Planned sewage improvements at Weston-Super Mare included a new, four-lane activated sludge plant.
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Upstream of the aeration lanes, an inlet zone referred to as an anoxic selector tank (AST) received settled sewage effluent from the primary tanks and return activated sludge (RAS) from the final settlement tanks.
Within the AST, the two streams of fluid and solids were mixed and distributed through two submerged penstock weirs. In order to achieve an even solids distribution through the penstock weirs, adequate mixing was required within the tank to homogenize the solids.
The RAS discharges through a bellmouth connected to a vertical pipe within the tank, and the settled sewage liquor discharges through a horizontal pipeline connected to the side of the tank close to the base. Geosyntec was asked to investigate the solids distribution and mixing of the solids using computational fluid dynamics (CFD).
The concept design showed that at high flow, when the momentum of the discharge through the RAS pipe was high, the RAS would tend to short circuit to Outlet 2 and therefore generate an uneven distribution of RAS. The flow bias towards Outlet 2 was due to a skewed velocity profile at the bellmouth which was caused by the upstream bend, and insufficient distance between the bend and bellmouth for a symmetrical velocity profile to develop. Insufficient mixing within the tank, as shown by the large variation in RAS concentration, results in an uneven distribution of the RAS.
To improve mixing within the tank, the vertical pipe was removed. This was so that the RAS discharge was located at the base of the tank, meaning that the two fluid streams interact close to the point of discharge where the kinetic energy (and hence mixing energy) is greatest. Two baffles were also included in order to generate an inner mixing zone, and to prevent any short circuiting of solids. A significantly more uniform solids distribution as a result of these changes, and hence a more uniform distribution of solids through each of the submerged penstock weirs.
The use of CFD, penstock weirs, and Geosyntec’s experience of mixing and solids distribution, aided the improvement of the mixing tank, resulting in a system with a good solids distribution over a range of flow rates.