Optimising Flow and Load Distribution at a Sewage Treatment Works
Civil engineering structures are used to distribute the flow and solid loads to multiple settling tanks or Activated Sludge Plant (ASP) lanes at a Sewage Treatment Works (STW). Achieving a good solids distribution requires adequate mixing to homogenise the solids, but this can be detrimental to achieving a good flow distribution, which normally requires quiescent conditions.
At Stoke Bardolph, a new chamber to mix the effluent from the primary flow and Return Activated Sludge (RAS) from the final tanks was investigated by MMI using Computational Fluid Dynamics (CFD). The mixture of RAS and primary flow was to be equally distributed from the chamber to ten ASP lanes.
The initial concept is shown in Figure 1, where the RAS enters at the centre of the chamber, and the primary flow through the side channel. It was found that due to the direction and momentum of the primary flow, and the lack of mixing between the two fluid streams, there was a good distribution of flow, but a poor distribution of the solids. The solids distribution is shown in Figure 2.
Figure 1: Concept Design
Figure 2: Poor Solids Distribution with the Initial Concept
In order to achieve a good solids distribution, adequate mixing of the two fluid streams could only be achieved when the RAS was fed into the channel upstream of the chamber. Figure 3 shows the location of the RAS feed and contours of RAS concentration, which demonstrates good mixing in the upstream channel. The solids distribution to each ASP lane was within ±5% of the ideal distribution. Figure 4 shows the chamber during construction at Stoke Bardolph STW’s.
Figure 3: Chamber with RAS Feed Upstream of the Chamber
Figure 4: Chamber During Construction (Photograph Courtesy of ukwaterprojectsonline.com)
MMI Engineering’s experience of flow and solids distribution aided the development of the distribution chamber, leading to a system with a good flow and solids distribution over a range of flow rates.
For more information surrounding our flow and solids distribution capabilities in the Water Industry, please contact us on 0117 960 2212 (Bristol) or email us.