I am wondering about this because I drank a diet coke the other day, it had no fat, no sugar, no calories... Is it actually healthy to drink?
Short: NO. It's not healthy neither for you nor for your environment. E.g. there is study (Jotham Suez et al., 2014), which indicates that there are problems with sugar substitutes when it comes to glucose metabolism. E.g. Acesulfame which comes often together with Aspartame is difficult to treat in wastewater. Just drink tap water or maybe mineral water.