Mixers sit near the bottom of the lagoon and pull water down from above and expell it radially. The results showed that the combination of aerators and mixers provided excellent mixing through out the lagoon.