Water hardness varies with geographic locations. Waste water is liable to be any thing. If you have any doubt about your water, conduct an O-ring immersion test prior to installing the mechanical seal. Look around the plant for present or past experience. Look for elastomers in valves, other seals, gages, filters, strainers, etc.
Test is the next step.
If possible, start with two elastomers of the same compound and immerse only one of them in the fluid and leave it there for one to two weeks. You can then compare that O-ring to the one that was not immersed. If the elastomer is not compatible with the fluid it will change weight, shape, or appearance. If neither is satisfactory you will have to use a non elastomer seal.