Generally, arsenic removal in conjunction with iron removal requires that the ratio of iron to arsenic be 20:1. The arsenic is actually adsorbed onto the iron precipitates. How much iron is present in your water?

In order for consistent arsenic removal to occur, oxidants generally need to be continuously fed upstream of the greensand filter, rather than just during regen. How is your system set up? A flow diagram would be helpful.

Thx so much for the reply. Raw water was:
Iron - .225 mg/l

Sorry for my lack of skill on flow diagram, I certainly appreciate the help.

First, I would start with a water test. It is possible your water has changed substantially, impeding arsenic removal. If pH, silicates, orthophosphates or organic matter has increased, aresenic removal will decrease.

If water conditions are fine, then: This is going to sound contrary to your iron-removal purposes, but if you add enough ferric chloride to your water prior to your greensand filter to bring the ratio of iron to arsenic up to 20:1, you should be able to remove 80-90% of the arsenic prior to the Layne system. You will need to consult with the folks that provided your greensand unit to ensure it is capable of removing the additional iron.