There are two types of basins used in softening and coagulation Water Treatment. You will only find the Horizontal Basin types on very old systems.
This is because of the much longer detention times (4 to 6 hours) versus the (1 to 2 hours) of the Up Flow type basin. What this means is that the size of Up Flow units are four to six times smaller for the same volume of water treated.
This enables quite a savings in the cost of the Basin. These images beside this text were created with Open Office Vector Drawing tool.
In the flow path of the Horizontal Settling basin water simply moves horizontal through each step until leaving the basin. In the upflow basin raw enters at the mixer turbine pump of the basin, about half way down the basin center, mixes with the chemicals , recirculated sludge. Then it is pumped up through the turbine overflowing the inter tube rolling down the shield. A sludge blanket is formed at the bottom of the basin around the reaction mixing shield part of the basin. water flows through this blanket then to the weirs going to the filters. Some water and the sludges goes under the shield to re-enter the mixing zone. Excess sludge is withdrawn .Mixing,coagulation,flocculation,reacting and sedimentation all take place in the upflow reactor at the same time. Reusing sludge accelerates the flock formation greatly speeding up sedimentation. The result is a much faster process for the volume of water treated.
When softening water you need two hours to make sure that all of your reactions have gone to completion. Other wise they could complete where you don't want say your clear well. Also if you test your water at different points in your basin you will notice a higher pH where the chemicals are initially mixed. As the water goes through the basin , longer detention time , the pH drops as more of the lime is reacted.
Doing jar testing for lime and coagulant dosages, the jar test should be made to mirror your basin as much as possible. RPM of your mixer should match your jar stirrer, rapid and slow mix should be made to match your basin and the same detention times used in the jar test as in your basin.
Turbidity testing will help you determine the best coagulant to use as well as the best dosage.
The next page explains how to calculate dosages for softening.