Keurig K-Cups that are pre-sealed with coffee grounds or mixes can be used only one time. Once the seal at the top of the cup is broken, it cannot be re-sealed.
K-Cups containing coffee grounds are specifically measured for one use only. Running an additional 8 to 10 ounces of water through the K-Cup will only produce a lightly flavored cup of water. If the K-Cup contains any other variety of drink mix, the powder mix will completely dissolve after the first cup.
Adding another scoop of grounds to a used K-Cup is not an option either. In order to add more grounds to the K-Cup the entire seal would need to be removed, which is in place to prevent overflow of water while brewing. In addition, the thin plastic of the K-Cup is not made for extended exposure to hot water. If re-used, the plastic may begin to break down and melt in the filter space.
To combat this, several environmentally friendly companies have created a K-Cup mold made of silicone and wire screen that can be washed and re-used. These cups are made to be filled with loose coffee grounds for multiple uses. The silicone and wire make them dishwasher-safe and long-lasting.