There's a second meter that tracks the irrigation usage and that's how they subtract the sewer charges for irrigation. Of course you have to pay extra for the

There's a second meter that tracks the irrigation usage and that's how they subtract the sewer charges for irrigation. Of course you have to pay extra for the meter to be installed and there's often an additional "standing charge" for that extra meter in every bill, but it's usually since the sewer rates are quite high, it's worth doing. My municipality uses 75% of the water usage (minus the reading from the meter used for irrigation) as the basis for the sewer charge.