9. Why is the two-tank Point-of-Entry (POE) system significantly better than a one-tank POE system?

  • A one-tank POE system is cheaper in the short term, but with no back-up/safety tank homeowners are at risk of drinking water with unhealthy levels of arsenic during the period after the arsenic begins to break through and before the next testing.
  • The water goes through the first tank and then through the second tank.  We call the first tank the "worker tank" because it does the most work removing arsenic. When the worker tank is new it will remove all the arsenic, but after about one year (depending on the arsenic level and how much water is used), the worker tank's arsenic removal efficiency will start to decline and some arsenic will start to break through the worker tank. When this occurs, the second tank will remove the arsenic, and this is why we call the second tank the "safety tank".
  • Without the safety tank you would be exposed to the arsenic getting through the worker tank.  With only a one-tank system you won't know you're being exposed to arsenic until the next water test is obtained.
  • A properly installed and maintained two-tank POE system will reduce your arsenic exposure to zero, which is the EPA maximum contaminant level goal for arsenic. A one-tank POE system can't meet this goal.
  • A two-tank POE system is also more economical over the life of the system. With one tank you'll need to change the tank as soon as the concentration gets near 5 mcg/l. Otherwise you will be exposed to arsenic levels above the state standard. However, with a two-tank POE system, you can safely conduct once per year sampling and not need to replace the worker tank until the concentration after the worker tank exceeds 5 mcg/l.  Even if the concentration after the worker tank goes up to 10 or 20 mcg/l, the safety tank will remove all of the arsenic before it reaches the taps in your home.