When treating well water containing soluble iron, when should polyphosphates be added?

The addition of polyphosphates before chlorine in the treatment of well water containing soluble iron is crucial for effective iron control. Polyphosphates act as sequestering agents that prevent soluble iron from precipitating and forming rust in the water. By adding polyphosphates first, they coat the iron particles and keep them in solution, which helps to ensure that the iron does not oxidize or precipitate out of the water as soon as chlorine is introduced.

Once chlorine is added afterward, it primarily acts as a disinfectant, but if iron is already oxidized, it can lead to the formation of iron deposits and clogging in the distribution system. Thus, by treating with polyphosphates first, it allows the subsequent chlorination process to function more effectively without immediate interference from iron compounds, resulting in clearer and safer water.

This sequence is important because if the chlorine is added first, it can react with the soluble iron, causing it to oxidize and potentially form sediment. Therefore, the timing of polyphosphate addition is a key factor in ensuring that the whole treatment system operates efficiently in managing soluble iron levels.