Offhand, I do not see any specific literature details for water/food process lines. I guess the micro. requirements for potable water represent a logical reference. I would expect yr local Public Heath Department to have such a set of micro. requirements ?
Opinions regarding Legionella action limits for water seem to vary significantly between locations and also with respect to application,eg cooling towers,domestic etc, for example, here is OSHA -
(note the caveat - "Remember that these numbers are only guidelines, and the goal is zero detectable Legionella in a water source.")
and 2 examples for UK -
The current UK HSE literature is accessible here -
You probably need to find out which reference source is regulatory applicable to yr location.