Peak flow handling and equalization tank

Peak flow handling

Unlike conventional activated sludge process, all the incoming wastewater must pass through membrane. Therefore, the flux loss during peak flow event can directly lead to the failure of the plant. Therefore, design engineers determine the number of membrane modules required considering the duration and the extent of the peak flow (or flux). However, installing enough membrane surface areas is often not cost effective especially when peaking factor (peak flow divided by average flow) is high. There are two known options.

  • Conventional activated sludge (CAS) process can be installed parallel to MBR, where incoming water is split between the two processes. During peak events, more wastewater can be directed to CAS temporarily, where the increased feed flow is treated with an expense of potential TSS and COD leak from the clarifier. This option is particularly feasible when wastewater needs to be partially recycled since MBR can produce recycle water while CAS produce discharge water.
  • Enhanced coagulation and clarification can be considered to treat the excess wastewater during peak flow events. Inorganic coagulant can be used to remove insoluble COD and phosphorous, but nitrogen removal can a concern.
  • Extra air can be supplied to membrane to scour membrane more vigorously as discussed in Energy Optimization.

Equalization tank size determination

Equalization tank is an effective mean to handle hourly peak flows in a day, which can reduce the amount of membrane modules significantly by alleviating the diurnal flow patterns commonly found in municipal plants. The optimum size of the equalization tank can be determined using various methods (Metcalf & Eddy, 2002), but economics, space availability, maximum holding time without affecting biology in the subsequent biological process, etc. are also dominant factors affecting the actual tank size in real world.

The one way of determining the optimum equalization tank size is a graphical method shown in Fig. 1.

  1. Cumulative volume of daily flow is plotted over a day or the desired time period.
  2. Draw a line from the origin to the last data point. The slope of the line indicates the average flow of the day.
  3. Draw two tangent lines parallel to the average flow line. One line must be farthest to the left and the other must be farthest to the right from the line.
  4. Measure the vertical distances from the point of tangency and the average flow line.
  5. The sum of the distances “a” and “b” is the equalization tank volume required.

Design3Fig. 1. Plot for equalization tank volume determination (Karia and Christian, 2006)


© Seong Hoon Yoon