From: Rabbi Moshe Sofer
To: Unknown recipient
A mikvah received rainwater through metal pipes permanently attached to the building, but with a small detached pipe segment inserted when needed. The ruling authority disqualified the mikvah because metal pipes are susceptible to tumah, violating the requirement of havayaso al yedei taharah, and separated a woman who had already immersed from her husband. The Chasam Sofer surveys five levels of Rishonic opinion on the requirement, rules one must be stringent regarding the three primary stringencies, but identifies possible bedieved grounds for leniency if the small pipe was not the sole means of conveyance or if the mikvah's ladder had perforated rungs qualifying it as pshutei klei eitz subject only to Rabbinic disqualification.
Source: Shu"t Chasam Sofer YD §199