Running on treadmills would be easier for your knees than running outside on concrete or software outside paths that might have stones and other obstacles that increase the rate of twisting your knees.
Besides the surface, however, there are other things to take into consideration: Weight and shoes, technique, but most importantly, a very conservative training schedule that allows your body to recover and become stronger.
Weight and shoes
When you run, the impact on your ankles, knees, and hips is up to 5 to 6 times your body weight. Logically, the less your weight, the less impact on your joints.
However, this is most important for people that are overweight. More cushioning support in your running shoes can absorb some of the impacts. It can really pay off to have shoes fitted to your feet in a professional running shop where you test several pairs of shoes before you decide which one is best.
Treadmill running technique
Altering and improving your running technique can help take the load off your knees.
There is some evidence that the new barefoot running shoes can help reduce ankle and knee problems because less support in the shoes causes people to run more on their forefoot instead of rolling the foot from heel to toe.
Do notice though that It's not the shoes that reduce the impact, but it's the technique.
Treadmill training schedule
In our experience, the number one reason for all running injuries is that most people who start running want too much too soon.
They only sprint, run too long too often, and are exhausted after every single workout.
This is not a healthy way to train your running skills, especially people who are generally in a good shape but not runners tend to overestimate their bodies.
We know how stupid it can feel to run very slowly and with walking breaks, but if you want your body to get used to the impact of running, you have to.
Otherwise, you have a really large risk to injure yourself. If you have knee problems, my advice would be to seek out a beginner's schedule stick to it for 3 to 4 months, and see if that works with your knees.
Don't jump the schedule. Stick to it.
You can exhaust yourself and push your limits in the pool or do other activities that don't have such an impact on your knees.
Hopefully, you will feel how your knees get stronger and stronger, and after a few months, you might be able to feel what it feels like to run 15 thousand without a hint of pain.