and either one of the following conditions is met:

pollutants are discharged into navigable waters through a man-made ditch, flushing system or other similar man-made device;

or pollutants are discharged directly into waters of the United States that originate outside of and pass over, across, or through the facility or otherwise come into direct contact with the animals confined in the operation.

Provided, however, that no animal feeding operation is a concentrated animal feeding operation as defined above if such animal feeding operation discharges only in the event of a 25 year, 24-hour storm event.

The term animal unit means a unit of measurement for any animal feeding operation calculated by adding the following numbers:

the number of slaughter and feeder cattle multiplied by 1.0, plus the number of mature dairy cattle multiplied by 1.4, plus the number of swine weighing over 25 kilograms (approximately 55 pounds) multiplied by 0.4, plus the number of sheep multiplied by 0.1, plus the number of horses multiplied by 2.0.

The term man-made means constructed by man and used for the purpose of transporting wastes.