a) Unless otherwise approved by the Agency under Section 604.145(b), a minimum chlorine contact time of 60 minutes must be provided at all plants treating surface water, groundwater under the direct influence of surface water, groundwater with basins open to the atmosphere, and groundwater obtained from unconfined, fractured bedrock. The equivalent baffling factor must be greater than or equal to 0.3 to prevent short-circuiting. The 60-minute contact time must be calculated based on the following formula:

b) For the purposes of this Section, contact time is measured as follows:

1. 1) When the treatment process includes filtration, contact time is measured as the time following filtration of the water until the water reaches the first user.
2. 2) When the treatment process does not include filtration, contact time is measured as the time following chlorination of water until the water reaches the first user.