Let's define both terms.  Septic systems are designed to handle sewage and waste water from structures, usually houses.  The main sewage drainpipe empties into an underground, empty concrete tank.  There, the solid waste settles to the bottom to be digested by bacteria before being discharged from the tank into the groundwater.  A tailing pond is a pond that is used to hold the waste products of metal ore mining and processing.  The heavy solid waste settles to the bottom of the pond and is essentially contained so it is not discharged into water systems or populated areas.  The common concern between the two is harmful waste being quickly discharged into the environment where it can adversely affect people.  The septic tank holds waste until it can be processed into harmless byproducts, and the tailing pond holds potentially harmful solid waste for the long term.