Sedimentation is one of the most widely used treatment processes in water and wastewater treatment, as well as in many other industries. It works because it is so simple: density works! That is, denser materials will sink in a column of less dense water, and low-density substances will come to rest on top of the water. By allowing a heterogeneous mixture of such substances to rest quietly for several minutes or a few hours, the sludge will settle to the bottom and the scum will rise to the top.
(As an aside, that latter fact also seems to apply to management in many cases!)
Sedimentation provides the time for this separation of densities to occur. However, it is up to the Operator to remove the non-water density substances from the process; if the Operator fails to do so, the process will fill up with sludge until the quality of the water leaving the process is the same as the quality entering. This would be a monumental waste of time, as well as a waste of our sedimentation process. And that would be the Operator’s fault.
Generally, sludge enters the process at a slow, steady rate, but must leave the process in batches. This is due mostly to the mechanical limitations of our sludge removal equipment. The Operator needs to establish a schedule of sludge withdrawal that addresses this difference between sludge inflow and outflow rates.
Subscriber Dan presented me with a math problem that explored this issue. Tomorrow, we’ll walk through an example of such a problem, so that you can see how to establish a proper sludge pumping schedule for your sedimentation process. Here’s what we’ll need:
1. Influent solids level (in mg/L)
2. Effluent solids level (in mg/L)
3. Sludge pumping rate
4. Sludge concentration, in percent solids, density, or specific gravity.
So look up this information for your plant, and we’ll walk through a calculation here at MOST tomorrow.