Is tetrapotassium pyrophosphate added before or after the filter
you are probably using it as a sequestring agent for corrosion and scale in your network. If it is a case, what I usually saw is dosage before distribution.
Do you mean polyphosphate to control corrosion in the distribution system. Tetrapotassium Pyrophosphate or TSPP should never be added to drinking water as it is toxic.
Corrosion controls are added after the filters.