pH Neutralization System Mixer Pumping Capacity

Good mixing is achieved by turning over the volume of the tank at least once every minute. The Turn-Over rate is calculated by dividing the tank volume by the difference of the mixer pumping capacity and the influent flow rate.

pH Neutralization Tank Turn-Over Rate

For example, a 500-gallon tank would require a mixer with a 550-gpm pumping capacity if the influent flow rate was 50 gpm.

pH Neutralization Tank Sizing

Engineers size treatment tanks to allow adequate retention of the influent for pH measurement and subsequent reagent addition. Good engineering practice for pH control requires 15 to 30 minutes retention time. Retention time is calculated by dividing the tank volume by the average influent flow rate.

pH Neutralization Tank Retention Time

pH Neutralization Tank Configuration

A typical pH neutralization tank configuration is shown in Figure 3-1a. The treatment tank inlet and outlet should be at opposite sides, one high and one low with respect to the bottom of the tank. This configuration prevents the influent from floating across the top of the tank without being mixed with the rest of the tank contents (commonly known as short circuiting). In addition, the path from inlet to outlet should be maximized to allow better mixing. Introducing the influent to the bottom of the tank maximizes the flow path through the tank, as shown in Figure 3-1b (Bottom In – Top Out). The opposite configuration is also acceptable but the flow path is shorter.

Figure 3-1a - Typical pH Neutralization Tank Piping

Figure 3-1b - pH Neutralization Tank Inlet & Outlet Location

Tank baffles and cubic tank structures are other design enhancements that can increase the mixing and retention time, and minimize short circuiting. Engineers typically install baffles on cylindrical tanks to prevent whirlpool effects. Offsetting the mixer can also prevent whirl pooling. Cubic tanks are tanks that measure the same diameter as height. Tanks too long and narrow or too wide and short lead to dead areas in the tank.