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Understanding and optimizing mixing in a large surface water mixing basin

After Image
Before Image
Surface wateraeration

Questions from the drinking water company

The Dutch drinking water company Waterbedrijf Groningen, produces drinking water from surface water and ground water. The surface water treatment side includes a large mixing basin that serves the following purposes:

  • Peak shaving (you don't want canal water fluctuations to directly impact treatment)
  • Natural purification (impact of sun and biological conversions during the time in the basin)

To reach those objectives, the basin should be optimally mixed. Unfortunately, the big investment was performed without prior CFD analysis. However, optimisation was possible. Specifically, the utility had the following questions:

  • How efficient is the mixing in the basin? (the basin was designed without the help of CFD),what are the major flow patterns, and how can we potentially optimize the mixing? (avoid shortcircuiting and dead zones)
  • What is the impact of the aeration (2 aerators installed),and are they optimally located and operated (e.g. air flow rates)

CFD modelling of the basins

We modelled the basin in 3D, realistically accounting for:

  • Aeration (2 coarse bubble aerators in the basin)
  • The basin specific geometry (recent depth measurements were available)
  • The real flow rates

The input needed to execute the project was very limited:

  • Geometry data (was recently recorded)
  • Inlet and outlet flow rates (known)
  • Air flow rates (known)
  • Placement of the aerators (known)

We injected a 'virtual tracer' in the inlet (not injected in reality!) and the CFD model visualised dead zones and shortcircuiting. 

4 scenarios were run:

  1. Influent flow rate 440 m³/h, aeration off
  2. Influent flow rate 440 m³/h, aeration on
  3. Influent flow rate 880 m³/h, aeration off
  4. Influent flow rate 880 m³/h, aeration on

As shown in the below figures, the impact of aeration is significant. Without aeration, the mixing performance of the basin is very poor, and the whole system changes. The strong coloured stream indicates the influent, shortcircuiting strongly without aeration. A quantitaive comparison of the mixing in all cases is given in the curves. The steep curves (more homogeneous system) show good mixing in the case of aeration. The impact of ifluent flow rate was limited, while higher flow rate led to better mixing.

Virtual tracer test using CFD in large surface water storage pond or basinVelocity vectors shiwing mixing patterns in drinking water storage basin

Dead zones and shortcircuiting in drinking water basin

Final impact of the study

  • Aeration was switched off in winter. We recommended to have also limited aeration in wintertime, as mixing without is very poor.
  • We noticed opportunity for energy saving, by reducing aeration rate
  • The water utility changed the sampling location, as samples were taken in the 'shortcircuiting zone', where the water quality is not representative

Questions about this project?

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