My 46 Year Old Son Is A Truck Driver And

My 46 year old son is a truck driver and often caries an empty plastic jug in his truck in case he has to pee and there are no facilities. While helping him unload the cab of his truck when he came home for a 2-day break from the road I noticed black segment like coarsely ground black pepper in the jug along with his urine. What could the black sediment be cause from?